Methyl and t-butyl reorientation in an organic molecular solid

• Published in: Solid state nuclear magnetic resonance 2009-10, Vol.36 (2), p.86-91
• Main Authors: Beckmann, Peter A., Dougherty, William G., Scott Kassel, W.
• Format: Article
• Language: English
• Subjects: Methyl group rotation; Nuclear spin–lattice relaxation; Organic solids; Solid state NMR; Tertiarybutyl group rotation; X-ray crystallography
• ISSN: 0926-2040; 1527-3326
• DOI: 10.1016/j.ssnmr.2009.06.002

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/2 π=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.