The reaction mechanism and the energetics of the C-2-C-7 and of the C-2-C-6 cyclization of (Z)-1,2,4-heptatriene-6-yne are studied employing large scale MR-CI calculations. In addition the influence of vibrational motions and of temperature corrections on the barrier height of both reactions is estimated. For both products the first excited electronic states are computed to estimate possible switches from diradical to zwitterionic reaction mechanism.