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

Abstract

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 C3h 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

Origin of the Second-Order Proton Catalysis of Ferriin Reduction in Belousov–Zhabotinsky Reactions: Density Functional Studies of Ferroin and Ferriin Aggregates with Outer Sphere Ligands Sulfate, Bisulfate, and Sulfuric Acid

“…There are six α-MOs (148α–153α) above the highest occupied d-orbital in the α-set (147α), and the analogous occurs in the β-set; that is, six occupied π-orbitals of the phenanthroline ligands are above the iron d-electrons. While textbooks routinely propagate the notion that the radical electron must, as a rule, occupy the HOMO, clearly this idea is misguided in the present case, as in many others. − …”

Stabilities and Spin Distributions of Benzannulated Benzyl Radicals

“…While ROHF theory completely neglects spin polarization, UHF theory suffers from spin contamination (i.e., an overestimation of spin polarization). Spin contamination of the UHF solution can be remedied, , but spin polarization intrinsically is a correlation effect, and correlated methods are required to compute meaningful spin density distributions. − In practice, this is often accomplished by post-Hartree−Fock treatments 14 either with perturbation methods (i.e., Møller−Plesset perturbation theory 15 ) or configuration interaction treatments (i.e., quadratic configuration interaction, QCI, theory 16 ). Studies of radicals have become more frequent with the availability of density functional methods, and the usual spin contaminations are moderate. , This knowledge has been the implicit justification for the use of density functional theory (DFT) methods even though problems with spin-projected density functional theory are known. , Aside from the theoretical challenges, computations of radicals push the limits of computational feasibility because unrestricted theory doubles the number of orbitals and thereby greatly increases the post-HF task.…”

“…Spin contamination of the UHF solution can be remedied, 9,10 but spin polarization intrinsically is a correlation effect, and correlated methods are required to compute meaningful spin density distributions. [11][12][13] In practice, this is often accomplished by post-Hartree-Fock treatments 14 methods (i.e., Møller-Plesset perturbation theory 15 ) or configuration interaction treatments (i.e., quadratic configuration interaction, QCI, theory 16 ). Studies of radicals have become more frequent with the availability of density functional methods, and the usual spin contaminations are moderate.…”