The effect of rotating substituent in 2,2,5,7,8-pentamethylchroman derivatives. X-ray, 13C CP MAS analysis and DFT analysis

The aim of this study was to analyze the influence of rotating substituent at position 6 in 2,2,5,7,8-pentamethylchroman derivatives on geometry of the chroman ring. Three substituents have been chosen which differed in electro donor/acceptor properties (-NO2, -OH and -NH2). The substituents were rotated by 10° step from 0° to 180° whereas the geometries were fully optimized at B3LYP/6-311G** level of theory. The rotation barrier is related with a size of the substituent, being largest for the nitro group (5.9 kcal/mol) and smallest for the hydroxyl group (2.0 kcal/mol). The largest changes on geometry were observed in closest proximity of the substituents in the phenyl ring (due to steric effect) and C4A-C8A-O1-O2 angle in the chroman fragment, related to the antioxidant activity of a compound. The latter varied much stronger for electron-accepting substituent (the range −12.6° to −16.3°) than for electron donating ones (the variation range −17.1° to −18.9°), showing the rather high sensitivity in the first case. Apart from theoretical studies the crystal structure of newly synthesized 6-nitro-2,2,5,7,8-pentamethylchroman was determined by X-ray diffraction and studied by 13C NMR including solid-state 13C CP-MAS NMR.