The 1H NMR spectra of 2,6-diisobornyl-4-methylphenol (I) revealed a concentration dependence of the hydroxyl proton signal, δH(OH), that is unusual for sterically hindered phenolic antioxidants. In inert cyclohexane, a small but reliably measured upfield shift of this signal is observed with increasing concentration of I. The same dependence is observed in aromatic solvents (toluene, mesitylene), but the effect is significantly larger. Based on these data, as well as the results of IR spectral measurements and quantum chemical DFT calculations, it is shown that in aromatic solvents, the hydroxyl group of phenol I engages in an OH···π interaction with the π-electrons of the solvent, which causes the upfield shift of δH(OH). In the inert solvent, the OH···π bond is formed as a result of the interaction of the OH group with the π-electrons of another molecule of I. With increasing concentration of the aromatic solvent, the δH(OH) value asymptotically approaches a limit of ~4.27 ppm in toluene and ~4.03 ppm in mesitylene. In the IR spectrum of a solution of I in carbon tetrachloride, the main OH stretching vibration band, ν(O–H), is observed at 3606.9 ± 0.1 cm–1 as a symmetric peak, corresponding to a hydroxyl group free from hydrogen bonding. In mesitylene, in addition to the main band, a broad shoulder appears in the low-frequency region of the spectrum. Decomposition of the complex signal made it possible to identify a new absorption band for the OH group at 3574.0 ± 0.7 cm–1, confirming the presence of an OH···π contact between I and the aromatic solvent. DFT modeling of the interaction of I with toluene and mesitylene indicates that the phenolic fragment is initially planar, while in the mesitylene complex, the OH group moves out of the phenol plane by 26°, which further confirms its participation in intermolecular interactions. The distance between the H atom of the hydroxyl group and the nearest three carbon atoms of the aromatic ring of mesitylene is only 2.7–2.9 Å, which is consistent with noticeable OH···π bonding. Similar structural motifs are observed for the toluene–I complex. The low-frequency shift of ν(O–H) by 33 cm–1 upon complex formation is well reproduced by the O–H vibration frequency calculated in the harmonic approximation, which yields Δν(O–H) = 42 cm–1. Finally, the GIAO calculation of the relative change in the chemical shift of the hydroxyl proton, ΔδH, correctly conveys the observed trend: the signal shifts upfield by 0.4 ppm more in mesitylene than in toluene, which is in reasonable agreement with the experimentally observed difference in the limiting chemical shifts, ΔδH = 0.24 ppm.
Sadykov et al. (Sun,) studied this question.