Host-guest interactions as a model for the solvent influence on the conformational populations in [2.2.2.2]paracyclophane

The interaction of a number of small organic molecules with the two lowest energy minimum conformations of [2.2.2.2] paracyclophane (4 degrees-PCP), 1 and 2, are studied by MMP2( 87) force field calculations. Formations of nesting as well as inclusion complexes were identified on the potential energy surface. With CH2Cl2 and CHCl3 used as guests, the nesting complexes are lower in energy than the inclusion ones with both conformations 1 and 2. Furthermore, the nesting complexes with 2 are found to be more stable than the nesting ones with 1. Formation of the double nesting complexes of CH2 Cl-2 and CHCl3 with 1 and 2 raise the difference in complexation energy in favour of 2. The preference of 4 degrees-PCP for the form 2 in solution is explained based on the above analysis, although 2 is calculated to be 0.2 kcal.mol(-1) higher in steric energy than 1 in the gas phase.