Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9746760 | International Journal of Mass Spectrometry | 2005 | 19 Pages |
Abstract
The standard enthalpy and entropy changes (ÎH° and ÎS°) for the formation of solvated SN2 complexes (S)Xâ(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 Clâ(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 (ÎH298â and ÎH298â¡) were determined at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d,p) level of theory. For the Clâ(H2O) + CH3Br and Brâ(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 Clâ + CH3Br and Clâ(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, Brâ(H2O) + CH3Cl and Brâ + (CH3Cl)(H2O) proceeds through two different surfaces. Water transfer to the leaving group can be facilitated by rotation of the Brâ(CH3Cl) part in the exit channel Brâ(CH3Cl)(H2O) complex. Finally, for the Clâ + CH3Cl reactions in the condensed phase, complexation and activation energies (ÎE(É) and ÎEâ¡(É)) 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 âÎE(É) and ÎEâ¡(É) 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.
Related Topics
Physical Sciences and Engineering
Chemistry
Analytical Chemistry
Authors
B. Bogdanov, T.B. McMahon,