Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3]+ and Radical Dication [C(SH)3]⋅,2+

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation C(SH)₃]⁺ ( a ) and of radical dication C(SH₃)]^⋅,2+ ( b ). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π-HOMOs of C_3h symmetric C(SH)₃]⁺ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MO's “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))-E(UHF) and E(PMP4(s+3))-E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near-quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))-E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 :1023–1035, 1997