Solid-state 2H NMR studies of methyl group dynamics in aspirin and aspirin • β-cyclodextrin

The dynamics of the methyl group in aspirin, and in the inclusion complex aspirin beta-cyclodextrin, have been studied by deuterium nuclear magnetic resonance spectroscopy and relaxation time measurements. The methyl group in aspirin undergoes fast thermally activated rotation about its axis of symmetry within the temperature range studied, 120 less than or equal to T less than or equal to 295 K, with an activation energy of 4.9 +/- 0.05 kJ/mol and a correlation time at 230 K of (2.47 +/- 0.3) . 10(-12) s. The motionally averaged electric field gradient tensor, however, remains axially asymmetric because of electronic effects of a neighbouring oxygen atom. In aspirin beta-cyclodextrin the methyl group rotation about its axis of symmetry is less hindered with an activation energy of 2.1 +/- 0.7 kJ/mol and a correlation time at 230 K of (2.5 +/- 0.4) . 10(-12) s. The axis of symmetry undergoes a slow reorientation with a higher activation energy of 21 +/- 9 kJ/mol and a correlation time at 230 K of (1.7 +/- 1.5) . 10(-6) s.