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The infrared spectrum of CO 2-(pH 2) 2 trimers is predicted by performing exact basis-set calculations on a global potential energy surface defined as the sum of accurately known two-body pH 2-CO 2 (J. Chem. Phys.2010, 132, 214309) and pH 2-pH 2 potentials (J. Chem. Phys.2008, 129, 094304). These results are compared with new spectroscopic measurements for this species, for which 13 transitions are now assigned. A reduced-dimension treatment of the pH 2 rotation has been employed by applying the hindered-rotor averaging technique of Li, Roy, and Le Roy (J. Chem. Phys.2010, 133, 104305). Three-body effects and the quality of the potential are discussed. A new technique for displaying the three-dimensional pH 2 density in the body-fixed frame is used, and shows that in the ground state the two pH 2 molecules are localized much more closely together than is the case for the two He atoms in the analogous CO 2-(He) 2 species. A clear tunneling splitting is evident for the torsional motion of the two pH 2 molecules on a ring about the CO 2 molecular axis, in contrast to the case of CO 2-(He) 2 where a more regular progression of vibrational levels reflects the much lower torsional barrier. © 2011 American Chemical Society.

The size of a series of typical substituents has been probed by dynamic NMR measurements of the barriers to aryl-aryl rotation of the corresponding biphenyls. The resulting B values are meaningful because only mono-ortho substituted compounds were investigated and thus the results are not compromised by the non-additivity of multiple steric effects. On the basis of the chosen model system ethynyl and cyano groups were found to be slightly smaller than a phenyl ring. In contrast, vinyl and, in particular, formyl groups proved to be larger than phenyl. The latter difference is due to the loss of conjugation forces at the planar transition state. Alpha-Hydroxyhexafluoroisopropyl is slightly more bulky than tert-butyl. Pentafluorophenyl and trifluoromethoxy exhibit nearly the same effective size as phenyl and methoxy, respectively. Trifluoromethyl is somewhat smaller than isopropyl.

The torsional barriers of 2'-substituted 2-arylpyridines have been probed experimentally (by using dynamic NMR spectroscopy) and computationally (by using density functional theory). Due to the compressibility of the lone pair, the torsional barriers of the arylpyridines are up to 4.2 kcal/mol smaller than those of the carba-analogous biphenyls. Furthermore, the ground states of the 2-arylpyridines are less twisted than those of the biphenyls. Finally, due to an out-of-collinearity distortion, the intramolecular repulsion is attenuated in both rotational transition states, in the syn coplanar conformer (in which the pyridine nitrogen and the substituent R face each other) and in the anti coplanar conformer (in which they are on opposite sides of the molecule).