Benzene-d6 and toluene-d8 as guest molecules in micropores of a layered zirconium phosphonate: 2 H, 13 C{1 H}, and 31 P{1 H} solid-state NMR, deuterium NMR relaxation, and molecular motions

Abstract

For the first time, pore spaces in the Zr (IV) phosphonate (1) as a representative of layered metal (IV) phosphonate materials have been investigated by studying mobility of guest molecules, benzene-d₆ , and toluene-d₈ . Guest molecules located in micropores of 1 have been characterized by solid-state ¹³ C{¹ H} and ² H NMR spectra in static samples with varying temperatures. At moderately low temperatures, the benzene and toluene molecules experience fast isotropic reorientations and show the motionally averaged liquid-like carbon and deuterium line shapes in the NMR spectra. At lower temperatures, two anisotropic motional modes have been found for benzene molecules by analyzing the ² H NMR line shapes: the well-known in-plane C₆ rotation and composite motions. Interpretation of the variable-temperature ² H T₁ relaxation times identifies the composite motions as 120° flips around the C₆ axis perpendicular to the molecular plane and the rotations around the molecular para-C-C axis. The data obtained resulted in the idealized (cylinder-shaped) model of micropores in compound 1 with the diameter of 20-30 Å. Furthermore, the activation energy of 20.1 kJ/mol determined for the benzene motions classifies the molecule-surface interactions as weak but enough for absorption.

Keywords: 13C; 2H; 31P; NMR; molecular motions; relaxation; zirconium phosphonate.