SUBSTITUENT EFFECTS ON THE STABILITY OF ARENE-ARENE COMPLEXES - AN AM1 STUDY OF THE CONFORMATIONAL EQUILIBRIA OF CIS-1,3-DIPHENYLCYCLOHEXANES

Molecular orbital calculations at the AM1 level have been performed on cis-1,3-diarylcyclohexane systems (1, 2) in order to provide a theore tical model and suggest novel experimental models for the investigatio n of arene-arene Jr-stacking. Energy minima were located for diaxial c onformers (a) in which aryl rings are coplanar and rr-stacked and (b) in which aryl rings adopt an edge-to-face or intermediate conformation . The average face-to-face distance varies fi om 4.1 to 4.4 Angstrom, outside the van der Waals contact distance of 3.4 Angstrom, and the mi nimum inter-ring distance varies from 3.1 to 3.5 Angstrom. The diaxial -diequatorial conformational equilibria (Delta E(ax-eq)) were calculat ed for a large series of para-substituted and meta-substituted 1,3-dia rylcyclohexanes and plotted against Sigma sigma(p), Delta sigma(p), an d E(HOMO) - E(LUMO). The best correlation was observed using electrost atic potentials calculated at the electron density surface on the, pi- face of individual arenes. Similar correlations were observed for conf ormational equilibria associated with the improved 1,3-dimethyl-1,3-di arylcyclohexane series (3). More limited correlations were drawn for t he diaxial-diequatorial conformational equilibria involving (i) edge-t o-face diaxial cis-1,3-diarylcyclohexanes and (ii) pi-stacked diaxial cis-1,3-diarylcyclohexanes in which one aryl ring is perfluorinated (5 , 6). In all cases, the results (a) showed good agreement with literat ure data on related experimental systems and (b) demonstrated the domi nance of arene-arene electrostatic contributions to conformational ene rgy over the negligible orbital mixing and charge-transfer interaction s.