NMR studies of molecular dynamics of some organic salts and charge transfer complexes
Creators: name: Williams, Donald Shanthakumaran
Publisher: name: University of British Columbia
Year: 2010

description: Nuclear magnetic resonance absorption and spin-lattice relaxation time measurements have been carried out on the tropolone salt of t-butylamine, (CH₃)₃CN⁺H₃Tr⁻ (Tr ⁻= tropolonate ion, C₇H₅0₂⁻), the choline salts, (CH₃)₃NCH₂CH₂OH. X⁻ (x⁻ = Cl⁻, Br , I , ClO₄ ⁻) and the trimethylamine-phosphorous penta-fluoride adduct, (CH₃)₃NPF₅, in order to study molecular motion and phase transitions in these systems in the solid state. Activation energies and rate parameters associated with the motional processes are reported. Proton magnetic resonance (pmr) absorption second moments and proton spin-lattice relaxation times in the Zeeman frame (T₁) in the temperature range 66K - 425K for the solid (CH₃)₃CNH₃Tr⁻ show that the molecule is rigid on the nmr timescale at the lowest temperature studied, while at higher temperatures rotation of methyls about their C₃ symmetry axes is found to set in first, followed by an additional composite motion involving reorientation of both the t-butyl group and the NH₃ group about the C-N bond. A proton study in the partially deuterated (-ND₃) analogue has enabled the relaxation effects of the latter two motions to be separated, and, by fitting the T₁, data for the two compounds to appropriate relaxation rate expressions, activational energy barriers for the abovementioned motional processes have been determined. It has also been suggested that the t-butyl group and the NH₃ group rotate independently about the C-N bond rather than as one unit. Proton spin-lattice relaxation time measurements in both the Zeeman and rotating frames of reference (T₁ and T₁[sub p]) for the four choline salts and second moments of the pmr absorption for the perchlorate have enabled the following motional processes to be identified: (i) rotation of the methyl groups at low temperatures followed successively by, (ii) the onset of motion of the NMe₃ moiety about the long chain C-N axis (denoted C₃), (iii) general reorientation of the whole choline cation, (iv) additional slow motion of the long chain (CH₂CH₂OH in the case of the chloride and bromide, and (v) diffusion of the choline ion in the case of the iodide and perchlorate. From a quantitative analysis of the and data, activation energies for the above types of motion are determined. A crystal-crystal phase transition known to occur at 353, 364 and 362K in the chloride, bromide and iodide, respectively, has been confirmed. A similar transition has been discovered in the perchlorate, and is found to occur at a much lower temperature (272K). Evidence has also been presented for a further crystal-crystal phase transition in choline iodide at 430K, at which point a "quenching" of the diffusional process is found in this structure. In the adduct (CH₃)₃NPF₅, studies of proton and fluorine nmr absorption spectra and measurements have shown that (i) at 4.2K the molecule is 'rigid1, (ii) C₃ reorientation of one of the methyls and reorientation of the PF₅ group about the P-N bond cause a ¹H and ¹⁹F nmr line narrowing, (iii) this is followed by the C₃ rotation of the other two methyl groups together with the rotation of the (CH₃)₃N group about the P-N bond. These are confirmed by a successful simulation of the observed pmr lineshapes at 4.2K and at 77K. The proton and fluorine T₁ data show the ¹H and ¹⁹F spins to be strongly coupled. A study of fluorine T₁ in the fully deuterated compound, (CD₃)₃NPF₅ has enabled the analysis of the overall T₁ data to be simplified. The observed trends in the T₁ data are seen to be well explained by the theory for a coupled spin system of two unlike spins.