ESR features of the bicyclobutyl radical revisited. A counterintuitive ordering of short- and long-range isotropic hyperfine coupling constants

A quantum mechanical protocol for the study of flexible open-shell systems has been applied in a reinvestigation of the isotropic hyperfine coupling constants of the bicyclobutyl radical. Our computations indicate that the most stable structure of the radical corresponds to the exo-form and is characterized by a strongly pyramidal radical center. The hyperfine coupling constants computed for this form are in good agreement with experiment, but indicate that the accepted assignment of Hα and Hγendo protons should be reversed. The energy difference between the exo and endo form is sufficiently large to avoid thermal averaging of observables at reasonable temperatures. Vibrational averaging effects in the potential well corresponding to the exo minimum are significant for the Cα atom, but do not alter results for H atoms. A striking outcome of this study is the demonstration that stereo-electronic can lead to the counterintuitive results that long range couplings are larger than α ones.