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Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH) 3 ] + and Radical Dication [C(SH) 3 ] ⋅,2+
Author(s) -
Glaser Rainer,
Chen Grace Shiahuy,
Grützmacher Hansjörg
Publication year - 1997
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199706)18:8<1023::aid-jcc6>3.0.co;2-v
Subject(s) - dication , chemistry , radical ion , atomic physics , annihilation , unpaired electron , electron , electronic correlation , bond dissociation energy , spin (aerodynamics) , excited state , ion , molecular physics , dissociation (chemistry) , molecule , physics , organic chemistry , quantum mechanics , thermodynamics
Theoretical level dependencies are discussed of relative isomerstabilities and rotational barriers of trithiomethyl cation[C(SH) 3 ] + ( a ) and of radicaldication [C(SH 3 )] ⋅,2+ ( b ).Spin polarization and dynamic electron correlation are very important forthe radical dictation. Removal of an electron from one of the degenerateπ‐HOMOs of C 3h symmetric[C(SH) 3 ] + stabilizes the remainingπ electron to such an extent that the unpaired electron is not in theHOMO of the dictation. The radial π MO's “diving below the Fermilevel” facilitates strong spin polarization because of its energeticproximity to σ MOs. Projection of the first three higher spin stateseliminates spin contaminations, and the terms E (PUHF(s+3))‐ E (UHF) and E (PMP4(s+3))‐ E (MP4) are discussed. The combination ofannihilation of spin contamination and electron correlation is essentialfor the determination of relative energies and rotational barriers of theradical dication. The results obtained at this level match the results ofhigh level QCISD(T) calculations in a near‐quantitative fashion.Perturbation theory alone does not correct for spin contamination even ifit is carried to full fourth order and includes triple excitations; the E (PMP4(s+3))‐ E (MP4) values are all negative and canexceed 5 kcal/mol in magnitude. Previous studies showed that annihilationof spin contaminations is important in regions of potential energy surfaceswhere σ bonds are broken (homolytic dissociation), formed (radicaladdition), or both (H abstraction by radical). Our findings stress that theannihilation of spin contaminations can be just as important for anyprocess that greatly alters spin polarization and even if that processproceeds without breaking or forming of σ bonds. For comparison,density functional theory also was employed in the potential energy surfaceanalyses. The results obtained with the B3LYP formalism were found to beless susceptible to spin contamination and resulted in rather goodagreement with the best pertubation and configuration interaction results.© 1997 John Wiley & Sons, Inc. J Comput Chem 18 :1023–1035, 1997