structural characterization and photochemistry of 2-chloro-6-fluorobenzoic acid isolated in a xenon matrix

2-Chloro-6-fluorobenzoic acid (ClFBA) was studied by low temperature solid state FTIR spectroscopy in a Xe matrix and complemented by DFT(B3LYP)/6-311++G(d,p) calculations. The ClFBA molecule exists in three different conformers, according to the theoretical calculations. The second and third conformers (II, III) are higher in energy than the most stable conformer (I) by ca. 17 kJ mol-1, and bear a trans carboxylic acid moiety, while form I has this group in the cis conformation. According to the energy data predicted theoretically only the conformer I was expected to be present in the cryogenic xenon matrix, a result that was confirmed experimentally. Attempts for in situ generation and detection of forms II and III by near-IR (2nOH; 6867 cm-1) excitation of conformer I indicated that in solid xenon, once produced, these conformers promptly relax by tunneling to the most stable conformer. Laser UV (l = 235 nm) excitation of matrix-isolated ClFBA led to prompt decarboxylation of the compound, with production of CO2 and 1-chloro-3-fluorobenzene (ClFB), whose vibrational signatures could be doubtlessly identified in the spectra of the photolysed matrix.