Rate Constants For Proton Transfer Reactions Research Articles

Quantification of Excited-State Brønsted–Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pKa. Knowledge of the excited-state pKa value, pKa*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pKa* is possible using the Förster cycle analysis, but only when the ground-state pKa is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pKa* (pKb*) > 7), because ground-state pKa (pKb) values are >14, making it difficult to quantify them accurately in water. Another method to determine pKa* relies on acid-base titrations with photoluminescence detection and Henderson-Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pKa* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.

Calculation of the sensitivity of proton-transfer-reaction mass spectrometry (PTR-MS) for organic trace gases using molecular properties

Proton-transfer-reaction mass spectrometry (PTR-MS) allows the detection of a large number of trace gases in air through proton-transfer reaction with H3O+ reagent ions and detection by a mass spectrometer. Measurement sensitivities can be experimentally determined using calibration gases or calculated using the rate constant for the proton-transfer reaction, but rate constants have only been measured for a subset of compounds. Numerous theoretical approaches that describe the ion-molecule collision processes have shown how to accurately calculate capture collision rate constants between an ion and neutral molecules using the polarizability and permanent dipole moment of the molecule. Here we show that polarizability, dipole moment, and resulting capture rate constants for proton-transfer reactions of H3O+ with various different volatile organic compounds (VOCs) can be obtained using the molecular mass, elemental composition, and functionality of VOCs. The polarizabilities of a class of VOCs possessing a specific number of electronegative atoms were linearly correlated with their molecular mass. The dipole moments in a series of VOCs, in which VOCs contain a specific functional group and arbitrary residual hydrocarbon parts, can be approximated as a constant value. The capture rate constants calculated using polarizability and dipole moment, as estimated from molecular mass, elemental composition, and functional group, agreed within 10% with measured values for most VOCs. Those capture rate constants were applied to the calculation of the sensitivities of VOCs detected by our PTR-MS, taking into account the ion transmission efficiency and the degree of fragmentation of protonated VOCs observed in that instrument as well as chemical properties of the VOCs. The resulting calculated sensitivities agreed within 20–50% of those measured by PTR-MS, but several notable exceptions exist. This result shows that the neutral concentration of a VOC detected as a protonated molecule in PTR-MS can be approximated using only molecular mass, elemental composition, and functionality of the VOC. The present study is useful for all PTR-MS instruments regardless of the type of mass analyzer; however, the identification of elemental composition by high mass resolution instrumentation is important.

Intersecting-state model calculations on fast and ultrafast excited-state proton transfers in naphthols and substituted naphthols

The intersecting-state model (ISM) is applied to the calculation of absolute rate constants for proton-transfer reactions of naphthols and substituted naphthols in the first singlet state and for ground states. ISM incorporates quantum–mechanical tunnelling, zero-point energy corrections and an electrophilicity parameter to account for the lowering of the binding energy of transition states. Good agreement with experimental rates is observed over 12 orders of magnitude. The lower reactivity of the photoacids in alcohols when compared to the behaviour in water can be accounted for the differences in the potential energy curves of the OH bonds. Patterns of reactivity with respect to free-energy relations and kinetic isotope effects are also discussed.

Kinetics and thermodynamics of protonation reactions: H 3O + (H 2O) h + B = BH + (H 2O) b + ( h − b + 1) H 2O, where B is a nitrogen, oxygen or carbon base

The extent of proton transfer from H 3O + (H 2O) h to compounds B due to proton transfer reactions (PT) H 3O + (H 2O) h + B = BH − (H 2O) b + ( h − b + 1)H 2O was studied with a pulsed-electron high-pressure mass spectrometer (PHPMS). The H 3O + (H 2O) h were equilibrium hydrate populations in a third gas (air or methane) containing water pressure in the torr range and B in the sub-millitorr range. When a limited reaction time was available ( t < 0.5 ms), the compounds B fell into three groups regarding the observed extent of PT. Compounds with gas-phase basicity (GB) higher than 200 kcal mol −1 experienced maximum proton transfer. For these B, which are mostly nitrogen bases, the extent of proton transfer was proportional to the rate constant k PTA (kinetic control) and the k PT were found close to the ion/molecule collision-rates. Compounds B with GB < 200 kcal mol −1 were found to reach PT equilibria (thermodynamic control). The oxygen bases in this group showed an extent of PT that decreased fairly regularly as GB(B) decreased. The extent of PT observed for a group of B under thermodynamic control was much less than for oxygen bases of the same GB. This group of compounds, which included the carbon bases pyrrole, furan, and thiophene, corresponds to BH + which form hydrates of very low stability. Hydration equilibria were measured for BH + of pyrrole, furan, and thiophene. The rate constants for PT involving the carbon base pyrrole were also measured and found to decrease as the PT exothermicity decreased.

Selected-ion flow tube methods applied to the bracketing of proton affinities. PA(C 2N 2) and PA(HC 3N)

A selected-ion flow tube (SIFT) apparatus fitted with an electron impact ion source has been used to produce protonated species XH + by direct electron impact, by reaction of X + with H 2, and by reaction of H 2 + with X. The three methods of XH + production were applied in the bracketing of the proton affinities of cyanogen, C 2N 2, and cyanoacetylene, HC 3N, with measurements of rate constants for proton-transfer reactions of XH + with C 2N 2 and HC 3N and of C 2N 2H + and H 2C 3N + with X. Comparisons with established proton affinities for X provide values for PA(C 2N 2) = 162±2 kcal mol −1 and PA(HC 3N) = 184±4 kcal mol −1. The latter result is in disagreement with a previous flowing afterglow determination.

Rate constants at 297 K for proton-transfer reactions with C 2H 2: An assessment of the average quadrupole orientation theory

Rate constants for proton-transfer reactions of the type XH + + C 2H 2 → C 2H 3 + + X (type 1) where X = H 2, D 2, CH 4, CO, N 2, CO 2 and N 2O, and the type C 2H 2 + X − → XH + C 2H − (type 2) where X = H, D, NH 2, CH 3NH, C 2H 5NH, (CH 3) 2N, OH, CH 3O, C 2H 5O and (CH 3) 2CHO have been measured at 297 ± 2 K using the flowing afterglow technique. The measured rate constants are compared with the predictions of classical theories on ion—molecule collisions, viz. the Langevin and Average Quadrupole Orientation theories. The Average Quadrupole Orientation theory provides collision rates generally to within ±5% of the measured reaction rates for reactions of type (1) but underestimates by ca. 10–60% the collision rates for several of the reactions of type (2). The results for reactions of type (2) are interpreted to suggest a small contribution of induced dipole—induced dipole interaction to the collision rate. Trends in k expt/ k AQO have been observed for homologous series of reactions involving alkoxide and amide ions. These trends have raised questions regarding the extent to which charge delocalization may affect the collision rate and the influence of reaction exothermicity on proton-transfer efficiency for these reactions.