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.