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 (2 n OH; 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 CO 2 and 1-chloro-3-fluorobenzene (ClFB), whose vibrational signatures could be doubtlessly identified in the spectra of the photolysed matrix.