The geometry and internal rotational barrier of carbamic acid and several derivatives

The geometries of the following carbamate-containing molecules have been optimized at the ab initio SCF level using the MINI-1, 3-21G, and 6-31G* basis sets: carbamic acid, methylcarbamate, ethylcarbamate, N -methyl methylcarbamate, N,N -dimethyl methylcarbamate, N -vinyl methyl-carbamate and N -phenyl methylcarbamate. The latter two basis sets yield geometries in best agreement with experimental data. Calculated energy barriers for rotation about the CN bond become progressively higher as the basis set is enlarged; electron correlation leads to a modest reduction. At the MP2/6-31G*//6-31G* level, the barriers lie in the range of 80–90 kJ mol −1. Alkyl substitution on either end of the carbamate group increases the + C=O − charge separation. N -substitution reduces the electron density on the nitrogen whereas the negative charge of this atom is increased by esterification of carbamic acid.