Resonance Effect in Formamide

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

Contribution from resonance structure 3, which contains a formal double bond between carbon and nitrogen, is considered to be primarily responsible for the coplanarity and the high rotational barrier about the amide bond [58], The introduction of resonance structure 3 also implies that there is significant charge-delocalization from the nitrogen lone pair to the carbonyl oxygen. 

Here we revisit the VB resonance model in formamide by taking all six resonance structures into account. Such a study allows us to compare the individual contributions from resonance structures 1-6 to the resonance effect in formamide. For comparison, the isoelectronic systems vinylamine and formamidine are also investigated to gain insights into the trends of resonance stabilization. A 6-31G(d) basis set is employed in the calculations, and the orbitals in the VB functions are self-consistently determined for each resonance structure, but restricted to be atomic orbitals. The structural weights of the six resonance structures are listed in Table 2. 

To derive the VRE s in these three conjugated systems, calculations with the three resonance structures 1, 2 and 5 are also performed and the results are presented in Table 2. 

Individual structural weights, total energies and VRE s from the VBSCF calculations with the 6-31G(d) basis set 

---