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Abstract: Hydrogen bond complexation between glycine and THF and between glycine and water involving four lowest-energy glycine conformers have been studied. The complexes have been investigated in the gas phase [...]

The present investigation deals with the study of the N-H bond dissociation enthalpies (BDEs) of the Y-substituted (N[H.sub.2]-C(=X)Y-R) and N-substituted ((R)(H)NC(=X)YH) carbamates (X, Y = O, S, Se; R = H, C[H.sub.3], F, Cl, N[H.sub.2]), which have been evaluated using ab initio and density functional methods. The variations in N-H BDEs of these Y-substituted and N-substituted carbamates as 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 electrodelocalization of the lone pairs of heteroatoms in the molecules and radicals affect the ME and RE values depending upon the type and site of substitution (whether N- or Y-). The variations in N-H BDEs depend upon the combined effect of molecule stabilization and radical stabilization by the various substituents. Key words: carbamates, substituent effect, molecule stabilization energy, radical stabilization energy, natural bond orbital, electron delocalization. La presente etude s'interesse aux enthalpies de dissociation des liaisons (EDL) N-H de carbamates Y-substitues (N[H.sub.2]-C(=X)Y-R) et N-substitues ((R)(H)NC(=X)YH) (X,Y=O, S, Se; R = H, C[H.sub.3], F, Cl, N[H.sub.2]), lesquels ont ete evalues a l'aide de methodes ab initio et de la fonctionnelle de la densite. Les variations des EDL N-H de ces carbamates, consequences de l'effet de substituant, ont ete apprehendees du point de vue de l'energie de stabilisation moleculaire (ESM) et de l'energie de stabilisation radicalaire (ESR) qui ont ete calculees a partir des reactions isodesmiques. L'analyse des orbitales naturelles de liaison a montre que la delocalisation electronique des paires isolees d'heteroatomes dans les molecules et les radicaux affectait les valeurs de l'ESM et de l'ESR, en fonction du type et du site de substitution (N- ou Y-). Les variations des EDL N-H dependent des effets combines des stabilisations moleculaire et radicalaire par divers substituants. [Traduit par la Redaction] Mots-cles: carbamates, effet de substituant, energie de stabilisation moleculaire, energie de stabilisation radicalaire, orbitale naturelle de liaison, delocalisation electronique., Introduction In reactions involving radical intermediates, no quantity is more important than the homolytic bond dissociation energy (BDE). (1) Thermodynamics of reactions involving the free radical is governed by enthalpy [...]

Ab initio MO and density functional method were used to study the electron delocalization in isocyanates, formamides, and ureas. An explanation is provided in terms of orbital interactions for the observed trends in the C-N rotational barriers and N-inversion barriers in these systems.