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Modified Gaussian‐2 level investigation of the identity ion‐pair S N 2 reactions of lithium halide and methyl halide with inversion and retention mechanisms
Author(s) -
Ren Yi,
Chu SanYan
Publication year - 2004
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/jcc.10394
Subject(s) - chemistry , halogen , halide , polarizability , ion , dipole , analytical chemistry (journal) , computational chemistry , molecule , inorganic chemistry , chromatography , alkyl , organic chemistry
Identity ion‐pair S N 2 reactions LiX + CH 3 X → XCH 3 + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian‐2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [Δ H cent ≠ (inv)] are found to be much higher than the corresponding values for the gas phase anionic S N 2 reactions, decreasing in the following order: F (263.6 kJ mol −1 ) > Cl (203.3 kJ mol −1 ) > Br (174.7 kJ mol −1 ) > I (150.7 kJ mol −1 ). The barrier gaps between the two mechanisms [Δ H cent ≠(ret) − Δ H cent ≠(inv)] increase in the order F (−62.7 kJ mol −1 ) < Cl (4.4 kJ mol −1 ) < Br (24.9 kJ mol −1 ) < I (45.1 kJ mol −1 ). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S N 2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole–dipole complexes CH 3 X · · · LiX (Δ H comp ) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol −1 for I up to 58.9 kJ mol −1 for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion‐pair S N 2 reactions. The energetic profiles are found to be still double‐well shaped for most of the ion‐pair S N 2 reactions in the solution, but the potential profile for reaction LiI + CH 3 I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [Δ H cent ≠(inv)] with the geometric looseness of the inversion transition state %C—X ≠ , the dissociation energies of the C—X bond (D C—X ) and Li—X bond (D Li—X ) are observed, respectively. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 461–467, 2004