Methyl and t-butyl group rotation in a molecular solid:¹H NMR spin-lattice relaxation and X-ray diffraction

Abstract

Methyl and t-butyl group rotation in a molecular solid: 1H NMR spin-lattice relaxation and X-ray diffraction. P A Beckmann, C E Moore, and A L Rheingold 2016 Physical Chemistry Chemical Physics, 18 1720-1726. Methyl and t-butyl AbstractWe report solid state 1 H nuclear magnetic resonance spin-lattice relaxation experiments and X-ray diffractometry in 2-t-butyldimethylsilyloxy-6-bromonaphthalene. This compound offers an opportunity to simultaneously investigate, and differentiate between, the rotations of a t-butyl group [C(CH 3 ) 3 ] and its three constituent methyl groups (CH 3 ) and, simultaneously, a pair of 'lone' methyl groups (attached to the Si atom). The solid state 1 H relaxation experiments determine activation energies for these rotations. We review the models for the dynamics of both 'lone' methyl groups (ones whose rotation axes do not move on the NMR time scale) and models for the dynamics of the t-butyl group and its constituent methyl groups (whose rotation axes reorient on the NMR time scale as the t-butyl group rotates).

Proton Spin‐Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

“…This phenomenon is a result of cross correlations among the three spin‐1/2 protons (or three spin‐1/2 19 F nuclei) and is well understood, and well established experimentally ,. The relaxation becomes more exponential if the methyl group rotation axis is reorienting, such as in a t ‐butyl group, or for whole‐molecule rotation ,. It also becomes more exponential if CH 3 – non‐CH 3 1 H‐ 1 H interactions play a significant role ,,.…”

Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra‐ and Intermolecular Potential Contributions to tert‐Butyl and Methyl Group Rotation

“…[16] Rapid spin diffusion assures that all 1 H spins in the molecule relax at the same rate. [13,26] At short times following the perturbation of the 1 H spin system, these cross correlations do not come into play and, with minor modifications, the dynamics can be treated as the relaxation resulting from three relaxing 1 H spin pairs in a methyl group. [17][18][19][20][21][22][23][24][25] If methyl group rotation is superimposed on tert-butyl group rotation (on the NMR time scale or faster), the relaxation becomes more exponential.…”

“…[17][18][19][20][21][22][23][24][25] If methyl group rotation is superimposed on tert-butyl group rotation (on the NMR time scale or faster), the relaxation becomes more exponential. [13,26] At short times following the perturbation of the 1 H spin system, these cross correlations do not come into play and, with minor modifications, the dynamics can be treated as the relaxation resulting from three relaxing 1 H spin pairs in a methyl group. [19] This makes the models used to interpret data much simpler and more meaningful.…”

Note: Methyl and t-butyl group rotation in van der Waals solids

“…The spin-lattice relaxation is caused by the modulation of 1 H-1 H spin-spin interactions by the rotations of the t-butyl groups and their constituent methyl groups. 1,2 The parameters that characterize the rotations, in turn, contribute to a better understanding of the anisotropies in the intramolecular and intermolecular potentials in these types of van der Waals solids composed of small organic molecules. The compound was purchased from Sigma-Aldrich (98%, mp 327-331 K).…”

“…A single crystal was taken directly from the supplier's sample and the crystal and molecular structure was determined using X-ray techniques as described elsewhere. 1 The reference number for the Cambridge Structural Database, where parameters characterizing the monoclinic structure can be found, is 1539154. The structure is shown in Fig.1a and the two molecules in the asymmetric unit can be seen in Fig.…”