Saddle-point geometries and barriers to internal rotation of formamide : an ab initio study

Abstract

By use of a direct transition-state program and the STO-3G minimal basis set, two saddle-points are detected on the energy surface for internal motion of formamide. These correspond mainly to rotation about the C-N bond along with some lengthening of this bond and increased pyramidal distortion at nitrogen as compared with that in the ground state. The STO-3G estimates of the barrier height (34-39 kJ mol^-1) are in very poor agreement with experimental values (70-90 kJ mol^-1), but 4- 31G energy evaluations for the STO-3G-optimized structures give much better estimates (62-80 kJ mol^-1). Contrary to a previous report, use of the 4-31G extended basis set for geometry optimization suggests that only the lower-energy member (NH₂ cis to CO) of the above pair is a true transition state for internal motion of formamide; its energy relative to that of the 4-31G-optimized ground state (planar) is 83.5 kJ mol^-1, very close to the midpoint of the experimental range. The transition state appears to lie in a region of the 4-31G energy surface that is relatively flat with respect to pyramidal distortion at nitrogen; constraining the amino group to planarity raises the calculated energy by only 6.5kJmol^-1.