Structures of the rotamers of nitrosomethanol: An ab initio study

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Ab initio Hartree— Fock (HF) and large scale configuration interaction (CI) calculations are performed on the nitrosomethanol molecule (HO(CH 2)NO). The potential energy surface is investigated at the HF level using a basis set of double-ζ (DZ) quality with an emphasis on the two internal rotational degrees of freedom. In agreement with experiment, two rotameric forms are predicted to exist, one of which displays an intramolecular hydrogen bond. In contrast to the results of previous calculations, neither of these structures is predicted to possess a plane of symmetry. Single point calculations have been performed at the HF level incorporating a single set of polarization functions on each of the atoms ( p on hydrogen: d on carbon, nitrogen and oxygen). The effect of electron correlation was investigated by performing CI calculations including all single and double substitutions from the HF reference function. This generates 115,961 singlet configurations using the DZ basis set. Electron correlation was found not to play a major role in determining the relative energies along the rotational degrees of freedom. Final estimates of the relative energies come from assuming additivity of basis set (DZ → DZ plus polarization) and electron correlation (HF → CI) improvements. Analytical second derivatives of the HF energy with respect to nuclear coordinates were calculated at the DZ level for all of the stationary points located. Harmonic vibrational frequencies were calculated for the two putative conformers of nitrosomethanol.