Structure of the cyclohexa-1,4-diene radical cation: an ab initio molecular orbital and electron paramagnetic resonance study

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The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.