The carbon-carbon indirect nuclear spin-spin coupling constants in cyclopropane, aziridine and oxirane were investigated by means of ab initio calculations at the RPA, SOPPA and DFT/B3LYP levels. We found that the carbon-carbon couplings are by far dominated by the Fermi contact term. Our best SOPPA and DFT results are in a very good agreement with each other and with the experimental values, whereas calculations at the RPA level of theory strongly overestimate the carbon-carbon couplings. Significant differences in the basis set dependence of the calculated carbon-carbon coupling constants obtained with either wavefunction method, RPA or SOPPA, or the density functional method, DFT/B3LYP, are observed. The SOPPA results depend much more strongly on the quality of the basis set than the results of DFT/B3LYP calculations. The medium-sized core-valence basis sets cc-pCVTZ and even cc-pCVDZ were found to perform fairly well at the SOPPA level for the one-bond carbon-carbon couplings investigated here.
- Basis set
- C NMR
- Carbon-carbon spin-spin coupling constant
- Electronic correlation
- Three-membered rings