H-BONDED AND STACKED DIMERS OF PYRIMIDINE AND P-BENZOQUINONE - A COMBINED MATRIX-ISOLATION INFRARED AND THEORETICAL AB-INITIO STUDY

Matrix isolation IR spectroscopy and high-level ab initio calculations were applied to investigate the structure and vibrational spectra of quinone-pyrimidine heterodimers formed in low-temperature Ar matrices. A specially developed experimental technique was used to separate ban ds of quinone-pyrimidine dimer from bands of quinone and pyrimidine mo nomers and homodimers in the IR spectra. As a result, nine bands assig ned to the quinone-pyrimidine heterodimer were identified. Ab initio c alculations at the MP2/6-31+G, MP2/6-31++G** and SCF/6-31++G** levels of theory have been carried out to determine the relative energies an d vibrational spectra of three stable configurations of the quinone-py rimidine dimer found theoretically. These configurations are two plana r complexes with two weak C-H ... O and C-H ... N hydrogen bonds and o ne stacked complex stabilized by dispersion forces. The effect of basi s set superposition error (BSSE) on the relative stabilities and the v ibrational spectra of the dimers was also investigated. The non-BSSE-c orrected calculations at the MP2/6-31+G and MP2/6-31++G** levels of t heory predict the stacked dimer to be the most stable conformer, but a ccounting for BSSE resulted in a reverse stability ordering of the sta cked and the planar dimers. The comparison of the observed frequency s hifts with the theoretically predicted shifts has shown that the plana r configuration is responsible for the experimentally observed bands. This is in agreement with the stability ordering derived from the BSSE -corrected relative energies. To account for the matrix effects on the stability of the planar and stacked dimers, additional calculations w ere carried out using the Onsager's reaction field model and the MP2/6 -31++G* level of theory. These calculations confirm that the planar H -bonded dimer is the most stable configuration.