Chemical Physics Letters 463 (2008) 330–333

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Chemical Physics Letters

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A rotational study of the molecular complex tert-butanol���NH3

Barbara M. Giuliano a, Mattea C. Castrovilli a, Assimo Maris a, Sonia Melandri a, Walther Caminati a,*,Edward A. Cohen b

a Dipartimento di Chimica, ‘G. Ciamician’, dell’Università, Via Selmi 2, I-40126 Bologna, Italyb Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA

a r t i c l e i n f o

Article history:Received 30 July 2008In final form 19 August 2008Available online 22 August 2008

0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.08.071

* Corresponding author. Fax: +39 051 209 9456.E-mail address: walther.caminati@unibo.it (W. Cam

a b s t r a c t

The rotational spectra of tert-butanol���14NH3 and tert-butanol���15NH3 have been investigated andassigned by pulsed jet Fourier transform microwave spectroscopy. According to the values of the 14Nquadrupole coupling constants, the complex (where NH3 acts as a proton acceptor) adopts a configura-tion with an almost linear O–H���N hydrogen bond. The NH3 moiety undergoes a nearly free rotation.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Intermolecular interactions are of well-known importance inunderstanding basic chemical, physical and biological properties.In particular, the hydrogen bonding is ubiquitous in biological sys-tems, solution chemistry and other chemical phenomena. Theinteractions involving oxygen, hydrogen and nitrogen atoms havebeen the subject of many experimental and theoretical works be-cause of their role in shaping DNA structure or in driving proteinfolding. The most common hydrogen bonds in these systems areof the types O–H���O, O–H���N, N–H���O and N–H���N. They are mod-erately strong, in the range 15–25 kJ/mol�1.

High resolution molecular spectroscopy, in particular pulsed jetFourier transform microwave spectroscopy, has been used todetermine in a very precise and univocal way the structure of com-plexes formed by hydrogen bonded molecules, and then to charac-terize the features of specific hydrogen bonds.

In this way, detailed information has been obtained on the pe-culiar role in hydrogen bonding networks played by water, whichcan easily change its role as proton donor/acceptor, as, for example,in the water dimer [1]. In complexes of water with organic/biolog-ical molecules, it has been found that water can play such a doublerole either with molecules containing oxygen or nitrogen atoms.For example, in the cases of phenol–water [2] and indole–water[3], water acts as proton acceptor, while in the cases of oxirane[4] and pyrrolidine [5] its role is that of a proton donor. Finally,water forms adducts with formamide either as a proton donor ora proton acceptor [6].

Ammonia, similarly to water, possesses slightly acidic protonsand a lone pair, and could play an analogous double role. We have

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inati).

already mentioned that such a double role can be played by aminesor imines (see Refs. [3,5]). However, the dimer of ammonia itselfdisplays an unexpected structure [7], and the rotational spectraof its complexes with organic molecules revealed only conformerswhere ammonia acts as a proton acceptor. This is the case forF3CH–NH3 [8], CH3OH–NH3 [9] and pyrrole–NH3 [10].

However, only a few molecular systems have been investigatedwhere ammonia has the possibility playoff playing its double roleof proton donor/acceptor. In the case of the tert-butanol���NH3

(TBA–NH3) complex, ammonia could bind to the alcohol moleculeeither through an OH���N or a NH���O linkage. For this reason westudied its rotational spectrum.

2. Experimental

The microwave spectra of the TBA–NH3 have been recorded inthe frequency range 6–18 GHz using a COBRA version [11] of aBalle–Flygare type [12] molecular beam Fourier transform micro-wave spectrometer already described elsewhere [13], recently up-dated with the FTMW++ set of programs [14].

A gaseous mixture of 1% anhydrous ammonia (99.99% pure, bySigma–Aldrich) diluted in helium was flowed over a sample of TBA(supplied by Sigma–Aldrich and used without further purification)at room temperature, and expanded through a solenoid valve(General Valve series 9) into the Fabry–Perot type resonator cham-ber. The backing pressure was kept at 3.5 bar in order to reach aconcentration of about 1% of t-butanol in the gas mixture priorto the expansion being the ideal experimental conditions for theformation of the complex a 1:1 ratio of the two moieties.

To produce TBA–15NH3, 15NH3 (Sigma–Aldrich) with a 15N atomfraction of 98% has been used.

The estimated accuracy of the measured frequency was about2 kHz and the resolution of the hyperfine components was about7 kHz.

Fig. 1. Hyperfine structure of the 31,2 20,2 transition of TBA–14NH3.

B.M. Giuliano et al. / Chemical Physics Letters 463 (2008) 330–333 331

3. Theoretical calculations

MP2/6-311++G** preliminary ab initio calculations were run toobtain information about the geometry and the relative bindingenergies of the complex. For this purpose, the GAUSSIAN 03 packageof programs [15] was used. Two stable structures, I and II, with anO–H���N and an N–H���O hydrogen bond, respectively, have beenfound. Their energies and calculated spectroscopic constants andshapes are shown in Table 1.

The equilibrium values of the dissociation energies have alsobeen calculated, giving De(I) = 31.76 and De(II) = 19.06 kJ/mol,respectively. MP2/6-311++G** vibrational frequencies in the har-monic approximation were used to confirm that the two stationarypoints found are minima and to calculate zero point dissociationenergies of D0(I) = 24.96 and D0(II) = 13.53 kJ/mol. The De and D0

values were also corrected for basis set superposition error (BSSE)using the counterpoise procedure [16] giving for D0e(I), D0e(II), D00(I)and D00(II) the values 23.20, 12,93, 16.40 and 7.39 kJ/mol,respectively.

The calculated rotational constants, dipole moment compo-nents, quadrupole coupling constants and relative energies (DE,DE0, DE0 and DE00‘ values) of the two conformers are shown in Table1.

4. Rotational spectra

The calculations indicate that the two forms have similar rota-tional constants and both spectra are predominantly la-type. Forthis reason the first search for the rotational transitions was fo-cused on the la–R type transitions. After a first scan the J = 3 2and 4 3 bands, Ka = 0 and 1 transitions have been assigned. Sub-sequently more la and the lc–R type transitions were measured. Inspite of a deep search, no lb type transitions were found.

The measured lines exhibit the hyperfine structure due to theinteraction between the 14N nuclear quadrupole and the overall

Table 1MP2/6-311++G** calculated spectroscopic parameters and relative energies of the two stab

I

A/MHz 4504B/MHz 1634C/MHz 1628la/D �2.8lb/D 0.0lc/D 1.9vaa/MHz �3.74vbb/MHz 1.93vcc/MHz 1.81DE/kJ mol�1a 0b

DE0/kJ mol�1 0DE0/kJ mol�1 0DE00/kJ mol�1 0

a DE, DE0, DE0 and DE00 are the equilibrium and zero point energy difference between tcorrections.

b Absolute energy of the complex (�289.478146 Ha).

rotational angular momenta. This structure is shown in Fig. 1 forthe 31,2 20,2 transition.

All the measured frequencies (given as Supplementary mate-rial) were fitted using SPFIT from Pickett’s suite of programs CALPGM

[17] within Watson’s S reduction and Ir representation [18]. Thederived spectroscopic parameters are reported in Table 2. Afterthe analysis of the most abundant species, the spectrum ofTBA–15NH3 was easily assigned. It is simpler than that ofTBA–14NH3 because 15N does not have quadrupole moment.

The measured lines, which are available as Supplementarymaterial, probably belong to the m = 0 state of the free or near-freeinternal rotation of the NH3 group as will be discussed below. Theobtained spectroscopic constants are reported in the second col-umn of Table 2.

le forms of TBA–NH3

II

456716691650�1.20.60.91.581.90�3.4812.7011.4310.279.01

he two conformers, without and with (primed values) basis set superposition error

Table 2Spectroscopic parameters of TBA–NH3

TBA–14NH3 TBA–15NH3

A/MHz 4599.961(1) 4599.933(1)B/MHz 1607.5260(2) 1561.7992(3)C/MHz 1600.1612(2) 1554.8461(4)DJ/kHz 1.233(5) 1.188(8)DJK/kHz 16.01(2) 15.44(6)vaa/MHz �3.188(5) –(vbb � vcc)/MHz 0.040(8) –r/MHz 0.004 0.003N 57 20

332 B.M. Giuliano et al. / Chemical Physics Letters 463 (2008) 330–333

5. Conformation and structure

There are two sets of experimental data which contain informa-tion on the orientation of NH3 with respect to the alcohol moiety.The first is given by the substitution coordinates [19] (shown inTable 3) of the nitrogen atom in the principal axes system ofTBA–14NH3 from analyzing the changes on the planar momentsof inertia when going to TBA–15NH3. Unfortunately these valuescannot discriminate between the two conformers, because theyare very similar to each other.

The second set of data, the quadrupole coupling constants, de-pends on the orientation of the 14N quadrupolar tensor with respectto the principal axes system of the complex. Since the orientation of

Table 4Experimental quadrupole coupling constants of TBA–NH3 are compared to thetheoretical values of species I and II

Exp. Calc. I Calc. II

vaa/MHz �3.188(5) �3.74 1.58(vbb � vcc)/MHz 0.04(8) 0.03 5.38

Fig. 2. Sketch of TBA–NH3, and para

Table 3Experimental substitution coordinates of TBA–NH3 are compared to the theoreticalvalues of species I and II

Exp. Calc. I Calc. II

a/Å ±3.0550(5) 3.0216 �3.0254b/Å ±0.0a 0.0a 0.022c/Å ±0.027(6) 0.040 �0.030

a Set to zero for symmetry reasons.

NH3 is very different in the two forms, the values of the quadrupolecoupling are also very different, and one can see from Table 4 thatthe experimental values correspond to species I.

However, there are still considerable differences between theobserved value of vaa and that calculated for species I. In theequilibrium structure of the complex as determined by ab initiocalculations, the symmetry axis of ammonia molecule (defined asthe z-axis, see Fig. 2) is almost collinear with the a-axis (the calcu-lated angle is 4.2�) and the predicted value for the vz constant ofthe NH3 molecule is �3.92 MHz. Thus, if we assume that the com-plex formation does not affect the electric field gradient at the Nnucleus, one can expect the vaa quadrupole coupling constant tobe very similar to the v0 constant of the isolated NH3 molecule,which is �4.08983(2) MHz [20]. On the contrary, the experimen-tally derived vaa value is 22% smaller than the v0 constant. Thisdiscrepancy could be interpreted as a reduction that arises fromzero point angular oscillations of the NH3 moiety relative to thea-axis of the complex. In particular, it is also possible to quantifythe oscillation amplitude from the equations below [8,9]

v0 ¼ vaahð3 cos2 a� 1Þ=2i ð1Þ

�a ¼ arccosðhcos2 ai1=2Þ ð2Þ

where �a is the effective bending angle of the motion. Taking intoaccount the experimental value vaa = �3.188(5) MHz, one canestimate �a = 22.5(1)�.

This result shows a good agreement with the values previouslyreported for similar adducts, such as F3CH–NH3 [8] and CH3OH–NH3 [9].

In principle, it would be possible to fit the six available rota-tional constants to determine a partial r0 structure, that is the Rand h parameters of Fig. 3. However, since we observed only them = 0 level of the presumably almost free internal rotation ofNH3, we have to take into account that in this state the NH3 moietyis not rotating. This means that the planar moment of inertia Pbb

[=1/2 (�Ibb + Iaa + Icc)] should correspond only to the TBA contribu-tions. Unfortunately, the MW spectrum of TBA is very complicatedbecause of the delocalisation of the hydroxyl hydrogen, and therotational constants are available only for a preliminary estimateof Pbb (�55.5 uÅ2 [21]). However, while the Pbb’s ab initio (equilib-rium) values of TBA and TBA–14NH3 (=55.476 and 56.707 uÅ2,respectively) differ of the ammonia contributions, the experimen-tal Pbb value of TBA–14NH3 (=55.656 uÅ2) is very close to that ofbare TBA, so supporting the hypothesis of a very low V3 barrier.

In conclusions, to lead to the effective change in the Pbb value,the rotational constants of the ground state must be correctedaccording to

meters of the NH3 orientation.

Fig. 3. Hydrogen bond parameters of TBA–NH3.

B.M. Giuliano et al. / Chemical Physics Letters 463 (2008) 330–333 333

A00 ¼ Ar þW ð2Þ00 Fq2

a

B00 ¼ Br

C00 ¼ Cr þW ð2Þ00 Fq2

c

ð3Þ

where Ar, Br and Cr are the ‘rigid’ rotational constants in the limit ofthe very high barrier. The W ðnÞ

00 are the Hersbach’s barrier-dependentperturbation sums relative to the A sublevels of the ground state[22] and qg = kgIa/Ig.

Since ka ffi cos(4.2�) and kc ffi cos(85.8�), one can expect a verylittle change on the rotational constant C with respect to the ‘rigid’value, while B remains unchanged. It was then possible to usethese rotational constants (for both isotopologues) to estimate apartial r0 structure, obtaining R = 2.03 Å and h = 171�. This is onthe order of the theoretically predicted values, R = 1.979 Å andh = 171.0�.

6. Dissociation energy

The intermolecular stretching motion, directed along the hydro-gen bond direction, is almost parallel to the a-axis of the complex.In this case we can use the pseudo diatomic approximation whichconsiders the two molecular subunits of the complex as two rigidparts. Using this model the hydrogen bond stretching force con-stant can be derived from the B and C rotational constants andthe DJ centrifugal distortion constant through the equation [23]

ks ¼ ð4pÞ4ðlRcmÞ2½4B4 þ 4C4 � ðB� CÞ2ðBþ CÞ2�=hDJ ð4Þ

where l is the reduced mass of the complex and Rcm is the distancebetween the centers of mass of the two subunits as shown in Fig. 2.

Contributions to the DJ parameter from the free internal rota-tion of the NH3 group were considered as negligible due to the nearco-linearity between the principal axis a of the complex and theinternal rotation axis.

From the partial structural fitting a Rcm value of 3.841 Å is de-rived, the corresponding ks is 7.87(3) N/m. From this value the cor-responding harmonic stretching frequency ms = 1/2p(ks/l)1/2 isdetermined to be 98 cm�1.

Assuming a Lennard-Jones type potential the zero point dissoci-ation energy of the complex can be derived applying the approxi-mate expression [24] ED = 1/72 ks � R2

cm which results in a value of10.0 kJ/mol. This value is rather close to the theoretical one.

7. Conclusions

The structural and energetic features of TBA–14NH3 have beeninvestigated through molecular beam microwave spectroscopy.The experimentally determined quadrupole coupling constantspermit discrimination between the two possible hydrogen bondstructures. The one in which the ammonia molecule acts as protonacceptor has been observed and appears much more stable thanthe one with inverted proton donor/acceptor role. The hydrogenbond geometry has been determined to have an H���N bond lengthof 2.03 Å which is in a very good agreement with the bond lengthsof the previous ammonia complexes studied. The dissociation en-ergy, force constants and vibrational frequency have been also esti-mated and the experimentally derived values are of the same orderof magnitude as the calculated ones. They are consistent with thevalues for the other ammonia complexes studied. These resultsshow that the three methyl groups in the TBA molecule do not af-fect the characteristics of the hydrogen bond. In particular, the val-ues determined are very similar to those of methanol-ammonia.

The analysis of the quadrupole coupling constants revealed alarge amplitude bending motion of the ammonia moiety.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2008.08.071.

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