Ab initio and DFT benchmark study for nucleophilic substitution at carbon (SN2@C) and silicon (SN2@Si)

• Published in: Journal of computational chemistry 2005-11, Vol.26 (14), p.1497-1504
• Main Authors: Bento, A. Patrícia, Solà, Miquel, Bickelhaupt, F. Matthias
• Format: Article
• Language: English
• Subjects: ab initio benchmark; Carbon; Chemical reactions; density functionals validation; Ions; nucleophilic substitution; potential energy surface; Silicon
• ISSN: 0192-8651; 1096-987X
• DOI: 10.1002/jcc.20261

To obtain a set of consistent benchmark potential energy surfaces (PES) for the two archetypal nucleophilic substitution reactions of the chloride anion at carbon in chloromethane (SN2@C) and at silicon in chlorosilane (SN2@Si), we have explored these PESes using a hierarchical series of ab initio methods [HF, MP2, MP4SDQ, CCSD, CCSD(T)] in combination with a hierarchical series of six Gaussian‐type basis sets, up to g polarization. Relative energies of stationary points are converged to within 0.01 to 0.56 kcal/mol as a function of the basis‐set size. Our best estimate, at CCSD(T)/aug‐cc‐pVQZ, for the relative energies of the [Cl−, CH3Cl] reactant complex, the [ClCH3Cl]− transition state and the stable [ClSiH3Cl]− transition complex is −10.42, +2.52, and −27.10 kcal/mol, respectively. Furthermore, we have investigated the performance for these reactions of four popular density functionals, namely, BP86, BLYP, B3LYP, and OLYP, in combination with a large doubly polarized Slater‐type basis set of triple‐ζ quality (TZ2P). Best overall agreement with our CCSD(T)/aug‐cc‐pVQZ benchmark is obtained with OLYP and B3LYP. However, OLYP performs better for the SN2@C overall and central barriers, which it underestimates by 2.65 and 4.05 kcal/mol, respectively. The other DFT approaches underestimate these barriers by some 4.8 (B3LYP) to 9.0 kcal/mol (BLYP). © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1497–1504, 2005