Damped Quantum Rotation of the Methyl Group in Liquid-Phase NMR Spectra of 9-Methyltriptycene Derivatives

摘要

Hindered rotation of the methyl group evidenced in NMR line shapes is normally viewed as a sequence of classical random jumps between the three equivalent orientations of the group.In solids at low temperatures,possible effects of the quantum tunneling,manifested as apparent spin-spin couplings between the methyl protons,are incorporated by force into this essentially classical picture.In the damped quantum rotation (DQR)theory formulated recently,the hindered rotation is entirely a non-classical phenomenon.It is described in terms of a consistent combination of the quantum tunneling and two quantum rate (coherence-damping)processes;the phenomenological behaviour of the methyl group becomes classical only when the two quantum rate processes occur with equal rates.Occurrences of the DQR effect,i.e.,differences between the two quantum rate constants,were earlier detected in solids at temperatures below 110 K and,surprisingly,also in liquids above 170 K.In this study,further examples of the DQR effects in liquid-phase NMR are reported.Like the previous observations for liquids,the present ones involve the strongly hindered methyl groups in 9-methyltriptycene derivatives.A particularly clear manifestation of the DQR effect is found for 1,4-dibromo-9-methy ltriptycene.With the ratio of the two quantum rate constants exceeding 1.25,this system shows the farthest departure from the classical behaviour,ever reported for liquids.Our earlier observations of the quantum tunneling of the methyl group in liquid phase,reflected in strong dependence on temperature of the J-couplings between the methyl protons,are now augmented by further evidence.