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

Abstract

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 1 H 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 1 H 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.

Proton Spin‐Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

“…Ln R * versus T −1 and β versus T −1 are shown as squares in Figure for samples EA1 and EA2 and in Figure for samples EAQ1, EAQ2, and EAQ3. Based on many previous experiments,, β versus T −1 can be expected to lie within the two purple lines in Figures b and 10b. For EA, β versus T −1 for both samples EA1 and EA2 in Figure b is as expected (or nearly so) but the different β versus T −1 data for all three samples of EAQ at low temperatures in Figure b is unusual.…”

“…All these images are very unusual and very difficult to interpret unambiguously. Normally, at the spatial resolution indicated in part (b) of these figures, at least some structures that are clearly single crystals can be seen in these types of organic solids,, but that is not the case here. All these images suggest structures that may or may not be single crystals and that, if they are single crystals, the crystals have a smallest dimension (thickness) that is 100s nm or less.…”

“…In both EA and EAQ, the dominant solid state 1 H spin‐lattice relaxation mechanism involves methyl group rotation modulating the six 1 H− 1 H spin−spin interactions among the three 1 H spins in a methyl group . In the solid state, the ethyl groups, like isopropyl groups, are static on the NMR time scale, meaning that the NMR activation energy for rotation is over approximately 60 kJ mol −1 which is to be compared to the methyl group activation energies in the 8–18 kJ mol −1 for molecules like those studied here ,,,. The quenching of ethyl and isopropyl group rotation in the solid state results from intermolecular interactions, and the rotational asymmetry of ethyl and isopropyl groups.…”

“…6 in ref 83). It is important that the H−H distance in a methyl group r as determined by electronic structure calculations is used since single‐crystal X‐ray diffraction finds distances that are approximately 10 % too short …”