Investigation of Solid State Matrices via 2H NMR on Mobile Deuterated Guest Molecules

Abstract

AbstractDeuterated benzene and hexamethylbenzene (HMB) molecules are used as probes to study molecular crystals and organic glasses via 2H NMR. The rotational mobility of the guest molecules causes fast nuclear spin polarization which enables spin‐lattice relaxation and line shape investigations in a temperature range of 10—300 K. The parameters of the molecular reorientation of the guests are investigated for the different matrices. HMB as a guest in hexachlorobenzene is characterized by an exponential relaxation and by an increase of the activation energy for the sixfold rotation by a factor of 2 as compared to neat HMB. A homogeneous mixing is found up to a concentration of about 7%. For the glassy matrices significant non‐exponential relaxation is observed corresponding to motional heterogeneities in the disordered systems. The relaxation is described by a convolution of exponential decays and a distribution of relaxation times which is related to a distribution of activation energies for the guest motion. Different shapes for the relaxation function at high and low temperatures prove an asymmetric distribution with its maximum on the low energy side. The deconvolution procedure leads to detailed information because two different Laplace transformations are involved. Below 40 K the 2H NMR line shape of HMB in the different matrices is determined by tunneling of the methyl groups. An increase of the tunneling frequency from about 10 kHz in neat HMB to more than 2 MHz in the other host systems is estimated.