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 s_C3C4-p*_C1C2 interaction in TS_out is much the same as in TS_in. It matches further with the acceptor ability based on the net of s_C3C4-p*_C1C2 and p_C1C2-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.