substituent effect on n-h bond dissociation of carbamates
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Une revue deNRC Research PressCanadian Journal of
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Substituent effect on NH bond dissociation enthalpies ofcarbamates: a theoretical studyRupinder preet Kaur, Damanjit Kaur, and Ritika Sharma
Abstract: The present investigation deals with the study of the NH bond dissociation enthalpies (BDEs) of the Y-substituted(NH2-C(=X)Y-R) and N-substituted ((R)(H)NC(=X)YH) carbamates (X, Y = O, S, Se; R = H, CH3, F, Cl, NH2), which have been evaluatedusing ab initio and density functional methods. The variations in NH BDEs of these Y-substituted and N-substituted carbamatesas the effect of substituent have been understood in terms of molecule stabilization energy (ME) and radical stabilization energy(RE), which have been calculated using the isodesmic reactions. The natural bond orbital analysis indicated that the electrode-localization of the lone pairs of heteroatoms in the molecules and radicals affect the ME and RE values depending upon the typeand site of substitution (whether N- or Y-). The variations inNHBDEs depend upon the combined effect ofmolecule stabilizationand radical stabilization by the various substituents.
Key words: carbamates, substituent effect, molecule stabilization energy, radical stabilization energy, natural bond orbital,electron delocalization.
Rsum : La prsente tude sintresse aux enthalpies de dissociation des liaisons (EDL) NH de carbamates Y-substitus(NH2-C(=X)Y-R) et N-substitus ((R)(H)NC(=X)YH) (X, Y =O, S, Se; R =H, CH3, F, Cl, NH2), lesquels ont t valus a` laide demthodesab initio et de la fonctionnelle de la densit. Les variations des EDLNHde ces carbamates, consquences de leffet de substituant,ont t apprhendes du point de vue de lnergie de stabilisation molculaire (ESM) et de lnergie de stabilisation radicalaire(ESR) qui ont t calcules a` partir des ractions isodesmiques. Lanalyse des orbitales naturelles de liaison a montr que ladlocalisation lectronique des paires isoles dhtroatomes dans les molcules et les radicaux affectait les valeurs de lESM etde lESR, en fonction du type et du site de substitution (N- ou Y-). Les variations des EDL NH dpendent des effets combins desstabilisations molculaire et radicalaire par divers substituants. [Traduit par la Rdaction]
Mots-cls : carbamates, effet de substituant, nergie de stabilisation molculaire, nergie de stabilisation radicalaire, orbitalenaturelle de liaison, dlocalisation lectronique.
IntroductionIn reactions involving radical intermediates, no quantity is
more important than the homolytic bond dissociation energy(BDE).1 Thermodynamics of reactions involving the free radical isgoverned by enthalpy change accompanying the formation ofthe free radical, i.e., BDE.2 The BDEs act as database for reectingthe intrinsic or instantaneous strength of the bond as well as thestability of the radicals obtained after cleavage. These are one ofthe important properties of molecules for considering their de-composition and chemical reactivities and for estimating theirheat of formation.3 The accurate prediction of BDEs has numer-ous applications, including the identication of sites for potentialfree radical attack in peptides, the assessment of the effectivenessof antioxidants, and the study of chain-transfer processes (such aslong-chain branching) in free radical polymerization.47 Most ofthe organic and biochemical reactions involve abstraction of thehydrogen atom that includes combustion, polymerization, atmo-spheric, and interstellar chemical pathways. The oxidation of ri-bonucleosides and deoxyribonucleosides by certain antibiotics,metal complexes, and redox active metalloenzymes has been sug-gested to occur through hydrogen abstraction.810 Hydrogen atomabstraction by the adenosyl radical is the key activation step incoenzyme B-12 mediated processes,11 whereas hydrogen atom ab-straction reactions involving peptide radicals are associated with
various physiological disorders12,13 such as arteriosclerosis,14 dia-betes, aging,15,16 and Alzheimers disease.1719
The NH bond is a key functional group in organic and biolog-ical chemistry. The class of compounds containing NH bonds hasattracted a large number of researchers due to their presence inpharmaceutical agents,2024 building blocks of biomolecules,synthetic commercial products and toxic substances, and2527
antioxidants and as22,2830 complexing agents,31 herbicides, andsurfactants.22,30,32 Nitrogen-centered radicals are of synthetic im-portance33 where they have been used in a number of cyclizationprocesses34 and homolytic amination reactions of aromatic mol-ecules. The NH bond cleavage is also reported in the protontransfer enzymatic reactions catalyzed by acetylcholinestrase. Ni-trogen derivatives of amides, lactams, carbamates, and imideshave been shown to be effective initiators for the metal-catalyzedliving radical polymerization of methacrylates.35
Determination of NH BDEs of carbamates is important frombiological, synthetic, medicinal as well as industrial perspec-tive, as they exhibit complex chemical and biological activi-ties as synthetic intermediates, protecting groups, chelatingagents, and free radical scavengers and being constituents ofinsecticides, pesticides, fungicides, antiviral, antifungal, anti-bacterial, antiparasitic, antiproliferative, antidermatophytic,antitumor agents, chemotherapeutic drugs, antioxidants, andindustrial chemicals.3643
Received 18 July 2014. Accepted 3 October 2014.
R. Kaur and R. Sharma. Guru Nanak Dev University College, Verka, Distt, Amritsar 143001, India.D. Kaur. Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India.Corresponding author: Rupinder preet Kaur (e-mail: firstname.lastname@example.org).
Can. J. Chem. 93: 279288 (2015) dx.doi.org/10.1139/cjc-2014-0326 Published at www.nrcresearchpress.com/cjc on 20 October 2014.
The strength of a bond is sensitive to the number of atoms/groups attached to the atom involved in bond dissociation. Asubstituent inuences the bond strength of a molecule in a quan-titative sense. The strength of the NH bond is sensitive to thenumber of atoms attached to the nitrogen.44,45 Substituents pres-ent on the radical because of their steric or electronic natureinuence both the kinetic and thermodynamic stability of theradical.46 Substituents through their steric effects can play asignicant role in the kinetic stabilization, whereas loweringof energy of the ground state plays an important role in the ther-modynamic stabilization of the radical. It is therefore the intrinsicproperty that is principally inuenced by the ability of the sub-stituents to delocalize the unpaired electron.47 This can reducethe reactivity by reducing the spin densities on radical center. Themeasure of stability of nitrogen-centered radicals is provided bythe NH homolytic BDE.
The effect of the substituent on the intrinsic structure, reactiv-ity, and energetics of carbamates has been analyzed by studyingthe BDE in NH2C(=X)Y-R and (R)(H)NC(=X)YH (X, Y = O, S, Se; R = H,F, Cl, CH3, NH2) molecules.
Computational detailsAll of the calculations reported in the present study were car-
ried out using the Gaussian 98 program suite.48 Full geometryoptimizations were performed on each species without anysymmetry constraint at the B3LYP/6-31+G* and MP2/6-31+G*theoretical levels. Each optimized structure was characterized byfrequency calculations to be a minimum without any imaginaryvibrational frequency. The geometries optimized at the B3LYP/6-31+G* level have been used to calculate single point energies at the6-31+G* and 6-311++G** basis sets using restricted open-shell for-malism. The CBS-Q composite level has also been used to calculatethe BDEs. The calculations have been performed using the Gauss-ian 09 program suite.
BDE is calculated as the enthalpy change of the following reac-tion in the gas phase at 298 K and 1 atm of pressure:
A-B g A g B g
The BDE at 298 K was calculated by using the thermochemicalscheme supplied by Gaussian as in the following equation:BDE298(R1R2) = [fH298(R1) + fH298(R2)] fH298(R1R2), in whichR1R2 is the neutral molecule and R1 and R2 are the correspond-ing radicals.49 By following the comprehensive paper of Merricket al., zero-point energies are scaled by a factor of 0.9153 at HF/6-31+G*, and these scaled zero-point vibration energy values areused for applying corrections to energies evaluated at the MP2/6-31+G* theoretical level and by 0.9806 at the B3LYP levels.50 Vibra-tional frequencies for calculating vibrational enthalpy are scaledby 0.8945 at HF/6-31+G*, and these scaled values are used for theMP2/6-31+G* theoretical level and by 0.9989 for the B3LYP level.51
The electronic energy of atomic hydrogen atom in DFT calcu-lations is basis set dependent, as can be observed from Jursicand Martins data (0.50027 hartree for the 6-31G basis set,0.50216 hartree for the 6-311G basis set, and 0.50226 hartree forthe 6-311++G basis set).52 Pople et al. suggested a high-level correc-tion for bound unpaired electrons and corrected the electronicenergy of the hydrogen atom to 0.50000 hartree in the G2method.53
DiLabio and Pratt suggested that for the hydrogen atom being aunique one-electron system, a similar correction should be ap-plied in DFT calculations for the evaluation of BDE by xing theelectronic energy of hydrogen to 0.50000 hartree.54 This hasbeen found to decrease the deviation of calculated BDEs frome