Importance of Electronic Delocalization on the C−N Bond Rotation in HCX(NH2) (X = O, NH, CH2, S, and Se)

Abstract

A block-localized wave function method, which in effect can switch off conventional conjugation and hyperconjugation effects, is employed to investigate the origin of the rotational barriers in formamide and its analogues. It is found that the resonance between the π electrons on the CX double bond and the nitrogen lone pair significantly stabilizes the planar conformation in HCXNH2 (X = O, NH, CH2, S, and Se). The absolute resonance energy follows the order of formamide < thioformamide < selenoformamide, with predicted vertical resonance energies of −25.5, −35.7, and −37.6 kcal/mol, respectively. The computed vertical resonance energies for X = O, NH, and CH2 are −25.5, −22.5, and −19.1 kcal/mol, respectively, which follow the decreasing trend of electronegativity. Although the rotational barrier about the C−N bond in vinylamine (4.5 kcal/mol) is much smaller than that of formamide (15.7 kcal/mol), the resonance energy in vinylamine is of the same order as that of formamide (−19.1 versus −25.5 kcal/mol)....