Quadrupole echo distortion as a tool for dynamic NMR: Application to molecular reorientation in solid trimethylamine

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Abstract
Deuterium NMR quadrupole echo spectra of trimethylamine-d9 (TMA-d9) were recorded in the solid phase from about −150 °C to the melting point (−117 °C). The spectra exhibit pronounced line shape variation with temperature which is interpreted in terms of two independent dynamic processes, (i) threefold methyl group reorientation about the respective C3 axes, and (ii) threefold whole molecule reorientation about the molecular C′3 axis. To derive quantitative kinetic parameters for these processes the experimental spectra are compared with simulated traces, placing particular emphasis on the effect of line shape distortion due to the delay time τ between the π/2 pulses in the quadrupole echo experiment. This approach improves the sensitivity of the fitting method and allows accurate determination of the kinetic parameters for both the C3 and C3 reorientation processes. The results are k(−150 °C)=3.0×104 s−1, k′(−150 °C)=0.1×104 s−1, ΔE=3.0±0.6 kcal/mol, and ΔE′=8.0±1.6 kcal/mol, respectively. In Appendix A a convenient procedure is described for calculating spin dynamics in I=1 systems and applied to a number of cases. In Appendix B the approach of Appendix A is applied to compute dynamic line shapes in the quadrupole echo experiment.