Polar effects and structural variation in 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives: a quantum chemical study.
The transmission of polar effects through the bicyclo[2.2.2]octane framework has been investigated by ascertaining how the geometry of a phenyl group at a bridgehead position is affected by a variable substituent at the opposite bridgehead position. We have determined the molecular structure of several Ph-C(CH(2)-CH(2))(3)C-X molecules (where X is a charged or dipolar substituent) from HF/6-31G and B3LYP/6-311++G molecular orbital calculations and have progressively replaced each of the three -CH(2)-CH(2)- bridges by a pair of hydrogen atoms. Thus the bicyclo[2.2.2]octane derivatives were changed first into cyclohexane derivatives in the boat conformation, then into n-butane derivatives in the anti-syn-anti conformation, and eventually into assemblies of two molecules, Ph-CH(3) and CH(3)-X, appropriately oriented and kept at a fixed distance. For each variable substituent the deformation of the benzene ring relative to X = H remains substantially the same even when the substituent and the phenyl group are no longer connected by covalent bonds. This provides unequivocal evidence that long-range polar effects in bicyclo[2.2.2]octane derivatives are actually field effects, being transmitted through space rather than through bonds. Varying the substituent X in a series of Ph-C(CH(2)-CH(2))(3)C-X molecules gives rise to geometrical variation (relative to X = H) not only in the benzene ring but also in the bicyclo[2.2.2]octane cage. The two deformations are poorly correlated. The rather small deformation of the benzene ring correlates well with traditional measures of long-range polar effects in bicyclo[2.2.2]octane derivatives, such as sigma(F) or sigma(I) values. The much larger deformation of the bicyclo[2.2.2]octane cage is controlled primarily by the electronegativity of X, similar to deformation of the benzene ring in Ph-X molecules. Thus the field and electronegativity effects of the substituent are well separated and can be studied simultaneously, as they act on different parts of the molecular skeleton.
Campanelli
Anna Rita
AR
Department of Chemistry, University of Rome La Sapienza, I-00185 Rome, Italy.
Domenicano
Aldo
A
Ramondo
Fabio
F