Philip M. Kiefer

How Acidic Is Carbonic Acid

Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK(a) of carbonic acid H2CO3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors. The time-dependent diffusion-assisted PT rate was analyzed using the Szabo-Collins-Kimball equation with a radiation boundary condition. The on-contact PT rates were found to follow the acidity order of the carboxylic acids: the stronger was the acid, the slower was the PT reaction to its conjugate base. The pK(a) of carbonic acid was found to be 3.49 ± 0.05 using both the Marcus and Kiefer-Hynes free energy correlations. This establishes H2CO3 as being 0.37 pK(a) units stronger and about 1 pK(a) unit weaker, respectively, than the physiologically important lactic and pyruvic acids. The considerable acid strength of intact carbonic acid indicates that it is an important protonation agent under physiological conditions.

Solvent-induced red-shifts for the proton stretch vibrational frequency in a hydrogen-bonded complex. 1. A valence bond-based theoretical approach.

A theory is presented for the proton stretch vibrational frequency νAH for hydrogen (H-) bonded complexes of the acid dissociation type, that is, AH···B ⇔ A(-)···HB(+)(but without complete proton transfer), in both polar and nonpolar solvents, with special attention given to the variation of νAH with the solvents dielectric constant ε. The theory involves a valence bond (VB) model for the complexs electronic structure, quantization of the complexs proton and H-bond motions, and a solvent coordinate accounting for nonequilibrium solvation. A general prediction is that νAH decreases with increasing ε largely due to increased solvent stabilization of the ionic VB structure A(-)···HB(+) relative to the neutral VB structure AH···B. Theoretical νAH versus 1/ε slope expressions are derived; these differ for polar and nonpolar solvents and allow analysis of the solvent dependence of νAH. The theory predicts that both polar and nonpolar slopes are determined by (i) a structure factor reflecting the complexs size/geometry, (ii) the complexs dipole moment in the ground vibrational state, and (iii) the dipole moment change in the transition, which especially reflects charge transfer and the solution phase proton potential shapes. The experimental proton frequency solvent dependence for several OH···O H-bonded complexes is successfully accounted for and analyzed with the theory.

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

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 hydrogens charge from approximately midway between that of a hydrogen atom and that of a proton.

Temperature‐Dependent Solvent Polarity Effects on Adiabatic Proton Transfer Rate Constants and Kinetic Isotope Effects

Temperature-dependent solvent polarity effects are presented for proton transfer (PT) reactions in a polar environment in the proton adiabatic regime, in which proton motion is quantum mechanical, but is not tunneling. Inthis adiabatic perspective, the reaction activation free energy barrier ΔG is in the solvent coordinate, with contributions due to proton and hydrogen bond vibration zero point energy, and is strongly dependent on the polarity of the solvent. In particular, the consequences of the feature that solvent polarity decreases with increasing temperature is explored within this nontraditional PT picture, via a continuum dielectric solvent model. For an acid ionization PT reaction, the activation free energy barrier increases with increasing temperature due to the solvent polaritys temperature dependence (although the so-called intrinsic barrier and reorganization energy for the thermodynamically symmetric reaction varies in the opposite direction). The resulting PT rate constant k has a reduced effective activation energy in an Arrhenius plot due to the temperature-dependent solvent polarity effect; this differs from the activation free energy via an activation entropy effect. The Arrhenius behavior of the kinetic isotope effect k H /k D is, however, largely unchanged by the solvent polaritys temperature dependence due to a cancellation of effects between isotopes. Some further consequences of the solvent polaritys temperature dependence for adiabatic PT systems are discussed.

Effect of Solvent Dielectric Constant and Acidity on the OH Vibration Frequency in Hydrogen-Bonded Complexes of Fluorinated Ethanols.

Infrared spectroscopy measurements were used to characterize the OH stretching vibrations in a series of similarly structured fluoroethanols, RCH2OH (R = CH3, CH2F, CHF2, CF3), a series which exhibits a systematic increase in the molecule acidity with increasing number of F atoms. This study, which expands our earlier efforts, was carried out in non-hydrogen-bonding solvents comprising molecules with and without a permanent dipole moment, with the former solvents being classified as polar solvents and the latter designated as nonpolar. The hydrogen bond interaction in donor-acceptor complexes formed in solution between the fluorinated ethanol H-donors and the H-acceptor base DMSO was investigated in relation to the solvent dielectric and to the differences ΔPA of the gas phase proton affinities (PAs) of the conjugate base of the fluorinated alcohols and DMSO. We have observed that νOH decreases as the acidity of the alcohol increases (ΔPA decreases) and that νOH varies inversely with ε, exhibiting different slopes for nonpolar and polar solvents. These 1/ε slopes tend to vary linearly with ΔPA, increasing with increasing acidity. These experimental findings, including the ΔPA trends, are described with our recently published two-state Valence Bond-based theory for acid-base H-bonded complexes. Lastly, the correlation of the alcohols conjugate base PAs with Taft σ* values of the fluorinated ethyl groups CH(n)F(3-n)CH2- provides a connection of the inductive effects for these groups with the acidity parameter ΔPA associated with the H-bonded complexes.

Solvent-Induced O–H Vibration Red-Shifts of Oxygen-Acids in Hydrogen-Bonded O–H···Base Complexes

Infrared spectroscopy has been used to characterize the solvent effect on the OH stretching vibrations νOH of phenol, 1-naphthol, 2-naphthol, 1-hydroxypyrene, and ethanol. We distinguish the dielectric (nonspecific) effect of the solvent on ΔνOH, the observed red-shifts in νOH, from the much larger red-shift caused by direct hydrogen (H)-bonding interactions with the solvents. To isolate the solvent dielectric constant ε effect on νOH, the OH oscillator was also studied when it is already H-bonded with an invariant oxygen base, dimethyl sulfoxide. We find that ΔνOH depends importantly on ΔPA, the difference between the proton affinities of the conjugate base of the proton donor and the proton acceptor. For a given H-bonded complex, νOH tends to vary inversely with ε, exhibiting different slopes for polar and nonpolar solvents, i.e., solvents comprising molecules with and without a permanent dipole moment, respectively. We use a two-state valence-bond-based theory to analyze our experimental data. This demonstrates that the OH oscillator acquires a more ionic-like character in the vibrational excited state, i.e., charge transfer; this results in a stronger H-bond in a more anharmonic potential for the OH vibration. The theory distinguishes between nonpolar and polar solvents and successfully accounts for the observed 1/ε and ΔPA variations.

Proton Transfer Reactions and Hydrogen Bonding in Solution

This chapter reviews some nontraditional theoretical views developed in this group on acid-base proton transfer reactions AH×××B → A-×××HB+ in solution, whose key ingredients are inclusion of the quantum character of the proton motion, the identification of a solvent coordinate as the reaction coordinate, and attention to the AB hydrogen (H-) bond in the reacting pair. A special emphasis is placed on the several distinct important roles played by H-bonding. These include, for the AB H-bond in the reacting pair, the determination of the appropriate quantum regime for the reaction and, for the reaction pair-solvent interactions, the H-bond rearrangements involved in the reaction coordinate. While the focus is on ground electronic state proton transfers, some remarks are given for excited state proton transfers, where ultrafast dynamics can be studied. In addition, a key ingredient of the novel theoretical perspective involving the AB H-bond is commented upon in connection with ultrafast infrared experimental studies of H-bond dynamics in water.