Ab initio studies of critical conformations in ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide

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Abstract
Ab initio calculations on conformations corresponding to the torsional energy minima and maxima of ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide were carried out with geometry optimizations using second‐order Mo/ller–Plesset perturbation theory (MP2) in the 6–311+G(3df,2p) basis. Compared with experiments the MP2/6–311+G(3df,2p) calculations yield absolute average deviations of 0.128 kcal/mol for 6 rotational barriers, 0.009 Å for 18 bond lengths, 0.7° for 16 bond angles, 0.5° for 2 dihedral angles, and 0.17 D for 5 dipole moments. Three smaller basis sets, 6–31G(d), 6–31+G(d,p), and 6–311G(d,p), were also used to study variations in the total energies and barrier heights as a result of basis set expansions. Several single‐point calculations were performed to estimate effects of electron correlation enhancement from MP2 to quadratic configuration interaction [QCISD(T)]. Simulating a high level calculation with lower level calculations in a procedure similar to the recent G2(MP2) theory was found very successful.