Gas-phase non-identity SN2 reactions at neutral nitrogen: a hybrid DFT study

The gas-phase non-identity nucleophilic substitution reactions at saturated nitrogen Y-+ NH2X --> NH2Y + X- (Y, X = F, Cl, Br and I) were evaluated at the level of MPW1K/6-31+G(d, p). The enthalpies of reactions are exothermic only when the nucleophile is the lighter halide. Central barriers (DeltaH(YX)(not equal)) for reactions in the exothermic direction are slightly higher than the corresponding barriers at carbon. The lower overall barriers relative to the reactants (DeltaH(YX)(b)) than the corresponding reactions at carbon suggest that S(N)2 reactions at nitrogen may be more facile than at carbon. Both the central barriers (AH(YX)(not equal)) and the overall barriers (DeltaH(YX)(b)) correlate well with reaction exothermicity. Further interesting features of the non-identity reactions at nitrogen are the reasonable correlation between the central barriers (DeltaH(YX)(not equal)) with the composite geometrical looseness (%L-not equal), geometrical asymmetry (%AS(not equal)), and charge asymmetry of the transition structures (Deltaq (X - Y)). The data for the central barriers and the overall barriers show good agreement with the prediction of the Marcus equation and its modification, respectively. Kinetic and thermodynamic investigations predict that the nucleophilicity of X- in the S(N)2 at nitrogen decreases in the order F- Cl > Br > I. (C) 2003 Elsevier B.V. All rights reserved.

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

[2]  Kihyung Song,et al.  A SN2 Reaction That Avoids Its Deep Potential Energy Minimum , 2002, Science.

[3]  S. Shaik,et al.  Is the avoided crossing state a good approximation for the transition state of a chemical reaction? An analysis of Menschutkin and ionic SN2 reactions , 1994 .

[4]  Frank Weinhold,et al.  Natural bond orbital analysis of near‐Hartree–Fock water dimer , 1983 .

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

[6]  Mikhail N. Glukhovtsev,et al.  Gas-Phase Identity SN2 Reactions of Halide Ions at Neutral Nitrogen: A High-Level Computational Study , 1995 .

[7]  Harold Basch,et al.  The periodic table and the intrinsic barrier in s(n)2 reactions. , 2002, The Journal of organic chemistry.

[8]  Yi Ren,et al.  The performance of density function theory in describing gas-phase SN2 reactions at saturated nitrogen , 2002 .

[9]  ab Mikhail N. Glukhovtsev,et al.  Gas-Phase Identity SN2 Reactions of Halide Anions and Methyl Halides with Retention of Configuration , 1996 .

[10]  Yi Ren,et al.  Hybrid DFT study on the gas-phase SN2 reactions at neutral oxygen , 2003 .

[11]  L. Curtiss,et al.  Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint , 1988 .

[12]  V. Bierbaum,et al.  Gas phase reactions of NH2Cl with anionic nucleophiles: Nucleophilic substitution at neutral nitrogen , 2001, Journal of the American Society for Mass Spectrometry.

[13]  Frank Weinhold,et al.  Natural hybrid orbitals , 1980 .

[14]  W. J. Albery,et al.  The Application of the Marcus Relation to Reactions in Solution , 1980 .

[15]  ac and Addy Pross,et al.  Acidities, Proton Affinities, and Other Thermochemical Properties of Hypohalous Acids HOX (X = F−I): A High-Level Computational Study , 1996 .

[16]  Charles A. Lieder,et al.  Gas-phase nucleophilic displacement reactions , 1974 .

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

[18]  P. Hiberty,et al.  Valence Bond Mixing and Curve Crossing Diagrams in Chemical Reactivity and Bonding , 1995 .

[19]  P. Beak,et al.  The endocyclic restriction test: experimental evaluation of transition-structure geometry for a nucleophilic displacement at neutral nitrogen , 1991 .

[20]  Henry F. Schaefer,et al.  SN2 reaction at neutral nitrogen: transition state geometries and intrinsic barriers. , 1993 .

[21]  Rudolph A. Marcus,et al.  Theoretical relations among rate constants, barriers, and Broensted slopes of chemical reactions , 1968 .

[22]  Robert L. Kuczkowski,et al.  Molecular structures of gas‐phase polyatomic molecules determined by spectroscopic methods , 1979 .

[23]  Rudolph A. Marcus,et al.  Chemical and Electrochemical Electron-Transfer Theory , 1964 .

[24]  R. J. Boyd,et al.  Charge development at the transition state: a second-order Moeller-Plesset perturbation study of gas-phase SN2 reactions. , 1991 .

[25]  F. Weinhold,et al.  Natural population analysis , 1985 .

[26]  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 .

[27]  D. Christen,et al.  Microwave spectrum, inversion, and molecular structure of monofluoramine, FNH2 , 1987 .

[28]  Jon K. Laerdahl,et al.  Gas phase nucleophilic substitution , 2002 .

[29]  W. Hase,et al.  Trajectory studies of SN2 nucleophilic substitution. II. Nonstatistical central barrier recrossing in the Cl−+CH3Cl system , 1992 .

[30]  Donald G. Truhlar,et al.  How Well Can Hybrid Density Functional Methods Predict Transition State Geometries and Barrier Heights , 2001 .