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.

Discovery of Deep-Seated Skeletal Rearrangements in the Photocyclizations of Some tert-Butyl-Substituted 1,2-Diarylethylenes.

The in-solution oxidative photocyclization of stilbenes to phenanthrenes is a well-known and synthetically valuable reaction. We report here our discovery that the oxidative photocyclization of several tert-butyl-substituted 1-styrylphenanthrenes resulted not only in the expected formation of tert-butyl-substituted picenes but also in the previously unknown rearrangement leading to the formation of tert-butyl-substituted pentahelicenes.

Phenyl groups versus tert-butyl groups as solubilizing substituents for some [5]phenacenes and [7]phenacenes.

In recent years, we have used the photocyclizations of diarylethylenes to synthesize a number of [n]phenacenes in the hope that they might be useful as the bridging groups for electron transfer processes in donor-bridge-acceptor molecules. Because [n]phenacenes with n > 5 are very insoluble, their synthesis and characterization has required the attachment of solubilizing substituents such as tert-butyl. The studies of Pascal and co-workers of some large polynuclear aromatic compounds having multiple phenyl substituents prompted us to explore the use of phenyls as alternative solubilizing groups for [n]phenacenes. Although phenyl groups turned out to provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.