A Density Functional, Infrared Linear Dichroism, and Normal Coordinate Study of Phenol and its Deuterated Derivatives:  Revised Interpretation of the Vibrational Spectra

The assignment of the vibrational spectra of phenol has been reexamined on the basis of Raman and new IR measurements and theoretical analysis of the normal modes of vibrations in the electronic ground state. The infrared spectra of C6H5OH, C6D5OD, and C6D5OH have been studied in solution and vapor phases, as well as has the Raman spectra in solutions. New experimental data were obtained from infrared linear dichroism (IR-LD) studies of phenol aligned in uniaxially oriented nematic liquid crystal solution. The measured dichroic ratios and orientation factors indicate an effective Cs symmetry of the molecule with coplanar orientation of OH bond with the benzene ring and supply unique information on the extent of symmetry lowering of benzene normal modes. The fundamental vibrational frequencies, force constants, and dipole derivatives have been calculated by ab initio quantum chemical methods applying the B3P86 density functional approximation with 6-311G** basis set. The force field optimized by means of a least-squares scaling procedure for phenol-d0 (using six scale factors) was used to calculate the frequencies (with a mean deviation from the observed values less than 1%), normal modes, potential energy distributions, transition moment vectors, and IR intensities for phenol-d0, -d1, -d5, and -d6 isotopomers. Compared to the deviations between the calculated and observed absorption intensities, a more satisfactory correlation was found between the calculated and experimentally determined vibrational transition moment directions. The results indicate unanimously that the perturbation of the normal modes of benzene by the asymmetric hydroxyl substituent is so great that the previous practice of assigning the normal vibrations of phenol to those of benzene or even to C2v symmetry species is not justified.