Raman and infrared spectra, conformational stability and vibrational assignment for 1,1,1‐trifluoro‐2‐methoxypropane

Abstract

AbstractThe Raman spectra of gaseous, liquid and solid 1, 1, 1‐trifluoro‐2‐methoxypropane, CF3(CH3)CHOCH3, and the corresponding methoxy‐d3 isotope, CF3(CH3)CHOCD3, together with the infrared spectra of the gases and solids were recorded from 3500 to 30 cm−1. A comparison of the vibrational spectra obtained for the fluid phases with those obtained for the amorphous and annealed solids indicates the predominance of two conformers in the fluid phases at ambient temperature. Owing to possible steric hindrance, it is suggested that the most stable conformer is the gauche‐2 form, which has the methoxy group trans with respect to the carbon‐carbon bond of the perfluoromethyl group. This conformer is the one present in the solid. The other conformer in appreciable abundance in the fluid states is probably the gauche‐1 form, which has the methoxy group trans to the carbon‐carbon bond of the methyl group. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between these conformers was determined to be 180 ± 100 cm−1 (515 ± 286 cal mol−1). The asymmetric torsional frequencies for the gauche‐2 and gauche‐1 conformers are 93 and 53 cm−1, respectively, and from these data and the enthalpy difference the values for the potential constants governing the internal rotation have been estimated. The normal modes have been assigned based on group frequencies, isotopic shifts, depolarization values and infrared band contours. The barrier to internal rotation of the methyl group is calculated to be 1275 cm−1 (3.65 kcal mol−1) from the torsional frequency of 234 cm−1 of the solid.