Effect of Electron Donating and Withdrawing Substitutents on the Ring Stability of Cyclobutadiene

Recent work has shown that the energy difference between the rectangular and square forms of cyclobutadiene is on the order of ∼25kJmol-1. In addn., the effect of electron correlation must be taken into account to obtain the correct exptl. geometry of a D2h rectangular singlet ground state. Given the very small energy differences between the two forms and that the p-system electrons play such a significant role, it is of interest to investigate the effect of electron withdrawing or donating substituents on the symmetry and energy of the ring structure. The influence of std. electron donors such as -NH2 and acceptors such as -NO2, as well as donor/acceptor strength will be examd