Tautomerization Rate Research Articles

Hot Carrier-Induced Tautomerization within a Single Porphycene Molecule on Cu(111).

Here, we report the study of tautomerization within a single porphycene molecule adsorbed on a Cu(111) surface using low-temperature scanning tunneling microscopy (STM) at 5 K. While molecules are adsorbed on the surface exclusively in the thermodynamically stable trans tautomer after deposition, a voltage pulse from the STM can induce the unidirectional trans → cis and reversible cis ↔ cis tautomerization. From the voltage and current dependence of the tautomerization yield (rate), it is revealed that the process is induced by vibrational excitation via inelastic electron tunneling. However, the metastable cis molecules are thermally switched back to the trans tautomer by heating the surface up to 30 K. Furthermore, we have found that the unidirectional tautomerization can be remotely controlled at a distance from the STM tip. By analyzing the nonlocal process in dependence on various experimental parameters, a hot carrier-mediated mechanism is identified, in which hot electrons (holes) generated by the STM travel along the surface and induce the tautomerization through inelastic scattering with a molecule. The bias voltage and coverage dependent rate of the nonlocal tautomerization clearly show a significant contribution of the Cu(111) surface state to the hot carrier-induced process.

Tautomerism in porphycenes: analysis of rate-affecting factors.

Double hydrogen transfer occurring in both ground and the lowest electronically excited singlet states was studied for a series of 19 differently substituted porphycenes. The rates of tautomerization have been determined using femtosecond pump-probe spectroscopy with polarized light. The values vary by over 3 orders of magnitude, suggesting the importance of tunneling. Good correlation exists between the values of the rates and the parameters characterizing the strength of two intramolecular hydrogen bonds: proton NMR shift, distance between the hydrogen-bonded nitrogen atoms, and the NH stretching frequency. While hydrogen-bond strength is the main factor determining the rate of double hydrogen transfer, other factors, such as static and dynamic symmetry breaking and the population of low-frequency vibrations also have to be taken into account.

Thermally and Vibrationally Induced Tautomerization of Single Porphycene Molecules on a Cu(110) Surface

We report the direct observation of intramolecular hydrogen atom transfer reactions (tautomerization) within a single porphycene molecule on a Cu(110) surface by scanning tunneling microscopy. It is found that the tautomerization can be induced via inelastic electron tunneling at 5K. By measuring the bias-dependent tautomerization rate of isotope-substituted molecules, we can assign the scanning tunneling microscopy-induced tautomerization to the excitation of specific molecular vibrations. Furthermore, these vibrations appear as characteristic features in the dI/dV spectra measured over individual molecules. The vibrational modes that are associated with the tautomerization are identified by density functional theory calculations. At higher temperatures above ∼75 K, tautomerization is induced thermally and an activation barrier of about 168meV is determined from an Arrhenius plot.

Controlling intramolecular hydrogen transfer in a porphycene molecule with single atoms or molecules located nearby

Although the local environment of a molecule can play an important role in its chemistry, rarely has it been examined experimentally at the level of individual molecules. Here we report the precise control of intramolecular hydrogen-transfer (tautomerization) reactions in single molecules using scanning tunnelling microscopy. By placing, with atomic precision, a copper adatom close to a porphycene molecule, we found that the tautomerization rates could be tuned up and down in a controlled fashion, surprisingly also at rather large separations. Furthermore, we extended our study to molecular assemblies in which even the arrangement of the pyrrolic hydrogen atoms in the neighbouring molecule influences the tautomerization reaction in a given porphycene, with positive and negative cooperativity effects. Our results highlight the importance of controlling the environment of molecules with atomic precision and demonstrate the potential to regulate processes that occur in a single molecule.

Accurate determination of the ultrafast proton transfer rate in porphycene using nuclear spin relaxation

The longitudinal (15)N relaxation rate and (15)N{(1)H} NOE enhancement were determined for porphycene in a CD2Cl2 solution. By invoking the rotational diffusion tensor of porphycene as previously determined, these data could be used to evaluate the tautomerization rate of protons in the NHN bridges. The proton transfer rate compares well to the results obtained by other authors by means of optical spectroscopy.

Corrole NH Tautomers: Spectral Features and Individual Protonation

Protonation of a free-base meso-pyrimidinyl-substituted AB(2)-corrole (H(3)AB(2)) in ethanol solution by stepwise addition of sulfuric acid has been studied in the temperature range from 293 to 333 K. The formation rate of protonated species was found to depend profoundly on the temperature at which the titration was undertaken. Two steps in the titration curve were identified at temperatures around 293-298 K, whereas one-step formation of protonated species was found to occur at temperatures above 308 K. The protonation product was the same in both cases, i.e., H(4)AB(2)(+) corrole, protonated at the macrocycle core nitrogen atoms. The two steps in the protonation kinetics at lower temperatures were attributed to protonation of individual tautomers of the free-base H(3)AB(2) corrole. To the best of our knowledge, this is the first well-illustrated (spectrophotometric) observation of individual properties of corrole NH tautomers in fluid solution. Concomitant increase in the NH tautomerization rate with increasing temperature is proposed to account for the one-step protonation. Evidences for the role of individual corrole NH tautomers in the protonation process as well as their optical features are discussed based on spectroscopic results and simulation data.

Unraveling the Fluorescence Features of Individual Corrole NH Tautomers

The fluorescence spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in the temperature range from 4.2 to 332 K. For the first time, the individual fluorescence profiles of the two corrole NH tautomers have been assigned over the whole temperature range. The pronounced temperature dependence of the fluorescence spectra of the meso-pyrimidinylcorrole under study was found to originate from switching between the fluorescence emissions of the two tautomers due to a reduced NH tautomerization rate with decreasing temperature. As a result, the long wavelength tautomer dominates the total emission spectrum at room temperature, whereas at low temperatures, the majority of the emission comes from the short wavelength tautomer. Energy level diagrams (involving the two NH tautomers) explaining the excitation energy deactivation channels in the meso-pyrimidinylcorrole at room temperature and below are presented. A significant H/D isotope effect on the NH tautomerization rate has been observed, resulting in an enhanced contribution of the short wavelength tautomer to the total fluorescence spectrum at the expense of that of the long wavelength tautomer. Substantially different fluorescence quantum yields have been determined for the individual NH tautomers, leading to a pronounced temperature dependence of the overall fluorescence quantum yield. The obtained results allow the unambiguous statement that the two NH tautomers of corroles coexist in fluid and solid solutions in a wide range of temperatures, with the proportion depending on the corrole substitution pattern. Moreover, this study shows that the (future) interpretation of the fluorescence properties of meso-pyrimidinylcorroles and all other corrole materials should be done (more) carefully, taking into account the coexistence of NH tautomers with individual spectral signatures.

Theoretical Studies for Excited-State Tautomerization in the 7-Azaindole–(CH3OH)n (n = 1 and 2) Complexes in the Gas Phase

The excited-state tautomerization of 7-azaindole (7AI) complexes bonded with either one or two methanol molecule(s) was studied by systematic quantum mechanical calculations in the gas phases. Electronic structures and energies for the reactant, transition state (TS), and product were computed at the complete active space self-consistent field (CASSCF) levels with the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The time-dependent density functional theory (TDDFT) was also used for comparison. The excited-state double proton transfer (ESDPT) in 7AI-CH(3)OH occurs in a concerted but asynchronous mechanism. Similarly, such paths are also found in the two transition states during the excited-state triple proton transfer (ESTPT) of the 7AI-(CH(3)OH)(2) complex. In the first TS, the pyrrole ring proton first migrated to methanol, while in the second the methanol proton moved first to the pyridine ring. The CASSCF level with the MRPT2 correction showed that the former path was much preferable to the latter, and the ESDPT is much slower than the ESTPT. Additionally, the vibrational-mode enhanced tautomerization in the 7AI-(CH(3)OH)(2) complex was also studied. We found that the excitation of the low-frequency mode shortens the reaction path to increase the tautomerization rate. Overall, most TDDFT methods used in this study predicted different TS structures and barriers from the CASSCF methods with MRPT2 corrections.

Bridging the Gap between Porphyrins and Porphycenes: Substituent‐Position‐Sensitive Tautomerism and Photophysics in meso‐Diphenyloctaethylporphyrins

2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin (2), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen-bonding strength as that in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen-bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.

Spectrophotometric and spectrofluorimetric investigation of different equilibria of a recently synthesized Schiff base with the aid of chemometric methods

In this study ground and excited states acidic dissociation constants of a recently synthesized Schiff base was obtained in a DMF:water mixture of 30:70 ( v/ v) using absorption and fluorescent spectra of the Schiff base in different pH values with the aid of chemometric methods. In addition, the fluorescent of the two kinds of tautomers of this Schiff base was investigated and the rate of tautomerization was obtained using rank annihilation factor analysis (RAFA). The effect of different kinds of surfactants such as sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and Triton X-100 on fluorescence spectrum of the Schiff base in a DMF:water mixture of 2:98 ( v/ v) was investigated. CTAB increased the fluorescence intensity of the Schiff base while SDS and Triton X-100 had no significant effect on it. β-Cyclodextrin increased the fluorescence intensity of the Schiff base. Also the sensing behavior of this Schiff base toward metal ions was studied in DMF and ethanol by fluorescence spectroscopy. The Schiff base showed prominent fluorescent signal in the presence of Zn 2+, whereas other metal ions failed to induce response and ground-state dissociation constant of the complex was determined by direct fluorimetric titration as a function of Zn 2+ concentration.

The Base-Catalyzed Keto−Enol Interconversion of 2-Nitrocyclohexanone in Ionic Liquids

While the solvent effect on the rate of tautomerization of 2-nitrocyclohexanone in several organic solvents appears to depend essentially on the permittivity of the solvent, a different behavior is observed in some ionic liquids. In particular the rate of the reaction in ionic liquids (ILs) is much faster than expected solely on the basis of the permittivity of the studied ILs. However, if more solvent parameters are taken into consideration (namely the polarizability, H-bond acidity, and cohesive pressure of the solvent) one comprehensive linear solvation energy relationship (LSER) for both organic and IL media can be obtained and no special "ionic liquid effect" can be highlighted. The kinetic results have also been discussed in terms of the Brønsted equation. It turns out that on passing from molecular solvents to ILs the transition state structure of the enolization reaction becomes markedly enolate-like (Brønsted beta value close to 1).

Tautomerization in Condensed Phases and in Isolated Molecules

AbstractWe review our recent investigations of systems revealing single or double proton (hydrogen) transfer in the ground or electronically‐excited states. The results obtained for condensed phases are compared with the data provided by experiments carried out in the regime of molecular isolation, mainly by using supersonic jet spectroscopy techniques.Both intra‐ and intermolecular processes have been studied. Four different bifunctional chromophores are discussed, all sharing a common feature of possessing a proton‐donating NH group and an acceptor nitrogen atom incorporated into a pyridine or pyrrolene ring.Porphycene and derivatives reveal, when isolated, coherent double hydrogen transfer manifested by vibration‐specific tunneling splittings. In condensed phases, tautomerization occurs as a rate process; its dynamics can be probed by techniques using polarized light. Huge dependence of the tautomerization rate on the strength of intramolecular hydrogen bonds has been detected.2‐(2′‐pyridyl)pyrrole provides a rare example of a molecule which undergoes both intramolecular excited‐state single proton transfer and a solvent‐assisted excited‐state double proton transfer reaction. Supersonic jet studies allowed us to identify the vibrational modes that enhance the tautomerization.Photoinduced intramolecular proton transfer in 7‐(2′‐pyridyl)indole is accompanied by twisting of the proton donor and acceptor subunits. The reaction occurs faster in the supersonic beam than in room temperature solutions.1H‐pyrrolo[3,2‐h]quinoline forms 1:1 and 1:2 complexes with alcohols and water. The stoichiometries and vibrational structure could be determined for jet‐isolated complexes. The 1:1 cyclic, doubly hydrogen‐bonded species reveal rapid photoinduced tautomerization both in condensed phases and when isolated in supersonic beams.

Ground‐ and Excited‐State Tautomerization Rates in Porphycenes

The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump-probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NHN distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and (1)H chemical shifts of the internal protons.

Thermodynamics and Kinetics of Intramolecular Water Assisted Proton Transfer in Na+-1-Methylcytosine Water Complexes

High-level ab initio predictions of the tautomerization equilibrium and rate constants of water-assisted proton transfer of 1-methyl-cytosine (MeC) to its MeC* imino tautomeric form in the presence of up to two water molecules (W) and the Na(+) cation were carried out. The calculated energy values were used to obtain the thermodynamic parameters and equilibrium concentration of MeC, its rare tautomer, and their complexes with up to two water molecules and the Na (+) cation. The rate constants for the tautomerization were obtained by using the instanton approach (a quasiclassical method based on the least-action principle). Hydration of MeC by one water molecule leads to an increase of the concentration of the MeC* tautomer in the equilibrium mixture and a decrease of the barrier of the MeC* formation (to 15.6 kcal/mol). If the Na(+) cation is present, the tautomeric form is much less favored, and the tautomerization barrier increases to 25.2 kcal/mol. It was found that MeC monohydrate has both the highest equilibrium (2.9 x 10(-2)) and rate (7.9 x 10(5) s(-1)) constants of tautomerization in comparison to the MeC*NaW and MeC*Na2W complexes containing the Na(+) cation. Moreover, this study also allows one to estimate the concentration of MeC present in the cell during DNA synthesis as the unwanted tautomer, which in forming a mismatched base pair can cause spontaneous point mutations. Kinetic simulations have demonstrated that the low values of equilibrium (10(-14)-10(-13)) and rate constants (10(-17)-10(-16) s(-1)) of tautomerization make contribution of the MeC*Na(+)W and MeC*Na(+)2W complexes to the point mutations in DNA unlikely. In contrast to these Na(+) complexes, MeC*W can reach an equilibrium concentration of 2.9 x 10(-2) within 10(-7) s; thus, it can increase the probability of the point mutations.

One-Electron Reduction of 8-Bromo-2-aminoadenosine in the Aqueous Phase: Radiation Chemical and DFT Studies of the Mechanism

Two tautomeric forms of one-electron oxidized 2-aminoadenosine (2AA) have been produced by reactions of hydrated electrons (e aq-) with 8-bromo-2-aminoadenosine (8-Br-2AA) at natural pH, whereas only one tautomer is formed by oxidation of 2AA. Tailored experiments by pulse radiolysis and time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations allowed the definition of the reaction mechanism in some detail. The electron adducts of 8-Br-2AA protonated at C8 eject Br- and produce the two short-lived tautomers (8 and 9). The first observable species decays by first-order kinetics to produce the second intermediate, which is also obtained by oxidation of 2AA by SO4*-. The rate of tautomerization (k taut = 4.5 x 104 s-1) is strongly accelerated by phosphate and is retarded in D2O (kinetic isotope effect 7). B1B95/6-31+G** calculations showed that the tautomerization is a water-assisted process. In acidic or basic solutions, the "instantaneous" formation of one-electron oxidized 2AA or its deprotonated forms has been produced by reactions of e aq- with 8-Br-2AA. gamma-Radiolysis of 8-Br-2AA in aqueous solutions followed by product studies led to the formation of 2AA as a single product.

Solvent effects on the rate of the keto–enol interconversion of 2-nitrocyclohexanone

The rates of tautomerization of 2-nitrocyclohexanone (2-NCH) have been measured spectrophotometrically at 25.0 +/- 0.1 degrees C in several organic aprotic solvents and their binary mixtures. In cyclohexane the reaction is effectively catalyzed by bases and inhibited by acids while the so-called "spontaneous reaction" appears essentially due to autocatalysis. Apparent second order rate constants (k(app)(B)) for the reaction catalyzed by triethylamine (TEA) and pyridine (Pyr) have been obtained. From the experimental k(app)(B) values rate constants for the enolization (k(1)(B)) and ketonization (k(-1)(B)) reactions have been calculated. A Kamlet-Taft type linear solvation energy relationship (LSER) adequately accounts for the observed solvent effects. Activation parameters for both reactions show that solvent effects are mainly entropic in origin and that there is a shift of the transition state from a ketone-like to an enol-like structure on passing from less to more polar solvents.

An evaluation of methyl 2‐oxocyclopentanecarboxylate as an iron(III) trap: food perspectives

SummaryThe 1,3‐ketoesters have great potential in the food industry, as they are suitable for trapping flavours because of their chelating effects. The results obtained for tautomerization rate constants, as a function of temperature, have allowed us to establish a fourth order enthalpic analytical equation of temperature, suggesting an overall entropic control of tautomerization. Iron, which is present as a trace element in foods and drinks, is of critical importance for the normal early growth and development of infants. The nutritional status of iron may affect the metabolism of other nutrients. In fact, iron chelates have significant beneficial effects in increasing iron bioavailability in human diets. As a result the abilities of methyl 2‐oxocyclopentanecarboxylate to chelate iron(III) in aqueous solutions was investigated. Thermodynamic and kinetic parameters associated with the double‐pathway mechanism of monochelate complexation have been determined and the advantages of kinetically and thermodynamically controlled processes are discussed.

Conservation of the Planar Chiral Information in the Tandem Oxy-Cope/Ene Reaction

[reaction: see text] We report the stereoselective synthesis of a Decalin unit using a tandem oxy-Cope/ene reaction. [3,3]-Shift of 1,2-divinylcyclohexenol 18 generates (1E, 3Z, 7E) cyclodecatrien-2-ol 23 which tautomerizes to ketone 21 (Figure 3). The chiral information embedded in 23 is transferred into the newly formed stereogenic carbon C6 in 21. The efficiency of the chirality transfer is a reflection of the difference in the rates of ring inversion versus tautomerization of 23. Ketone 21 undergoes a carbonyl-ene reaction to give Decalin 20. Assuming a rapid equilibrium between macrocyclic diastereomers, the diastereomeric ratio of the transannular ene reaction is governed by the Curtin-Hammett principle. Analysis of the conservation of the planar chiral information in the tandem oxy-Cope/ene reaction is presented.

Solvent effects versus concentration effects in determining rates of base-catalyzed keto-enol tautomerization

Solvent effects of homogeneous media (such as solvent–water mixtures) on chemical reactivity may be interpreted as due to solvent polarity and/or molecular structure of solvent molecules. In microheterogeneous media (such as aqueous micellar solutions), solvent effects on reaction rates must include concentration effects, in addition to changes in the solvent polarity of the micelle interface where the reaction is assumed to occur. In this work, we measured the rates of keto-enol tautomerization of the 2-acetylcyclohexanone (ACHE) and 2-acetyl-1-tetralone (ATLO) systems in dimethylsulfoxide (DMSO)–water mixtures and in aqueous micellar solutions with both anionic and cationic surfactants and in the presence of buffers. The results appear as an ideal framework to understand the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent polarity or even concentration effects.

Tautomerization of 2-acetylcyclohexanone in assemblies of cationic surfactants

The kinetic study of the keto-enol interconversion of 2-acetylcyclohexanone (ACHE) has been performed in organic solvents such as dimethylsulfoxide, 1-propanol, 2-propanol, methanol, dioxane, tetrahydrofuran and acetonitrile, as well as in aqueous micellar solutions of the cationic surfactants tetradecyltrimethylammonium chloride and tetradecyltrimethylammonium bromide at 25 °C. Either the solvent-assisted or H+-catalyzed reaction rates of tautomerization are reduced in both organic solvents and in micellar medium. In 70% v/v solvent–water or at fixed concentration of any cationic surfactant, the observed rate constant moderately increases with [H+]. The nitrosation of the enol has also been studied in micellar solutions of both cationic and anionic surfactants. Under experimental conditions of first-order dependence on [H+], [nitrite], and [X−] (X = Cl or Br), the presence of cationic micelles reduces strongly the rate of nitrosation, whereas in the presence of anionic micelles, the first-order rate constant, ko, goes through a maximum on increasing the surfactant concentration.