Conformational stability, barriers to internal rotation, ab initio calculations, and vibrational assignment of 2-butanone

Abstract

The Raman (3200–10 cm−1) and infrared spectra (3200–30 cm−1) have been recorded for 2-butanone (ethyl methyl ketone), CH3CH2C(O)CH3, for the gas and solid. Additionally, the Raman spectrum and qualitative depolarization ratios have been obtained for the liquid. Evidence for the existence of two conformations in the gas and liquid is observed, with the trans form (the two CH3 moieties oriented trans about the central C—C bond) much more abundant than the high-energy form, presently identified tentatively to be the gauche rotamer. The trans form is the only form remaining in the spectrum of the solid. A very weak infrared band at 65 cm−1 has been assigned to the asymmetric torsion of the trans conformer. Using this frequency, ΔH, and the dihedral angle of the gauche rotamer, a potential function is proposed for the conformational interchange. Spectral features observed at 195.5 and 181.0 cm−1 in the far infrared spectrum of the gas have been assigned to the [Formula: see text] and [Formula: see text] of the methyl torsion of the ethyl group of the trans form, from which a barrier of 868 ± 3 cm−1 (2.48 ± 0.01 kcal/mol) is calculated. Similarly, the fundamental CH3—C(O) torsion has been observed at 94.5 cm−1 for the trans conformer from which a threefold barrier to internal rotation has been calculated to be 233 cm−1 (666 cal/mol). Utilizing the rotational constants for two isotopic species, which were reported in an earlier microwave study, the heavy atom r0 structural parameters have been determined. A complete vibrational assignment is proposed, which is supported with the results from a normal coordinate analysis performed with the force fields calculated with the 3-21G basis set. Full geometry optimizations have been carried out at the 6-31G* basis set for both the trans and gauche conformations as well as for the transition states to determine the structural parameters, energy difference, and barriers to internal rotation about the central C—C bond. Key words: 2-butanone, vibrational spectrum; ab initio calculations, 2-butanone; conformational stability, 2-butanone.