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

The standard enthalpy and entropy changes (DeltaHdegrees and AS') for the formation of solvated S(N)2 complexes (S)X-(RY) (X, Y = Cl. Br; R =: (CH3)(2)CH: S = CH3OH, CH3CN, (CH3)(2)CO, 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 S(N)2 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 (DeltaH(298)(degrees), and DeltaH(298)(double dagger)) were determined at the MP2/6-311 + G(3df,2p)//MP2/6-31 + G(d,p) level of theory. For 29 the Cl-(H2O) + CH3 Br 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 set; 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-(CH3O) part in the exit channel Br(CH3O)(H2O) complex. Finally, for the Cl- + CH3Cl reactions in the condensed phase, complexation and activation energies (DeltaE(epsilon) and DeltaE(double dagger)(epsilon)) 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 - DeltaE(epsilon) and DeltaE(double dagger)(epsilon) 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. (C) 2004 Elsevier B.V All rights reserved.