Computational and Theoretical Chemistry 1036 (2014) 61–71

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Computational and Theoretical Chemistry

journal homepage: www.elsevier .com/locate /comptc

On carbon dioxide capture: An accurate ab initio study of the Li3N + CO2

insertion reaction

http://dx.doi.org/10.1016/j.comptc.2014.02.0222210-271X/� 2014 Elsevier B.V. All rights reserved.

E-mail address: varandas@uc.pt

A.J.C. VarandasDepartamento de Química, and Centro de Química, Universidade de Coimbra, 3004-535 Coimbra, Portugal

a r t i c l e i n f o

Article history:Received 3 February 2014Accepted 19 February 2014Available online 15 March 2014

Keywords:Carbon dioxide captureLithium nitrideMøller–Plesset perturbation theoryCoupled-clusterDensity functional theory

a b s t r a c t

The picture of an insertion reaction with formation of a fully lithiated carbamate is presented inspired inrecent experimental information and based on accurate electronic structure calculations. A study is per-formed of the Li3N + CO2 reaction, which was recently suggested as a candidate to capture carbon dioxideand yield useful chemicals. Investigated in the gas phase, the results predict Li3N to leave its planar equi-librium form when capturing CO2 that bends itself during the capture process. A species with the sumformula Li3NCO2 is formed which, jointly with its fragments, is characterized in detail by Møller–Plessetperturbation theory and the coupled cluster method. The practical interest of the title reaction and workahead are also assessed.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Carbon dioxide is the major product in combustion of organicmatter, and possibly the major atmospheric contaminant due toenergy production from combustion of fossil fuels [1–3] (coal,petroleum, and natural gas). With the prospect of runaway globalwarming due to CO2 increase in the atmosphere, research directedto reduction of this predominant greenhouse gas is crucial. Widelyapplied options include the sequestration of atmospheric CO2 intoother global pools [4,5], including oceanic injection, geologicalinjection, and scrubbing and mineral carbonation [6,7]. However,the cost and leakage as well as effects on the sea biota are majorissues of the geological and oceanic sequestration. Another possi-bility is to make carbon-based products from CO2. Unfortunately,due to its well known stability and hence poor reactivity, mostsuch ways tend to be unaffordable. Attempts to activate CO2 havefocused on the carboxylation and carbonation of organic substrates[8,9], synthesis of energy-rich C compounds [10,11], photochemi-cal conversion [11,12], and electrochemical reduction [13,14]. Butall these are endothermic processes, and hence energy input isrequired. Carbon capture emerges therefore as a multifacetedproblem requiring shared vision and worldwide collaborativeefforts from governments, policy makers and economists, as wellas scientists, engineers and venture capitalists [4]. In fact, it isregarded as one of the grand challenges for the 21st century [15].Yet, despite the incentives given and the number of existinghigh-profile collaborative programs, the Intergovernmental Panel

on Climate Change (IPCC) has recently pointed out that the avail-able methods of capture are energy intensive and not cost-effectivefor carbon emissions reduction [16].

In 2011, Hu and Huo [17] suggested a chemical reaction thatsoaks up CO2 and appears to yield useful chemicals along with sig-nificant amounts of energy. The suggested heat-releasing reactioninvolves CO2 and lithium nitride (Li3N), also known as trilithioni-trogen, the only stable alkali metal nitride that can be made byreacting lithium with nitrogen at room temperature [18] (see p.165), which seems to proceed by producing the ionic (salt-like)lithium nitride [19]:

6LiðsÞ þ N2ðgÞ ! 2Li3NðsÞ

Recalling that Li3N is often used as a reactive N-source for mate-rial synthesis, Hu and Huo [17] suggested its use for formation ofC3N4 from CO2, a reaction that has been expressed as

3CO2ðgÞ þ 4Li3NðsÞ ! 4C3N4ðsÞ þ 6Li2OðgÞ

This reaction has a favorable thermodynamics, showing negativevalues of the enthalpy change (795.9 kJ mol�1) and Gibbs freeenergy change (737.2 kJ mol�1). From the thermodynamic analysis,they were led to conclude that the reaction between CO2 and Li3Nwould be a viable approach to convert CO2 into valuable solid mate-rials. They further observed [17] the formation of amorphous car-bon nitride (C3N4), a semiconductor, and lithium cyanamide(Li2CN2), a precursor to fertilizers. Using their own words [17],‘‘when CO2 is added to less than a gram of Li3N at 330 �C, the sur-rounding temperature shots up almost immediately to 1000 �C,which is roughly the temperature of lava flowing from a volcano!’’

62 A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71

A further remark to note that Li3N, which appears to be the onlyknown stable alkali metal nitride is reddish colored, with a highmelting point [20] and an unusual cristal structure formed by twotypes of layers: one of composition Li2N with 6-coordinate Li cen-ters, the other with only lithium cations [21].

In this work, we study the Li3N + CO2 reaction in the gas phase,which to the best of our knowledge has not thus far been studied,although a density-functional theory (DFT) study of CO2 fixation bylithium amides yielding lithium carbamates has been reported[22]. Note that carbamates are organic compounds derived fromcarbamic acid (NH2COOH), with the most important carbamatebeing perhaps the one involved in the capture of CO2 by plantssince this process is necessary for their growth. Because the super-molecule formed in the title reaction shares with carbamates theNCOO skeleton, we will use occasionally this designation in thepresent work, although in the above sense. Note that the NCOOgroup here reported for the supermolecule assumes a form similarto the N-cyc-CO2 isomer of the NCO2 radical [23,24], but with somedifferences mainly on the larger opening OCO angle of the Y-shaped structure which is here of 121 deg compared to [24]�71 deg. Such a species can be produced in low-temperature envi-ronments or in gas phase via reaction of CN with O2, thence playingan important role in combustion chemistry [25] and receivingintensive attention in kinetic studies [26,27]. Nevertheless, studieson this radical are themselves by no means numerous. Briefly, cal-culations at various levels up to quadratic configuration interactiontheory with perturbative correction for triples have been utilizedby Benson and Francisco [25] to characterize the structure, vibra-tional frequencies, and energy of ONCO. Another study is by Wuand Lee [24] who employed DFT to predict geometries of five iso-mers of NCO2: ONCO, O-cyc-CNO, NCOO, N-cyc-CO2, and CNOO,listed in increasing energy order as obtained with the UCCSD(T)/6-311+G(d) method with inclusion of the zero-point vibrational-energy corrections calculated with Becke’s three-parameter hybridexchange functional and the correlation functional of Lee, Yang,and Parr (B3LYP) [28].

The paper is organized as follows. Section 2 addresses the ab ini-tio methods employed in the present work, while the major find-ings are discussed in Section 3. A summary and prospect forfuture work are on Section 4.

2. Methodology

The study here reported involved two basic steps. In the first, thereactant and product species are characterized using high levelmethods of ab initio electronic structure theory. Because both Li3Nand CO2 are closed-shell in their ground-singlet states, we haveadopted second-order Møller–Plesset perturbation theory (MP2)and the coupled-cluster method, this in its most popular version ofcoupled-cluster single and double electronic excitations with a per-turbative correction for triples, i.e., CCSD(T). In all calculations, corecorrelation effects have been ignored [29]. Then, a search has beencarried out for the supermolecular (intermediary) species. Recallingthat the structure of the akin, well studied, NH3–CO2 complex is con-sistent with the structure of other complexes of CO2 with Lewisbases in that their stereochemistry can be rationalized in terms ofthe interaction between the lone pair(s) of electrons of the base withthe lowest unoccupied p� orbital of CO2 [30], we have searched forthe optimal structure of Li3NCO2 by considering an insertion pathwhereby the N atom of Li3N and the C atom of CO2 faced each other.Because the number of minima in a potential energy surface isknown to increase drastically with the number of atoms, we shouldthen qualify that we focused on the one obtained through the aboveinsertion process. Yet, a reasonable amount of test work led us toconclude that this is likely to be the true minimum of the ground-state Li3NCO2 potential energy surface.

Two basis sets of the correlation consistent type family havebeen mostly utilized: cc-pVXZ and aug-cc-pVXZ, denoted shortlyas VXZ and AVXZ [31]. The simplest VXZ basis has been usedmostly for exploratory purposes, while the augmented basis con-tains polarization functions and is supposed to give quantitativeresults. Indeed, although inclusion of diffuse functions is seen to af-fect modestly the molecular parameters, they may have an impor-tant role due to the ionic character revealed by many of the bondsthat are formed. For affordability, the calculations have employedmostly basis with the two lowest cardinal numbers, X ¼ D; T ,although a higher member of the hierarchy (X ¼ Q) were occasion-ally also utilized. Note that AVQZ is the largest available basis set ofDunning’s augmented family for the Li atom [31]. Moreover, use ofa hybrid basis (V5Z for the Li atoms and AV5Z for all others) wouldmake the calculations too costly and possibly hardly justified. Itshould be pointed out that no attempt has been made to use com-posite procedures, in particular CCSD(T)/AVTZ//MP2/VDZ or evenCCSD(T)/AVTZ//DFT/VDZ, where as usual the notation impliesCCSD(T) calculations at a geometry or optimized path calculatedat a lower level of theory. Although such schemes are popular inmodern quantum chemistry [29] (they stem from the observationthat the geometry is often less sensitive to the theoretical levelthan relative energies), reasons were given for being cautious inpracticizing the approach [32,33].

Regarding the reliability of single-reference CCSD(T), the gold-en approach of quantum chemists, we have examined it by look-ing at two popular diagnostics. The T1-diagnostic [34] computedfrom single-substitution amplitudes amounts to 0.018, 0.041and 0.018 near the equilibrium geometries of CO2, Li3N and Li3-

NCO2, respectively, when using the AVTZ basis. As seen, they tendto satisfy the usual requirement of [0.02 for the CCSD(T) meth-od. However, the T1-diagnostic cannot by itself be a sign thatCCSD(T) is reliable. Since a doubles diagnostic has been reportedonly in nuclear structure theory, [35] another [36] (D1) fromsingle-substitution amplitudes has also been utilized. In the sameorder, one gets D1 ¼ 0:047; 0:078, and 0.055 which, used in tan-dem [34,36,37], yields ratios T1=D1 of 0.4, 0.5, and 0.3 accordingto common requirements. It is therefore reasonable to say thatthe CC approach performs well [37], particularly close to equilib-rium geometries. An additional item for judgement refers to thelargest T1 and T2 amplitudes: the print threshold value of 0.05has only sporadically been attained and in no case for more thanone amplitude at the stationary points. However, such values oc-curred at times and in some cases even frequently (as it may beanticipated from the results to be shown in Section 3) during thecalculation of the optimized reaction path (ORP), signaling someinadequacy of the single reference CC method at such regions.In summary, the CCSD(T) should give a solution as close as onemight ambition with a single-reference method to the full config-uration interaction limit, particularly at the stationary points herereported.

For enhanced accuracy, the electronic energy along the opti-mized reaction path (ORP) to be described later has been split intoits basic components, with each separately extrapolated to thecomplete basis set (CBS) limit, i.e., the Hartree–Fock (HF) and cor-relation (cor) contributions were extrapolated separately. Becausethe procedure has been described in detail in the literature [38–40](and references therein), it is here only briefly addressed. Recallthat the HF energy converges faster than the correlation energy,and hence poses no serious difficulties. We use here the protocolof Karton and Martin [41] for HF extrapolation from TZ and QZ. Thisis an empirical two-point extrapolation of the form Aþ BX�5:34,which is believed to yield converged energies with a root meansquare error of 0.12 kcal mol�1 (0.5 kJ mol�1) or so, thence per-fectly satisfactory for the present purposes; X is the cardinal num-ber defined above.

Table 1Optimal geometric properties and vibrational frequencies of CO2 at MP2 and CCSD(T)levels of theory.a

Prop. MP2 CCSD(T) Exp.d

VDZb AVTZb VDZb AVTZb AVQZc

RCO 1.1771 1.1702 1.1745 1.1670 1.1631 1.1621\OCO 180 180 180 180 180 180x1

e 647 659 650 664 670 667x3 1328 1326 1338 1341 1349 1333x4 2440 2400 2397 2372 2387 2349ZPE 2532 2522 2518 2520 2538

a Energies in Eh, distances in Å, angles in deg, frequencies in cm�1.b See total energy (E) in Table 5.c E ¼ �188:389596 Eh.d From Ref. [56].e Double degenerate.

A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71 63

Extrapolation of the dynamical correlation finds a basis on stud-ies of its dependence on the partial wave quantum number fortwo-electron atomic systems and second-order pair energies inmany-electron atoms [42,43], with a popular CBS scheme (see alsoelsewhere [38,44]) being [45] Ecor

X ¼ Ecor1 þ A3=ðX þ aÞ3, where Ecor

X isthe correlation energy obtained with the cardinal number X, andEcor1 and A3 are parameters determined from calculations for the

two highest affordable values of X; the offset parameter a is fixedfrom an auxiliary condition [38]. The above rule is reliable onlywhen based on X values typically larger than Q, which makes theprocedure quite expensive since the number of basis functionsscales with X3. Indeed, one may argue that enlarging the calcula-tion to a X ¼ 5 would justify the introduction of core-correlationeffects as the expected changes are of similar magnitude [46].We have overcome such a difficulty with the uniform singlet-pairand triplet-pair extrapolation (USTE [38]) scheme. This assumesthe form

EcorX ¼ Ecor

1 þ A3Y�3 þ A5Y�5 ð1Þ

where Y ¼ X þ a, with the constant A5 related to A3 via the auxiliaryrelation A5 ¼ A�5 þ cAm

3 , and A�5; c, and m are ‘universal’ like parame-ters for a chosen level of theory. For MP2 energies:A�5 ¼ 0:0960668; c ¼ �1:582009 Eh, and m ¼ 1, with a ¼ �3=8.Thus, for m ¼ 1 (e.g.: MP2, and CC methods) and two basis sets X1

and X2, one obtains:

Ecor1 ¼ Ecor

X2� A�5Y�5

2 þEcor

X1� Ecor

X2þ A�5 Y�5

2 � Y�51

� �

c Y�52 � Y�5

1

� �þ Y�3

2 � Y�31

Y�32 þ cY�5

2

� �

ð2Þ

where Yi ¼ Xi þ að Þ, with i ¼ 1;2. Note that the above two-parame-ter Ecor

1 ;A3� �

scheme has been shown [47] to yield accurate resultsfor a variety of systems and theoretical approaches [38,47–49];see Ref. [38] for the values of the relevant parameters A�5 and c.We emphasize that the method contains no parameters alien tothe theory for which they have been defined, with the coefficientsbeing expected to show only a minor dependence for methodsand basis sets that belong to related families. Rather than usingextrapolated frequencies, the zero-point energy corrections willbe calculated with the best available CCSD(T)/AVTZ frequencies,usually taken as highly reliable. Other contributions such as relativ-istic effects, and spin–orbit corrections are expected to be small forlight atoms, and hence assumed here to be negligible.

As noted above, a reaction dissociation profile has also beenobtained by moving backwards from the equilibrium seven-atomspecies to reactants via single-point calculations at every point ofa predefined grid such that the bond is successively lengthened un-til rupture takes place and reactants are reformed. All electronicstructure calculations were done using MOLPRO [50], with MOLD-EN and locally written codes being subsequently used to processgraphically the calculated data. In this approach, similar to oneutilized in a recent multi-reference situation [32,51], all other coor-dinates are optimized thence yielding an optimized reaction path(ORP). As for equilibrium attributes, they were converged typicallydown to a gradient of 610�5 Eh �1, thence smaller than default[50]. Note that the basis set superposition error [52] is ignored inobtaining the ORP. In fact, use of counterpoise [52] (CP) is deemedto be unjustified [39,53,54] in the MP2/AVXZ case due to havingCBS extrapolated the calculated energies. Note further that CP isnot warranted with simple basis functions [55], with CBS beingpossibly the only logical and reliable alternative [39,53,54]. Thishas not been pursued at the CCSD(T) level of theory since calcula-tions with even VTZ and VQZ basis would be rather computation-ally expensive.

3. Results and discussion

The optimum energy and geometry as well as harmonic fre-quencies calculated for the reactants at the MP2 and CC levels oftheory are reported in Tables 1 and 2, while the correspondingattributes of Li3NCO2 are in Table 3. Also calculated are the attri-butes at the restricted Hartree–Fock (HF) and unrestricted-HF(UHF) levels of theory (not given) as well as unrestricted Kohn–Sham (UKS) density functional theory. Whenever available, exper-imental results are also given for comparison. Due to space limita-tions, we leave in some cases aside the geometrical attributes ofthe larger species, which can nevertheless be easily obtained fromthe cartesian coordinates collected in the Supplementary Informa-tion (SI). Not surprisingly, the HF and UHF results for Li3NCO2 areessentially indistinguishble due to the closed shell nature of thesystem at equilibrium. For CO2, the agreement with the experi-mental values is good, which lets us expect a similar accuracy forthe predictions where such a comparison is unavailable.

Fig. 1 shows ball-and-stick plots of the equilibrium structures ofground-state CO2, Li3N, and Li3NCO2 as predicted from theCCSD(T)/AVTZ calculations. Regarding Li3N, it is interesting to ob-serve that in all cases but CCSD(T)/VDZ, where it shows a slightlypyramidal shape, the equilibrium geometry is predicted to be pla-nar with perfect D3h point group symmetry within the error of thenumerical optimization procedure (the threshold has been set at10�6 Eh �1). We further observe that the predicted LiN bond dis-tances agree reasonably well with the value of 1.773 Šavailablein the literature [57] at the MP2/6-311++G(d,p) level.

Besides the energy, there are other properties that may be de-rived once the wave function has been calculated. In particular,we may examine how the charge is distributed through the atomsthat integrate the molecule [29]. Because the in situ charge on anatom is not an observable, any attempt to assign an atomic chargemust involve a model of some sort. Some methods such as Mullik-en’s [58] population analysis and Stone’s [59,60] distributed multi-pole analysis allocate the density to various atoms on the basis ofan algorithm that is basis set dependent. While Mulliken’s methodis known to be very basis-set sensitive [29], Stone’s is more robustparticularly after his introduction of a modification that alleviatesthe basis set dependence [60]. There are also methods that focus onthe electron density directly and hence are less prone to basis setdependence: they assign regions of space to an individual atomusing a Voronoi [61] or Bader [62] partitioning. Furthermore, theHirshfeld [63] (or stockholder) method assigns charges based onusing atomic densities for partitioning the molecular electron den-sity, and it has been much utilized [64–66]. Despite the shortcom-ings of Mulliken’s [29] scheme, it will be chosen here due to itssimplicity and popularity. For Li3N, the charges predicted on the

Table 2Optimal geometric properties and vibrational frequencies of Li3N at MP2 and CCSD(T)levels of theory.a

Prop. MP2 CCSD(T)

VDZb AVTZb AVQZc VDZb,d AVTZb AVQZe

RLiN 1.8090 1.7708 1.7656 1.8016 1.7498 1.7448RLiLi 3.1333 3.0668 3.0582 3.0656 3.0305 3.0216bf 120 120 120 117 120 120/g <0.01 <0.1 0 21 <0.1 0x1 59 90 92 99 71 75x2

h 203 192 197 210 191 196x4 594 610 616 624 651 657x5

h 780 814 823 796 849 859ZPE 1310 1356 1374 1369 1401 1421

a Units as in Table 1.b See total energy (E) in Table 5.c E ¼ �77:07140951 Eh.d The planar structure with D3h symmetry is the saddle point for the umbrella

motion which has been converged down to a gradient of 1 l Eh Å�1 atE ¼ �76:983983 Eh, with geometrical parameters: RLiN ¼ 1:7910 Å andRLiLi ¼ 3:1019 Å. The harmonic frequencies are: 67.2 i, 200.9, 201.2, 632.7, 810.3,and 810.4 cm�1.

e E ¼ �77:072364 Eh .f\LiNLi.

g Dihedral angle.h Double degenerate.

Table 3Dipole moment and harmonic vibrational frequencies of Li3NCO2 at MP2 and CCSD(T)levels of theory.a

Prop. UKS MP2 CCSD(T)

AVTZb,c VDZd AVTZe VDZf AVDZg AVTZh

li 5.0 4.8 5.2 5.5 5.7 5.6x1 182 176 191 177 190 192x2 192 203 192 203 194 194x3 335 331 340 331 344 343x4 348 351 367 348 370 366x5 358 360 373 358 370 375x6 470 466 474 466 474 478x7 539 578 577 577 570 580x8 596 587 598 585 593 601x9 597 609 615 608 609 620x10 643 666 668 663 656 670x11 747 765 765 766 757 773x12 825 863 874 862 854 878x13 893 957 959 958 942 967x14 1131 1219 1221 1222 1220 1236x15 1343 1463 1433 1453 1410 1434ZPE 4600 4797 4824 4789 4776 4853

a Units as in Table 1. For the geometrical parameters, see the SI.b Using density functional group B-LYP = B88 + LYP.c Total energy, E ¼ �266:06932510 Eh.d E ¼ �265:233686 Eh.e E ¼ �265:504337 Eh.f E ¼ �265:259023 Eh.g E ¼ �265:325783 Eh.h E ¼ �265:535643 Eh.i Dipole moment norm, in debyes (D).

Table 4Mulliken charges in Li3NCO2 at MP2 and CCSD(T) levels of theory.

Atom # MP2 CCSD(T)

VDZ AVTZ VDZ AVDZ AVTZ

C 1 0.7073 1.0493 0.7101 1.4612 1.0406O 2 �0.5806 �0.8793 �0.5817 �1.0167 �1.1275Li 3 0.4454 0.8324 0.4451 0.3575 0.8342Li 4 0.4454 0.8324 0.4451 0.3575 0.8342O 5 �0.6496 �1.1278 �0.6491 �0.8966 �0.8793N 6 �0.7588 �1.3761 �0.7598 �0.6367 �1.3732Li 7 0.3909 0.6692 0.3904 0.3748 0.6711

+

a b

c

d

e

f

g

h

i

2

1

5

6

3

4

7

Fig. 1. Ball-and-stick plots of optimized CCSD(T)/AVTZ (MP2/AVTZ values given inbrackets) structures of CO2, Li3N, and Li3NCO2. For the latter, the bond distances in Åare: (a and b) 1.889 (1.893); (c) 1.361 (1.363); (d) 1.359 (1.358); (e) 1.292 (1.292);(f) 1.848 (1.854); (g) 1.911 (1.918); and (h and i) 1.943 (1.947). The dashed linesindicate other interactions than do not appear connected by default with MOLDEN.Despite the slightly shorter LiO bond lengths, we have maintained the designationof carbamate as if the OH and NH2 groups of a carbamate had been fully lithiated[note that other designations such as lithium carboxylatonitride or lithium(dioxidemethylene) amide cannot be excluded given the nearly equal sharing ofthe lithium atoms by the N and O atoms]. For other geometrical parameters, see theSI.

64 A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71

Li atoms at CCSD(T) level of theory are 0.2586 (0.2588), 0.1487, and0.0569 with the VDZ, AVTZ and AVQZ basis, respectively (in paren-thesis is the corresponding MP2 value). In the same order, they areat the N center: �0.7767 (�0.7761), �0.4471, and � 0.1712. Thedifferences are minimal with respect to a structure with perfectD3h (or C3v ) symmetry as might be expected from the calculatedvalues of the dipole moment; this is predicted to vanish up to 3decimal figures in CCSD(T), and is also rather small in MP2(0:0007 D and 0:0004 D at MP2/VDZ and AVQZ levels of theory).Despite the above basis-set dependence, an apparent decrease inionicity of Li3N is observed with increasing basis set, a trend thatparallels the decrease in the equilibrium bond length.

Li3NCO2 is predicted to have a three-dimensional equilibriumstructure with C1 symmetry at the MP2 level (‘‘quasi-Cs’’ in thesense that it loosely shows a plane of symmetry; see later), andCs at CCSD(T). Although the geometry is not explicitly given (itmay be easily computed from the data given as SI), we just remarkthat \OCO � 120.7 deg, which conforms well with the commonexpectation that bending in CO2 is a highly energetic process[67]. Note also that there is apparently an enhancement in ioniccharacter of Li3NCO2 when compared to Li3N. For the former, theMulliken charges on the Li atoms are 0.39 (atom #7) and 0.44(#3 and #4), while those of N (#6) and C (#1) are �0.76 and0.71, respectively; the full set is gathered in Table 4. Note that

Fig. 2. The HOMO of Li3NCO2 as obtained from the CCSD(T)/AVTZ calculationsreported in the present work.

A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71 65

the two CO bond lengths differ by about 0.1 Å, which may explainthe slightly different charges in the O atoms # 2 and #5: �0.58 and�0.65. This asymmetry on the O charges can perhaps be best ratio-nalized from the plot of the highest occupied molecular orbital(HOMO) at the CC/AVTZ level of theory, which is shown in Fig. 2.Note that during the approach of CO2 to Li3N, the threefold symme-try of the latter is broken thus placing two Li atoms in a differentcontext of the third one. This makes the two CO bonds distinct asmentioned above.

In turn, the atomic charges at MP2/AVTZ level are 0.67 (0.83),�1.38, 1.05, and �0:88 (�1:13) for Li, N, C, and O, respectively;the values in parenthesis refer to the second set of Li and O values.Although one should recall the above remarks on basis-set depen-dence, the present results suggest an enhancement of ionicity rel-ative to the MP2/VDZ results, a prediction comparable to the oneobserved with the CCSD(T) method. Apparently, the basis set effectis more relevant than the method employed; see Table 3. Weemphasize that the optimum CCSD(T) structure shows a highersymmetry (Cs) than the MP2 one (C1), as probably best seen fromthe isodensity contour maps in Fig. 3; note that the contours inthe left-hand-side plot (MP2) are less symmetrical with respectto the vertical axis than in the right-hand-side CCSD(T) one.

Fig. 3. Isodensity contour maps of Li3NCO2 highlighting the molecule highest symmetry:hand-side one. Key for colors in atomic units of charge: cyan, �0:1 up to 0:0; yellow,references to color in this figure legend, the reader is referred to the web version of thi

Indeed, this seems to be the case with all employed basis sets.Although we should note the slightly distinct tolerances used forconvergence (10�5 vs 10�6 Eh/Å for MP2/AVTZ and CCSD(T)/AVTZ,respectively), such a small difference can hardly be expected tohave any significant effect on the charge distribution. Of course,it should also have no impact on the reported large dipolemoments.

Illustrated in Fig. 4 is the ORP calculated as described above atthe CCSD(T) and MP2 levels of theory using affordable basis sets:VDZ for the former, VDZ and AVDZ for the latter. The same gridof 45 geometries has been considered in all VDZ calculations. Amovie showing the structural evolution along the ORP is shownin Fig. 5 at the MP2/VDZ level and Fig. 6 at CCSD(T)/VDZ. As seenfrom Fig. 4, the ORP is relatively smooth except at occasions wheresudden changes in energy occur. The first of these occurs at a re-gion where the structure of Li3NCO2 transits from 3D to planar.The notable observation refers to the ‘‘scar’’ left due to such astructural change. Interestingly, this is followed by corresponding‘‘scars’’ on the D1; T1 and D1=T1 diagnostics. This may then implythat such regions of the potential energy surface show distinctattributes as far the closed-shell nature of Li3NCO2 is concerned. In-deed, except close to equilibrium, some singles and doubles ampli-tudes are found to be larger than the threshold value of 0.05. Interms of the forces involved, the innermost ‘‘scar’’ is tentativelyrationalized as due to passing from a bonding dictated by strongionic interactions at near equilibrium Li3NCO2 to a somewhat lessionic character controlled by dipole–dipole interactions (plus high-er-order electrostatic contributions as well as the less directionalinduction and dispersion ones) involving a bent CO2 and aC2v-distorted Li3N molecules. This is best visible from the moviesfor the backwards reaction (rupture of the CN bond) in Figs. 5and 6. A remark is due to point out that the ORP is shown inFig. 4 only up to CN distances where convergence has beenachieved. As a result, the last stage of the calculated ORP showsneither a strictly planar Li3N structure nor a linear CO2 moleculeat the MP2/VDZ level. Of course, the patterns of T1;D1 and T1=D1

may suggest that the accuracy of the reported curves should notbe taken as quantitative all over the space since the wave functioncharacter seems to point out difficulties in being handled by asingle reference method. Moreover, sizeable electrostatic longrange interactions are to be expected in the Li3N–CO2 interaction

C1 for MP2/AVTZ in the left-hand-side panel, and Cs for CCSD(T)/AVTZ in the right-0:0 up to 0:05; magenta, 0:05 up to 0:1; red, above 0:1. (For interpretation of thes article.)

-265.30

-265.28

-265.26

-265.24

-265.22

-265.20

-265.18

-265.16

-265.14

-265.12

2 3 4 5 6 7 8 90.0

0.1

0.2

0.3

0.4

0.5

ener

gy/h

artr

ee

D1,

T1,

D1/

T 1 (u

nitle

ss)

C-N bond distance/bohr

Li3N-CO2

D1

T1

D1/T1CCSD(T)/VDZ

MP2/VDZMP2/AVDZ MP2/AVTZ

MP2/AVQZCBS(T,Q)/MP2

A B C D E F G H I J

Fig. 4. Calculated ORP for the reaction Li3NCO2 ? Li3N + CO2 as predicted at the CCSD(T) (in black) and MP2 (red) levels of theory with VDZ and AVDZ basis. The reaction pathobtained via single point calculations at the MP2/AVTZ, MP2/AVQZ, and CBS(T;Q)/MP2 levels of theory along a predefined grid of points (shown by the open circles in red) areindicated by the blue, green and orange lines which have been shifted upwords by 0.2069735, 0.2785331, and 0.33298282 Eh (respectively) such as to match the actuallyoptimized MP2/AVDZ ORP at the equilibrium geometry. Indicated are the corresponding D1; T1 and D1=T1 diagnostics [in black, together with the corresponding CCSD(T)energies], and the fully optimized MP2/VDZ energy (solid dot in magenta). Also indicated in the top axis are the distances associated to panels A–J in Fig. 5. (For interpretationof the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Movie showing the structures along the ORP for breaking the CN bond in Li3NCO2 at MP2/VDZ level of theory. The CN bond distances in panels A–J are indicated inFig. 4. Note the sudden changes in orientation over the region where the binding character changes.

Fig. 6. As in Fig. 5 but at CCSD(T)/VDZ level of theory. In panels A–E (top), the predefined reaction coordinate (CN distance) assumes values identical to Fig. 5, while in thebottom row (panels F–J) it is of 5.25, 6.25, 7.0625, 7.125, and 7:5 a0.

66 A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71

A B

C D

E F

Fig. 7. Stationary structures at other asymptotes of the Li3NCO2 potential energysurface at the CCSD(T)/AVTZ level of theory. Structures A–D represent minima ofthe fragments shown, E is the isomerization saddle point connecting structures Cand D, and F the planar saddle point structure connecting B and its reflection on theNCO plane. For minimum B, the same bond-length assumption for connection asbeen utilized as in Fig. 4, with the LiO bond distance being 1.805 Å and the LiN one1.930 Å; the reader is addressed to the SI for other geometrical parameters.

A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71 67

which would suggest that longer separations between the frag-ments should be considered for a more realistic description ofthe dissociation process. This reflects on the fact that the exother-micity of reaction extracted from the reported curves differs some-what from the value obtained by subtracting the tabulatedenergies of the fragments from the energy of the supermoleculeat equilibrium. Recall that both the CCSD(T) and MP2 methodsare size-consistent [68], and hence it is appropriate to calculatethe exothermicities by subtracting those two energy values. Theresults so obtained for the processes Li3N + CO2 ? Li3NCO2 andLi2O + LiNCO ? Li3NCO2 (and other isomers of LiOCN) are indicatedas DE in Table 5 with inclusion of the ZPE correction, i.e., by addingthe energy corresponding to the difference between the ZPE ofLi3NCO2 and the sum of the ZPEs of the relevant fragments. Notethat no account has been taken of the BSSE [52], which is expectedto be sizeable for small basis sets. The exothermicity predictedfrom the CBS/MP2 ORP is therefore expected to be more realistic,yielding �77.8 kcal �1 (�75.1 kcal mol�1 if accounting for ZPE cor-rection with the calculated MP2/AVTZ frequencies). In any case,small differences are to be expected since the bond-dissociationcurves do not extend to sufficiently large separations to warrantthat the optimized geometries of the fragments coincide with theones of the isolated species (e.g., linear vs nonlinear CO2 or planarvs pyramidal Li3N). It should be emphasized that the calculation ofa CBS ORP at the CCSD(T)/AVDZ level of theory would be extremelytime consuming and probably unjustified by the T1 and D1 diag-nostics, and hence not attempted. In any case, the CBS/MP2/AVXZvalue obtained above is probably within the range of DE onesreported from the various levels of theory here employed, asreported in Table 5. We emphasize that the ZPE correction has inthis case been estimated at MP2/AVTZ level, which should not bea source of any significant error.

Of course, one may anticipate the existence of other stationarypoints on the 15D potential energy surface of Li3NCO2, but their fullcharacterization is beyond the scope of the present work as we justfocused on the possibility of reaction itself. For an assessment ofthe possible reactive processes, we summarize in Fig. 7 just an-other set of closed-shell fragments that correlate with ground-state Li3NCO2, and which may be formed by rupturing the longestof the two CO bonds at equilibrium, namely Li2O, and isomers of

Table 5Calculated energies and exothermicities at MP2 and CCSD(T) levels of theory.

Feature MP2 CCSD(T)

VDZ AVTZ VDZ AVTZ

CO2a �0.133553 �0.321641 �0.148255 �0.340595

Li3Nb �0.981095 �1.052780 �0.984139 �1.056087Li3NCO2

c �1.233686 �1.504337 �1.259023 �1.535643Li2Od �0.015012 �0.107461 �0.015330 �0.109778LiOCNe �1.004154 �1.178635 �1.036846 �1.212607LiOCNf �1.153099 �1.329055 �1.172348 �1.352150LiNCOg �1.171092 �1.349234 �1.189184 �1.371352DEh �72.0 �78.8 �76.9 �84.5DEi �129.7 �132.0 �124.6 �128.6

�38.0 �39.4 �41.8 �43.2�27.2 �27.2 �31.5 �31.5

a Total energy, after addition of 188 Eh to the calculated value.b Adding 76 Eh.c Adding 264 Eh.d Adding 90 Eh.e Total energy of structure B, once added 175 Eh.f Of C, once added 174 Eh.g Of D, once added 174 Eh.h Exothermicity of reaction Li3N + CO2 ? Li3NCO2, in kcal mol�1 including ZPE

correction.i Exothermicity of reaction Li2O + LiOCN ? Li3NCO2, in kcal mol�1 including ZPE

correction. Entries refer from top to bottom to the species B to D of LiOCN.

LiOCN. Of course, this will not guarantee that the reverse associa-tive process can spontaneously occur as we have not performedany calculations of the relevant ORP, and hence did not insure thatrupture or formation of the CO bond will not involve any barrier asit appears to be the case of CN. Of such products, the former is lith-iated water, which is here predicted to be linear. Although, to ourknowledge, the diatomic molecule LiO has been the only one char-acterized to date in the gas phase [69,70], Wang and Andrews [74]have shown that laser-ablated lithium atoms can react with oxy-gen molecules, as do thermal lithium atoms, to form the LiO2 andLiO2Li ionic molecules. Moreover, the excess energy associatedwith laser ablation also fosters the endothermic reaction to giveLiO and ultimately Li2O [74]. By trapping the above species in solidneon and identifying them via isotopic shifts, comparison with ear-lier argon and nitrogen matrix results has been done, as well asDFT frequency calculations which pinpointed [74] the highly ionicnature of the electron distribution in LiO2 and LiO2Li. Clearly, ourresults in Table 6 for Li2O show good agreement with the availableinformation [75–77] (see also Ref. [74]). In particular, the \LiOLiangle of 180 deg is the only consistent value with the vanishingelectric dipole moment of this system [71]. The other product mol-ecules are lithium cyanate, lithium isocyanate, and other isomericspecies with the same sum formula. They all are part of an impor-tant class of compounds that have found a large variety of applica-tions in organic, bioorganic, and polymer chemistry [78]. In short,while organic isocyanates are commonly prepared from phosgen-ation of amines, imines, carbamates, and ureas [79], the generallyless stable cyanates are usually prepared from thermolysis of thia-triazoles or reactions of alcohols with cyanogen halides. They are

Table 6Optimal geometric and energetic parameters of Li2O at MP2 and CCSD(T) levels oftheory.a

Prop. MP2 CCSD(T) Exp.

VDZ AVTZ VDZ AVTZ AVQZb

RCO 1.6667 1.6487 1.6592 1.6355 1.6306 1.56c, 1.58d

\OCO 180 180 180 180 180 180x1

e 103 120 98 123 106 118f

x3 756 747 771 773 780 782f, 783g

x4 1006 990 1025 1017 1024 954f, 1011.2g

ZPE 984 988 996 1018 1008

a Units as in Table 1. See total energies in Table 4.b Total energy, E ¼ �90:132280 Eh.c Ref. [71].d Ref. [72].e Double degenerate.f Ref. [73].g Ref. [74]; the gas phase band position for Li2O is predicted to be 1018 cm�1, and

believed to be good to better than 10 cm�1.

Table 7Harmonic vibrational frequencies of minima in LiOCN potential energy surface at MP2and CCSD(T) levels of theory.a

Prop. MP2 CCSD(T)

VDZ AVTZ VDZ AVTZ

Blb 6.8 6.8 6.7 6.6x1 171 162 178 169x2 394 400 396 401x3 554 563 467 482x4 664 655 623 638x5 754 767 696 698x6 1651 1640 1578 1592ZPE 2094 2094 1969 1990

Clb 10.7 11.2 11.2 11.4x1

c 90 79 88 78x3

c 529 551 525 554x5 732 705 734 711x6 1247 1251 1250 1258x7 2236 2224 2277 2268ZPE 2726 2720 2744 2750

Dlb 8.6 9.0 9.0 9.3x1

c 70 97 69 97x3

c 616 634 619 641x5 690 678 692 684x6 1366 1359 1372 1370x7 2303 2266 2269 2247ZPE 2866 2883 2854 2888

a Units as in Table 1. See total energies in Table 4. For the geometrical parameters,see the SI.

b Dipole moment norm, in D.c Double degenerate.

68 A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71

known to contain two double bonds and exhibit strong chemicalreactivity [78].

In the present search for stationary points, we have encoun-tered three stable isomers corresponding to the chemical sum for-mula LiOCN: a nonplanar structure (labeled B in Fig. 7) and twolinear ones (C and D) where the Li atom is attached either to theO or the N atoms. Of them, the linear LiNCO one (lithium isocya-nate) corresponds to the minimum energy form. This is best seenfrom the schematic energy diagram in Fig. 8. As it is shown, ofthe above three minima, the one corresponding to structure B lieshighest in energy, and D lowest. With the saddle point connectingLiOCN and LiNCO being here predicted to be structure E, the oneconnecting the minimum B to its equivalent (obtained by reflectionof the Li atom on the NCO plane) corresponds to structure F. Notsurprisingly therefore, the saddle point for transit from C to Dhas a structure closer to C, since this lies highest in energy. Specif-ically, ignoring the ZPE correction, our gas phase calculations indi-cate that form D is 12.0 kcal mol�1 lower than C, while thetransition state structure E lies 1.9 kcal mol�1 above the minimumC. Note that three structures, apparently B, C, and D (the formercalled T-shaped p-complex), had already been predicted [79–82]to be minima at the MP2/6-31Gww level of theory, but with Bapparently inexistent at the MP2(full)/6-31+G(d,p)⁄⁄ level whereit was instead characterized as a transition state. Parenthetically,besides the MP2(full)//6-31G⁄⁄//RHF/6-31+G⁄⁄ calculations, thereader may find B3LYP//RHF/6-31+G⁄⁄ and B3LYP//B3LYP/6-

−100

−80

−60

−40

−20

0

20

40

60

ener

gy/k

cal m

ol−1

reaction coordinate

Li2O+LiOCN (B)

Li2O+LiOCN (C)Li2O+LiNCO (D)

Li3NCO2

Li3N+CO2

Fig. 8. Energy diagram of Li3N + CO2 reaction (CN and CO bond rupture in Li3NCO2)at CCSD(T)/AVTZ level of theory. Results are shown both with consideration of ZPE(in red), and without it (0 K, in black); the reference energy is for Li3N + CO2 at 0 K.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

31+G⁄⁄ results [79], while an earlier Hartree–Fock study is in Ref.[80]. A systematic study of substituent effects on the structuresand energies of isocyanates and imines by ab initio calculations isalso available [83]. Notably, the difference predicted in this workbetween the C and D isomers agrees well with the MP2(full)//6-31G⁄⁄//RHF/6-31+G⁄⁄ value of 11.77 kcal mol�1 reported by Leungand Streitwieser [79]. A fair agreement is also observed for the va-lue of 2214.5 cm�1 reported for the infrared stretching frequencyof the N@C@O group in LiNCO at the MP2/6-31G⁄ level [83]. Weemphasize from Table 7 that the isomer B of LiOCN is here unam-biguously predicted to be a minimum, although lying in energyabove the two linear ones for cyanate and isocyanate, this standinglowest. Unlike claimed elsewhere [81,82], the only saddle pointshere located correspond to structures E and F of Fig. 7, these toounambigously characterized at the MP2 and CCSD(T) levels of the-ory in Table 8.

A final remark on the distribution of charges. The Mullikencharge on Li calculated for lithium isocyanate (D) has been pre-dicted in earlier work [83] to be 0.67, with this highly positive va-lue suggested to indicate that the r-withdrawal effect were themost important for this linear molecule. We obtain values of0.72, 0.49, 0.71, and 0.49 at MP2/VDZ, MP2/AVTZ, CCSD(T)/VDZand CCSD(T)/AVTZ levels of theory, respectively. Given the basisset dependence, the agreement may be considered as fair, withthe value of the positive charge diminishing somewhat withincreasing basis. For the isocynate, there is also good agreementon the optimal geometries as reported by McAllister and Tidwell[83] with their best MP2//6-311G⁄⁄//MP2//6-311G⁄⁄ method: theyreport 1.210, 1.195, and 1.748 Å for the NC, OC, and LiN bond dis-tances which compare in the same order with our predictions of1.2026, 1.1944, and 1.7479 Å at the CCSD(T)/AVTZ level. As ex-pected from the high ionicity, all isomers including saddle points(Table 8) show large values of the dipole moment (for comparison,

Table 8Harmonic vibrational frequencies of saddle points in LiOCN potential energy surfaceat MP2 and CCSD(T) levels of theory.a

Prop. MP2 CCSD(T)

VDZb AVTZc VDZd AVTZe

Elf 9.0 9.4 9.0 9.3x1 126i 93i 125i 92ix2 534 558 535 563x3 558 566 557 572x4 723 712 724 717x5 1149 1165 1141 1162x6 2171 2175 2208 2214ZPE 2568 2588 2582 2614

Flf 7.5 7.5 7.2 7.1x1 137i 124i 138i 125ix2 338 344 337 342x3 485 492 394 405x4 667 645 613 634x5 730 757 686 684x6 1708 1692 1647 1659ZPE 1964 1965 1838 1862

a Units as in Table 1. For the geometrical parameters, see the SI.b The following are total energies for structures E and F, respectively:

E ¼ �175:147779, and �175:000591 Eh.c E ¼ �175:326187, and �175:175565 Eh.d E ¼ �175:166870, and �175:033638 Eh.e E ¼ �175:209760, and �175:349153 Eh.f Dipole moment norm, in D.

A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71 69

the dipole moment of LiO is [69] 6:8 D) that agree well with eachother via both used methods.

4. Final remarks

Due to the threat of the observed warming over the past fewdecades and a generalized understanding that the increase ofatmospheric CO2 may be a primary cause, it would obviously helpif it could be removed at our advantage but transforming into use-ful compounds. In the present work, we have investigated the gasphase homologue of the reaction of lithium nitride with carbondioxide that has been recently suggested for that purpose. We con-cluded that fixation of CO2 should occur promptly (without anyapparent barrier) with formation of a lithium carbamate type mol-ecule. Mechanistically, the process may be rationalized as involv-ing two steps: (a) formation of a van der Waals type speciesdictacted by the electrostatic interaction between the strong quad-rupoles of the two reactants [2] which tends to orient face-to-facethe C and N atoms, a process that is subsequently enhanced by aneven stronger dipole–dipole attraction involving a planar interac-tion of a bent CO2 and a distorted Li3N and (b) formation of Li3-

NCO2, a ionic species characterized by reasonably strong covalentand ionic bonds. Whether such a reaction can be experimentallystudied in the gas phase and eventually used at our advantagefor capturing CO2, this is yet to be demonstrated, with an answerfalling outside the scope of the present work. Indeed, various issuesremain to be settled. First and foremost, one wonders how to studyexperimentally the title reaction. Tentatively, one may guess that itsuffices to react Li with N2 (as during the preparation of Li3N) butin an atmosphere doped with CO2. Even so, will Li3NCO2 be formedin the gas phase? Given the reduced number of highly ionic speciesknown in the gas phase, such a possibility seems unlike. There isthough the possibility that Li3NCO2 may be prepared in a matrixenvironment as long as the reaction can take place there. This of-fers an interesting possibility that justifies consideration. Anotherissue refers to the presence of further Li3N molecules: would other

species be formed? Of course, a computational investigation of thetitle reaction in a cluster environment could pave the way to ourunderstanding of the title reaction under heterogeneous condi-tions. Now, if Li3NCO2 is formed in the gas phase, would it be morebenign than CO2 itself? Again, the answer shows little promisesince LiNCO is also likely to be formed (given the relatively smallenergy difference to the minimum of Li3NCO2) and one recallsthe tragic history [84] involving the akin methyl isocyanate. A finalremark goes to the role of water, an ubiquitous component of theatmosphere. Would the presence of water molecules influencethe title reaction? The answer is likely to be positive, as this isthe case [2] for the reaction of tertiary amides with CO2 in the pres-ence of H2O. Despite such a greyish scenario, further investigationon the title reaction would help assessing the practical importanceof capturing CO2 with lithium nitride [17].

Acknowledgments

I wish to thank my Colleagues Rui Fausto and Abílio Sobral forstimulating remarks. This work is supported by Fundação para aCiência e a Tecnologia, Portugal, under contracts PTDC/CEQ-COM3249/2012 and PTDC/AAG-MAA/4657/2012.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.comptc.2014.02.022.

References

[1] K.M.K. Yu, I. Curcic, J. Gabriel, S.C.E. Tsang, Recent advances in CO2 capture andutilization, ChemSusChem 1 (2008) 893–899.

[2] D. D’Alessandro, B. Smit, J. Long, Carbon dioxide capture: prospects for newmaterials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082.

[3] A. Maiti, Atomistic modeling toward high-efficiency carbon capture: a briefsurvey with a few illustrative examples, Int. J. Quantum Chem., in press, http://dx.doi.org/10.1002/qua.24579.

[4] R. Lal, Carbon sequestration, Philos. Trans. Roy. Soc. B 363 (2008) 815–830.[5] R. Lal, Sequestering atmospheric carbon dioxide, Crit. Rev. Plant Sci. 28 (2009)

90–96.[6] D.I. Auerbach, J.A. Caulfield, E.E. Adams, H.J. Herzog, Impacts of ocean CO2

disposal on marine life: I. A toxicological assessment integrating constant-concentration laboratory assay data with variable-concentration fieldexposure, Environ. Model. Assess. 2 (1997) 333–343.

[7] J.L. Kendall, D.A. Canelas, J.L. Young, J.M. DeSimone, Polymerizations insupercritical carbon dioxide, Chem. Rev. 99 (1999) 543564.

[8] M. Aresta, A. Dibenedetto, Carbon dioxide as building block for the synthesis oforganic carbonates: behavior of homogeneous and heterogeneous catalysts inthe oxidative carboxylation of olefins, J. Mol. Catal., A: Chem. 182 (2002) 399–409.

[9] M. Aresta, A. Dibenedetto, in: C.S. Song, A.M. Gaffney, K. Fujimoto (Eds.), CO2

conversion and utilization, American Chemical Society, Washington, DC, 2002.[10] Y.H. Hu, E. Ruckenstein, Catalytic conversion of methane to synthesis gas by

partial oxidation and CO2 reforming, Adv. Catal. 48 (2004) 297–345.[11] E. Fujita, Photochemical carbon dioxide reduction with metal complexes,

Coord. Chem. Rev. 186 (1999) 373–384.[12] T. Ogata, S. Yanagida, B.S. Brunschwig, E. Fujita, Mechanistic and kinetic

studies of cobalt macrocycles in a photochemical CO2 reduction system:evidence of Co–CO2 adducts as intermediate, J. Am. Chem. Soc. 117 (1995)6708–6716.

[13] J.P. Collin, J.P. Sauvage, Electrochemical reduction of carbon dioxide mediatedby molecular catalysts, Coord. Chem. Rev. 93 (1989) 245–268.

[14] C. Caix, S. Chardon-Noblat, A. Deronzier, Electrocatalytic reduction of CO2 intoformate with [(g5 �Me5C5) M(L)CI] + complexes (L = 2, 20-bipyridine ligands;M = Rh(III) and Ir(III)), J. Electroanal. Chem. 434 (1997) 163–170.

[15] M.Z. Jacobson, Review of solutions to global warming, air pollution, and energysecurity, Energy Environ. Sci. 2 (2009) 148–173.

[16] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage, Tech. Rep.,Cambridge University Press, Cambridge, 2005.

[17] Y.H. Hu, Y. Huo, Fast and exothermic reaction of CO2 and Li3N into C–N-containing solid materials, J. Phys. Chem. A 115 (2011) 11678–11681.

[18] B. Martel, Chemical Risk Analysis: A Practical Handbook, Butterworth,Heinerman, 2004.

[19] E. Döneges, Handbook of Preparative Inorganic Chemistry, Academic Press, NY,1963.

70 A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71

[20] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Butterworth,Heinemann, 1997.

[21] M.G. Barker, A.J. Blake, P.P. Edwards, D.H.Q. Gregory, T.A. Hamor, D.J. Siddons,S.E. Smith, Novel layered lithium nitridonickelates; effect of li vacancyconcentration on N co-ordination geometry and Ni oxidation state, Chem.Commun. 13 (1999) 1187–1188.

[22] S.O.N. Lill, U. Khn, E. Anders, Carbon dioxide fixation by lithium amides: DFTstudies on the reaction mechanism of the formation of lithium carbamates,Eur. J. Org. Chem. (2004) 2868–2880.

[23] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, A matrix isolation study of the Cs

symmetric OCNO(X2A00) radical, Phys. Chem. Chem. Phys. 7 (2005) 4089–4095.[24] Y.-J. Wu, Y.-P. Lee, Isomers of NCO2: IR-absorption spectra of ONCO in solid Ne,

J. Chem. Phys. 123 (2005) 174301.[25] B.D. Benson, J.S. Francisco, An ab-initio study of the structure, vibrational

frequencies and heat of formation of ONCO radical, Chem. Phys. Lett. 233(1995) 335–339.

[26] J.J. Liu, Y.H. Ding, Y.G. Tao, J.K. Feng, C.C. Sun, Theoretical study on themechanism of the 1CHCl + NO reaction, J. Comput. Chem. 23 (2002) 625–649.

[27] J.X. Zhang, J.Y. Liu, Z.S. Li, C.C. Sun, Theoretical study on the mechanism of the1CHCl + NO2 reactions, J. Comput. Chem. 25 (2004) 1184–1190.

[28] C. Lee, W. Yang, R.G. Parr, Development of the Colle–Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37 (1988)785–789.

[29] F. Jensen, Introduction to Computational Chemistry, second ed., Wiley,Chichester, 2006.

[30] D.I. Auerbach, D. Nelson Jr., G.T. Fraser, W. Klemperer, Does ammoniahydrogen bond?, Science 238 (1987) I670–1674

[31] Basis sets were obtained from the Extensible Computational ChemistryEnvironment Basis Set Database, Version 02/25/04, as developed anddistributed by the Molecular Science Computing Facility, Environmental andMolecular Sciences Laboratory which is part of the Pacific NorthwestLaboratory, P.O. Box 999, Richland, Washington 99352, USA, and funded bythe U.S. Department of Energy. The Pacific Northwest Laboratory is a multi-program laboratory operated by Battelle Memorial Institute for the U.S.Department of Energy under contract DE-AC06-76RLO 1830. Contact KarenSchuchardt for further information.

[32] A.J.C. Varandas, Accurate determination of the reaction course in HY2 �Y + YH(Y = O, S): detailed analysis of the covalent- to hydrogen-bonding transition, J.Phys. Chem. A 117 (2013) 7393–7407.

[33] L.P. Viegas, A.J.C. Varandas, Coupled-cluster reaction barriers ofHO2 + H2O + O3 : an application of the coupled-cluster//Kohn-Sham densityfunctional theory model chemistry, J. Comput. Chem. 35 (2014) 507–517.

[34] T.J. Lee, P.R. Taylor, A diagnostic for determining the quality of single-referenceelectron correlation methods, Int. J. Quantum Chem. S23 (1989) 199–207.

[35] M. Horoi, J.R. Gour, M. Włoch, M.D. Lodriguito, B.A. Brown, P. Piecuch, Coupled-cluster and configuration-interaction calculations for heavy nuclei, Phys. Rev.Lett. 98 (2007) 112501.

[36] M.L. Leininger, I.M.B. Nielsen, T.D. Crawford, C.L. Janssen, Chem. Phys. Lett. 328(2000) 431–436.

[37] T.J. Lee, Comparison of the t1 and d1 diagnostics for electronic structuretheory: a new definition for the open-shell d1 diagnostic, Chem. Phys. Lett. 372(2003) 362–367.

[38] A.J.C. Varandas, Extrapolating to the one-electron basis-set limit in electronicstructure calculations, J. Chem. Phys. 126 (2007) 244105.

[39] A.J.C. Varandas, Extrapolation to the complete basis set limit withoutcounterpoise. the pair potential of helium revisited, J. Phys. Chem. A 114(2010) 8505–8516.

[40] A.J.C. Varandas, Helium–fullerene pair interactions: an ab initio study byperturbation-theory and coupled-cluster methods, Int. J. Quantum Chem. 111(2010) 416–429.

[41] A. Karton, J.M.L. Martin, Comment on Estimating the Hartree–Fock limit fromfinite basis set calculations, Theor. Chem. Acc. 115 (2006) 330–333.

[42] C. Schwartz, Importance of angular correlations between atomic electrons,Phys. Rev. 126 (1962) 1015.

[43] W. Kutzelnigg, J.D. Morgan III, Rates of convergence of the partial-waveexpansions of atomic correlation energiesrates of convergence of the partial-wave expansions of atomic correlation energies, J. Chem. Phys. 96 (1992)4484–4508.

[44] A.J.C. Varandas, Generalized uniform singlet- and triplet-pair extrapolation ofthe correlation energy to the one electron basis set limit, J. Phys. Chem. A 112(2008) 1841–1850.

[45] T. Helgaker, W. Klopper, H. Koch, J. Noga, Basis-set convergence of correlatedcalculations on water, J. Chem. Phys. 106 (1997) 9639–9646.

[46] K.A. Peterson, A.K. Wilson, D.E. Woon, T.H. Dunning Jr., Benchmarkcalculations with correlated molecular wave functions. 12. Core correlationeffects on the homonuclear diatomic molecules B2–F2, Theor. Chem. Acc. 97(1997) 251–259.

[47] A.J.C. Varandas, Accurate ab initio-based molecular potentials: fromextrapolation methods to global modelling, Phys. Scr. 76 (2007) C28–C35.

[48] A.J.C. Varandas, Accurate global ab initio potentials at low-cost by correlationscaling and extrapolation to the one-electron basis set limit, Chem. Phys. Lett.443 (2007) 398–407.

[49] A.J.C. Varandas, Accurate ab initio potentials at low cost via correlation scalingand extrapolation: application to CO(A 1P), J. Chem. Phys. 127 (2007) 114316.

[50] H.-J. Werner, P.J. Knowles, G. Knizia, F.R. Manby, M. Schütz, P. Celani, T. Korona,R. Lindh, A. Mitrushenkov, G. Rauhut, K.R. Shamasundar, T.B. Adler, R.D. Amos,A. Bernhardsson, A. Berning, D.L. Cooper, M.J.O. Deegan, A.J. Dobbyn, F. Eckert,E. Goll, C. Hampel, A. Hesselmann, G. Hetzer, T. Hrenar, G. Jansen, C. Köppl, Y.Liu, A.W. Lloyd, R.A. Mata, A.J. May, S.J. McNicholas, W. Meyer, M.E. Mura, A.Nicklass, D.P. O’Neill, P. Palmieri, K. Pflüger, R. Pitzer, M. Reiher, T. Shiozaki, H.Stoll, A.J. Stone, R. Tarroni, T. Thorsteinsson, M. Wang, A. Wolf, Molpro, version2010.1, A Package of Ab initio Programs, 2010. <http://www.molpro.net>.

[51] A.J.C. Varandas, Ab initio treatment of bond-breaking reactions: accuratecourse of HO3 dissociation and revisit to isomerization, J. Chem. TheoryComput. 8 (2012) 428–441.

[52] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by thedifferences of separate total energies. some procedures with reduced errors,Mol. Phys. 19 (1970) 553–566.

[53] A.J.C. Varandas, Can extrapolation to the basis set limit be an alternative to thecounterpoise correction? A study on the helium dimer, Theor. Chem. Acc. 119(2008) 511–521.

[54] A.J.C. Varandas, Extrapolation to the complete basis set limit withoutcounterpoise. The pair potential of helium revisited (vol. 114, pg 8505,2010), J. Phys. Chem. A 115 (2011) 2668.

[55] D.W. Schwenke, D.G. Truhlar, Systematic study of basis set superpositionerrors in the calculated interaction energy of two HF molecules, J. Chem. Phys.82 (1985) 2418–2426.

[56] G. Herzberg, Molecular Spectra and Molecular Structure. III Electronic Spectraand Electronic Structure of Polyatomic Molecules, Van Nostrand, New York,1966.

[57] M.H. Park, H. Kim, J.J. Urm, J.H. Lee, Y.-K. Han, Y.S. Lee, Hydrogen desorptionmechanism of a LiNH hydrogen storage system, J. Mol. Struct. (Theochem) 962(2010) 68–71.

[58] R.S. Mulliken, Criteria for the construction of good self-consistent-fieldmolecular orbital wave functions, and the significance of LCAO-MOpopulation analysis, J. Chem. Phys. 36 (1962) 3428–3439.

[59] A.J. Stone, The Theory of Intermolecular Forces, Clarendon Press, Oxford, U.K.,1996.

[60] A.J. Stone, Distributed multipole analysis: Stability for large basis sets, J. Chem.Theory. Comput. 1 (2005) 1128–1132.

[61] B. Rousseau, A. Peeters, C.V. Alsenoy, Atomic charges from modified voronoipolyhedra, J. Mol. Struct. Theochem. 538 (2001) 235–238.

[62] R.F.W. Bader, Atoms in Molecules, Clarendon Press, Oxford, 1990.[63] G. Hirsch, P.J. Bruna, S.D. Peyerimhoff, R.J. Buenker, Ab initio CI study of the

stability and electronic spectrum of the HOCl molecule, Chem. Phys. Lett. 52(1977) 442–448.

[64] E.R. Davidson, S. Chakravorty, A test of the Hirshfeld definition of atomiccharges and moments, Theor. Chim. Acta 83 (1992) 319–330.

[65] C.F. Guerra, J. Handgraaf, E.J. Baerends, F.M. Bickelhaupt, Voronoi deformationdensity (VDD) charges: assessment of the Mulliken, Bader, Hirshfeld,Weinhold, and VDD methods for charge analysis, J. Comput. Chem. 25(2003) 189–210.

[66] J.F. Harrison, Relationship between the charge distribution and dipole momentfunctions of CO and the related molecules CS, SiO, and SiS, J. Phys. Chem. A 110(2006) 10848–10857.

[67] J.W. Rabalais, J.M. McDonald, V. Scherr, S.P. McGlynn, Electronic spectroscopicof isoelectronic molecules. 11. Linear triatomic groupings containing sixteen,Chem. Rev. 71 (1971) 73–107.

[68] T. Helgaker, P. Jørgensen, J. Olsen, Molecular Electronic-Structure Theory,Wiley, Chichester, 2000.

[69] S.M. Freund, E. Herbst, R.P. Mariella Jr., W. Klemperer, Radio frequencyspectrum of the X2P state of 7Li16O⁄, J. Chem. Phys. 56 (1972) 1467–1476.

[70] C. Yamada, E. Hirota, Infrared diode laser spectroscopy of the LiO radical, J.Chem. Phys. 99 (1993) 8489–8497.

[71] A. Buchler, J. Stauffer, W. Klemperer, L. Wharton, Determination of thegeometry of lithium oxide, Li2O(g), by electric deflection, J. Chem. Phys. 39(1963) 2299–2303.

[72] D. White, K.S. Seshadri, D.F. Dever, D.E. Mann, M.J. Linevsky, Infrared spectraand the structures and thermodynamics of gaseous LiO, Li2O, and Li2O2, J.Chem. Phys. 39 (1963) 2463–2473.

[73] D. Bellert, D.K. Winn, W.H. Breckenridge, Rovibrational energy levels of theLiOLi molecule from dispersed fluorescence and stimulated emission pumpingstudies, Chem. Phys. Lett. 348 (2002) 3139–3148.

[74] X. Wang, L. Andrews, Infrared spectra, structure and bonding in the LiO2,LiO2Li, LiO and Li2O molecules in solid neon, Mol. Phys. 107 (2009) 739–748.

[75] J. Koput, K.A. Peterson, The ab initio potential energy surface and vibrational–rotational energy levels of dilithium monoxide, Li2O, J. Chem. Phys. 116 (2002)9255–9260.

[76] D. Bellert, D.K. Winn, W.H. Breckenridge, Dispersed fluorescence studies oflinear, LiOLi: a strongly bound, but very ‘floppy’ ionic molecule, Chem. Phys.Lett. 348 (2001) 39–46.

[77] Y. Yuan, L. Cheng, Theoretical prediction for the structures of gas phase lithiumoxide clusters: (Li2O)n (n = 1–8), Int. J. Quantum Chem. 113 (2013) 1264–1271.

[78] S. Patai (Ed.), The Chemistry of Cyanates and Their Thio-Derivatives, JohnWiley & Sons, New York, 1977.

[79] S.S.-W. Leung, A. Streitwieser, Theoretical study of structure of alkali metalcyanates and isocyanates and their related ion pair SN2 reactions, J. Comput.Chem. 19 (1998) 1325–1336.

A.J.C. Varandas / Computational and Theoretical Chemistry 1036 (2014) 61–71 71

[80] D. Poppinger, L. Radom, A theoretical study of substituted CHNO isomers, J.Am. Chem. Soc. 100 (1978) 3674–3685.

[81] T. Veszpremi, T. Pasinszki, M. Feher, The structure of pseudohalides-theexistence of a new isomer, J. Am. Chem. Soc. 116 (1994) 6303–6306.

[82] H. jie Zhu, Y. Ren, J. Ren, A theoretical study of the ion pair SN2 reactionbetween lithium isocyanates with methyl fluoride with inversion andretention mechanism, J. Mol. Struct. (Theochem.) 686 (2004) 65–70.

[83] M.A. McAllister, T.T. Tidwell, Substituent effects on the structures and energiesof lsocyanates and imines by ab initio molecular orbital calculations, J. Chem.Soc. Perkin Trans. 2 (1994) 2239–2248.

[84] E. Broughton, The Bhopal disaster and its aftermath: a review, Environ. Health:Global Access Sci. Sour. 4 (2005) 6.