Ab Initio Comparison of Identity-Reaction Proton Transfers from Carbon Acids Yielding Localized vs. Delocalized Conjugate Bases

Valence bond calculations on the transition state for the E2 reaction of fluoride ion with ethyl fluoride — Implications for the More O'Ferrall – Jencks diagram

“…By these measures, the transition state is much closer to product than to reactant, and the positive charge on H-CH 2 CH 2 is about 0.2 units greater than in the reactant, mainly due to the increased charge on the proton in transit. The large positive charge on the proton in transit and the large negative charge on the β-CH 2 are reminiscent of the behavior observed in proton transfers from acids that yield resonance-stabilized anions (14,21,(25)(26)(27)(28), which are the examples of the principle of nonperfect synchronization (29)(30)(31).…”

Experimental and Computational Determination of the Effect of the Cyano Group on Carbon Acidity in Water

“…We would like to understand why there are chemical barriers for many strongly thermodynamically favorable reactions of unstable carbocations and carbanions in solution and how these activation barriers change with changing thermodynamic driving force for the reaction . The observation of a relatively large Marcus intrinsic rate constant of 10 7 M -1 s -1 2a for thermoneutral protonation of the dicyanomethyl carbanion by secondary ammonium cations has been interpreted as reflecting only a small transfer of negative charge from the α-carbon to the cyano substituents (Scheme ). , This is reasonable because there is good evidence that the intrinsic barriers to the protonation of carbanions , and to nucleophilic additions to carbocations 1,2b increase with increasing stabilization of charge resulting from its transfer to neighboring substituents. By contrast, a recent comparison of Hirshfeld atomic charges at acetonitrile and the cyanomethyl carbanion shows that deprotonation of acetonitrile results in a ca.…”

Proton Transfers from ZCH3 to ZCH2- (Z = F, Cl, Br, OH, SH, SeH). An ab Initio Investigation

“…As in our previous work, our criterion for the adequacy of the level of the calculations was their ability to reproduce experimental gas-phase acidities. Although the present systems are small enough that they could have been studied at higher levels than those chosen, we wanted to be able to compare them with results from our earlier studies. , The results are given in Table . It is immediately apparent that the results at MP2/6-31+G*//MP2/6-31+G* agree better with experiment than those at MP4/6-31+G**//MP2/6-31+G*.…”

“…In early work, Wolfe examined four reactions (Z = H, F, Cl, and SH) at the 3-21G(3-21G*) level and reported that the barrier was “essentially constant”. Actually it varies over a range of nearly 4 kcal, and it is now also known that 3-21G does not give an accurate account of proton-transfer barriers, so we decided to proceed with studies using more elaborate basis sets (6-31+G* and 6-31+G**) and correlation corrections (MP2 to MP4), which we had shown gave good values of gas-phase acidities and satisfactory agreement with experimental results. ,, …”

“…Pioneering work on simple systems was reported by Scheiner and his coworkers. − Later, Gronert applied the G2 method to methane and other simple first and second row hydrides . Evidence that acids yielding delocalized anions showed high barriers to deprotonation was found by work on acetaldehyde, − acetonitrile, , and propene . That the barriers for identity reactions of two of these species are unusually high was shown by the fact that a plot of acidity vs barrier height for identity reactions of three-carbon acids yielding localized anions, methane, ethylene, and acetylene, was linear, whereas the points for acetaldehyde and propene fell well above the line …”