THE VIBRATIONAL-SPECTRA INCLUDING MATRIX-ISOLATION AND CONFORMATIONS OF 1-CHLORO-1-FLUOROCYCLOBUTANE AND 1-CHLORO-1,2,2-TRIFLUOROCYCLOBUTANE

The IR and Raman spectra of 1-chloro-1-fluorocyclobutane (CFCB) and 1- chloro-1,2,2-trifluorocyclobutane (CTFCB) were recorded in various pha ses and at different temperatures, and included IR matrix spectra in a rgon and nitrogen matrices using the hot nozzle method. Both compounds have two conformers in the vapour and liquid states. CFCB crystallize d as one distinct conformer, while two conformers were present in all solid state spectra of CTFCB. The argon matrix spectrum of CFCB contai ned only one conformer at 13 K, suggesting a barrier lower than 5 kJ m ol-1 between the conformers. In both the argon and nitrogen matrix spe ctra of CTFCB two conformers were observed, although the concentration s of the less stable conformer were lower than in the vapour. This sug gests a partial conversion to the stable conformer during deposition a nd a barrier slightly higher than 6 kJ mol-1. The enthalpy difference DELTAH-degrees 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, respectivel y, for CTFCB. Ab initio calculations were carried out for both compoun ds using the 3-21G and 6-31G* basis sets. For CFCB, both calculations gave the conformer with the fluorine atom equatorial as the more stab le. For CTFCB, the smaller basis set also gave the conformer with equa torial fluorine as the more stable while the opposite was predicted wi th the larger basis set. Fairly complete assignments for both conforme rs of both compounds were made with the aid of normal coordinate calcu lations employing scaled ab initio force constants. Agreement was exce llent between calculation and experiment for the case in which the mor e stable conformer was that with fluorine equatorial but impossible to achieve with fluorine axial.