Routes of π-Electron Delocalization in 4-Substituted-1,2-benzoquinones

The substituent effect in 4-substituted-1,2-benzoquinone is investigated by means of modeling using B3LYP hybrid functional in conjunction with the 6-311+G(d,p) basis set. The interrelation between different types of substituents, X = NO, NO2, CN, CHO, H, Me, OMe, OH, NH2, NHMe and N(Me)(2), and both CO groups has been characterized both qualitatively and then quantitatively by means of several measures of pi-electron delocalization (HOMA, MCI, DI, FLU) based on structural and electronic properties of 4-substituted-1,2-benzoquinones chosen for analysis. Results of this analysis clearly show that only the meta-placed CO group is affected by substituents, whereas the para-placed CO group is rather insensitive to substitution. These observations may help to explain diversified chemical properties (including reactivity) of CO centers in o-benzoquinone derivatives. Among others, they may explain differences in proton-accepting properties of carbonyl 0 atoms, as it is shown for simple models in which carbonyl groups in o-benzoquinone act as proton acceptors in H-bonds of O center dot center dot center dot H-F type.