Elsevier

Chemical Physics

Volume 277, Issue 2, 15 March 2002, Pages 211-224
Chemical Physics

Dynamics of camphor sulphonic acid: a quasielastic neutron scattering study

https://doi.org/10.1016/S0301-0104(02)00313-0Get rights and content

Abstract

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 120° 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 ΔH=12.0±0.2kJmol−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.

Introduction

Numerous assemblies of camphor-based molecules in the solid state have been largely studied for many reasons and some of them usually form molecular solids exhibiting rich molecular dynamics. Among these, we brought a particular attention to camphor sulphonic acid, C10H16SO4, hereafter denoted as CSA (Fig. 1(a)) because it has been used as a doping agent of polyaniline (PANI) in order to prepare highly conducting polymer films [1], [2], [3], [4]. One of the most privileged techniques for the investigation of molecular dynamics in condensed phases is certainly inelastic scattering of slow neutrons which provides information about the geometry of the relevant motions and also about their characteristic times. Recently we performed by quasielastic neutron scattering a detailed analysis of the molecular dynamics of CSA inserted in between PANI chains [5], [6], [7]. We discovered an apparent relation between this dynamics and the electronic properties of the films. At this stage, it appeared crucial to characterise more precisely the molecular dynamics of CSA itself, taken in its stable hydrated phase. Specially, in order to better understand the interaction between the polymeric chain and CSA counter-ions it is necessary to differentiate any dynamical characteristics which would be intrinsic to the molecule from those arising from intermolecular effects. More precisely, in the study of the PANI/CSA system, the CSA methyl groups were found non-equivalent. To conclude definitely that these groups had their dynamics directly influenced by their local environment, for which they could constitute a microscopic probe we wanted to check whether that effect was not an intramolecular characteristic resulting from the chiral character of the CSA, as already observed with other molecules [8], [9].

The time-scale range accessible to a given neutron spectrometer is usually restricted to one or two orders of magnitude. To get information over a wider energy range, it often required the same specimen to be investigated with several instruments having complementary characteristics. This paper reports on several series of neutron quasielastic experiments by using three different spectrometers and performed with racemic (±)CSA hydrated solid phase containing one H2O molecule per CSA. As far as we know the crystallographic structure of this compound has not been published, but the structure of the mono-hydrated enantiomeric form is well known [10] (Fig. 1(b)).

Section snippets

Neutron scattering theory

This section aims at giving a minimum of information about neutron scattering and especially about incoherent quasielastic neutron scattering (IQNS), referring to standard textbooks for a general survey of the neutron technique [11], [12], [13], [14] or a description of quasielastic scattering [15], [16]. We want to point out a few general characteristics to which we shall refer in the analysis of the data.

In a typical neutron scattering experiment monochromatic neutrons exchange both energy, ℏω

Experimental

The IQNS experiments were performed at the high-flux reactor of the ILL (Grenoble, France). In a first series of experiments, the dynamics of CSA was investigated with the time-focusing time-of-flight spectrometer IN6 [17], operating with an incident wavelength λ=5.12Å. 89 spectra were recorded simultaneously for different scattering angles ranging from 14.7° to 113.5°. CSA powder was held in a flat circular aluminium container, 50 mm diameter and 0.3 mm thickness. Two series of experiments

Fixed-window measurements

With backscattering spectrometers, the fixed-window method is commonly used. In this technique the neutrons incoming on the sample have exactly the energy that is selected by the analysers. So only the neutrons scattered without energy change (within the instrument resolution) are recorded. In the same time an external parameter (generally the sample temperature) is varied. The occurrence of a molecular motion on the resolution time-scale results in the onset of quasielastic broadening and thus

Summary and discussion

The technique of incoherent neutron scattering has proved to be very powerful by allowing us to obtain clear information about the dynamics of the CSA molecule in its hydrated crystalline state. Several spectrometers with different characteristics giving access to complementary energy and momentum transfer ranges were used to cover a wide temperature interval. Fixed-window measurements carried out with the high-resolution spectrometer IN10 revealed the onset of a single molecular motion. The

Acknowledgements

We are thankful to B. Suchod (LSP) for giving us detailed information on crystallographic characteristics of (1s)-(+)-10-camphor sulphonic acid monohydrate. We are also grateful to M. Johnson (ILL) for communicating his first results of calculations of vibrational spectra.

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