Ultrasonic Relaxation and Cis—Trans Isomerization in Isopropyl Formate

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Abstract
The ultrasonic absorption of dilute solutions of isopropyl formate has been measured over the frequency range 0.6 to 45 Mc/sec at temperatures from 30° to 80°C. The frequency dependence of the absorption is that of a single relaxation process. The concentration dependence of the relaxation parameters is consistent with the mechanism being the interconversion of trans and cis isomers by rotation about the carboxyl C–O bond, while the temperature dependence yields a difference in enthalpy and entropy between the isomers of 3.7±0.5 kcal/mole and 1.5±0.6 cal/mole/deg, respectively, and an activation enthalpy of 5.8±0.4 kcal/mole. It is suggested that a large effect in addition to dipolar interaction and steric repulsion is necessary to explain the differences in enthalpy between formate isomers. The ultrasonic absorption in pure isopropyl formate was also measured at 45 Mc/sec over the temperature range −60° to +60°C, and the results have been interpreted in terms of a second relaxation caused by internal rotation about the methoxyl C–O bond.

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