Rotational Isomerism in Fluoroacetyl Halides

Abstract

Infrared and Raman spectra have been recorded for fluoroacetyl fluoride, chloride, and bromide. The data show that each compound exists as an equilibrium mixture of two isomers in the vapor and liquid states. Normal coordinate analysis calculations based on a Urey–Bradley potential function indicate that the two preferred conformations are a trans and a staggered conformation which is close to the cis. In all three compounds the trans isomer is found to be the more stable in both liquid and vapor phases, with its stability being relatively diminished in the vapor state. Approximate, but nearly complete vibrational frequency assignments are made for each isomer. The vapor phase enthalpy difference ΔH° = (H°trans − H°staggered) was measured as − 2.05 ± 0.25 kcal/mole for fluoroacetyl bromide.