Monday, April 26, 2010

J. Am. Chem. Soc., 2010, 132 (14), pp 5179–5185

Elucidation of Structure and Dynamics in Solid Octafluoronaphthalene from Combined NMR, Diffraction, And Molecular Dynamics Studies
Andrew J. Ilott, Sebastian Palucha, Andrei S. Batsanov, Mark R. Wilson and Paul Hodgkinson

X-ray diffraction (XRD), molecular dynamics simulations (MD), and 19F NMR have been used to investigate structure and dynamics in solid octafluoronaphthalene, C10F8. Two distinct processes are observed via measurements of 19F relaxation times as a function of temperature; a faster process from T1 relaxation with a correlation time of the order of ns at ambient temperature (fitting to Arrhenius-type parameters Ea = 20.6 ± 0.4 kJ mol−1 and τ0 = 8 ± 1 × 10−14 s) and a much slower process from T1ρ relaxation with a correlation time of the order of μs (fitting to Ea = 55.1 ± 1.3 kJ mol−1 and τ0 = 4 ± 2 × 10−16 s). Atomistic molecular dynamics reveals the faster process to involve a small angle jump of 40° of the molecules, which is in perfect agreement with the X-ray diffraction study of the material at ambient temperature. The MD study reveals the existence of more extreme rotations of the molecules, which are proposed to enable the full rotation of the octafluoronaphthalene molecules. This explains both the T1ρ results and previous wide-line 19F NMR studies. The experimental measurements (NMR and XRD) and the MD computations are found to be strongly complementary and mutally essential. The reasons why a process on the time scale of microseconds, and associated with such a large activation barrier, can be accessed via classical molecular dynamics simulations are also discussed.

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