Structural variation and transmission of substituent effects

In our studies on the transmission of electronic substituent effects in Ph-G-X molecules [1], where G is a framework (or spacer) and X a variable substituent, we use a phenyl group (Ph) as a probe, by virtue of its structural variation under the impact of the substituent. The geometry of the molecules is determined by MO calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. The geometrical changes occurring in the Ph probe are best represented by a linear combination of the internal ring angles, termed SF. In 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives (1), as well as in other molecular systems having a saturated framework as a spacer, the SF term expresses an essentially pure field effect. The SFBCO parameter correlates well with various experimental indicators of the field effect, and can itself be used as a measure of such effect. The correlation between SFBCO and the corresponding structural parameter for diamantane derivatives (2), SFDIAM, reveals a more pronounced attenuation of the field effect with distance in the case of uncharged substituents as compared with charged substituents. In those molecular systems where the spacer is involved in resonance interactions with substituent and probe (e.g. (E)-β-substituted styrenes (3), para-substituted biphenyls (4), and substituted polyenes (5)) the SF parameter expresses a composite polar effect. Multiple regression analysis, using appropriate indicators of electronic substituent effects (SFBCO, SR, and SE for the field, resonance, and electronegativity effects, respectively), is then employed to dissect SF into its components. The main components originate from the long-range effect of the substituent, enhanced by field-induced π-polarization of the spacer (SFBCO term) and resonance-induced field effect (SR term). The SR and SE parameters have been derived from the structural variation of monosubstituted benzene rings. Analysis of π-charge distribution in the coplanar conformations of 3 and 4 shows that the vinylene spacer is 57% more effective than the phenylene spacer in exchanging π-charge with the phenyl probe. The SF parameters of 4 and 5 are closely related to the electron density distribution in the π-system of the molecules. [1] A. R. Campanelli, A. Domenicano, Struct. Chem. 2015, 26, 1259−1271, and references therein.