Methyl reorientation in methylphenanthrenes. I. Solid state proton spin–lattice relaxation in the 3‐methyl, 9‐methyl, and 3,9‐dimethyl systems

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

We report solid state ¹H nuclear magnetic resonance spin-lattice relaxation experiments and X-ray diffractometry in 2-t-butyldimethylsilyloxy-6-bromonaphthalene.

Nonexponential (1)H spin-lattice relaxation and methyl group rotation in molecular solids

We report a quantitative measure of the nonexponential (1)H spin-lattice relaxation resulting from methyl group (CH3) rotation in six polycrystalline van der Waals solids. We briefly review the subject in general to put the report in context. We then summarize several significant issues to consider when reporting (1)H or (19)F spin-lattice relaxation measurements when the relaxation is resulting from the rotation of a CH3 or CF3 group in a molecular solid.

Amphidynamic crystals of a steroidal bicyclo[2.2.2]octane rotor: a high symmetry group that rotates faster than smaller methyl and methoxy groups

The synthesis, crystallization, single crystal X-ray structure, and solid state dynamics of molecular rotor 3 provided with a high symmetry order and relatively cylindrical bicyclo[2.2.2]octane (BCO) rotator linked to mestranol fragments were investigated in this work. By use of solid state (13)C NMR, three rotating fragments were identified within the molecule: the BCO, the C19 methoxy and the C18 methyl groups. To determine the dynamics of the BCO group in crystals of 3 by variable temperature (1)H spin-lattice relaxation (VT (1)H T1), we determined the (1)H T1 contributions from the methoxy group C19 by carrying out measurements with the methoxy-deuterated isotopologue rotor 3-d6. The contributions from the quaternary methyl group C18 were estimated by considering the differences between the VT (1)H T1 of mestranol 8 and methoxy-deuterated mestranol 8-d3. From these studies it was determined that the BCO rotator in 3 has an activation energy of only 1.15 kcal mol(-1), with a barrier for site exchange that is smaller than those of methyl (E(a) = 1.35 kcal mol(-1)) and methoxy groups (E(a) = 1.92 kcal mol(-1)), despite their smaller moments of inertia and surface areas.

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.

A proton spin-lattice relaxation rate study of methyl and t-butyl group reorientation in the solid state

We have measured the solid state nuclear magnetic resonance (NMR) 1H spin-lattice relaxation rate from 93 to 340 K at NMR frequencies of 8.5 and 53 MHz in 5-t-butyl-4-hydroxy-2-methylphenyl sulfide. We have also determined the molecular and crystal structures from X-ray diffraction experiments. The relaxation is caused by methyl and t-butyl group rotation modulating the spin-spin interactions and we relate the NMR dynamical parameters to the structure. A successful fit of the data requires that the 2-methyl groups are rotating fast (on the NMR time scale) even at the lowest temperatures employed. The rotational barrier for the two out-of-plane methyl groups in the t-butyl groups is 14.3+/-2.7 kJ mol(-1) and the rotational barrier for the t-butyl groups and their in-plane methyl groups is 24.0+/-4.6 kJ mol(-1). The uncertainties account for the uncertainties associated with the relationship between the observed NMR activation energy and a model-independent barrier, as well as the experimental uncertainties.

Methyl and t-butyl reorientation in an organic molecular solid

We have determined the molecular and crystal structure of 4,5-dibromo-2,7-di-t-butyl-9,9-dimethylxanthene and measured the (1)H spin-lattice relaxation rate from 87 to 270K at NMR frequencies of omega/2pi=8.50, 22.5, and 53.0MHz. All molecules in the crystal see the same intra and intermolecular environment and the repeating unit is half a molecule. We have extended models developed for (1)H spin-lattice relaxation resulting from the reorientation of a t-butyl group and its constituent methyl groups to include these rotors and the 9-methyl groups. The relaxation rate data is well-fitted assuming that the t-butyl groups and all three of their constituent methyl groups, as well as the 9-methyl groups all reorient with an NMR activation energy of 15.8+/-1.6kJmol(-1) corresponding to a barrier of 17.4+/-3.2kJmol(-1). Only intramethyl and intra-t-butyl intermethyl spin-spin interactions need be considered. A unique random-motion Debye (or BPP) spectral density will not fit the data for any reasonable choice of parameters. A distribution of activation energies is required.

Solid-state NMR and 2,3-dicyano-5,7-dimethyl-6H-1,4-diazepine

Crystalline 2,3-dicyano-5,7-dimethyl-6H-1,4-diazepine (A) was investigated by solid-state NMR spectroscopy, X-ray diffraction, and spectral simulations. The solid-state 13C NMR spectra of A display peculiar splittings for the methyl and cyano resonances. The crystal structure of A indicates that the methyl doublet is a consequence of two crystallographically inequivalent environments. The methyl motions associated with each site was examined via spin-lattice relaxation time (T1) measurements, and the carbon relaxation times (T(1)C) were used to calculate energy barriers to methyl rotation. The energy barriers to rotation were then used to correlate each methyl 13C shift with a particular crystallographic environment. The complex cyano splittings, however, are a result of both crystallographic inequivalence and residual 13C-14N dipolar coupling. The multiplet patterns of the isotropic shifts (centerbands) are dependent upon the magic-angle spinning (MAS) rate. Spectral simulations, using the perturbation method, of the centerbands and first-order sidebands were used to demonstrate, and elucidate, the observed MAS rate-dependent multiplet patterns of the cyano signals.

Unusual proton Zeeman spin relaxation in an organic solid: several crystal polymorphs or different glass structures?

Solid state proton Zeeman relaxation rate R1z measurements in two isomers of an organic solid (1- and 2-ethylnaphthalene) are reported. The samples are liquids at room temperature and the temperature T and Larmor frequency omega dependence of R1z depends strongly on how the sample is solidified. Methyl group (CH3) rotation is responsible for the proton spin relaxation and the methyl groups serve as probes of the local environment. The R1z measurements clearly distinguish between different solid states due to the differences in local structure at the several-molecule level. The experiments cannot be used to determine the states of these Van der Waals solids although interpreting the relaxation rate data suggests the states are unusual. We propose that these systems might exist in two (2-ethylnaphthalene) or more (1-ethylnaphthalene) polycrystalline polymorphs or that we are observing distinguishable glassy states, or, both. A phase transition is observed in 1-ethylnaphthalene. Variable temperature X-ray studies of organic systems that solidify well below room temperature are difficult, or at least not routine, and proton spin relaxation measurements serve as a convenient starting point for investigating such systems.