The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3kcalmol−1, which is higher than the best earlier theoretical estimate of 4.0kcalmol−1. This result is confirmed by including into the reference space the orbitals of the CC σ bonds beyond the standard π orbital space. It places the present value into the middle of the range of the measured data (1.6–10kcalmol−1). An adiabatic singlet-triplet energy gap of 7.4kcalmol−1 between the transition state Btg1 and the first triplet A2g3 state is obtained. A low barrier height for the CBD automerization and a small ΔE(A2g3,B1g1) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.

Multireference configuration interaction (MRCI) calculations have been performed for pyrrole with the aim of providing an explanation for the experimentally observed photochemical deactivation processes. Potential energy curves and minima on the crossing seam were determined using the analytic MRCI gradient and nonadiabatic coupling features of the COLUMBUS program system. A new deactivation mechanism based on an out-of-plane ring deformation is presented. This mechanism directly couples the charge transfer π1π* and ground states. It may be responsible for more than 50% of the observed photofragments of ππ*-excited pyrrole. The ring deformation mechanism should act complementary to the previously proposed NH-stretching mechanism, thus offering a more complete interpretation of the pyrrole photodynamics.