ESR features of the bicyclobutyl radical revisited. A counterintuitive ordering of short- and long-range isotropic hyperfine coupling constants
✍ Scribed by Vincenzo Barone; Carlo Adamo; Andre Grand; Robert Subra
- Publisher
- Elsevier Science
- Year
- 1995
- Tongue
- English
- Weight
- 373 KB
- Volume
- 246
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
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 H~, and l"]'end° protons should be reversed. The energy difference between the exo and endo form is sufficiently large to avoid ---y thermal averaging of observables at reasonable temperatures. Vibrational averaging effects in the potential well corresponding to the exo minimum are significant for the C~ atom, but do not alter results for H atoms. A striking outcome of this study is the demonstration that stereo-electronic effects can lead to the counterintuitive result that long range couplings are larger than a ones.