Substituent effects on "hyperconjugative" aromaticity and antiaromaticity in planar cyclopolyenes.

Computed aromatic stabilization energies (ASEs) and dissected nucleus independent chemical shifts (NICSπzz) quantify the effect of hyperconjugation on the (anti)aromaticities of the planar conformations of three, five, seven, and nine membered (CnHn)CR2 (R = H, SiH3, F) rings. CH2 and especially C(SiH3)2 groups supply two "pseudo" π electrons hyperconjugatively along with the olefinic π electrons in the ring, whereas a CF2 group acts like a partially vacant p orbital. Following the Hückel rule, compounds with 4n+2 (or 4n) pseudo π electrons are "hyperconjugatively" aromatic (or antiaromatic).