Activation Energy and NBO Interaction Approaches to the Determination of the Relative Resonance Acceptor Strengths of CN, NO, COMe, CHO and NO2 Groups from the Ring-Opening of 1,2-Benzo-3-Carbomethoxycyclobutenes

The relative resonance-acceptor ability based on the activation energies for the outward ring-openings of 1,2-benzo-3-carbomethoxycyclobutenes bearing CN, NO, COMe, CHO and NO₂ as C6 and C7 substituents is different from that based on the s_C3C4-p*_C1C2 interactions in the inward opening transition structures. The differential activation energy (∆G^‡) predicts outward ring-opening and, in contrast, the differential s_C3C4-p*_C1C2 interaction energy predicts inward opening, throughout. The relative resonance-acceptor ability estimated from the s_C3C4-p*_C1C2 interaction energies is more realistic than that from the activation energies.