RESONANCE INTERACTIONS IN ACYCLIC SYSTEMS .3. FORMAMIDE INTERNAL-ROTATION REVISITED - CHARGE AND ENERGY REDISTRIBUTION ALONG THE C-N BOND ROTATIONAL PATHWAY

The changes that occur during the rotation of the amino group of formamide have been studied in some detail. Geometry optimizations at the MP2/6-31 G* level confirmed the relatively large increase in C-N bond length but small decrease in the C-O length on going from the planar structure to the rotational transition state. A calculation of the force constants for formamide and for the transition state showed that the carbonyl force constant changed relatively little, but the C-N constant changed by about 30%. The path followed in the rotation was studied starting with the saddle point geometry and following it computationally down to the ground state. The geometrical changes are discussed. The electron populations were calculated for a number of structures along the reaction coordinate by numerical integration of the charge density within uniquely defined atomic volumes. The oxygen population was little affected by the rotation and the main charge shift was between carbon and nitrogen. The electrostatic potentials for the structures also were examined and converted to effective charges for spherically symmetrical atoms. All of the analyses indicated that essentially all of the interactions leading to the rotational barrier originate in the C-N bond and that the oxygen does not participate to a significant extent. The direction of the charge shift between C and N was in opposite directions for the electron populations derived by integration of the charge density, and by fitting the electrostatic potentials. However, this was due to the difference in the definition of the atoms, being anisotropic in the first case and spherically symmetrical in the second. All of the observations can be rationalized on the basis of the assumption that stabilization of the lone pair on nitrogen is the most important factor in determining both structures and energies.