Thermochemistry and structures of solvated SN2 complexes and transition states in the gas phase: experiment and theory

Abstract The standard enthalpy and entropy changes (ΔH° and ΔS°) for the formation of solvated SN2 complexes (S)X−(RY) (X, Y = Cl, Br; R = (CH3)2CH; S = CH3OH, CH3CN, (CH3)2CO, CH3CF2H) have been determined by pulsed-ionization high pressure mass spectrometry. Not surprisingly, solvent effects are evident even at this mono-salvation level. Structures of solvated SN2 complexes and transition states for the Cl−(S) + CH3Cl SN2 reaction (S = H2O, H2S, NH3, PH3, SO2) have also been determined at the MP2/6-31+G(d) level of theory. A large variety of solvent dependent structures have been obtained, showing solvent reorganization upon going from the complex to the transition state. Standard binding and activation enthalpies ( Δ H 298 ∘ and Δ H 298 ‡ ) were determined at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d,p) level of theory. For the Cl−(H2O) + CH3Br and Br−(H2O) + CH3Cl reactions, structures and enthalpies were calculated at the MP2/[6-31+G(d)/LanL2DZ(spd)] and MP2/[6-311+G(3df,2p)/LanL2DZ(spdf)]//MP2/[6-31+G(d)/LanL2DZ(spd)] level of theory. For the Cl− + CH3Br and Cl−(H2O) + CH3Br reactions potential energy surface scans were performed at the MP2/[6-31+G(d)/LanL2DZ(spd)] level of theory. Formation of the two possible sets of solvated products, Br−(H2O) + CH3Cl and Br− + (CH3Cl)(H2O) proceeds through two different surfaces. Water transfer to the leaving group can be facilitated by rotation of the Br−(CH3Cl) part in the exit channel Br−(CH3Cl)(H2O) complex. Finally, for the Cl− + CH3Cl reactions in the condensed phase, complexation and activation energies (ΔE(ɛ) and ΔE‡(ɛ)) were determined for a variety of solvents at the MP2/6-31+G(d) level of theory using the isodensity polarized continuum model. A linear correlation between −ΔE(ɛ) and ΔE‡(ɛ) was obtained, and a similar correlation exists for the mono-solvated gas phase SN2 reaction, indicating that mono-solvation already exhibits some features of the condensed phase reaction.

[1]  M. Bowers,et al.  Vibrational Excitation in Products of Nucleophilic Substitution: The Dissociation of Metastable X-(CH3Y) in the Gas Phase , 1994 .

[2]  ab Mikhail N. Glukhovtsev,et al.  Gas-Phase Non-Identity SN2 Reactions of Halide Anions with Methyl Halides: A High-Level Computational Study , 1995 .

[3]  L. Radom,et al.  The performance of B3-LYP density functional theory in describing SN2 reactions at saturated carbon , 1996 .

[4]  Michael J. Frisch,et al.  The performance of the Becke-Lee-Yang-Parr (B-LYP) density functional theory with various basis sets , 1992 .

[5]  D. Truhlar,et al.  Deuterium kinetic isotope effects and their temperature dependence in the gas-phase S{sub N}2 reactions X{sup -} + CH{sub 3}Y {yields} CH{sub 3}X + Y{sup -} (X, Y = Cl, Br, I) , 1995 .

[6]  Scott F. Smith,et al.  Theoretical examination of the SN2 reaction involving chloride ion and methyl chloride in the gas phase and aqueous solution , 1985 .

[7]  Timothy Clark,et al.  Efficient diffuse function‐augmented basis sets for anion calculations. III. The 3‐21+G basis set for first‐row elements, Li–F , 1983 .

[8]  Clemens C. J. Roothaan,et al.  New Developments in Molecular Orbital Theory , 1951 .

[9]  A. A. Viggiano,et al.  Nucleophilic displacement as a function of hydration number and temperature: rate constants and product distributions for OD-(D2O)0,1,2,3 + CH3Cl at 200-500 K , 1986 .

[10]  Gillian C. Lynch,et al.  Use of an improved ion–solvent potential‐energy function to calculate the reaction rate and α‐deuterium and microsolvation kinetic isotope effects for the gas‐phase SN2 reaction of Cl−(H2O) with CH3Cl , 1992 .

[11]  Jan E. Szulejko,et al.  High-Pressure Mass Spectrometric Investigations of the Potential Energy Surfaces of Gas-Phase SN2 Reactions , 1996 .

[12]  D. Laria,et al.  Solvation effects on a model SN2 reaction in water clusters , 1996 .

[13]  R. Dougherty,et al.  SN2 reactions in the gas phase. Structure of the transition state , 1974 .

[14]  M. Plesset,et al.  Note on an Approximation Treatment for Many-Electron Systems , 1934 .

[15]  Takehiko Shimanouchi,et al.  Tables of molecular vibrational frequencies. Consolidated volume II , 1972 .

[16]  S. Samdal,et al.  Effect of the crystalline environment on molecular parameters —an ab initio study of cyanoformamide (carbonocyanidic amide) , 1983 .

[17]  K. Wiberg,et al.  Solvent Effects. 5. Influence of Cavity Shape, Truncation of Electrostatics, and Electron Correlation on ab Initio Reaction Field Calculations , 1996 .

[18]  Frank Jensen,et al.  Steric Effects in SN2 Reactions. The Influence of Microsolvation , 2001 .

[19]  Y. Okuno.  Microscopic description of nonadiabatic, nonequilibrium, and equilibrium solvations for solvated cluster reactions: (H2O)nCl−+CH3Cl→ClCH3+Cl−(H2O)n , 1996 .

[20]  Stephan E. Barlow,et al.  The gas-phase displacement reaction of chloride ion with methyl chloride as a function of kinetic energy , 1988 .

[21]  A. Viggiano,et al.  Temperature Dependences of the Rate Constants and Branching Ratios for the Reactions of OH-(H2O)0-4 + CH3Br , 1997 .

[22]  Haobin Wang,et al.  Temperature Dependence of the Rate Constants and Branching Ratios for the Reactions of Cl-(D2O)1-3 with CH3Br and Thermal Dissociation Rates for Cl-(CH3Br) , 1997 .

[23]  Alan J. Parker,et al.  Protic-dipolar aprotic solvent effects on rates of bimolecular reactions , 1969 .

[24]  D. Truhlar,et al.  Solvent and secondary kinetic isotope effects for the microhydrated SN2 reaction of Cl-(H2O)n with CH3Cl , 1991 .

[25]  L. Radom,et al.  Extension of Gaussian‐2 (G2) theory to bromine‐ and iodine‐containing molecules: Use of effective core potentials , 1995 .

[26]  J. J. Fisher,et al.  A pulsed ionization high-pressure mass spectrometric study of methyl cation transfer and methyl cation-induced clustering in dimethyl ether-acetone mixtures , 1988 .

[27]  F. Matthias Bickelhaupt,et al.  The Effect of Microsolvation on E2 and SN2 Reactions: Theoretical Study of the Model System F− + C2H5F + nHF , 1996 .

[28]  R. Dougherty,et al.  SN2 reactions in the gas phase. Nucleophilicity effects , 1974 .

[29]  Brian D. Wladkowski,et al.  Application of Marcus theory to gas-phase SN2 reactions: experimental support of the Marcus theory additivity postulate , 1993 .

[30]  J. Hacaloglu,et al.  Measurements of Solvent and Secondary Kinetic Isotope Effects for the Gas-Phase SN2 Reactions of Fluoride with Methyl Halides , 1994 .

[31]  A. Viggiano,et al.  Rate constants and product distributions as functions of temperature for the reaction of OH−(H2O)0,1,2 with CH3CN , 1987 .

[32]  B. Bogdanov,et al.  Influence of fluorine substitution on the structures and thermochemistry of chloride ion–ether complexes in the gas phase , 2001 .

[33]  D. Clary,et al.  Chemistry as a function of solvation number. Solvated-ion reations in the gas phase and comparison with solution , 1988 .

[34]  D. Truhlar,et al.  Modeling Transition State Solvation at the Single-Molecule Level: Test of Correlated ab Initio Predictions against Experiment for the Gas-Phase SN2 Reaction of Microhydrated Fluoride with Methyl Chloride , 1994 .

[35]  Diethard K. Bohme,et al.  Gas-phase measurements of the influence of stepwise solvation on the kinetics of SN2 reactions of solvated F− with CH3Cl and CH3Br and of solvated Cl− with CH3Br , 1985 .

[36]  H. Maskill.  Structure and reactivity in organic chemistry , 1999 .

[37]  Krishnan Raghavachari,et al.  Gaussian-2 theory for molecular energies of first- and second-row compounds , 1991 .

[38]  J. I. Brauman,et al.  Intramolecular Microsolvation of Thermoneutral Gas-Phase SN2 Reactions , 1996 .

[39]  R. Alexander,et al.  Solvation of ions. XIV. Protic-dipolar aprotic solvent effects on rates of bimolecular reactions. Solvent activity coefficients of reactants and transition states at 25°. , 1968 .

[40]  Leo Radom,et al.  Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors , 1996 .

[41]  Michael Henchman,et al.  Nucleophilic displacement vs. proton transfer: the system hydrated hydroxide ion + chloromethane [OH-.cntdot.(H2O)0,1,2 + CH3Cl] in the relative energy range 0.03-5 eV , 1985 .

[42]  Michael J. Frisch,et al.  Self‐consistent molecular orbital methods 25. Supplementary functions for Gaussian basis sets , 1984 .

[43]  D. Truhlar,et al.  A Six-Body Potential Energy Surface for the SN2 Reaction ClN-(g) + CH3Cl(g) and a Variational Transition-State-Theory Calculation of the Rate Constant , 1990 .

[44]  J. Pople,et al.  Self‐consistent molecular orbital methods. XX. A basis set for correlated wave functions , 1980 .

[45]  P. C. Hariharan,et al.  The influence of polarization functions on molecular orbital hydrogenation energies , 1973 .

[46]  E. A. Moelwyn-Hughes,et al.  530. The kinetics of certain ionic exchange reactions of the four methyl halides in aqueous solution , 1959 .

[47]  K. Giles,et al.  Measurements of equilibria and reactivity of cluster ions at atmospheric pressure: reactions of Cl-(CHCl3)0-2 with methyl bromide and methyl iodide , 1993 .

[48]  S. Hartshorn.  Aliphatic Nucleophilic Substitution , 1973 .

[49]  W. R. Wadt,et al.  Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi , 1985 .

[50]  Shinichi Yamabe,et al.  Solvation of halide ions with water and acetonitrile in the gas phase , 1988 .

[51]  D. Truhlar,et al.  Effect of nonequilibrium solvation on chemical reaction rates. Variational transition-state-theory studies of the microsolvated reaction Cl-(H2O)n + CH3Cl , 1990 .

[52]  P. Kebarle,et al.  SN2 reactions in the gas phase. Temperature dependence of the rate constants and energies of the transition states. Comparison with solution , 1984 .

[53]  Scott F. Smith,et al.  SN2 reaction profiles in the gas phase and aqueous solution , 1984 .

[54]  Keiji Morokuma,et al.  POTENTIAL ENERGY SURFACE OF THE SN2 REACTION IN HYDRATED CLUSTERS , 1982 .

[55]  William L. Jorgensen,et al.  Energy profile for a nonconcerted SN2 reaction in solution , 1985 .

[56]  W. B. Knighton,et al.  Gas-phase SN2 reactions of chloride ion with alkyl bromides at atmospheric pressure. Temperature dependence of the rate constants and energies of the transition states , 1993 .

[57]  Mark S. Gordon,et al.  Self‐consistent molecular orbital methods. XXIII. A polarization‐type basis set for second‐row elements , 1982 .

[58]  J. Pople,et al.  Self—Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules , 1972 .

[59]  B. Bogdanov,et al.  Stepwise solvation of halides by alcohol molecules in the gas phase 1 1 Dedicated to Professor Micha , 1999 .

[60]  H. Tachikawa.  Collision Energy Dependence on the Microsolvated SN2 Reaction of F-(H2O) with CH3Cl: A Full Dimensional Ab Initio Direct Dynamics Study , 2001 .

[61]  A. Castleman,et al.  The association of ammonia with halide ions in the gas phase , 1987 .

[62]  Jan M.L. Martin,et al.  Benchmark ab Initio Energy Profiles for the Gas-Phase SN2 Reactions Y- + CH3X → CH3Y + X- (X,Y = F,Cl,Br). Validation of Hybrid DFT Methods , 2000 .

[63]  Tom Ziegler,et al.  Potential Energy Surfaces of the Gas-Phase SN2 Reactions X- + CH3X = XCH3 + X- (X = F, Cl, Br, I): A Comparative Study by Density Functional Theory and ab Initio Methods , 1994 .

[64]  G. Cardini,et al.  Microsolvation effect on chemical reactivity: The case of the Cl−+CH3Br SN2 reaction , 2001 .

[65]  Diethard K. Bohme,et al.  Gas-phase measurements of the influence of stepwise solvation on the kinetics of nucleophilic displacement reactions with CH3Cl and CH3Br at room temperature , 1984 .

[66]  J. I. Brauman,et al.  Intramolecular Microsolvation of SN2 Transition States , 1999 .