Nuclear-spin relaxation in molecular solids with reorienting methyl and<i>t</i>-butyl groups: The spectral density and the state of the solid

Phys. Chem. Chem. Phys.

Moore

Rheingold

2016

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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).

Section: The Two Si Methyl Groups Might Very Well Have Different Valumentioning

confidence: 99%

“…38 and some of the A inter ) can all be related to A intra as well as other numerical factors. 33,38,39 The best fit of the high temperature R versus T data in Fig. 5 is found for all the constants C i set to their calculated values.…”

Section: The Two Si Methyl Groups Might Very Well Have Different Valumentioning

confidence: 99%

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

Phys. Chem. Chem. Phys.

Moore

Rheingold

2016

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“…This equality is a coincidence. The theoretically calculated value of this parameter is given by 3 Ã t ϭ n t N ͑ 2.41ϫ10 10 s Ϫ2 ͒, ͑8͒…”

Section: ͑2͒mentioning

confidence: 99%

“…The C k j are numerical factors of order unity, 0 is the permeability of free space, and the r k j are various distances found in methyl and tert-butyl groups. 3 Under the condition that the intramethyl spin-spin interactions are included in the calculation exactly, but the extra-methyl, intra-t-butyl spin-spin interactions are approximated, A 1 ϭA 2 ϭA t . This equality is a coincidence.…”

Section: ͑2͒mentioning

confidence: 99%

H 1 nuclear magnetic resonance spin-lattice relaxation, C13 magic-angle-spinning nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and x-ray diffraction of two polymorphs of 2,6-di-tert-butylnaphthalene

The Journal of Chemical Physics

Burbank

Clemo

et al. 2000

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Polymorphism, the presence of structurally distinct solid phases of the same chemical species, affords a unique opportunity to evaluate the structural consequences of intermolecular forces. The study of two polymorphs of 2,6-di-tert-butylnaphthalene by single-crystal x-ray diffraction, differential scanning calorimetry ͑DSC͒, 13 C magic-angle-spinning ͑MAS͒ nuclear magnetic resonance ͑NMR͒ spectroscopy, and 1 H NMR spin-lattice relaxation provides a picture of the differences in structure and dynamics in these materials. The subtle differences in structure, observed with x-ray diffraction and chemical shifts, strikingly affect the dynamics, as reflected in the relaxation measurements. We analyze the dynamics in terms of both discrete sums and continuous distributions of Poisson processes.

“…A class of molecules that has been rewarding to study are single-and fused-ring aromatic structures containing one or two t-butyl groups. [1][2][3][4][5][6][7][8] We have synthesized the organic molecule 3-t-butylchrysene, determined the molecular ͑Fig. 1͒ and crystal ͑Figs.…”

Section: Introductionmentioning

confidence: 99%

Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction

The Journal of Chemical Physics

Buser

Gullifer

et al. 2003

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We have synthesized 3-t-butylchrysene and measured the Larmor frequency /2 ͑ϭ 8.50, 22.5, and 53.0 MHz͒ and temperature T ͑110-310 K͒ dependence of the proton spin-lattice relaxation rate R in the polycrystalline solid ͓low-frequency solid state nuclear magnetic resonance ͑NMR͒ relaxometry͔. We have also determined the molecular and crystal structure in a single crystal of 3-t-butylchrysene using x-ray diffraction, which indicates the presence of a unique t-butyl group environment. The spin-1/2 protons relax as a result of the spin-spin dipolar interactions being modulated by the superimposed reorientation of the t-butyl groups and their constituent methyl groups. The reorientation is successfully modeled by the simplest motion; that of random hopping describable by Poisson statistics. The x-ray data indicate near mirror-plane symmetry that places one methyl group nearly in the aromatic plane and the other two almost equally above and below the plane. The NMR relaxometry data indicate that the nearly in-plane methyl group and the entire t-butyl group reorient with a barrier of 24.2 Ϯ0.9 kJ mol Ϫ1 , and the two out-of-plane methyl groups reorient with a barrier of 14.2Ϯ0.6 kJ mol Ϫ1 . Following a brief review of methyl group rotation in simple ethyl-, and isopropyl-substituted one-and two-ring aromatic van der Waals molecular solids, the barriers for the out-of-plane methyl groups and the t-butyl group in 3-t-butylchrysene are compared with those barriers in three related molecular solids whose crystal structure is known: 4-methyl-2,6-di-t-butylphenol, 1,4-di-t-butylbenzene, and polymorph A of 2,6-di-t-butylnaphthalene. A trend is observed in the reorientational barriers for the t-butyl and the out-of-plane methyl groups across this series of four compounds: as the t-butyl barriers decrease, the out-of-plane methyl barriers increase.