ChemInform Abstract: Methyl Vinyl Ketone in the Gas Phase, Investigated by Electron Diffraction, Infrared Band Contour Analysis and Microwave Spectroscopy, Supplemented with ab initio Calculations of Geometries and Force Fields.

Abstract

Abstract The structure of methyl vinyl ketone was studied by the analysis of gas phase electron diffraction, microwave and infrared (IR) data, including band profiles. The experimental data are supplemented with vibrational constraints taken from force-field calculations at the 4-21G ab-initio level and with geometrical constraints arising from molecular mechanics (MMP 1 force field), and geometry relaxed 4-21G and 4-21G** ab-initio calculations. An independent vibrational analysis was performed leading to an assignment of the IR frequencies. A contour analysis allowed unequivocal identification of the ν(CO) band centres and shapes of the two conformers. All data are in accord with the gas phase consisting of ap and sp conformers, at room temperature in the ratio ap : sp = 80 : 20. An enthalpy difference of ΔH = 1.07(10) kcal mol−1 was obtained. A rationalization of why microwave spectroscopy failed to detect the sp conformer was found in the direction and magnitude of its dipole moment as well as in the value of its partition function. The experimental data favour the ab-initio constrained geometrical model, the MMP1 based model could be rejected on statistical grounds. Furthermore, the data are in accord with planar heavy atom skeletons for both the ap and sp forms. Subject to the ab-initio constraints, the internal coordinates (rg distances, r0α angles) were refined to: CO 1.219 A; CC 1.336 A; C(sp2)C(sp2) 1.494 A; C(sp2)C(sp3) 1.520 A; 〈C(sp2)H〉 1.089 A; 〈C(sp3)H〉 1.109 A; CCC(ap) 125.0°; CCC(sp) 120.6°; C(sp2)C(O)C(sp3)(ap) 118.9°; C(sp2)C(O)C(sp3)(sp) 116.1°.