Proton spin relaxation, internal motion and structure in solid 1,2,4,5-tetraisopropylbenzene

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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.

Section: Discussionmentioning

confidence: 99%

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

Beckmann

Buser

Gullifer

et al. 2003

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“…The parameter E NMR should be in the range 8-16 kJ mol −1 for the types of solids of which 4,4 -dimethoxybiphenyl is an example. 20,49,50,[58][59][60][61][62][63] The parameter C/C should be approximately 1, or perhaps somewhat larger if the time dependence of methyl 1 H nuclei−nonmethyl 1 H nuclei vectors is significant. The parameter, τ ∞ /τ • , should be within an order of magnitude of unity.…”

Section: Theoretical Expressions For the Relaxation Rate And The Omentioning

confidence: 99%

Methyl group rotation, 1H spin-lattice relaxation in an organic solid, and the analysis of nonexponential relaxation

Beckmann

Schneider

2012

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We report 1 H spin-lattice relaxation measurements in polycrystalline 4,4 -dimethoxybiphenyl at temperatures between 80 and 300 K at NMR frequencies of ω 0 /2π = 8.50, 22.5, and 53.0 MHz. The data are interpreted in terms of the simplest possible Bloch-Wangsness-Redfield methyl group hopping model. Different solid states are observed at low temperatures. The 1 H spin-lattice relaxation is nonexponential at higher temperatures where a stretched-exponential function fits the data very well, but this approach is phenomenological and not amenable to theoretical interpretation. (We provide a brief literature review of the stretched-exponential function.) The Bloch-Wangsness-Redfield model applies only to the relaxation rate that characterizes the initial 1 H magnetization decay in a high-temperature nonexponential 1 H spin-lattice relaxation measurement. A detailed procedure for determining this initial relaxation rate is described since large systematic errors can result if this is not done carefully.

“…In a large class of planar aromatic compounds, the rotational barrier for methyl group rotation in ethyl [1][2][3][4][5][6][7][8][9] and isopropyl 1,7,[10][11][12] groups tends to be dominated by intramolecular interactions in both the isolated molecule [6][7][8][9] and in the crystal. [1][2][3][4][5][10][11][12] The barriers for ethyl and isopropyl group rotations in these compounds, however, are much larger in the crystal than in the isolated molecule because these groups lack rotational symmetry.…”

Section: Introductionmentioning

confidence: 99%

“…[1][2][3][4][5][10][11][12] The barriers for ethyl and isopropyl group rotations in these compounds, however, are much larger in the crystal than in the isolated molecule because these groups lack rotational symmetry. As such, the interactions that determine the rotational barrier are dominated by intermolecular interactions.…”

Section: Introductionmentioning

confidence: 99%

“…Ethyl and isopropyl group rotational barriers in isolated molecules similar to that studied here [6][7][8][9] are in the 1 -8 kJ mol −1 range, but in the solid state they are effectively infinite. [1][2][3][4][5][10][11][12] In the solid state, rotational barriers in the 5-50 kJ mol −1 range can be determined by observing the temperature and the nuclear magnetic resonance ͑NMR͒ frequency dependence of 1 H spin-lattice relaxation rates. Previously published experiments with solid 3-isopropylchrysene 1 were fitted with a model that suggested that the methyl groups rotated in a barrier of 11Ϯ 1 kJ mol −1 .…”

Section: Introductionmentioning

confidence: 99%

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The quenching of isopropyl group rotation in van der Waals molecular solids

Wang¹,

Rheingold

DiPasquale

et al. 2008

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X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol −1 and the isopropyl group barrier is found to be about 240 kJ mol −1 , meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol −1 for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR 1 H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11Ϯ 1 kJ mol −1 and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.