The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments of the two planar O-cis and O-trans rotomers of 2-, 3- and 4-trifluoromethylbenzaldehydes have been calculated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6311 ++G(d, p) basis set level. The calculations were adapted to the C(s) symmetries of all the molecules. The O-trans rotomers with lower energy of all the compounds have been found as preferential rotomers in the ground state. The mean vibrational deviations between the calculated vibrational frequency values of the two conformers of all the compounds have been shown to increase while the relative energies increase, and so it has been concluded that the higher the relative energy between the two conformers the bigger is the mean vibrational deviation. It has also been found that HF method is superior to B3LYP for the geometric parameters and vibrational frequencies. For comparison electron correlation effect the geometries have also been optimized using Moller-Plesset (MP2) theory with 6-311 ++G(d, p) basis set level. The energies and bond lengths calculated at MP2 level are found to be slightly bigger than those calculated at HF level. This was attributed that the inclusion of the electron correlation effect causes the bond lengths more close to the experimental values. (C) 2008 Elsevier B.V. All rights reserved.