Conformational in the IR spectra of 1′-hydroxyethyl derivatives of 1,4-benzo- and 5,8-dihydroxy-1,4-naphthoquinones: A DFT study
Detailed conformational analysis of the molecule of ′-hydroxyethyl-1,4-benzoquinone (3) by the B3LYP/cc-pVTZ method revealed predominance of rotamers with the free 1′-OH group in the gas phase. B3LYP/cc-pVTZ calculations with inclusion of solvent (cyclohexane) effect in the framework of the polarizable continuum model predict an increase in the percent-age of such rotamers compared to the corresponding gas-phase values. The results obtained are in qualitative agreement with the experimentally observed pattern of v(OH) bands in the IR spectrum of compound 3 in cyclohexane (hexane) solution. Conformational analysis, including tautomerism and rotamerism, of 2-ethyl-1′,5,8-trihydroxy-1,4-naphthoquinone (2) was performed by the B3LYP method with the 6-31G(d), 6-311G(d), 6-311G(d,p), and cc-pVDZ basis sets. The most abundant tautomeric form of compound 2 is form A in which the substituent bearing 1′-OH group is in the quinonoid nucleus. In the gas phase, the percentage of all rotamers in form A is about 86% (among them, the proportion of rotamers with the free 1′-OH group is more than 60%). The main reason for splitting of the v(OH) bands in the IR spectra of compounds 2 and 3 in solutions in nonpolar solvents is the equilibrium between rotamers with a relatively weak intramolecular hydrogen bond between the 1′-OH group and the carbonyl group and those having no this bond.