Conformational barriers to internal rotation and vibrational assignment of methyl vinyl ketone

Abstract

The infrared (3500 to 50 cm−1) and Raman (3200 to 10 cm−1) spectra have been recorded for the gaseous and solid states of methyl vinyl ketone. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. The asymmetric torsion for the s-trans conformer was observed at 116 cm−1 in the infrared spectrum of the gas with two accompanying hot bands and the corresponding torsion of the s-cis conformer was observed at 87 cm−1 with an additional hot band occurring at 84 cm−1. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1 = 180±9, V2 = 827±107, V3 = 113±8, and V4 = 150±34 cm−1. From these data it has been determined that the s-trans conformer is the predominant form at ambient temperature and the enthalpy difference between the s-trans and s-cis conformers is 280 cm−1 (800 cal/mol) for the vapor. The calculated trans–cis barrier is 827 cm−1 (2.36 kcal/mol) and the cis–trans barrier is 547 cm−1 (1.56 kcal/mol). From the relative intensities of the Raman lines of the liquid at 607 (s-cis) and 537 cm−1 (s-trans) as a function of temperature, the enthalpy difference was found to be 172 cm−1 (492 cal/mol). A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. These results are compared to similar quantities in related molecules.