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Journal of Chemical Physics / Volume 128 / Issue 11 / ARTICLES / Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

J. Chem. Phys. 128, 114503 (2008); http://dx.doi.org/10.1063/1.2890042 (9 pages)

A microscopic view of substitution reactions solvated by ionic liquids

Guilherme M. Arantes and Mauro C. C. Ribeiro

Laboratório de Espectroscopia Molecular, Instituto de Química, Universidade de São Paulo, C.P. 26077, CEP 05513-970 São Paulo, Brazil

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(Received 11 January 2008; accepted 7 February 2008; published online 18 March 2008)

The solvation effect of the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate on nucleophilic substitution reactions of halides toward the aliphatic carbon of methyl p-nitrobenzenesulfonate (pNBS) was investigated by computer simulations. The calculations were performed by using a hybrid quantum-mechanical/molecular-mechanical (QM/MM) methodology. A semiempirical Hamiltonian was first parametrized on the basis of comparison with ab initio calculations for Cl− and Br− reaction with pNBS at gas phase. In condensed phase, free energy profiles were obtained for both reactions. The calculated reaction barriers are in agreement with experiment. The structure of species solvated by the ionic liquid was followed along the reaction progress from the reagents, through the transition state, to the final products. The simulations indicate that this substitution reaction in the ionic liquid is slower than in nonpolar molecular solvents proper to significant stabilization of the halide anion by the ionic liquid in comparison with the transition state with delocalized charge. Solute-solvent interactions in the first solvation shell contain several hydrogen bonds that are formed or broken in response to charge density variation along the reaction coordinate. The detailed structural analysis can be used to rationalize the design of new ionic liquids with tailored solvation properties.

© 2008 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. METHODS
    1. Electronic structure calculations
    2. Parametrization of the semiempirical Hamiltonian
    3. Potential energy function
    4. Free energy profile
    5. Simulation details
  3. RESULTS AND DISCUSSION
    1. Parametrization of the semiempirical Hamiltonian
    2. Structure and dynamics of IL solvating pNBS
    3. Free energy profiles for substitution reactions
    4. Structure of the IL solvating the reaction species
  4. CONCLUSIONS

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KEYWORDS and PACS

Keywords

ab initio calculations, bonds (chemical), chemical exchanges, free energy, hydrogen bonds, organic compounds, reaction kinetics theory, solvation

PACS

  • 82.30.Hk

    Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

  • 82.20.Db

    Transition state theory and statistical theories of rate constants

  • 82.20.Hf

    Product distribution

  • 82.20.Wt

    Computational modeling; simulation

  • 82.20.Yn

    Solvent effects on reactivity

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.2890042

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

For access to fully linked references, you need to log in.
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