Dynamics of camphor sulphonic acid:: a quasielastic neutron scattering study

,Molecular motions in mono-hydrated racemic camphor sulphonic acid (+/-)-10-C10H16O4S--H3O+ which is abbreviated as (CSA-H2O) were investigated using incoherent neutron scattering techniques. Analysis of the intensity of the purely elastic scattering over a wide temperature range (4-340 K) carried out with a high-resolution backscattering spectrometer revealed the onset of molecular motions at ca. 100 K which could be observed on the 10-(10) s time-scale up to T = 180 K. These motions were identified as 120degrees jumps of the methyl groups. Quasielastic measurements using both the backscattering and the time-of-flight techniques enabled to study this movement from 150 to 340 K. The corresponding characteristic time was found to follow an Arrhenius law with an activation energy DeltaH = 12.0 +/- 0.2 kJ mol(-1). All the methyl groups appear as dynamically equivalent. That result is at contrast with earlier studies on conducting polymers where CSA was introduced as a counter-ion and for which the intermolecular effects were found to strongly influence the dynamics. Inspection of the low frequency, part of the vibrational spectrum evidences deformations of the C-C-S angle and rotational oscillations of the hydration molecules. (C) 2002 Elsevier Science B.V. All rights reserved.