On the Relative Resonance-Acceptor Abilities of CN, NO, COMe, CHO and NO2 Groups
The relative resonance-acceptor ability based on ∆G‡out is different from that based on ∆G‡in for the conrotatory ring-opening of 3-carbomethoxycyclobutene bearing CN, NO, COMe, CHO and NO2 as C1-substituent. In contrast, the relative resonance-acceptor ability based on sC3C4-p*C1C2 interaction in TSout is much the same as in TSin. It matches further with the acceptor ability based on the net of sC3C4-p*C1C2 and pC1C2-s*C3C4 interactions. The overall resonance-acceptor ability is estimated to be CN < COMe < CHO < NO2 < NO from calculations at MP2/6-31+G(d,p) level of theory.