The spin state of the biradical obtained upon heating 1,6-diisopropyl-1,6-diazacyclodeca-3,8-diyne (I) was investigated. Compd. I was heated in Ph2, naphthalene, and PhBr at 150 Deg in the presence of a mixt. of [d4]- and [d0]-9,10-dihydroanthracene (DHA) as H source. The relative contributions of the cage (C) and cage-escape (E) reaction paths could be detd. by analyzing the products 2,6-diisopropyl-1,2,3,5,6,7-hexahydro-2,6-naphthyridine and 2,6-diisopropyl-4-(9,10-dihydroanthracenyl)-1,2,3,5,6,7-hexahydro-2,6-naphthyridine as well as their deuterated analogs. The C/E ratio was detd. by GC-MS anal. and was found to be 0.30 in Ph2, 0.40 in naphthalene, and 0.22 in PhBr, and to be more or less independent of the DHA concn. over a wide range. The energetics of the cyclization of 1,6-diazacyclodeca-3,8-diyne (II) to 1,2,3,5,6,7-hexahydro-2,6-naphthyridine-2,6-diyl (III) were investigated at several levels of theory. Calcns. of the various structures along the reaction coordinate at the CASPT2/6-31G* level yield an energy difference of 10.7 kcal/mol between II and III and of 17.8 kcal/mol between II and the transition state. The transition state shows only weak biradical character, indicating that the electronic structure of II is retained as long as possible. The energy difference between the 1Ag and 3Au states of III is predicted to be 13.0 kcal/mol.