Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation
2006 (English)In: Organic and biomolecular chemistry, ISSN 1477-0520, E-ISSN 1477-0539, Vol. 4, no 22, 4241-4250 p.Article in journal (Refereed) Published
Three skeletal rearrangement channels for the norbornadiene (N center dot+) to the 1,3,5-cycloheptatriene (CHT center dot+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N center dot+, 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 center dot+), either through a concerted path (N center dot+-NCD center dot+), or by a stepwise mechanism via a stable intermediate (N center dot+-I1- NCD center dot+). 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 center dot+ rearranges further to CHT center dot+ with significantly less activation energy. The third channel proceeds from N center dot+ through I1 and then directly to CHT center dot+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N center dot+ to CHT center dot+ 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 center dot+ and NCD center dot+ 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 center dot+ with MP2/6-311+G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.
Place, publisher, year, edition, pages
2006. Vol. 4, no 22, 4241-4250 p.
IdentifiersURN: urn:nbn:se:uu:diva-96117DOI: 10.1039/b612791fISI: 000241720500029PubMedID: 17312982OAI: oai:DiVA.org:uu-96117DiVA: diva2:170586