Solid State 1 H Spin - lattice Relaxation and Isolated - Molecule and Cluster Electronic Structure Calculations in Organic Molecular Solids: The Relationship Between Structure and Methyl Group and t - Butyl Group Rotation

We report ab initio density functional theory electronic structure calculations of rotational barriers for t-butyl groups and their constituent methyl groups both in the isolated molecules and in central molecules in clusters built from the X-ray structure in four t-butyl aromatic compounds. The X-ray structures have been reported previously. We also report and interpret the temperature dependence of the solid state 1H nuclear magnetic resonance spin-lattice relaxation rate at 8.50, 22.5, and 53.0 MHz in one of the four compounds. Such experiments for the other three have been reported previously. We compare the computed barriers for methyl group and t-butyl group rotation in a central target molecule in the cluster with the activation energies determined from fitting the 1H NMR spin-lattice relaxation data. We formulate a dynamical model for the superposition of t-butyl group rotation and the rotation of the t-butyl group’s constituent methyl groups. The four compounds are 2,7-di-t- butylpyrene, 1,4-di-t-butylbenzene, 2,6-di-t-butylnaphthalene, and 3-t-butylchrysene. We comment on the unusual ground state orientation of the t-butyl groups in the crystal of the pyrene and we comment on the unusually high rotational barrier of these t-butyl groups

Solid State 1 H Spin - lattice Relaxation and Isolated - Molecule and Cluster Electronic Structure Calculations in Organic Molecular Solids: The Relationship Between Structure and Methyl Group and t - Butyl Group Rotation

We report ab initio density functional theory electronic structure calculations of rotational barriers for t-butyl groups and their constituent methyl groups both in the isolated molecules and in central molecules in clusters built from the X-ray structure in four t-butyl aromatic compounds. The X-ray structures have been reported previously. We also report and interpret the temperature dependence of the solid state 1H nuclear magnetic resonance spin-lattice relaxation rate at 8.50, 22.5, and 53.0 MHz in one of the four compounds. Such experiments for the other three have been reported previously. We compare the computed barriers for methyl group and t-butyl group rotation in a central target molecule in the cluster with the activation energies determined from fitting the 1H NMR spin-lattice relaxation data. We formulate a dynamical model for the superposition of t-butyl group rotation and the rotation of the t-butyl group’s constituent methyl groups. The four compounds are 2,7-di-t- butylpyrene, 1,4-di-t-butylbenzene, 2,6-di-t-butylnaphthalene, and 3-t-butylchrysene. We comment on the unusual ground state orientation of the t-butyl groups in the crystal of the pyrene and we comment on the unusually high rotational barrier of these t-butyl groups

Methoxy and Methyl Group Rotation: Solid State NMR 1H Spin-lattice Relaxation, Electronic Structure Calculations, Xray Diffractometry, and Scanning Electron Microscopy

We report solid-state (1) H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments, X-ray diffractometry, field-emission scanning electron microscopy, and both single-molecule and cluster ab initio electronic structure calculations on 1-methoxyphenanthrene (1) and 3-methoxyphenanthrene (2) to investigate the rotation of the methoxy groups and their constituent methyl groups. The electronic structure calculations and the (1) H NMR relaxation measurements can be used together to determine barriers for the rotation of a methoxy group and its constituent methyl group and to develop models for the two coupled motions

Methoxy and Methyl Group Rotation: Solid State NMR 1H Spin-lattice Relaxation, Electronic Structure Calculations, Xray Diffractometry, and Scanning Electron Microscopy

We report solid-state (1) H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments, X-ray diffractometry, field-emission scanning electron microscopy, and both single-molecule and cluster ab initio electronic structure calculations on 1-methoxyphenanthrene (1) and 3-methoxyphenanthrene (2) to investigate the rotation of the methoxy groups and their constituent methyl groups. The electronic structure calculations and the (1) H NMR relaxation measurements can be used together to determine barriers for the rotation of a methoxy group and its constituent methyl group and to develop models for the two coupled motions

The Quenching of Isopropyl Group Rotation in Van Der Waals Molecular Solids

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 11 +/- 1 kJ mol(-1) and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale

CF3 Rotation in 3-(Trifluoromethyl)phenanthrene. X-ray Diffraction and ab Initio Electronic Structure Calculations

The molecular and crystal structure of 3-(trifluoromethyl)phenanthrene has been determined by X-ray diffraction. The structure of the isolated molecule has been calculated using electronic structure methods at the HF/3-21G, HF/6-31G*, MP2/6-31G* and B3LYP/6-31G* levels. The potential energy surfaces for the rotation of the CF3 group in both the isolated molecule and cluster models for the crystal were computed using electronic structure methods. The barrier height for CF3 rotation in the isolated molecule was calculated to be 0.40 kcal mol-1 at B3LYP/6-311+G**//B3LYP/6-311+G**. The B3LYP/6-31G* calculated CF3 rotational barrier in a 13-molecule cluster based on the X-ray data was found to be 2.6 kcal mol-1. The latter is in excellent agreement with experimental results from the NMR relaxation experiments reported in the companion paper (Beckmann, P. A.; Rosenberg, J.; Nordstrom, K.; Mallory, C. W.; Mallory, F. B. J. Phys. Chem. A 2006, 110, 3947). The computational results on the models for the solid state suggest that the intermolecular interaction between nearest neighbor pairs of CF3 groups in the crystal accounts for roughly 75% of the barrier to rotation in the solid state. This pair is found to undergo cooperative reorientation. We attribute the CF3 reorientational disorder in the crystal as observed by X-ray diffraction to the presence of a pair of minima on the potential energy surface and the effects of librational motion

Methyl and T-Butyl Group Reorientation in Planar Aromatic Solids: Low-Frequency Nuclear Magnetic Resonance Relaxometry and X-Ray Diffraction

We have synthesized 3-t-butylchrysene and measured the Larmor frequency omega/2pi (= 8.50, 22.5, and 53.0 MHz) and temperature T (110-310 K) dependence of the proton spin-lattice relaxation rate R in the polycrystalline solid [low-frequency solid state nuclear magnetic resonance (NMR) relaxometry]. 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 +/- 0.9 kJ mol(-1), and the two out-of-plane methyl groups reorient with a barrier of 14.2 +/- 0.6 kJ mol(-1). 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-butyl- naphthalene. 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

Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: Electronic structure calculations and solid state NMR relaxation measurements

The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8–13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (Eclust) range from 0.6 to 3.4 kcal/mol. Solid-state 1H NMR spin–lattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (ENMR) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences Eclust – ENMR for each of the ten methyl groups range from −0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (Eclust) and the corresponding computed barriers in the isolated molecules (Eisol) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of Eclust – Eisol range from 0.0 to 1.0 kcal/mol

Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: Electronic structure calculations and solid state NMR relaxation measurements

The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8–13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (Eclust) range from 0.6 to 3.4 kcal/mol. Solid-state 1H NMR spin–lattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (ENMR) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences Eclust – ENMR for each of the ten methyl groups range from −0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (Eclust) and the corresponding computed barriers in the isolated molecules (Eisol) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of Eclust – Eisol range from 0.0 to 1.0 kcal/mol

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

We report a variety of experiments and calculations and their interpretations regarding methyl group (CH3) rotation in samples of pure 3-methylglutaric anhydride (1), pure 3-methylglutaric acid (2), and samples where the anhydride is slowly absorbing water from the air and converting to the acid [C6H8O3(1) + H2O → C6H10O4(2)]. The techniques are solid state 1H nuclear magnetic resonance (NMR) spin-lattice relaxation, single-crystal X-ray diffraction, electronic structure calculations in both isolated molecules and in clusters of molecules that mimic the crystal structure, field emission scanning electron microscopy, differential scanning calorimetry, and high resolution 1H NMR spectroscopy. The solid state 1H spin-lattice relaxation experiments allow us to observe the temperature dependence of the parameters that characterize methyl group rotation in both compounds and in mixtures of the two compounds. In the mixtures, both types of methyl groups (that is, molecules of 1 and 2) can be observed independently and simultaneously at low temperatures because the solid state 1H spin-lattice relaxation is appropriately described by a double exponential. We have followed the conversion 1 → 2 over periods of two years. The solid state 1H spin-lattice relaxation experiments in pure samples of 1 and 2 indicate that there is a distribution of NMR activation energies for methyl group rotation in 1 but not in 2 and we are able to explain this in terms of the particle sizes seen in the field emission scanning electron microscopy images