ASSIGNMENT OF THE FUNDAMENTAL FREQUENCIES OF PARA-BENZOQUINONE - A SCALED QUANTUM-MECHANICAL FORCE-FIELD STUDY

The recently developed unrestricted Hartree-Fock natural orbital-complete active space (UNO-CAS) method is applied to the determination of the geometry and force field of p-benzoquinone. With an active space consisting of eight electrons distributed in eight-pi orbitals (8 x 8) and the 6-31G* basis set, UNO-CAS predicts CC and CO distances in good agreement with gas electron diffraction results. The full quadratic force field is obtained by an empirical scaling of the 8 x 8 UNO-CAS/4-21G force constants evaluated at the 6-31G* UNO-CAS geometry. This force field produces frequencies in good agreement with reliable experimental results. Based on the calculations a positive assignment of all fundamentals of p-benzoquinone is given. The in-phase and out-of-phase C = O and C = C coupled stretching fundamentals are confirmed to be at 1665 and 1614 cm-1. The two a(u) modes are predicted to be at 1004 and 298 cm-1, and the lowest b3u mode nu(26) is predicted to be at 88 cm-1. These predictions are in excellent agreement with the latest experimental results. The fundamental frequencies of the D2h symmetry isotopomers of p-benzoquinone are also calculated and compared with experiments. Some of the isotopomer fundamentals are reassigned. The UNO-CAS method is shown superior to RHF in predicting equilibrium geometries and quantum mechanical force fields in strongly correlated systems like quinone.