Elucidation of Structure and Dynamics in Solid Octafluoronaphthalene from Combined NMR, Diffraction, And Molecular Dynamics Studies

Abstract

X-ray diffraction (XRD), molecular dynamics simulations (MD), and (19)F NMR have been used to investigate structure and dynamics in solid octafluoronaphthalene, C(10)F(8). Two distinct processes are observed via measurements of (19)F relaxation times as a function of temperature; a faster process from T(1) relaxation with a correlation time of the order of ns at ambient temperature (fitting to Arrhenius-type parameters E(a) = 20.6 +/- 0.4 kJ mol(-1) and tau(0) = 8 +/- 1 x 10(-14) s) and a much slower process from T(1rho) relaxation with a correlation time of the order of mus (fitting to E(a) = 55.1 +/- 1.3 kJ mol(-1) and tau(0) = 4 +/- 2 x 10(-16) s). Atomistic molecular dynamics reveals the faster process to involve a small angle jump of 40 degrees 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 T(1rho) results and previous wide-line (19)F 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.