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 (E(clust)) range from 0.6 to 3.4 kcal/mol. Solid-state (1)H NMR spin-lattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (E(NMR)) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences E(clust) - E(NMR) 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 (E(clust)) and the corresponding computed barriers in the isolated molecules (E(isol)) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of E(clust) - E(isol) range from 0.0 to 1.0 kcal/mol.

[1]  J. Baudry.  Van der waals interactions and decrease of the rotational barrier of methyl-sized rotators: a theoretical study. , 2006, Journal of the American Chemical Society.

[2]  C. W. Mallory,et al.  CF3 rotation in 3-(trifluoromethyl)phenanthrene. X-ray diffraction and ab initio electronic structure calculations. , 2006, The journal of physical chemistry. A.

[3]  C. W. Mallory,et al.  Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction , 2003 .

[4]  Chick C. Wilson.  Barrier heights to methyl group rotation in dimethylnaphthalenes , 2002 .

[5]  R. Kimmich.  NMR: Tomography, Diffusometry, Relaxometry , 2001 .

[6]  Frank Weinhold,et al.  UNDERSTANDING BARRIERS TO INTERNAL ROTATION IN SUBSTITUTED TOLUENES , 1995 .

[7]  J. Glusker,et al.  Molecular orbital studies of methylation effects in aromatic hydrocarbons. 1. Ab initio calculations of the structure, electronic properties and energy of toluene, 1-methylnaphthalene, and 2-methylnaphthalene , 1988 .

[8]  A. Saika,et al.  Crystal structures of 4,5-dimethyl- and 4-methylphenanthrenes. Planarity of congested phenanthrene molecules , 1987 .

[9]  C. W. Mallory,et al.  Methyl reorientation in methylphenanthrenes. I. Solid state proton spin–lattice relaxation in the 3‐methyl, 9‐methyl, and 3,9‐dimethyl systems , 1987 .

[10]  T. Liljefors,et al.  A note on torsional force constants in molecular mechanics for a methyl group attached to a conjugated system , 1985 .

[11]  A. Saika,et al.  1H and 13C NMR study on rotation of congested methyl groups in methyl substituted phenanthrenes, fluorenes, and fluorenones , 1984 .

[12]  C. Blomberg,et al.  Brownian motion description of activation energies from NMR-relaxation times for rotating molecular-groups , 1981 .

[13]  T. Liljefors,et al.  On the relationship between the potential barrier and the activation energy for the internal rotation of a methyl group , 1979 .

[14]  E. Berliner,et al.  Internal Strain and Reactivity. Some Reactions of Acenaphthene , 1965 .

[15]  H. Carr,et al.  The Principles of Nuclear Magnetism , 1961 .

[16]  Roy W. H. Tess,et al.  Synthesis of Phenanthrene Derivatives. III.19-Methylphenanthrene , 1939 .

[17]  Chick C. Wilson,et al.  Methyl group librations in sterically hindered dimethylnaphthalene molecules: neutron diffraction studies of 1,8-dimethylnaphthalene between 50 and 200 K , 2000 .

[18]  Chick C. Wilson.  LOCKED-IN METHYL GROUPS IN 1,5-DIMETHYLNAPHTHALENE CLOSE TO THE MELTING POINT , 1997 .

[19]  C. W. Mallory,et al.  Methyl reorientation in methylphenanthrenes—II. solid state proton spin-lattice relaxation in the 1-CH3, 9-CH3 and 1-CD3, 9-CH3 systems , 1990 .

[20]  O. Johnson,et al.  The crystal and molecular structure of 9,10-dimethylphenanthrene , 1989 .

[21]  G. Bodenhausen,et al.  Principles of nuclear magnetic resonance in one and two dimensions , 1987 .

[22]  H. Wolf,et al.  Bewegungen der CH3-Gruppen in Methy-naphthalin-Kristallen I. Protonen-Spin-Relaxation in Kristallinen Methy-Derivaten des Naphthalins , 1972 .

[23]  R. D. Haworth,et al.  408. Synthesis of alkylphenanthrenes. Part V. 9-Methyl-, 1 : 9-dimethyl-, and 1 : 2 : 8-trimethyl-phenanthrenes , 1932 .