Assignment of the Fundamental Frequencies of Para-Benzoquinone - a Scaled Quantum-Mechanical Force-Field Study

by Liu, R. F.; Zhou, X. F.; Pulay, P.

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.

Journal
Journal of Physical Chemistry
Volume
96
Issue
11
Year
1992
Start Page
4255-4261
URL
https://dx.doi.org/10.1021/j100190a028
ISBN/ISSN
0022-3654
DOI
10.1021/j100190a028