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 http://onlinelibrary.wiley.com/doi/10.1002/cphc.201101067/abstract Xianlong Wang, *[a] Lolita Rotkina, [b] Hong Su, [c] and Peter A. Beckmann, Mawr, Pennsylvania 19010-2899, USA Fax: (+1) 610-526-7489 E-mail: pbeckman@brynmawr.edu Accepted for publication in ChemPhysChem February 2012This paper brings together field emission scanning electron microscopy, single-crystal Xray diffraction, and ab initio electronic structure calculations in both an isolated molecule and a cluster of 7 whole and 14 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 O in the isolated molecule and 16 O 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, 1-9.] Wang et al 2 IntroductionSmall methyl-substituted organic molecules in the solid state are good systems for investigating (1) the relationship between methyl group rotation and molecular and crystal structure [1][2][3][4][5][6][7][8][9][10][11][12] and (2) for developing models for intramolecular motion. [7,11,[13][14][15][16] They are also good test cases for comparing calculated and measured barriers to intramolecular motion. [17][18][19][20][21][22] In this paper we bring together single crystal X-ray crystallography, isolated molecule and molecular cluster ab initio electronic structure calculations, and field emission scanning electron microscopy with 4,4'-dimethoxybiphenyl (Figures 1 and 2). This allows us to relate the properties describing methyl group rotation and methoxy group libration to the structure on both the microscopic and macroscopic scales. By "methyl group rotation", we mean a rotation from a ground state to a transition state followed by a return to the ground state either by rotating in the same direction or by rotating in the opposite direction. By "methoxy group libration", we mean a rotation from the ground state only part way to the transition state followed by a reversal of the rotation back to the ground state. We compare the calculated barrier for this coupled met...