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

Abstract

Energy profiles for the identity-reaction carbon-to-carbon proton transfers from carbon acids of the type ZCH3 to their conjugate bases ZCH2 - have been studied by ab initio methods. Gas-phase acidities of ZCH3 species are reproduced well at MP2/6-31+G*//MP2/6-31+G*. The barriers to proton transfer relative to the separated reactants (ΔH TS) in kcal mol-1 are F (2.2), Cl (−4.2), Br (−8.2), OH (−2.1), SH (−5.7), SeH (−11.0) at MP2/6-31+G*//MP2/6-31+G*. Values at MP4/6-31+G**//MP2/6-31+G* are very similar. The orders of acidities and barrier heights suggest that the polarizability of Z is the dominant effect on both. That two or more ion−dipole complexes of similar energy exist for each system implies that the complexes will be fluxional. It is doubtful that these proton transfers could be observed experimentally. The dominant process for HYCH2 - + CH3YH would almost certainly be the transfer of the YH proton, and calculations on the SN2 reactions ZCH2 - + CH3Z → ZCH2CH3 + Z- (Z = F, Cl, Br) reveal them to be strongly exothermic with barriers 6−10 kcal mol-1 below those for the corresponding proton transfer processes.

Computational Comparison of SN2 Substitution Reactions of CHX•- and CH2X- with CH3X (X = Cl, Br). Do Open-Shell and Closed-Shell Anions React Differently?

“…As part of a recent study on proton-transfer reactions, Van Verth et al calculated the ion−molecule complex ( 1-c ) and backside transition state ( 1-tsb ). Their activation energy at the MP4/6-31+G(d)//HF/6-31+G(d) level (1.2 kcal/mol) is very close to the barrier calculated in this work at the [QCISD(T)/6-311+G(2d,p)]//MP2/6-31+G(d)+ZPC level (1.0 kcal/mol).…”

Proton Transfer to and from Carbon in Model Reactions

“…1.2A). This has led to the determination of the Marcus intrinsic barriers for a variety of proton transfer reactions by experiment and through calculations [58][59][60][61][62][63][64][65]. 1.2A) [46,50,51], carbocation-nucleophile addition [52], bimolecular nucleophilic substitution [53,54] and other reactions [55][56][57] by assuming that their reaction coordinate profiles may also be constructed from the intersection of parabolas that describe the reactant and product states.…”

Carbon-to-Carbon Identity Proton Transfers from Propyne, Acetimide, Thioacetaldehyde, and Nitrosomethane to Their Respective Conjugate Anions in the Gas Phase. An ab Initio Study

“…Since even for the transition states there is significant negative charge on the Y group, the polarizability effect on their stability is rather modest. This contrasts with the reactions of the type ZCH 3 + ZC ⇄ ZC + ZCH 3 with Z = F, Cl, Br, OH, SH where the polarizability of Z has a strong barrier reducing effect …”

“…Although the correlation based just on field and resonance effects was remarkably good ( r 2 = 0.986), it neglected polarizability effects. Because such effects can potentially be quite important, especially for gas-phase anions , but also in solution, we have now added the polarizability term σ α to our correlation (eq 11). This is shown in Figure .…”