The vibrational spectra including matrix isolation and conformations of 1-chloro-1-fluorocyclobutane and 1-chloro-1,2,2-trifluorocyclobutane

Abstract

Abstract The IR and Raman spectra of 1-chloro-1-fluorocyclobutane (CFCB) and 1-chloro-1,2,2-trifluorocyclobutane (CFCB) were recorded in various phases and at different temperatures, and included IR matrix spectra in argon and nitrogen matrices using the hot nozzle method. Both compounds have two conformers in the vapour and liquid states. CTFCB crystallized as one distinct conformer, while two conformers were present in all solid state spectra of CTFCB. The argon matrix spectrum of CFCB contained only one conformer at 13 K, suggesting a barrier lower than 5 kJ mol−1 between the conformers. In both the argon and nitrogen matrix spectra of CTFCB two conformers were observed, although the concentrations of the less stable conformer were lower than in the vapour. This suggests a partial conversion to the stable conformer during deposition and a barrier slightly higher than 6 kJ mol−1. The enthalpy difference ΔH° between the conformers was 2.2 kJ mol−1 for CFCB in the liquid and 2.7 and 2.9 kJ mol−1 in the vapour and liquid, respectively, for CTFCB. Ab initio calculations were carried out for both compounds using the 3-21G* and 6-31G* basis sets. For CFCB, both calculations gave the conformer with the fluorine atom equatorial as the more stable. For CTFCB, the smaller basis set also gave the conformer with equatorial fluorine as the more stable while the opposite was predicted with the larger basis set. Fairly complete assignments for both conformers of both compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. Agreement was excellent between calculation and experiment for the case in which the more stable conformer was that with fluorine equatorial but impossible to achieve with fluorine axial.