Intermolecular Proton Transfer Reactions Research Articles

Information carriers and (reading them through) information theory in quantum chemistry

This Perspective discusses the reduction of the electronic wave function via the second-order reduced density matrix to the electron density ρ(r), which is the key ingredient in density functional theory (DFT) as a basic carrier of information. Simplifying further, the 1-normalized density function turns out to contain essentially the same information as ρ(r) and is even of preferred use as an information carrier when discussing the periodic properties along Mendeleev's table where essentially the valence electrons are at stake. The Kullback-Leibler information deficiency turns out to be the most interesting choice to obtain information on the differences in ρ(r) or σ(r) between two systems. To put it otherwise: when looking for the construction of a functional F(AB) = F[ζ(A)(r),ζ(B)(r)] for extracting differences in information from an information carrier ζ(r) (i.e. ρ(r), σ(r)) for two systems A and B the Kullback-Leibler information measure ΔS is a particularly adequate choice. Examples are given, varying from atoms, to molecules and molecular interactions. Quantum similarity of atoms indicates that the shape function based KL information deficiency is the most appropriate tool to retrieve periodicity in the Periodic Table. The dissimilarity of enantiomers for which different information measures are presented at global and local (i.e. molecular and atomic) level leads to an extension of Mezey's holographic density theorem and shows numerical evidence that in a chiral molecule the whole molecule is pervaded by chirality. Finally Kullback-Leibler information profiles are discussed for intra- and intermolecular proton transfer reactions and a simple S(N)2 reaction indicating that the theoretical information profile can be used as a companion to the energy based Hammond postulate to discuss the early or late transition state character of a reaction. All in all this Perspective's answer is positive to the question of whether an even simpler carrier of information than the electron density function ρ(r) can be envisaged: the shape function, integrating to 1 by construction fulfils this role. On the other hand obtaining the information (or information difference) contained in one (or two) systems from ρ(r) or σ(r) can be most efficiently done by using information theory, the Kulback-Leibler information deficiency being at the moment (one of) the most advisable functionals.

Femtosecond Fluorescence Dynamics of a Proton-Transfer Dye Interacting with Silica-Based Nanomaterials

2-[5′-N-(3-triethoxysilyl)propylurea-2′-hydroxyphenyl]-benzothiazole (HBTNH2) was covalently bonded to the inner surface of amorphous silica nanoparticles and unmodified and Al-doped mesoporous structured silicate, MCM-41 and (Al)MCM-41, respectively. The photodynamics of these materials was investigated using steady-state and femtosecond fluorescence up-conversion techniques. The guest molecule shows emission dependence on the excitation wavelength. Covalently bonding HBTNH2 to silica nanoparticles affects both the spectral and the dynamical behavior of the dye. The dual-band fluorescence and ultrafast dynamics indicate the possibilities for an excited-state intermolecular proton-transfer reaction between the dye and the silica framework. The observed behavior is assigned to contribution from excited populations of enol, keto, and anion forms. The excited-state dynamics and the emission spectra of the HBTNH2−MCM-41 material clearly reflect the confinement effect imposed by the MCM-41 nanochannels. Furthe...

A DFT-Based Theoretical Study on the Photophysics of 4-Hydroxyacridine: Single-Water-Mediated Excited State Proton Transfer

Study of intra- and intermolecular hydrogen-bonding interaction and excited state proton transfer reaction has been carried out in 4-hydroxyacridine (4-HA) and its hydrated clusters theoretically. Density functional theory [B3LYP/6-311++G(d,p)] has been exploited to calculate structural parameters and relative energies of different conformers of 4-HA and its hydrates. The substantial impact of solvent reaction field on hydrogen-bond energies, conformational equilibrium, and tautomerization reaction in aqueous medium have been realized by employing Onsager and PCM reaction field methods, and the stability of the conformers of 4-HA is found to be profusely modulated by the electrostatic influence of the solvent. A deeper insight into the nature of H-bonding in 4-HA and its hydrated clusters has been achieved under the provision of natural bond orbital and atoms in molecule analysis. Elucidation of potential energy curves for proton transfer reaction reveals that an intrinsic and two-water-molecule-assisted proton transfer (PT) reaction in 4-HA is hindered by high energy barrier in the S(1) surface, whereas single-water-assisted PT reaction is practically rendered barrierless. At the same time, the appreciably high barrier height of the ground state potential energy curve in all the cases unambiguously rules out the possibility of ground state proton transfer reaction.

Analyzing Kullback–Leibler information profiles: an indication of their chemical relevance

The Kullback-Leibler information deficiency is evaluated along molecular internal rotational or vibrational coordinates and along the intrinsic reaction coordinate for several reactions (intra- and intermolecular proton transfer and SN2 reaction). For the first time an in depth analysis of the information deficiency along the reaction path is reported. The results are consistent with the Hammond postulate, indicating that the information profiles contain relevant chemical information. A local version of the information deficiency is defined by considering Hirshfeld's partitioning of atoms in molecules. The analysis of the local information profiles permits the identification of the atoms taking part in the electron reorganization processes.

On the Role of Water in Intermolecular Proton-Transfer Reactions

We study the mechanism of proton transfer (PT) between the photoacid (8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS)) and the base acetate in aqueous solution using femtosecond vibrational spectroscopy. By probing the vibrational resonances of the photoacid, the accepting base, and the hydrated proton we find that intermolecular PT in this model system involves the transfer of the proton across several water molecules linking the donor-acceptor pair by hydrogen bonds (H-bonds). We find that at high base concentration the rate of PT is not determined by the mutual diffusion of acid and base but rather by the rate of Grotthuss-like conduction of the proton between molecules. This long-range PT requires an activated solvent configuration to facilitate the charge transfer.

Intramolecular Proton-Transfer Processes Starting at Higher Excited States: A Fluorescence Study on 2-Butylamino-6-methyl-4-nitropyridine N-Oxide in Nonpolar Solutions

This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations.

Theoretical study of intermolecular proton transfer reaction in isolated 5-hydroxyisoxazole–water complexes

A systematic investigation in isolated 5-hydroxyisoxazole–water complexes (5-HIO · (H 2O) n n = 1–3) is performed at the DFT level, employing B3LYP/6-31G(d, p) basis set. Single-point energy calculations are also performed at the MP2 level using B3LYP/6-31G(d, p) optimized geometries and the 6-311++G(d, p) basis set. The computational results show that the keto tautomer K 2 is the most stable isomer in the gas phase, and the tautomer K 1 to be the next most stable tautomer. Hydrogen bonding between HIO and the water molecule(s) will dramatically lower the barrier by a concerted multiple proton transfer mechanism. The proton transfer process of 3WE cis ↔ 3WK 1 and 2WE trans ↔ 2WK 2 is found to be more efficient in two tautomerization, and the barrier heights are 7.03 and 14.15 kcal/mol at B3LYP/6-31G(d, p) level, respectively. However, the proton transfer reaction between E cis and K 1 cannot happen without solvent-assisted.

A Quantum Chemical Study of Cu2+ Interacting with Guanine−Cytosine Base Pair. Electrostatic and Oxidative Effects on Intermolecular Proton-Transfer Processes

The influence of metal cations (M = Cu+, Ca2+ and Cu2+) coordinated to N7 of guanine on the intermolecular proton-transfer reaction in guanine−cytosine base pair has been analyzed using the B3LYP density functional approach. Gas phase metal cation interaction stabilizes the ion pair structure derived from the N1−N3 single-proton-transfer reaction, the effects being more pronounced for the divalent cations than for the monovalent one. For Cu2+GC the reaction is largely favored due to both electrostatic and oxidative effects. Hydration of the metal cation disfavors the reaction due to the screening of electrostatic effects. However, for Cu2+ the reaction can still be easily produced, especially for certain local environments of the metal cation for which Cu2+ induces the oxidation of guanine. Therefore, the ability of Cu2+ to oxidize guanine turns out to be a key factor for this mutagenic process.

Excited state intermolecular proton transfer reactions in few N-heterocyclic systems

Excited state intermolecular proton transfer reactions in few N-heterocyclic systems

Water-Induced Quenching of Salicylic Anion Fluorescence

Salicylic anion absorption and emission are studied in a variety of solvents and solvent mixtures. The large Stokes shift observed for this anion is taken to be indicative of a rapid excited state proton transfer reaction to its keto form. The changes in the Stokes shift in the various solvents can be well-correlated with changes in polarity/polarizability and hydrogen-bonding acidity. The time-resolved data can for the most part also be well-correlated with these properties. A notable exception is the behavior in water and water-rich mixtures. A significant decrease in fluorescence lifetime is observed, and the influence of temperature in pure water is much larger than in other neat liquids. As an explanation for these effects, an excited state intermolecular proton transfer reaction is suggested, from larger-sized water clusters, to the anion.

Excited-state intermolecular proton transfer reaction of 6-hydroxyquinoline in protic polar medium

Steady state and time resolved characteristics of 6-hydroxyquinoline (6-HQ) in alcoholic solvents and in a mixture of solvent (methanol:water) are studied. In neat alcoholic solvents, 6-hydroxyquinoline (6-HQ) shows emission around 376 nm. The emission corresponds to normal species and shows a single exponential decay (1.45 ns). However, in methanol containing water, dual emission i.e., normal one at ∼376 nm and a large Stokes shifted emission at ∼570 nm is observed. In presence of small percentage of water the decay time shows a decreasing trend and at higher percentage of water (>10% in v) a clear departure from the single exponential is observed. The results are interpreted in terms of multiple proton transfer through methanol and water chain.

Solvent reorganization controls the rate of proton transfer from neat alcohol solvents to singlet diphenylcarbene.

Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singlet diphenylcarbene was detected and characterized for the first time. Similar band shapes were observed in acetonitrile and in cyclohexane with lambda(max) approximately 370 nm. The singlet absorption decays by intersystem crossing to triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band is completely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, which is tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solvents the growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transfer from an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds each nascent carbene, the intrinsic rate of protonation in the absence of diffusion could be measured. In methanol, proton transfer occurs with a time constant of 9.0 ps, making this the fastest known intermolecular proton-transfer reaction to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanol the rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanol that geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect was observed for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometry the quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reaches a maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of the photoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficiencies are observed in the other alcohols. Because the primary quantum yield for formation of singlet diphenylcarbene is still unknown, the importance of reaction channels that might exist in addition to protonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases that open new possibilities for fundamental studies of proton transfer in solution.

Femtosecond Dynamics of a Simple Merocyanine Dye: Does Deprotonation Compete with Isomerization?

The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazolium betaine type 1-methyl-4-(4‘-hydroxytyryl)pyridinium betaine (Mtrans) and its conjugate acid MH+trans in aqueous solution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 × 1012 s-1 for Mtrans at pH 10 and 0.8 × 1012 s-1 for MH+trans at pH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actual isomerization takes place. Wavelength excitation studies give support for the former assignment. These results are discussed in terms of the recent results found for the primary processes of retinal in bacteriorhodopsin. Time-resolved transient measurements show that no excited-state deprotonation of MH+trans occurs in aqueous solutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecond time domain. This is in agreement with present theories of intermolecular proton-transfer reactions, which require solvent reorganization as well as the time of deprotonation estimated from the pKa value of this molecule in the excited state. The results of our MO calculations on the electronic structure of these two compounds could account for the fact that, while MH+trans photoisomerizes, its deprotonated form does not.

Mechanisms of photoinduced protolytic interactions of 2,5-diphenyloxazoleortho-hydroxy derivatives in media of various acidities

The mechanism of photoinduced intra- and intermolecular protolytic interactions ofortho-hydroxy derivatives of 2,5-diphenyloxazole in an aqueous alcohol medium was studied over a wide range of acidity from pH ∼13 toH 0 of ∼−7. The spectral parameters of protolytic forms and equilibrium constants were obtained, and rate constants for the primary photochemical processes (excited-state intra- and intermolecular proton transfer reactions) were evaluated. It was shown that the spectral characteristics of oxazoleortho-hydroxy derivatives in an acidic medium are formed as a result of competition and interchange of intra- and intermolecular protolytic interactions. The phototautomeric form in strongly acidic solutions was found to be produced by dissociation of the cationic form with the protonated oxazole ring.

Experimental observation of light-induced solitary waves of analyte bands in capillary electrophoresis.

The phenomenon of electrophoresis in free solution has been studied theoretically down to the molecular level for decades. In addition, intermolecular photo-induced proton transfer reactions, which occur in a wide class of molecules (phenols and aminoarenes) as well as proteins (green fluorescent protein), were also studied extensively. However, the study of the effect of light-induced electrophoretic mobility changes of the analytes in electrophoresis was begun only recently. In the present work, capillary zone electrophoresis was chosen as the environment to measure the magnitude of these electrophoretic mobility shifts induced by light. Background electrolytes (running electrolytes) with high refractive indices were developed, allowing the capillary to work like an optical fiber. The experimental conditions for obtaining stable coupling and guided laser light along the liquid core are discussed. Experimental evidence of band compression is observed, leading to a solitary wave behavior of the analyte band (2-naphthol). These solitary waves result from competition between thermal diffusion (dispersion mechanism) and a nonlinear (band compression) effect due to the combined electrophoresis phenomenon and absorption of guided light by the molecules of the band (which are subjected to a "reversible intermolecular proton transfer reaction" as one of their decay routes). The possibilities of applying this effect to different methods and techniques are also discussed.

Evidence for an Intermolecular Proton-Transfer Reaction Induced by Collision in Gas-Phase Noncovalently Bound Complexes

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTEvidence for an Intermolecular Proton-Transfer Reaction Induced by Collision in Gas-Phase Noncovalently Bound ComplexesFei He, Javier Ramirez, and Carlito B. LebrillaView Author Information Department of Chemistry, University of California Davis, California 95616 Cite this: J. Am. Chem. Soc. 1999, 121, 19, 4726–4727Publication Date (Web):May 1, 1999Publication History Received6 October 1998Published online1 May 1999Published inissue 1 May 1999https://doi.org/10.1021/ja983535pCopyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views141Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (29 KB) Get e-AlertsSUBJECTS:Macrocyclic compounds,Monomers,Oligosaccharides,Peptides and proteins,Reaction mechanisms Get e-Alerts

Cyclic Voltammetric and E.P.R. Spectroscopic Studies on Apical Substituent Effects in Radical Anions of 9-Substituted and 9,10-Disubstituted Nitroethanoanthracenes

Cyclic voltammetry and electron paramagnetic resonance spectroscopy were used to examine apical substituent effects on the properties of Me2SO solutions of the radical anions from 9-substituted and 9,10-disubstituted 2- and 3-nitro-9,10-dihydro-9,10-ethanoanthracenes (1)-(24). The reductions of the nitro group are, in general, reversible at 100 mV s-1 and at 20°, except where there are coupled intra-or inter-molecular electron or proton transfer reactions with aliphatic bridgehead substituents, such as a carboxylic acid or iodine. The substituent effects for the meta- and para-nitroethanoanthracene systems are both similar and additive. This similarity in the meta and para polar substituent effects is attributed to the orthogonality of the π*(ArNO2/ArNO2-) orbital with the σ* orbital of the carbon substituent (C-X) groups at the bridgehead positions. Overall, the meta substituent effect was slightly lower than that for corresponding simple meta-nitrobenzyl systems, but the para-nitro systems showed substituent effects that are a factor of 2 smaller than that for corresponding simple para-nitrobenzyl systems. These linear correlations between the substituent effects and redox potentials have been used to estimate the redox potentials of irreversible systems, which are required for digital simulations of reactions involving nitroaryl radical anions. Only small substituent effects are present in hyperfine coupling with the aliphatic and aromatic protons of the nitroaromatic radical anions of ethanoanthracenes (1)-(13), but a clear trend to lower nitrogen hyperfine values was observed with increasing electron-withdrawing ability of the apical substituent. In addition, no spin density was transferred to the benzylic bridgehead substituent in any of the nitro radical anions studied, clearly demonstrating that the bond between the bridgehead substituent and the carbon at a benzylic position is orthogonal to the π-system of the nitroaromatic ring bearing the odd electron.

Time and internal energy dependent fluorescence spectra of naphthol⋅water clusters

Time-resolved fluorescence spectra of 1-naphthol⋅(H2O)n clusters formed in a supersonic jet were measured under conditions of strong and weak cooling. Wavelength selectivity was used to excite very similar size distributions in both cases, thereby allowing controlled study of temperaturelike effects of internal energy in clusters. In both warm and cold clusters, long wavelength fluorescence from 1-naphtholate formed via an intermolecular excited state proton transfer (ESPT) reaction is observed, but this emission was significantly more red-shifted in warmer clusters. The fluorescence spectra of warmer clusters also shift strongly to the red after excitation, on a time scale of about 10 ns. Colder clusters showed no spectral shifts on this time scale. A weak solvent isotope effect was also observed. Similar clusters of 2-naphthol with water were not observed to undergo ESPT under any conditions. The time-dependent spectra indicate that ESPT in warm 1-naphthol⋅water clusters is not prompt, but continues on a ∼10 ns time scale, as does dynamic stabilization of previously reacted clusters. The ESPT rate is believed to be controlled by the rate at which the water relaxes around the naphthol, via interaction with two electronic excited states. The 10 ns and the two faster [R. Knochenmuss, G. R. Holtom, and D. Ray, Chem. Phys. Lett. 215, 188 (1993)] time scales now known in the spectral dynamics are discussed in terms of water solvation relaxation, and connected to the bulk by means of molecular dynamics simulations.

Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

The present review describes the salient features of inter- and intramolecular proton transfer reactions of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl)-, 2-(4′-aminophenyl)-, 2-(2′-hydroxyphenyl)-, 2-(3′-hydroxyphenyl)- and 2-(4′-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4′-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.

Proton scavenging in photoacid geminate recombination processes

Proton geminate recombination is observed in reversible excited-state intermolecular proton transfer reactions. Introducing a proton scavenger into the solution reduces the proton geminate recombination yield. At low scavenger concentrations the experimental data are in excellent agreement with a newly developed diffusive model.