The spin state of the biradical obtained upon heating 1,6-and to be more or less independent of the DHA concentration over a wide range. The energetics of the cyclization of 1,6-diisopropyl-1,6-diazacyclodeca-3,8-diyne (9b) has been investigated. 9b was heated in biphenyl, naphthalene, and diazacyclodeca-3,8-diyne (9a) to 1,2,3,5,6,7-hexahydro-2,6naphthyridine-2,6-diyl (10b) have been investigated at bromobenzene at 150 °C in the presence of a mixture of [D 4 ]and [D 0 ]-9,10-dihydroanthracene (DHA) as hydrogen source. several levels of theory. Calculations of the various structures along the reaction coordinate at the CASPT2/6-31G* level The relative contributions of the cage (C) and cage-escape (E) reaction paths could be determined by analysing the yield an energy difference of 10.7 kcal/mol between 9a and 10a and of 17.8 kcal/mol between 9a and the transition state products 2, 6-diisopropyl-1,2,3,5,6,7-hexahydro-2,6-naphthyridine (11b) and 2,6-diisopropyl-4-(9,10-dihydroanthra-(14a). The transition state shows only weak biradical character, indicating that the electronic structure of 9a is cenyl)-1,2,3,5,6,7-hexahydro-2,6-naphthyridine (12b) and their deuterated analogues. The C/E ratio could be retained as long as possible. The energy difference between the 1 A g and 3 A u states of 10a is predicted to be 13.0 kcal/mol. determined by GC-MS analysis and was found to be 0.30 in biphenyl, 0.40 in naphthalene, and 0.22 in bromobenzene,