A b i n i t i o calculation of spin densities in hydrocarbon radicals

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Abstract
Ab initio calculations are reported for the isotropic hyperfine coupling constants arising from contact spin density distributions in hydrocarbon radicals. Representative sigma radicals (vinyl and ethynyl), nonconjugated pi radicals (methyl and ethyl), and conjugated pi radicals (allyl and cyclohexadienyl) are examined in the unrestricted Hartree‐Fock approximation with a wide variety of Gaussian basis sets, ranging from crude minimal sets to a large uncontracted set. The unrestricted Hartree–Fock method itself gives poor results for this property, but is found to yield results of semiquantitative accuracy after a spin projection is performed. Some significant differences are found between results after full quartet annihilation and after complete spin projection. Due to a systematic cancellation of errors, the crude minimal STO‐3G basis set gives generally better results than more sophisticated minimal basis sets. Extended basis sets give only a slight overall improvement on the STO‐3G results. The standard contraction schemes for large Gaussian basis sets are found to be quite appropriate for spin density calculations. It is concluded that the projected unrestricted Hartree–Fock method can be a useful tool for study of spin density distributions in hydrocarbon radicals even when deployed with highly contracted Gaussian basis sets.