Intrinsic Barriers for Proton Transfer Reactions Involving Electronegative Atoms, and the Water Mediated Proton Switch:  An Analysis in Terms of Marcus Theory

1996 ◽  
Vol 118 (51) ◽  
pp. 12886-12890 ◽  
Author(s):  
J. Peter Guthrie
Keyword(s):  
Proton Transfer ◽  
Marcus Theory ◽  
Transfer Reactions ◽  
Proton Transfer Reactions

Marcus theory and the existence or non-existence of transition states in gas phase deprotonation of carbon acids

1984 ◽  
Vol 62 (8) ◽  
pp. 1465-1469 ◽  
Author(s):  
Saul Wolfe
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Transition States ◽  
Marcus Theory ◽  
Energy Change ◽  
Transfer Reactions ◽  
Gas Phase Reactions ◽  
Proton Transfer Reactions ◽  
Carbon Acids ◽  
Computational Level

At the 3-21G (3-21G*) computational level, the intrinsic barriers associated with proton transfer between XCH2− and CH3X have been found to be essentially constant (ca. 10 kcal/mol) for X = H, F, SH, Cl. According to the Marcus rate-equilibrium treatment of proton transfer reactions, this result means that transition states should not exist for gas phase reactions [Formula: see text], when the energy change exceeds 20 kcal/mol. This prediction has been confirmed for two cases (X = H, F) in which the energy change is less than 20 kcal/mol, and two cases (X = SH, Cl) in which the energy change is greater than 20 kcal/mol.

Normal acid/base behaviour in proton transfer reactions to alkoxy substituted carbenes: estimates for intrinsic barriers to reaction and pKa values

1999 ◽  
Vol 77 (7) ◽  
pp. 1230-1240 ◽  
Author(s):  
John Paul Pezacki
Keyword(s):  
Proton Transfer ◽  
Key Words ◽  
Marcus Theory ◽  
Transfer Reactions ◽  
Acid Base ◽  
Pka Values ◽  
Proton Transfer Reactions ◽  
Normal Acid

Theoretical Eigen curves can be drawn through Brønsted data for dimethoxycarbene (1), phenyltrimethyl-siloxycarbene (3a), 4-methylphenyl(trimethylsiloxy)carbene (3b), 4-methoxyphenyl(trimethylsiloxy)carbene (3c), and β-naphthyl(trimethylsiloxy)carbene (3d). The Brønsted plots for these data are clearly curved with α values near 1 when proton transfer is thermodynamically unfavourable and α values near 0 when proton transfer is thermodynamically favourable, suggesting that these carbenes behave as "normal" Brønsted bases. Estimates of the intrinsic barriers (ΔG0‡) for proton transfer reactions and of the pKa values for the conjugate acids of the carbenes, extracted from these theoretical curves, have been made. The magnitudes of the intrinsic barriers (ΔG0‡) for these proton transfer reactions determined by Eigen and Marcus theories are all 1-5 kcal mol-1, suggesting that these reactions are intrinsically fast. Small intrinsic barriers also imply "normal" acid/base behaviour. Extrapolated pKa values are also the first estimates for the pKa values of the conjugate acids of carbenes 1 and 3a-3d. Key words: carbenes, proton transfer reactions, carbocations, Marcus theory, Brønsted plots.

Sign in / Sign up

Export Citation Format

Share Document