Time-resolved resonance raman spectroscopic studies on the radical anions of methyl-1,4-benzoquinone and 2,6-dimethyl-1,4-benzoquinone

Abstract

Time-resolved resonance Raman spectroscopy (TR3) and density functional calculations have been used to study the effect of asymmetric substitution of methyl group on the structure and vibrational spectra of the radical anion of benzoquinone. The asymmetrically substituted benzoquinones that have been studied are methyl-1,4-benzoquinone (MBQ) and 2,6-dimethyl-1,4-benzoquinone (2,6 DMBQ). Specific vibrational mode assignments have been made to all the vibrational frequencies recorded in the experiment. From the structural changes upon reduction to the radical anion, it has been observed that the unpaired electron in the antibonding π* orbital is more localized on the C=O bond proximal to the methyl group which results in increase in proximal carbonyl bond length compared to the distal bond. The coupling between the C=O bonds has been analyzed on the basis of the (DFT results) shift in the vibrational frequency upon isotopic substitution of carbonyl carbon and oxygen atoms. Comparison of the (¹⁸O) isotopic shift and shift in their C=O stretching frequencies between MBQ and 2,6 DMBQ suggests in the case of 2,6 DMBQ that coupling of the C=O modes with other internal coordinates is considerable for the symmetric C=O stretch compared to asymmetric C=O stretch. Further studies based on site-specific isotopic substitution of the carbonyl carbon and oxygen atoms suggest that symmetric C=O stretch is predominantly from the proximal carbonyl bond and the extent of contribution from the proximal carbonyl bond increases in case of 2,6 DMBQ. The asymmetric C=O stretch has slightly more contribution from the distal carbonyl stretch than the proximal.