Basis set and electron correlation effects on the internal rotational barrier heights of formamide and acetamide

Abstract

The internal rotational barrier heights of formamide and acetamide were studied by ab initio molecular orbital calculations using several basis sets up to 6-311 G (2d,2p) with electron correlation correction. The calculated barrier heights depended on the basis set used. The calculated barrier heights with the MP3 (Moller–Plesset 3rd order perburbation) electron correlation correction were decreased by the augmentation of polarized functions by as much as 5–6 kcal mol–1. Further augmentation of multiple polarized functions and diffuse functions had little effect on the calculated barrier heights. The calculated barrier heights using polarized basis sets were decreased by the incorporation of electron correlation by as much as 1–2 kcal mol–1 from the values obtained by HF method. The calculated barrier heights of formamide and acetamide using polarized basis sets with electron correlation correction were 14.5–15.4 and 12.5–13.2 kcal mol–1, respectively. Whereas these barrier heights were 1–8 kcal mol–1 lower than the experimental values measured in the liquid phase, the calculated values were close to the barrier heights measured in a polystyrene matrix. Zero point and thermal vibrational energies of formamide were calculated at HF/6-31 G* level. The calculated vibrational energy correction for the barrier height was only –0.75 kcal mol–1.