CCSD(T) study of the far-infrared spectrum of ethyl methyl ether

摘要

Band positions and intensities for the far-infrared bands of ethyl methyl ether are variationallydetermined from a three-dimensional (3D) potential energy surface calculated with CCSD(T)/cc-pVTZ theory. For this purpose, the energies of 181 selected geometries computed optimizing3n-9 parameters are fitted to a 3D Fourier series depending on three torsional coordinates. The zeropoint vibrational energy correction and the search of a correct definition of the methyl torsionalcoordinate are taken into consideration for obtaining very accurate frequencies. In addition, secondorder perturbation theory is applied on the two molecular conformers, trans and cis-gauche, in orderto test the validity of the 3D model. Consequently, a new assignment of previous experimentalbands, congruent with the new ab initio results, is proposed. For the most stable trans-conformer,the v30, v29, and v28 fundamental transitions, computed at 115.3, 206.5, and 255.2 cm-1, arecorrelated with the observed bands at 115.4, 202, and 248 cm-1. For the cis-gauche the three bandpositions are computed at 91.0, 192.5, and 243.8 cm-I. Calculations on the —d3 isotopomer confirmour assignment. Intensities are determined at room temperature and at 10 K. Structural parameters,potential energy bathers, anharmonic frequencies for the 3n-9 neglected modes, and rotationalparameters (rotational and centrifugal distortion constants), are also provided.