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 reexamine d on the basis of Raman and new IR measurements and theoretical analys is of the normal modes of vibrations in the electronic ground state. T he infrared spectra of C6H5OH, C6D5OD, and C6D5OH have been studied in solution and vapor phases, as well as has the Raman spectra in soluti ons. New experimental data were obtained from infrared linear dichrois m (IR-LD) studies of phenol aligned in uniaxially oriented nematic Liq uid crystal solution. The measured dichroic ratios and orientation fac tors indicate an effective C-s symmetry of the molecule with coplanar orientation of OH bond with the benzene ring and supply unique informa tion on the extent of symmetry lowering of benzene normal modes. The f undamental vibrational frequencies, force constants, and dipole deriva tives have been calculated by ab initio quantum chemical methods apply ing the B3P86 density functional approximation with 6-311G* basis set . The force field optimized by means of a least-squares scaling proced ure for phenol-d(0) (using six scale factors) was used to calculate th e frequencies (with a mean deviation from the observed values less tha n 1%), normal modes, potential energy distributions, transition moment vectors, and IR intensities for phenol-d(0), -d(1), -d(5), and -d(6) isotopomers. Compared to the deviations between the calculated and obs erved absorption intensities, a more satisfactory correlation was foun d between the calculated and experimentally determined vibrational tra nsition moment directions. The results indicate unanimously that the p erturbation of the normal modes of benzene by the asymmetric hydroxyl substituent is so great that the previous practice of assigning the no rmal vibrations of phenol to those of benzene or even to C-2v symmetry species is not justified.