Nonexponential Solid State ¹H and ¹⁹F Spin–Lattice Relaxation, Single-crystal X-ray Diffraction, and Isolated-Molecule and Cluster Electronic Structure Calculations in an Organic Solid: Coupled Methyl Group Rotation and Methoxy Group Libration in 4,4′-Dimethoxyoctafluorobiphenyl

Abstract

We investigate the relationship between intramolecular rotational dynamics and molecular and crystal structure in 4,4'-dimethoxyoctafluorobiphenyl. The techniques are electronic structure calculations, X-ray diffractometry, and (1)H and (19)F solid state nuclear magnetic resonance relaxation. We compute and measure barriers for coupled methyl group rotation and methoxy group libration. We compare the structure and the structure-motion relationship in 4,4'-dimethoxyoctafluorobiphenyl with the structure and the structure-motion relationship in related compounds in order to observe trends concerning the competition between intramolecular and intermolecular interactions. The (1)H spin-lattice relaxation is nonexponential in both the high-temperature short-correlation time limit and in the low-temperature long-correlation time limit, albeit for different reasons. The (19)F spin-lattice relaxation is nonexponential at low temperatures and it is exponential at high temperatures.

1H and 19F spin-lattice relaxation and CH3 or CF3 reorientation in molecular solids containing both H and F atoms

“…Methoxy group (OCH 3 ) reorientation in 2 is quenched 8 so the CH 3 reorientation axis is not reorienting on the NMR time scale; it will undergo small-angle, highfrequency vibrations. 8 The same will be true for fluoromethoxy group (OCF 3 ) reorientation in 1.…”

“…8 Half a molecule of 2, which is the asymmetric unit in the crystal, is shown in Fig. 1 (CSD-WOQFAL 8 ).…”

“…One occurs predominantly at high temperatures 28,29 and the other occurs predominantly at low temperatures. 8,11 In Sec. II we discuss the experimental procedure needed to characterize the nonexponential 1 H and 19 F spin-lattice relaxation.…”

“…(3) There is a phenomenological dimensionless parameter that is a measure of the strength of the interactions between 1 H and 19 F spins. 8,11 Although the 19 …”

“…2,3,6,8,11,15,16,101 If E NMR were frozen at its central value, the uncertainty in τ ∞ would be significantly smaller. It seems that most practitioners quote this smaller uncertainty.…”