On the Electronic Nature of a Butadienyl Biradical – Experiments and ab initio MO Calculations

Abstract

The spin state of the biradical obtained upon heating 1,6-diisopropyl-1,6-diazacyclodeca-3,8-diyne (9b) has been investigated. 9b was heated in biphenyl, naphthalene, and bromobenzene at 150 °C in the presence of a mixture of [D4]- and [D0]-9,10-dihydroanthracene (DHA) as hydrogen source. The relative contributions of the cage (C) and cage-escape (E) reaction paths could be determined by analysing the products 2,6-diisopropyl-1,2,3,5,6,7-hexahydro-2,6-naphthy-ridine (11b) and 2,6-diisopropyl-4-(9,10-dihydroanthra-cenyl)-1,2,3,5,6,7-hexahydro-2,6-naphthyridine (12b) and their deuterated analogues. The C/E ratio could be determined by GC-MS analysis and was found to be 0.30 in biphenyl, 0.40 in naphthalene, and 0.22 in bromobenzene, and to be more or less independent of the DHA concentration over a wide range. The energetics of the cyclization of 1,6-diazacyclodeca-3,8-diyne (9a) to 1,2,3,5,6,7-hexahydro-2,6-naphthyridine-2,6-diyl (10b) have been investigated at several levels of theory. Calculations of the various structures along the reaction coordinate at the CASPT2/6-31G* level 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 (14a). The transition state shows only weak biradical character, indicating that the electronic structure of 9a is retained as long as possible. The energy difference between the 1Ag and 3Au states of 10a is predicted to be 13.0 kcal/mol.