title: Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation.
abstract: Three skeletal rearrangement channels for the norbornadiene (N*+) to the 1,3,5-cycloheptatriene (CHT*+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N*+, are investigated using the quantum chemical B3LYP, MP2 and CCSD(T) methods in conjunction with the 6-311 +G(d,p) basis set. Two of the isomerizations proceed through the norcaradiene radical cation (NCD*+), either through a concerted path (N*+ - NCD*+), or by a stepwise mechanism via a stable intermediate (N*+ - I1 - NCD*+). At the CCSD(T)/6-311 +G(d,p)//B3LYP/6-311 +G(d,p) level, the lowest activation energy, 28.9 kcal mol(-1), is found for the concerted path whereas the stepwise path is found to be 2.3 kcal mol(-1) higher. On both pathways, NCD*+ rearranges further to CHT*+ with significantly less activation energy. The third channel proceeds from N*+ through I1 and then directly to CHT*+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N*+ to CHT*+ via the quadricyclane and the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cations, is 4.6 kcal mol(-1) lower than the most favorable path of the present study. If the methylene group is substituted with C(CH3)2, however, the concerted path is estimated to be 5.6 kcal mol(-1) lower than the corresponding substituted multi-step path at the B3LYP/6-311+(d,p) level. This shows that substitution of particular positions can have dramatic effects on altering reaction barriers in the studied rearrangements. We also note that identical energies are computed for CHT*+ and NCD*+ whereas, in earlier theoretical investigations, the former was reported to be 6-17 kcal mol(-1) more stable than the latter. Finally, a bent geometry is obtained for CHT*+ with MP2/6-311 +G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.
authors.affiliation: Department of Quantum Chemistry, Uppsala University, Box 518, 751 20, Uppsala, Sweden.
authors.forename: Daniel
authors.initials: D
authors.lastname: Norberg
authors.forename: Per-Erik
authors.initials: PE
authors.lastname: Larsson
authors.forename: Nessima
authors.initials: N
authors.lastname: Salhi-Benachenhou
mesh_terms: D002412:Cations / Q000737:chemistry; D003198:Computer Simulation; D003508:Cycloheptanes / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D009636:Norbornanes / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D002412:Cations; D003508:Cycloheptanes; D005609:Free Radicals; D009636:Norbornanes