Ab initio calculations of the rotational barriers in formamide and acetamide: The effects of polarization functions and correlation

Ab initio calculations have been used to determine the gas-phase rotational barrier about the CN bond in formamide and acetamide. The results indicate that the inclusion of polarization functions in the basis set leads to a substantial decrease (ca. 5 kcal mol⁻¹) in the calculated barrier height at the SCF level. Electron correlation effects decrease the barrier by less than 1 kcal mol⁻¹, while the addition of zero point energy corrections changes the barrier height only slightly. Based upon the current calculations, the 0 K rotational barriers for isolated formamide and acetamide are predicted to be 14.2 and 12.5 kcal mol⁻¹, respectively.