title: A proton spin-lattice relaxation rate study of methyl and t-butyl group reorientation in the solid state.
abstract: We have measured the solid state nuclear magnetic resonance (NMR) 1H spin-lattice relaxation rate from 93 to 340 K at NMR frequencies of 8.5 and 53 MHz in 5-t-butyl-4-hydroxy-2-methylphenyl sulfide. We have also determined the molecular and crystal structures from X-ray diffraction experiments. The relaxation is caused by methyl and t-butyl group rotation modulating the spin-spin interactions and we relate the NMR dynamical parameters to the structure. A successful fit of the data requires that the 2-methyl groups are rotating fast (on the NMR time scale) even at the lowest temperatures employed. The rotational barrier for the two out-of-plane methyl groups in the t-butyl groups is 14.3+/-2.7 kJ mol(-1) and the rotational barrier for the t-butyl groups and their in-plane methyl groups is 24.0+/-4.6 kJ mol(-1). The uncertainties account for the uncertainties associated with the relationship between the observed NMR activation energy and a model-independent barrier, as well as the experimental uncertainties.
authors.affiliation: Department of Physics, Bryn Mawr College, 101 North Merion Ave., Bryn Mawr, PA 19010-2899, USA.
authors.forename: Laura C
authors.initials: LC
authors.lastname: Popa
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold
authors.forename: Peter A
authors.initials: PA
authors.lastname: Beckmann
mesh_terms: D002073:Butanes / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008697:Methane / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D013696:Temperature
chemical_list: D002073:Butanes; D011522:Protons; C046888:butane; D008697:Methane