Probability Of Proton Transfer Research Articles

Theoretical Insights into Excited-State Intermolecular Proton Transfers of 2,7-Diazaindole in Water Using a Microsolvation Approach.

The detailed excited-state intermolecular proton transfer (ESInterPT) mechanism of 2,7-diazaindole with water wires consisting of either one or two shells [2,7-DAI(H2O)n; n = 1-5] has been theoretically explored by time-dependent density functional theory using microsolvation with an implicit solvent model. On the basis of the excited-state potential energy surfaces along the proton transfer (PT) coordinates, among all 2,7-DAI(H2O)n, the multiple ESInterPT of 2,7-DAI(H2O)2+3 through the first hydration shell (inner circuit) is the most easy process to occur with the lowest PT barrier and a highly exothermic reaction. The lowest PT barrier resulted from the outer three waters pushing the inner circuit waters to be much closer to 2,7-DAI, leading to the enhanced intermolecular hydrogen-bonding strength of the inner two waters. Moreover, on-the-fly dynamic simulations show that the multiple ESInterPT mechanism of 2,7-DAI(H2O)2+3 is the triple PT in a stepwise mechanism with the highest PT probability. This solvation effect using microsolvation and dynamic simulation is a cost-effect approach to reveal the solvent-assisted multiple proton relay of chromophores based on excited-state proton transfer.

Proton-transfer dynamics in ionized water chains using real-time time-dependent density functional theory

In density functional-theoretic studies of photoionized water-based systems, the role of charge localization in proton-transfer dynamics is not well understood. This is due to the inherent complexity in extracting the contributions of coupled electron-nuclear non-adiabatic dynamics in the presence of exchange and correlation functional errors. In this work, we address this problem by simulating a model system of ionized linear H-bonded water clusters using real-time Time Dependent Density Functional Theory (rt-TDDFT)-based Ehrenfest dynamics. Our aim is to understand how self-interaction error in semilocal exchange and correlation functionals affects the probability of proton transfer. In particular, we show that for H-bonded (H$_2$O)$_n^+$ chains (with $n>3$), the proton-transfer probability attains a maximum, becoming comparable to that predicted by hybrid functionals. This is because the formation of hemibonded-type geometries is largely suppressed in extended H-bonded structures. We also show how the degree of localization of the initial photo-hole is connected to the probability of a proton-transfer reaction, as well as to the separation between electronic and nuclear charge. These results are compared to those obtained with adiabatic dynamics where the initial wavefunction is allowed to relax to the ground state of the ion cluster, explaining why different functionals and dynamical approaches lead to quantitatively different results.

PROTON TRANSFER IN PHOSPHORUS-CONTAINING ACID–N,N-DIMETHYLFORMAMIDE SYSTEM WITH GLANCE OF ENVIRONMENT

Proton transfer processes in the molecular and ion-molecular complexes of phosphorus acids (phosphoric H3PO4, phosphorous H3PO3, methylphosphonic СН3Н2РО3) with N,N-dimethylformamide (DMF) was studied. The potential energy surface (PES) for proton transfer was studied using the B3LYP/6-31++G(d,p) level of theory, and the solvent effect (here DMF) on the PES was included using the conductor polarized continuum model (CPCM). For all cases, the energy profile for proton transfer represents a double well curve, if intermolecular O…Odistance for the hydrogen bond considered has a fixed length equal to 2.7 Å. The solvent effect favors a proton transfer in the molecular complexes, but no shift of the equilibrium towards ionic pairs is observed. As a result, the energy values associated with proton transfer are significantly reduced in comparison with those found for the gas phase. The proton transfer in the complexes of H3PO4 with DMF is more favored than this process for the cases with H3PO3 and СН3Н2РО3. The probability of proton transfer in the Н3РО4–DMF and (Н3РО4)2–DMF is nearly identical. On the contrary, the barrier height for transfer in Н3РО4–(DMF)n for n=1÷3 increases with increasing number of DMF molecules. The energy barrier for proton transfer in the DMFH+–DMF and H3PO4–H2PO4– is lower than the ones for the molecular complexes.

The effect of protic solvents on the excited state proton transfer of 3-hydroxyflavone: A TD-DFT static and molecular dynamics study

The effect of intermolecular hydrogen bonding played by protic solvents (ammonia, methanol and water) on the excited state proton transfer (ESPT) reaction of 3-hydroxyflavone (3HF) was theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The formation of intermolecular hydrogen bond induced by protic solvents indicates that the intramolecular hydrogen bond may be interrupted in favor of a complex causing low quantum yield of keto emission and exhibiting dual emission (both enol and keto) in experiment. The strengthening of intermolecular hydrogen bond in the S1 state has been confirmed by the red-shift of IR vibrational spectra and shorter bond distances involving proton transfer (PT) process in comparison with those of the S0 state. From potential energy curves (PECs) of PT coordinate, PT process is likely to proceed in S1 state and PT in 3HF(NH3) occurs more easily than those of 3HF(CH3OH) and 3HF(H2O) due to its lower barrier. Moreover, on-the-fly dynamics simulations of all complexes were carried out to provide the detailed information on the PT mechanism. The dynamic results show that ESPT process of 3HF with protic solvent takes place through intermolecular hydrogen bond with slower PT time (259, 117 and 104fs for 3HF(NH3), 3HF(CH3OH) and 3HF(H2O), respectively) than that of 3HF (76fs) via intramolecular hydrogen bond. Furthermore, the ultrafast PT time is found to be nicely correlated with polarity of solvent and PT probability is also anti-correlated with PT barrier.

Theoretical study on influence of geometry controlling over the excited-state intramolecular proton transfer of 10-hydroxybenzo[h]quinoline and its derivatives

A structural modification on quinoline as a proton donor of 10-hydroxybenzo[h]quinoline (HBQ) giving different HBQ derivatives greatly affects their photophysical properties. In this study, the excited-state intramolecular proton transfer (ESIPT) reactions of HBQ and its derivatives with different geometries have been systematically investigated using DFT and TD-DFT at B3LYP/TZVP. Calculated absorption and emission spectra are used to describe the photophysical changes in which the absorption spectra of HBQ derivatives are blue-shifted compared with that of HBQ while their emission spectra are blue-shifted except those of 3,4-dihydro indene[1,2-b]pyrrole-8-ol (IPRO) and 2-(4H-pyrrol-2-yl)phenol (PRP) compounds with different proton donor and connecting moiety are red-shifted. From results of potential energy curves along the proton transfer (PT) coordinate, PT is favorable in the excited-state but not in the ground state. On-the-fly dynamics simulations in the excited-state are further employed to determine reaction mechanisms and the time evolution of PT. The ESIPT process easily occurs in most of the compounds except the IPRO with much high PT barrier. The ESIPT times in most compounds take place within 100fs and PT probability is nicely anti-correlated with the PT barrier. Thus, the geometry changes alter the electronic spectra but do not affect ESIPT of HBQ derivatives. Moreover, once the PT is complete, the internal conversion is initiated by twisted skeleton, leading to lower intensity of tautomer emission.

Ab initio molecular dynamics study of H-bonding and proton transfer in the phosphoric acid-N,N-Dimethylformamide system.

Car-Parrinello molecular dynamics simulations of phosphoric acid (H3PO4)-N,N-dimethylformamide (DMF) mixtures over the whole composition range have been carried out. It has been found that the neutral molecules are the dominant species in this system. The concentration dependences of the average number of H-bonds per proton acceptor atom in P=O and C=O groups as well as per proton donor atom in DMFH+ ions towards phosphate species have been discussed. The H-bonding between components in all investigated mixtures of H3PO4 and DMF is possible. A significant fraction of the protonated DMF forms appears at phosphoric acid mole fraction higher than 0.37, indicating a high probability of proton transfer from phosphate species to oxygen atoms in C=O groups. The intermolecular proton transfer between phosphate species themselves is mainly observed when xH3PO4 > 0.19. Satisfactory agreement with available experimental data for structural characteristics of the investigated system was obtained.

Chemical dynamics simulations of the monohydrated OH(-)(H2O) + CH3I reaction. Atomic-level mechanisms and comparison with experiment.

Direct dynamics simulations, with B97-1/ECP/d theory, were performed to study the role of microsolvation for the OH(-)(H2O) + CH3I reaction. The SN2 reaction dominates at all reactant collision energies, but at higher collision energies proton transfer to form CH2I(-), and to a lesser extent CH2I(-) (H2O), becomes important. The SN2 reaction occurs by direct rebound and stripping mechanisms, and 28 different indirect atomistic mechanisms, with the latter dominating. Important components of the indirect mechanisms are the roundabout and formation of SN2 and proton transfer pre-reaction complexes and intermediates, including [CH3--I--OH](-). In contrast, for the unsolvated OH(-) + CH3I SN2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH(-)(H2O) + CH3I SN2 reaction are in good agreement with experiment with respect to reaction rate constant, product branching ratio, etc. Differences between simulation and experiment are present for the SN2 velocity scattering angle at high collision energies and the proton transfer probability at low collision energies. Equilibrium solvation by the H2O molecule is unimportant. The SN2 reaction is dominated by events in which H2O leaves the reactive system as CH3OH is formed or before CH3OH formation. Formation of solvated products is unimportant and participation of the (H2O)CH3OH---I(-) post-reaction complex for the SN2 reaction is negligible.

'Uncertainty' region of acid-base ΔpKa: Salt versus co-crystal formation

The hydrogen bond is a very important interaction which demonstrates a wide variety in strength and shape and can be held as the central point in the crystal engineering process of a great number of compounds. In the formation of salts versus co-crystals, pH control is necessary in order to obtain one form or the other. In the pharmaceutical industry which is actively developing Active Pharmaceutical Ingredients (APIs) with desirable properties, such consideration is of major concern. In 2012, Cruz-Cabeza[1] carried out a survey of over 6 000 structures and found a linear relationship between ΔpKa and the probability of proton transfer for the pKa range of -1 to +4. The cross-over point was determined to be at ΔpKa = 1.3. Beyond this point the probability to obtain a salt rather than a co-crystal is higher than 50%. Parallel to this work, Gilli & Gilli[2] have built a sliding rule which predicts the strength of a Donor – Acceptor hydrogen bond based on the ΔpKa of the system. Their device predicts the formation of salts if ΔpKa > 3, and co-crystals when ΔpKa < -3. In their paper, Gilli & Gilli point out that the interval of ΔpKa matching should be shifted by 1.5 units when interpreting crystal structures,[3] which agrees with the findings of Cruz-Cabeza. In this work we discuss the bonding of 22 structures made up of acid-base pairs which either form salts or co-crystals. A series of five substituted benzoic acids with 10 substituted pyridines and quinolines have been crystallized so that their ΔpKa, defined as pKa(base) – pKa(acid), spans the range -1.14 to +4.16. This covers the `uncertainty' region for the formation of salt versus co-crystals. Although most of our results confirmed that structure formation of co-crystal versus salt parts at ΔpKa ≍ 2, we report here a structure that does not follow the general rule and serves as a cautionary tale.

Acid–base crystalline complexes and the pKa rule

Differences in the predicted aqueous pKa values (ΔpKa) have been calculated for 6465 crystalline complexes containing ionised and non-ionised acid–base pairs in the Cambridge Structural Database. A linear relationship between ΔpKa and the probability of proton transfer between acid–base pairs has been derived for crystalline complexes with ΔpKa between −1 and 4. The pKa rule is validated and quantitated.

Excited-State Intermolecular Proton Transfer Reactions of 7-Azaindole(MeOH)n (n = 1–3) Clusters in the Gas phase: On-the-Fly Dynamics Simulation

Ultrafast excited-state intermolecular proton transfer (PT) reactions in 7-azaindole(methanol)(n) (n = 1-3) [7AI(MeOH)(n=1-3)] complexes were performed using dynamics simulations. These complexes were first optimized at the RI-ADC(2)/SVP-SV(P) level in the gas phase. The ground-state structures with the lowest energy were also investigated and presented. On-the-fly dynamics simulations for the first-excited state were employed to investigate reaction mechanisms and time evolution of PT processes. The PT characteristics of the reactions were confirmed by the nonexistence of crossings between S(ππ*) and S(πσ*) states. Excited-state dynamics results for all complexes exhibit excited-state multiple-proton transfer (ESmultiPT) reactions via methanol molecules along an intermolecular hydrogen-bonded network. In particular, the two methanol molecules of a 7AI(MeOH)(2) cluster assist the excited-state triple-proton transfer (ESTPT) reaction effectively with highest probability of PT.

Effects of hydration level, temperature, side chain and backbone flexibility of the polymer on the proton transfer in short-side-chain perfluorosulfonic acid membranes at low humidity conditions

Ab initio molecular dynamics (MD) simulations are done to elucidate the electronic structure properties of a short-side-chain perfluorosulfonic acid (SSC PFSA) membrane at low humidity conditions. The artificial neural network (ANN) approach along with statistical methods is then employed to model and analyze these properties. The ANN method substantially speeds up the ab initio electronic structure calculations and has superior accuracy in mimicking the results of such calculations. The aim of this study is to understand the effects of hydration level, temperature, side chain flexibility of the SSC PFSA membranes, and backbone flexibility of the SSC PFSA membranes on the proton transfer in these membranes. Statistical analysis of results using analysis of means (ANOM) and analysis of variance (ANOVA) methods shows that no proton transfer from the SSC PFSA to the neighboring water molecules occurs at considerably low hydration levels. However, an increase in the probability of proton transfer is observed when the SSC PFSA membrane is sufficiently saturated. This process is more favorable at low temperatures. Moreover, the flexibility of either the side chains or the backbone of the SSC PFSA membrane has a great influence on the proton transfer phenomenon in such a way that allowing them to move freely causes an increase in the affinity of the SSC PFSA membrane to share its protons with water molecules. Further investigation is performed concerning the combined effect of the independent parameters (i.e., hydration level, temperature, side chain flexibility of the SSC PFSA membrane, and backbone flexibility of the SSC PFSA membrane) on the proton transfer process and the results are reported in detail.

Fluorescent probe based on proton transfer in 4′-(diethylamino)-3-hydroxyflavone: Spectral and luminescent characteristics upon selective excitation

The spectral characteristics of acetonitrile solutions of 4′-(diethylamino)-3-hydroxyflavone dye with dual fluorescence are studied under selective excitation. This dye is a structural analog of 3-hydroxyflavone and exhibits excited-state proton transfer, which, in contrast to 3-hydroxyflavone, has a thermodynamic rather than a kinetic character. The fluorescence spectra at different excitation photon energies and the excitation spectra of different fluorescence bands are studied. It is found that the intensity ratio of the normal and tautomeric fluorescence bands lying near 507 and 570 nm, respectively, depends on the excitation wavelength, namely, this ratio is 1.45 and almost does not change in the region of the main absorption band (370–420 nm), while, in the region of the second singlet band (near 280 nm), it decreases to 1.15. This can be explained by an increase in the probability of proton transfer with formation of a tautomeric form in the case of excitation into the second band. Another interesting feature is the existence of a latent third emission band peaked at 535 nm, which was found and reliably recorded upon excitation at wavelengths of 470–500 nm. Addition of water quenches this emission, which indicates that it belongs to the anionic form of the dye.

On the Mechanism of Electron-Capture-Induced Dissociation of Peptide Dications from 15N-Labeling and Crown-Ether Complexation

15N-labeling of di- and tripeptides reveals that electron capture to doubly protonated peptides results almost exclusively in ammonia loss from the N-terminal end, which clearly shows that a significant fraction of electron capture occurs at this end. In accordance with this finding, the competing channel of N-Calpha bond breakage leads to z+* ions and neutral c fragments after electron capture to small dications. In larger peptides that live long enough for internal proton exchanges to occur, c+ ions are also formed and in some cases in dominant yield. Attachment of one or two crown ethers to ammonium groups is likely to reduce the probability of proton transfer, which enhances the formation of z+* relative to c+. The total yield of z+* and c+ is, however, more or less unchanged, which indicates that proton transfer or hydrogen transfer from a NH3 group to the amide group is not required for the N-Calpha bond breakage.

Photoionization of methanol dimer using a tunable vacuum ultraviolet laser

Methanol dimer (CD3OH)2 ion–molecule reaction is initialized by VUV (vacuum ultraviolet) laser photoionization. The proton and deuteron transfers are the dominant reactions. The relative probabilities of deuteron transfer from the methyl group and proton from the hydroxyl group were measured as a function of VUV photon energy between 10.91 to 10.49 eV. According to those results, the probability of proton transfer from the hydroxyl group increases with the VUV photon energy. Isotopic scrambling is not complete before dissociation of the ion complex in the photon energy used. In addition, ab initio calculations are performed and four stable structures of the methanol dimer ion are found. One of these structures is an unreported complex, CD3OHD+⋯CD2OH, which has a very unusual type of hydrogen bond. This complex plays a significant role in the deuteron transfer reaction in the range of excitation energies used in this study.

The ion–molecule reaction after multiphoton ionization in the binary cluster of ammonia and methanol

The binary cluster (CH 3OH) n (NH 3) m was studied by using a multiphoton ionization time-of-flight mass spectrometer (MPI-TOFMS). The measured two series of protonated cluster ions: (CH 3OH) n H + and (CH 3OH) n NH 4 + (1≤ n≤14) were attributed to the ion–molecule reaction in the binary cluster ions. The mixed cluster of CH 3OD with ammonia was also studied. The results implied that the proton transfer probability from the OD group was larger than that from CH 3 group. The ab initio calculation of the binary cluster was carried out at the HF/STO-3G and MP2/6-31G ∗∗ levels of theory, and indicated that the latter process of the proton transfer must overcome a barrier of ∼29 kcal/mol.

Fluorescence of Retinal Schiff Base in Alcohols

Spectra, quantum yields, and kinetics of the unprotonated n-butyl Schiff base of retinal (SBR) fluorescence are investigated in n-alcohols at room temperature. While the absorption spectra in H-bonding solvents are only slightly shifted to longer wavelengths compared to the absorption in hexane, the emission spectra are strongly influenced by H-bond formation. The radiative S1 state lifetimes are found to be in the 100−200 ns range, indicating that the S1 → S0 transition is strongly forbidden. With decreasing polarity the rate of the S1 state decay decreases and in pure alcohols the rate exhibits an exponential dependence on the dielectric constant. Semiempirical calculations indicate a larger probability of proton transfer from alcohol to SBR in the excited state than in the ground state, which may result in protonation of the SBR upon excitation. The theoretical model also shows that the polar environment increases the probability of such a proton transfer from alcohol to SBR. The observed experimental ...

Effect of nuclear absorption on nucleon transfer probabilities in heavy-ion reactions.

We perform a classical dynamics study of the proton transfer reactions measured for the $^{12}\mathrm{C}$${+}^{197}$Au and $^{16}\mathrm{O}$${+}^{197}$Au systems. The nuclei are assumed to move along classical trajectories, and the proton transfer probability is considered as a tunneling process around the point of closest approach. The absorption due to the imaginary part of the optical potential also is included. At the highest energies considered, several trajectories contribute to each scattering angle. The contributions associated to the different trajectories are added to obtain the proton transfer probability. We find that, for a properly selected value of the strength parameter in the imaginary part of the optical potential, the theoretical results fit adequately the experimentally measured values. \textcopyright{} 1996 The American Physical Society.

Quantum effects in many-body transport theory

Quantum extension of the many-body transport theory in the Wigner representation formalism is considered. Satisfactory description of the ground state of nuclei recovering the super-shell structure is obtained. The method is applied to analyse the role of quantum effects in low-energy nucleus-nucleus collisions. The dynamics of super-heavy systems formation and recently reported anomalies in proton transfer probability in heavy-ion reactions are discussed.

Double proton transfer and charge-transfer transitions in biological hydrogen-bonded systems guaninecytosine (GC) and GCMg 2+ systems

Potential energy curves for intermolecular proton transfer have been calculated within a modified INDO scheme. Analysis of the nature of the excited states, with emphasis on charge-transfer transitions, has been performed. The proton-transfer probability was found to be markedly greater in some excited states than for the ground state. With the metal ions the double proton transfer should be most effective in charge-transfer states.

Potential energy curves in complementary base pairs and in model hydrogen bonded systems

AbstractThe potential energy curves for the intermolecular proton transfer in complementary base pairs and respective hydrogen bonded model systems have been studied at the semiempirical all‐valence and ab initio STO‐3G level. The proton transfer probability was found to be markedly greater in excited electronic states than that of the ground state. Substantial lowering of the potential barrier height for the coupled double proton transfer in model (HCOOH)2 system has been observed when the dynamics of zero‐point vibrational deformations of the heavy atom skeleton and the environmental effects were taken into account in nonempirical STO‐3G calculations. The results of quantum‐chemical calculations indicate on the possibility of the double proton transfer in complementary base pairs corroborating the submolecular Löwdin hypothesis on the mechanism of spontaneous mutations.