Ab initio determination of molecular geometries and vibrational frequencies of CX3 COOH (X = H, F, Cl, Br)

Using Gaussian 03 Revision C.02 version of the quantum chemical program ab initio and DFT computations have been carried out at the rhf/6-31+g*, b3lyp/6-31+g*, b3lyp/6-31++g** and b3lyp/6-311++g** levels to compute optimized geometries, harmonic vibrational frequencies along with intensities in IR and Raman spectra and atomic charges for the acetic (ethanoic) acid and its 1,1,1-tri-halo (fluoro, chloro and bromo) derivatives. The optimized molecular structures for all the four molecules are found to possess Cs point group symmetry. The symmetric stretching mode is found to have lowest magnitude of the three CX3 stretching modes for all the four molecules, whereas the symmetric deformation mode is found to have the lowest magnitude for EA and TFEA and the highest magnitude for TCEA and TBEA. The parallel rocking mode of the CX3 group is found to have lower magnitude than the perpendicular rocking mode for EA and TFEA where reverse is found for TCEA and TBEA. The modes of the COOH group are substituent sensitive except the OH stretching mode. Moreover, the maximum effect is found for the TFEA molecule. The CF3 group is found to have the characteristic frequencies as 235–505, 787, 1150–1190 and 1400 cm−1 which are due to the modes δs(CF3), νs(CF3),νas(CF3) and ν(C–CF3), respectively.