4-Chloro-3-fluorobenzaldehyde: Experimental (XRD, FT-IR and Raman) and DFT studies

The structure of 4-chloro-3-fluorobenzaldehyde (CFB, C7H4ClFO) has been characterized by single-crystal X-ray diffraction, FT-IR and Raman techniques. The conformational isomers, optimized geometrical parameters, normal mode frequencies and corresponding vibrational assignments of CFB have been examined using the density functional theory method, with the Becke-3-Lee–Yang–Parr functional and the 6-311+G(3df,p) basis set. Reliable vibrational assignments and molecular orbitals have been investigated by the potential energy distribution. CFB crystallizes in monoclinic space group P21/c with the O-cis conformation. In order to examine the effect of fluorine on the conformational preference, the chloro- and bromo-analogs of CFB have also been studied theoretically. It is observed that CFB prefers the O-cis conformation while the chloro- and bromo-analogs prefer the O-trans conformation. Although the free energy difference between the O-cis and O-trans conformers is less than 0.2 kcal/mol, the free energy rotational barrier is at least 8.4 kcal/mol [B3LYP/6-311+G(3df,p)]. MP2/aug-cc-pVDZ computations reveal that these free energies remain almost unaffected. The atypical characteristic of fluorine affecting conformational isomerism is observed. In addition, there is a good agreement between the experimentally determined structural parameters and vibrational frequencies of CFB and those predicted theoretically.