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The formal similarity between meshed alkyl or aryl groups in crowded molecules and meshed mechanical gears has given rise to speculation that the availability of free spaces between the ligands of the central atoms might lead to a reduction in the steric requirements of the groups and hence to effects on rates of conformational interconversion. In a discussion of this notion, we recently concluded that there was no compelling evidence to support the operation of this dynamic gear effect in the case of methyl and tert-butyl groups (1). However, molecules in which two triptycyl (Tr) groups are bonded to a common center (X) at their bridgehead positions (C9 and C9'), and with a C9-X-C9' angle substantially less than 180°, bear an obvious resemblance to bevel gear systems. A molecule of this class, whose first representative, bis(9-Tr) selenide, was described by Wittig and Tochtermann in 1962 (2), might therefore be expected to exhibit facile cogwheeling. Indeed, the recent observation (3) of a low barrier to internal rotation (-8 kcal molh') in bis(9-Tr)methane (1) and bis(9-Tr) ether is consistent with rapid cogwheeling of the intermeshed Tr groups. This observation can of course be equally well accounted for by rapid rotation of the Tr groups without cogwheel rotation and therefore fails to provide direct evidence for dynamic gearing.However, strong support for the gearing hypothesis is obtained from empirical force-field calculations. By using a previously described force field (4), we have found that the C2 transition state for gearing in 1 (see Fig. 1) is only t1.0 kcal molh' above the Cs ground state (Fig. 1A). On the other hand, the gear-clashed conformation (Fig. 1B) is t20 kcal molhThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

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Analysis of molecular symmetry is combined with empirical force field calculations to provide a basis for describing internal motions in three types of molecules comprised of two threefold rotors on a C , frame: propane (l), di-tert-butylmethane (2), and bis(9-triptycy1)methane (3). The molecular symmetry group of these systems is G36 = (C, X C3) A V (of order 36).A representation of this group in terms of the rotor conformational angles shows a close relationship to a crystallographic plane group and furnishes a pictorial map of changes in geometry during conformational interconversions. Potential energy surfaces relevant to such changes were calculated by the empirical force field (EFF) method for all three systems by using full matrix Newton-Raphson optimization together with the force field from the MM2 program. Salient features of the internal motions in 1-3 are described. The calculations reveal that 3 undergoes correlated disrotation (dynamic gearing) of the 9-triptycyl (Tp) groups as a virtually unhindered process, with a barrier of 0.19 kcal mol-', whereas gear slippage (net conrotation of the Tp groups) requires at least 30.1 kcal mol-'. The mechanism of gear slippage is found by calculation to involve a transition state in which two benzene rings within one Tp moiety are squeezed together and tucked into the notch between two rings in the other Tp group. The stereochemical consequences of these interconversion mechanisms on labeled variants of all three compounds are analyzed by using group theory. The results are given in terms of the type and number of residual stereoisomers expected for each type of labeled molecule under the operation of those interconversion processes that are found to be energetically feasible according to the EFF calculations. The minimal labeling of 3 is deduced that allows a distinction between gearing with and without gear slippage. 24A, 2013. ( e ) Grant, D. M.; Pugmire, R. J.; Livingston, R. C.; Strong, K. A.; McMurray, H. L.; Brugger, R. M. Compton, D. A. C.; Rizzolo, J. J.; Jalilian, M. R.; Zozulin, A. J.; Odom, J. D. J. Mol. Struct. 1981, 77, 195. (9) As a notable example, we single out the study of Kwart and Alekmanlo of internal rotations in the dimesitylcarbinyl system, which led to the postulation of a 'cogwheel effect". The correlated rotation of aryl rings in systems containing two such rings attached to a single atom (Ar2Z) have been described and classified." The permutational consequences of the one-ring flip are satisfied by an internal motion in which the two aryl rings rotate in synchrony, so that when one ring is in the C-Z-C plane, the other is perpendicular to it. In crowded systems, i.e., in systems containing phenyl rings with ortho substituents, this cogwheeling motion corresponds to the lowest energy pathway, with ground and transition states assuming helical and perpendicular conformations," respectively.(10) Kwart, H.; Alekman, S.