Single-crystal X-ray diffraction, isolated-molecule and cluster electronic structure calculations, and scanning electron microscopy in an organic solid: models for intramolecular motion in 4,4'-dimethoxybiphenyl.

This paper brings together field emission scanning electron microscopy, single-crystal X-ray diffraction, and density functional electronic structure calculations in both an isolated molecule and a cluster of seven whole and fourteen half molecules of 4,4'-dimethoxybiphenyl to investigate coupled methyl-group rotation (over a barrier) and methoxy-group libration (meaning a rotation from the ground state not all the way to the transition state and back again). The structure of the isolated molecule, determined by the electronic structure calculations, is compared with the structure of the molecule found in the crystal. As the methyl group rotates from its ground state to its transition state, the methoxy group rotates 30° in the isolated molecule and 16° in the cluster. The calculated barriers for this coupled methyl-group rotation and methoxy group libration in the isolated molecule and in the crystal are 12.8 kJ mol(-1) and 10.3 kJ mol(-1) respectively, suggesting that intermolecular interactions in the crystal lower the barrier. These barriers are compared with the value of 11.5±0.5 kJ mol(-1) obtained from solid-state (1)H spin-lattice relaxation measurements [P. A. Beckmann and E. Schneider, J. Chem. Phys. 2012, 136, 054508.].