Vibrational absorption intensities in chemical analysis. 4. An ab initio study of intensities for methyl ethanoate

The molecular structure and the vibrational spectra (infrared and Raman) of methyl ethanoate have been calculated with a 3-21G basis set. The structure, atomic polar tensors (dipole derivatives with respect to Cartesian displacements of the nuclei), and King's effective atomic charges have also been computed with 3-21G** (p functions on H atoms) and with a 3-21G basis set supplemented by hybrid sp orbits at the center of (a) all non-CH bonds and (b) all bonds (3-21G/cbsp). Inclusion of center of bond functions leads to a considerable improvement in CH bond lengths even when the functions are restricted to non-CH bonds. The dipolar properties, however, are substantially affected as the functions are added to the CH bonds themselves. With the 3-21G basis the King's effective atomic charges cover only a narrow range (0.147 e-0.158 e) for the hydrogen atoms of the CH3(CO) and for the OCH3 groups. Only small changes result from inclusion of polarization p functions on hydrogen atoms. With 3-21G/cbsp (all bonds) the CH3(CO) hydrogen effective charges drop to 0.135 e (CH in-plane of skeleton) and 0.127 e (CH out-of-plane) whereas the OCH3 rise to 0.185 e (in-plane) and 0.169 e (out-of-plane). The transformed atomic polar tensors give more insight into the polar behavior of molecular deformation. The squares of the CH dipole stretching gradients (proportional to IR intensities in the isolated bond model) of the CH3(CO) are one-third of those of the OCH3 whereas the in-plane bending intensity of the OCH3 is reduced to a very small value. These are in accord with known characteristics of infrared CH3 bands in the CH3(CO) and OCH3 environments. Differences in tensor components for the bonds in the skeletal plane and out-of-plane are rationalized in terms of the interaction of the bending bond with the electrons of the adjacent oxygen atom. Comparison with earlier results on 1,3,5-trioxane indicates a contribution proportional to the number of lone pair electrons in trans configuration. The ab initio quadratic force constants are scaled to product optimum fit to experimental wavenumbers. The absorption intensities agree well with measurements in CCl4 solutions of methyl ethanoate.