Conformational Studies of Fluoromethylcyclopropane from Temperature-Dependent FT-IR Spectra of Xenon Solutions and Ab Initio Calculations

Abstract

Variable-temperature (−55 to −100 °C) studies of the infrared spectra (3500 to 400 cm-1) of fluoromethylcyclopropane, c-C3H5CH2F, dissolved in liquefied xenon have been carried out, and the Raman (liquid and solid) and infrared (gas and solid) spectra have been recorded from 3500 to 60 cm-1. By utilizing four conformer pairs, an enthalpy difference of 262 ± 26 cm-1 (3.13 ± 0.31 kJ/mol) was obtained, with the gauche rotamer the more stable conformer and the only form present in the solid. The abundance of cis conformer present at ambient temperature is 12 ± 1%. On the basis of the far-infrared spectral data along with the experimental enthalpy and gauche dihedral angle, the potential function governing conformational interchange has been obtained, and the determined potential constants are V1 = −245 ± 23, V2 = −414 ± 17, V3 = 1263 ± 6, V4 = 272 ± 17, and V5 = 101 ± 2 cm-1, with the cis-to-gauche barrier of 1223 cm-1 (14.63 kJ/mol) and the gauche-to-gauche barrier of 1362 cm-1 (16.29 kJ/mol). From MP2 ab in...