Theoretical study of the structure of 2-methylpropanal in the ground electronic state: Three-dimensional model for torsional vibrations

The geometric structure and conformational dynamics of the 2-methylpropanal molecule in the ground electronic state was studied using ab initio methods (MP2, CASSCF, and QCISD). The region of the potential energy surface (PES) corresponding to transitions between conformers by the rotation of the isopropyl and two methyl tops was examined in one-, two-, and three-dimensional approximations. The influence of the atomic orbital basis set and the method for the treatment of correlation effects on the accuracy of the calculation of PES characteristics was considered. To describe large-amplitude motion on the basis of calculated PES cross-sections, one-, two-, and three-dimensional vibrational problems were solved in the anharmonic approximation. The comparison of the results makes it possible to estimate coupling between torsional vibrations, and the most rigorous and informative technique for estimating this coupling seems to be the analysis of nodal surfaces of multidimensional vibrational wavefunctions.