SOLVENT EFFECT ON THE CONFORMATIONAL BEHAVIOR OF SUBSTITUTED SPIRO[4.5]DECANES AND SPIRO[5.5]UNDECANES

We studied the relatively complex polar systems 6-substituted-1,4-diox ospiro[4.5]decanes and 7-substituted-1, 5-dioxospiro [5.5]undecanes wi th substituents X = CH3, F, Cl, CN, OH, OCH3, and NO2. Solvent effects on the equilibrium have been analysed by means of a Self-Consistent-R eaction-Field model and the PM3 method. Complete geometry optimization s have been carried out for all the structures in the gas phase and in solution. For some substituents, a set of rotamers have been separate ly optimized. The discussion of the results is focussed on the effects arising from structural aspects and from steric and electrostatic int eractions on the axial/equatorial relative stability. The role played by multipole moment is considered. In general, good agreement with ava ilable experimental data and with previous theoretical studies has bee n obtained. Though the use of semiempirical methods and simple solvent models prevents us from reaching definitive conclusions, this approac h seems to be very useful in predicting the main role of solute-solven t interactions in conformational equilibria of complex systems for whi ch ab initio calculations cannot be performed.