"Methyl and t-butyl reorientation in an organic molecular solid" by Peter A. Beckmann, William G. Dougherty Jr. et al.
 

Document Type

Article

Version

Author's Final Manuscript

Publication Title

Solid State Nuclear Magnetic Resonance

Volume

36

Publication Date

2009

Abstract

We have determined the molecular and crystal structure of 4,5-dibromo-2,7-di-t-butyl-9,9-dimethylxanthene and measured the 1H spin–lattice relaxation rate from 87 to 270 K at NMR frequencies of ω/2π=8.50, 22.5, and 53.0 MHz. All molecules in the crystal see the same intra and intermolecular environment and the repeating unit is half a molecule. We have extended models developed for 1H spin–lattice relaxation resulting from the reorientation of a t-butyl group and its constituent methyl groups to include these rotors and the 9-methyl groups. The relaxation rate data is well-fitted assuming that the t-butyl groups and all three of their constituent methyl groups, as well as the 9-methyl groups all reorient with an NMR activation energy of 15.8±1.6 kJ mol−1 corresponding to a barrier of 17.4±3.2 kJ mol−1. Only intramethyl and intra-t-butyl intermethyl spin–spin interactions need be considered. A unique random-motion Debye (or BPP) spectral density will not fit the data for any reasonable choice of parameters. A distribution of activation energies is required.

DOI

http://www.sciencedirect.com/science/article/pii/S0926204009000587

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