Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Kiefer, Philip M.; Hynes, James T.

doi:10.1002/9783527611546.ch10

Cited by 11 publications

(32 citation statements)

References 59 publications

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“…These levels were calculated as in this group’s previous PT work. 27–29,76 While we obviously do not present a full quantum treatment of the reaction, these levels would depict a quantum adiabatic PT reaction (rather than a quantum tunneling reaction) 23–29,77 for the eq 1 PT, although of a barrierless, nonactivated type, consistent with H 2 CO 3 ’s acid strength and the reaction’s Δp K a difference, as anticipated in the Introduction.…”

Section: Proton Transfer Resultsmentioning

confidence: 92%

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

et al. 2016

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Protonation by carbonic acid H2CO3 of the strong base methylamine CH3NH2 in a neutral contact pair in aqueous solution is followed via Car–Parrinello molecular dynamics simulations. Proton transfer (PT) occurs to form an aqueous solvent-stabilized contact ion pair within 100 fs, a fast time scale associated with the compression of the acid–base hydrogen-bond (H-bond), a key reaction coordinate. This rapid barrierless PT is consistent with the carbonic acid-protonated base pKa difference that considerably favors the PT, and supports the view of intact carbonic acid as potentially important proton donor in assorted biological and environmental contexts. The charge redistribution within the H-bonded complex during PT supports a Mulliken picture of charge transfer from the nitrogen base to carbonic acid without altering the transferring hydrogen’s charge from approximately midway between that of a hydrogen atom and that of a proton.

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Section: Proton Transfer Resultsmentioning

confidence: 92%

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

et al. 2016

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“…In order to include the water solvent in the dynamical description (and as in previous activated PT studies 6–10 ), we employ an energy gap coordinate Δ E . This will prove useful, despite the present PT reaction’s lack of any activation barrier.…”

Section: Trajectory Analysis Detailsmentioning

confidence: 99%

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution II: Solvent Coordinate-Dependent Reaction Path

et al. 2016

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The protonation of methylamine base CH3NH2 by carbonic acid H2CO3 within a hydrogen (H)-bonded complex in aqueous solution was studied via Car–Parrinello dynamics in the preceding paper (Daschakraborty, S.; Kiefer, P. M.; Miller, Y.; Motro, Y.; Pines, D.; Pines, E.; Hynes, J. T. J. Phys. Chem. B 2016, DOI: 10.1021/acs.jpcb.5b12742). Here some important further details of the reaction path are presented, with specific emphasis on the water solvent’s role. The overall reaction is barrierless and very rapid, on an ∼100 fs time scale, with the proton transfer (PT) event itself being very sudden (<10 fs). This transfer is preceded by the acid–base H-bond’s compression, while the water solvent changes little until the actual PT occurrence; this results from the very strong driving force for the reaction, as indicated by the very favorable acid-protonated base ΔpKa difference. Further solvent rearrangement follows immediately the sudden PT’s production of an incipient contact ion pair, stabilizing it by establishment of equilibrium solvation. The solvent water’s short time scale ∼120 fs response to the incipient ion pair formation is primarily associated with librational modes and H-bond compression of water molecules around the carboxylate anion and the protonated base. This is consistent with this stabilization involving significant increase in H-bonding of hydration shell waters to the negatively charged carboxylate group oxygens’ (especially the former H2CO3 donor oxygen) and the nitrogen of the positively charged protonated base’s NH3+.

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“…Despite the wealth of investigations on excited-state proton-transfer (ESPT) reactions, − several fundamental aspects are still under discussion. For example, the influence of the solvent environment on ESPT is recognized from both experimental , and theoretical − viewpoints but only a few studies have explicitly accounted for solvent relaxation and its influence on the ESPT. − There is, however, some experimental evidence that suggests that the initial proton-transfer step of strong photoacids is controlled by solvent relaxation. , Second, ESPT reactions are usually modeled according to the Eigen-Weller model (Scheme ), which consists of an initial short-range proton-transfer producing contact ion pairs, followed by a diffusion-controlled separation into free anions. , However, this model is often discussed only qualitatively, and detailed investigations resolving all individual rate constants and relaxation pathways are scarce. Few studies have demonstrated that the ESPT dynamics of several photoacids are consistent with the Eigen-Weller model, but the contact ion pairs were suggested to be spectrally indistinguishable from the free anions. ,,− Hence studies resolving all excited-state species both kinetically and spectrally are still missing.…”

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Influence of Solvent Relaxation on Ultrafast Excited-State Proton Transfer to Solvent

Kumpulainen

Rosspeintner

Dereka

et al. 2017

J. Phys. Chem. Lett.

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A thorough understanding of the microscopic mechanism of excited-state proton transfer (ESPT) and the influence of the solvent environment on its dynamics are of great fundamental interest. We present here a detailed investigation of an ESPT to solvent (DMSO) using time-resolved broadband fluorescence and transient absorption spectroscopies. All excited-state species are resolved spectrally and kinetically using a global target analysis based on the two-step Eigen-Weller model. Reversibility of the initial short-range proton transfer producing excited contact ion pairs (CIP*) is observed unambiguously in fluorescence and must be explicitly considered to obtain the individual rate constants. Close inspection of the early dynamics suggests that the relative populations of the protonated form (ROH*) and CIP* are governed by solvent relaxation that influences the relative energies of the excited states. This constitutes a breakdown of the Eigen-Weller model, although the overall agreement between the data and the analysis using classical rate equations is excellent.

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Theoretical Aspects of Proton Transfer Reactions in a Polar Environment

Cited by 11 publications

References 59 publications

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution II: Solvent Coordinate-Dependent Reaction Path

Influence of Solvent Relaxation on Ultrafast Excited-State Proton Transfer to Solvent

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