ABSTRACT
A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.
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.
Subject(s)
Cycloheptanes/chemistry , Models, Chemical , Norbornanes/chemistry , Cations/chemistry , Computer Simulation , Free Radicals/chemistry , Models, Molecular , Molecular Structure , Quantum Theory , StereoisomerismABSTRACT
An alternative skeletal rearrangement of the quadricyclane radical cation (Q*+) explains the side products formed in the one-electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, with an activation energy of 14.9 kcal mol(-1), is formed. Second, this species can further rearrange to 1,3,5-cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta-2,6-diene radical cation is an intermediate. The multistep rearrangement has a rate-limiting step with an estimated activation energy of 16.5 kcal mol(-1), which is 2.8 kcal mol(-1) lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q*+ cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol(-1), which agrees well with the experimental estimate of 9.3 kcal mol(-1). The computational estimates of activation energies were done using the CCSD(T)/6-311+G(d,p) method with geometries optimized on the B3LYP/6-311+G(d,p) level, combined with B3LYP/6-311+G(d,p) frequencies.
ABSTRACT
[structure: see text] A natural bond orbital analysis of the distonic bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation interprets its structure and radical character by a three-center two-electron bond between C2, C3, and C7 (a bishomoaromatic stabilization) and a singly occupied orbital on C5, n(5). Moreover, B3LYP/6-311+G(d,p) ESR parameters, which agree excellently with experiment, are interpreted in terms of spin polarization in the natural hybrids of sigma(C5-H5), and a dual hyperconjugative effect involving n(5), sigma(C1-H1a), sigma(C1-H1b), and antibonding counterparts.
