METHACROLEIN IN THE GAS-PHASE, INVESTIGATED BY ELECTRON-DIFFRACTION, MICROWAVE AND VIBRATIONAL SPECTROSCOPY, SUPPLEMENTED WITH AB-INITIO CALCULATIONS OF GEOMETRIES AND FORCE-FIELDS

The structure of methacrolein was studied by the analysis of gas-phase electron diffraction, Microwave and infrared (IR) data. The experimental data are supported with geometrical constraints taken from geometry relaxed ab initio calculations and vibrational constraints taken from force field calculations, both at the 4-2 1 G level. All data are in accord with the gas phase consisting of ap and sp conformers. An independent vibrational analysis was performed leading to an improved assignment of the IR frequencies. Assignments were found to depend upon the scaling of the ab initio force field, particularly on the grouping of the symmetry coordinates. The scaled force field reproduced the IR spectra with root-mean-square deviations of 15.5 cm-1 (ap) and 18.5 cm-1 (sp), and largest discrepancies of 36 cm-1 (ap) and 39 cm-1 (sp). Differences between individual force constants of the two forms could be rationalized. At room temperature the ratio ap: sp = 89 : 11 was found, but we consider the resulting energy (enthalpy) difference of DELTAH = 1.15 kcal mol-1 as a lower limit and the experimental value of 2.17 kcal mol-1 [Durig et al., Spectrochim. Acta, Part A, 42 (1986) 891 as an upper limit to the true value. The data are in accord with planar heavy atom skeletons for both the ap and sp forms. Subject to the ab initio constraints, the r(alpha)-degrees and r(g) geometry are reported. Least-squares refinements showed equal applicability of two approaches to bring 4-21 G geometries to an r(g) basis, one using individual, additive correction parameters and the other using regression-based correction parameters.