Proton Transfer In Hydrogen Bonds Research Articles

Hydrogen bonding network in the electrical double layer mediates fast proton skipping to Get Rid of Fenton reaction pH dependence enables highly efficient alkaline water purification

A fundamental scientific quandary in environmental has always been the pH dependence in Fenton reaction, that is, the reaction kinetics decreases by approximately 1–3 orders of magnitude upon transitioning from an acidic to an alkaline state. Here, we discovered that protons significantly contribute to the Fenton reaction through an examination of the reaction's interfacial behavior characteristics. Proton transfer mediated by hydrogen bond network connectivity in the electric double layer is responsible for the pH dependence of Fenton reaction kinetics. On this basis, the surface potential of Fenton reaction catalyst was modified to optimize the distribution of H2O in the electrical double layer, enhancing the connectivity of the interfacial hydrogen bond network and providing a fast channel for rapid proton transfer. The consecutive hydrogen bond network mediates rapid proton hopping, increasing the proton concentration in the Helmholtz layer, and consequently promoting the proton conductivity from 5.38×10−7 S·cm−2 to 4.35×10−6 S·cm−2 in alkaline conditions for Fenton reaction. Meanwhile, the kinetics reaction rate was improved 20 times, and the pH dependence was reduced from 70.9 % to 12.6 %. This discovery clarifies the key role of the interfacial hydrogen bond network and proton transfer in Fenton reaction kinetics pH dependence. It provides new theories and methods for achieving alkaline high Fenton reaction kinetics.

Very Strong Hydrogen Bond in Nitrophthalic Cocrystals.

This work presents the studies of a very strong hydrogen bond (VSHB) in biologically active phthalic acids. Research on VSHB comes topical due to its participation in many biological processes. The studies cover the modelling of intermolecular interactions and phthalic acids with 2,4,6-collidine and N,N-dimethyl-4-pyridinamine complexes with aim to obtain a VSHB. The four synthesized complexes were studied by experimental X-ray, IR, and Raman methods, as well as theoretical Car-Parrinello Molecular Dynamics (CP-MD) and Density Functional Theory (DFT) simulations. By variation of the steric repulsion and basicity of the complex' components, a very short intramolecular hydrogen bond was achieved. The potential energy curves calculated by the DFT method were characterized by a low barrier (0.7 and 0.9 kcal/mol) on proton transfer in the OHN intermolecular hydrogen bond for 3-nitrophthalic acid with either 2,4,6-collidine or N,N-dimethyl-4-pyridinamine cocrystals. Moreover, the CP-MD simulations exposed very strong bridging proton dynamics in the intermolecular hydrogen bonds. The accomplished crystallographic and spectroscopic studies indicate that the OHO intramolecular hydrogen bond in 4-nitrophthalic cocrystals is VSHB. The influence of a strong steric effect on the geometry of the studied cocrystals and the stretching vibration bands of the carboxyl and carboxylate groups was elaborated.

Effects of hydroxychromone rings with chalcogen atoms on ESIPT for 3-hydroxy-2-(5-(5-(5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)-4H-chromen-4-one derivatives: A theoretical study

Inspired by fascinating photochemical and photophysical characteristics of novel organic molecules with chalcogenide substitution, this study focuses on exploring the effects of atomic electronegativity of chalcogens (O, S, and Se) on hydrogen bond interactions and proton transfer reaction mechanisms for 3-hydroxy-2-(5-(5-(5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)-4H-chromen-4-one (FT). By comparing structural changes and infrared (IR) spectra, combined with preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we conclude S1-state hydrogen bond is strengthened, which is favorable for occurrence of ESIPT reaction. Charge recombination and constructed potential energy surfaces (PESs) reveal atomic electronegativity has a regulatory effect on ESIPT behavior for FT derivatives.

Effects of substituted chalcogen atoms on excited state reaction for 1-(benzo[d]thiazol-2-yl)-6-substituted-naphthalen-2-ol derivatives

Inspired by the brilliant photophysical properties of novel organic molecules containing chalcogenide substitution, in this work, effects of atomic electronegativity of chalcogen (O, S and Se) on hydrogen bond interactions and proton transfer (PT) reaction are focused. We present the characteristic CN-BTN is the main objective to explore the influence of atomic electronegativity on the hydrogen bond interaction and excited state intramolecular proton transfer (ESIPT) behaviour by photoexcitation. By comparing the structural changes and infrared (IR) vibrational spectra of the CN-BTN derivatives (CN-BTN-O, CN-BTN-S and CN-BTN-Se) fluorophores in S0 and S1 states, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that hydrogen bond is strengthened in S1 state, which is favourable for the occurrence of ESIPT reaction. The charge recombination behaviour of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimisation and searching transition state (TS) configuration, we finally clarify the ESIPT mechanism for CN-BTN derivatives. Specially, we confirm change of atomic electronegativity has a regulatory effect on the ESIPT behaviour of CN-BTN derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.

Design and characterization of a binary CT complex of imidazole-oxyresveratrol: exploring its pharmacological and computational aspects

A new binary charge transfer (CT) complex between imidazole (IMZ) and oxyresveratrol (OXA) was synthesized and characterized experimentally and theoretically. The experimental work was carried out in solution and solid state in selected solvents such as chloroform (CHL), methanol (Me-OH), ethanol (Et-OH), and acetonitrile (AN). The newly synthesized CT complex (D1) has been characterized by various techniques such as UV-visible spectroscopy, FTIR, 1H-NMR, and powder-XRD. The 1:1 composition of D1 is confirmed by Jobs’ method of continuous variation and spectrophotometric (at λmax 554 nm) methods at 298 K. The infrared spectra of D1 confirmed the existence of proton transfer hydrogen bond beside charge transfer interaction. These findings indicate that the cation and anion are joined together by the weak hydrogen bonding as N+–H–O–. Reactivity parameters strongly recommended that IMZ behaves as a good electron donor and OXA an efficient electron acceptor. Density functional theory (DFT) computations with basis set B3LYP/6-31G (d,p) was applied to support the experimental results. TD-DFT calculations gives HOMO (−5.12 eV) → LUMO (−1.14 eV) electronic energy gap ( Δ E ) to be 3.80 eV. The bioorganic chemistry of D1 was well established after antioxidant, antimicrobial, and toxicity screening in Wistar rat. The type of interactions between HSA and D1 at the molecular level was studied through fluorescence spectroscopy. Binding constant along with the type of quenching mechanism, was investigated through the Stern-Volmer equation. Molecular docking demonstrated that D1 binds perfectly with human serum albumin and EGFR (1M17) and exposes free energy of binding (FEB) values of −295.2 and −283.3 kcal/mol, respectively. The D1 fits successfully into the minor groove of HAS and 1M17, the results of molecular docking show that the D1 binds perfectly with the HAS and 1M17, the higher value of binding energy shows stronger interaction between HAS and 1M17 with D1. Our synthesized complex shows good binding results with HAS compared to 1M17. Communicated by Ramaswamy H. Sarma

Antioxidant activity and enzymatic of lichen substances: A study based on cyclic voltammetry and theoretical

The antioxidant activity of nine lichen substances, including methylatrarate (1), methyl haematommate (2), lobaric acid (3), fumarprotocetraric acid (4), sphaerophorin (5), subsphaeric acid (6), diffractaic acid (7), barbatolic acid (8) and salazinic acid (9) has been determined through cyclic voltammetry. The compounds 1–4 presented slopes close to the Nernst constant of 0.059 V, indicating a 2H+/2e− relation between protons and electrons, as long as the compounds 5, 6, 7, 8, and 9 present slopes between 0.037 V and 0.032 V, indicating a 1H+/2e− relation between protons and electrons. These results show a high free radical scavenging activity by means of the release of H+, suggesting an important antioxidant capacity of these molecules. Theoretical calculations of hydrogen bond dissociation enthalpies (BDE), proton affinities (PA), and Proton Transfer (PT) mechanisms, at M06-2x/6-311+G(d,p) level complement the experimental results. Computations support that the best antioxidant activity is obtained for the molecules (3, 4, 5, 6, 7 and 8), that have a carboxylic acid group close to a phenolic hydroxyl group, through hydrogen atomic transfer (HAT) and sequential proton loss electron transfer (SPLET) mechanisms. Additional computations were performed for modelling binding affinity of the lichen substances with CYPs enzymes, mainly CYP1A2, CYP51, and CYP2C9*2 isoforms, showing strong affinity for all the compounds described in this study.

Model for vibrationally enhanced tunneling of proton transfer in hydrogen bond

Theoretical analysis of the effect of an external vibration on proton transfer (PT) in a hydrogen bond (HB) is carried out. It is based on the two-dimensional Schrödinger equation with trigonometric double-well potential. Its solution obtained within the framework of the standard adiabatic approximation is available. An analytic formula is derived that provides the calculation of PT rate with the help of elements implemented in Mathematica. We exemplify the general theory by calculating PT rate constant for the intermolecular HB in the Zundel ion H5O2+ (oxonium hydrate). This object enables one to explore a wide range of the HB lengths. Below some critical value of the frequency of the external vibration the calculated PT rate yields extremely rich resonant behavior (multiple manifestations of bell-shaped peaks). It takes place at symmetric coupling of the external vibration to the proton coordinate. This phenomenon is absent for anti-symmetric and squeezed mode couplings.

Dielectric, electro-optical and viscoelastic properties of nematic phase driven by hydrogen-bonding between cyano group and fluoroalkoxybenzoic acid

ABSTRACT Hydrogen-bonded liquid crystals (HBLC) formed by mixing 4-alcyloxy-2-fluorobenzoic acid (nOBAF) and the 4-pentyl-4-biphenylcarbonitrile (5CB) at (1:1), (1:3) and (3:1) (mol/mol) ratios have been investigated. Four HBLC series are prepared by changing the carbon chain length from 7 to 10 designated 5CB/nOBAF. The fluorobenzoic acids (nOBAF) are the proton donors while the pyridine unit (5CB) is the proton acceptors. Thermal behaviour and phase diagrams of the individual series have been established using differential scanning Calorimetry (DSC) and polarising optical microscopy (POM). It was found that all complexes exhibit nematic (N) and smectic G (SmG) phases. For the nematic phase, the dielectric anisotropy was increased while the thresholds voltage was decreased. This can be explained by the strong longitudinal dipole moment of the nematic phase compared to the nOBAF compounds. Furthermore, as the alkoxy chain length increases, the birefringence, response time, rotational viscosity, and splay elastic constant all decrease by the way of the reduction of the order parameter for the longer chains. In addition, hydrogen-bonded proton transfer between cyano and carboxylic acid groups have been explored by Fourier transform infrared spectroscopy (FTIR) for the first time.

A phosphonic acid anion and acid dimer dianion stabilized by proton transfer in OHN hydrogen bonds - models of structural motifs in blend polymer membranes.

The structure of intermolecular hydrogen-bonded complexes formed between tert-butylphosphonic acid and trimethylpyridine molecules has been experimentally studied as the simplest model system of the structural motifs in blend proton-conducting polymer membranes based on phosphonic acid residues. The stoichiometry of the formed complexes and proton positions in OHO and OHN hydrogen bonds were established by the H/D isotope effects and temperature dependences of the signals in 1H and 31P NMR spectra. Two structural motifs, namely, 1 : 2 and 2 : 2 acid-base complexes, were identified at the low temperature in a polar aprotic environment. In the 1 : 2 complex, one proton has passed through the hydrogen bond center creating a chain of two cooperatively coupled OHN bonds, while in the 2 : 2 complex both OHN bonds are zwitterionic and anti-cooperatively coupled to each other via a dianionic cyclic dimer of phosphonic acid in the middle. The dianionic cyclic dimer is metastable by itself, but under the used experimental conditions it is stabilized by complexation with two trimethylpyridinium cations. Additionally, quantum chemical calculations using the DFT method were carried out to support the experimental data.

The influence of base pair tautomerism on single point mutations in aqueous DNA.

The relationship between base pair hydrogen bond proton transfer and the rate of spontaneous single point mutations at ambient temperatures and pressures in aqueous DNA is investigated. By using an ensemble-based multiscale computational modelling method, statistically robust rates of proton transfer for the A:T and G:C base pairs within a solvated DNA dodecamer are calculated. Several different proton transfer pathways are observed within the same base pair. It is shown that, in G:C, the double proton transfer tautomer is preferred, while the single proton transfer process is favoured in A:T. The reported range of rate coefficients for double proton transfer is consistent with recent experimental data. Notwithstanding the approximately 1000 times more common presence of single proton transfer products from A:T, observationally there is bias towards G:C to A:T mutations in a wide range of living organisms. We infer that the double proton transfer reactions between G:C base pairs have a negligible contribution towards this bias for the following reasons: (i) the maximum half-life of the G*:C* tautomer is in the range of picoseconds, which is significantly smaller than the milliseconds it takes for DNA to unwind during replication, (ii) statistically, the majority of G*:C* tautomers revert back to their canonical forms through a barrierless process, and (iii) the thermodynamic instability of the tautomers with respect to the canonical base pairs. Through similar reasoning, we also deduce that proton transfer in the A:T base pair does not contribute to single point mutations in DNA.

A few insensitive energetic nitrogen rich compounds composed of substituted azoles and 3,5-dinitrobenzoic acid: Synthesis, characterization, physicochemical, detonation properties and pyrolytic products

Hydrogen bonded proton transfer crystals 1–4 were synthesized, using substituted azoles and 3,5-dinitrobenzoic acid, in a ratio 1:1 and ethanol as solvent at room temperature. The structure elucidation for the synthesized energetic crystals was carried out using spectral techniques such as, IR, 1H, and 13C NMR spectroscopy. The formation of proton transfer complexes occurred due to the transfer of lone pair of electrons from the donor azoles such as 3-nitropyrazole, 1,2,4-triazole, 3-amino-1,2,4-triazole, and 5-aminotetrazole to the acceptor 3,5-dinitrobenzoic acid through N+-H…O− type of hydrogen bond and was ascertained by X-ray crystallographic and proton NMR studies. Mathematical calculations propounded by various scientists are used to determine the explosive properties. The thermal stability was carried out using DSC-TGA studies. Sensitivity parameters were also calculated for the synthesized compounds. The results indicated that the proton transfer complexes are more stable, insensitive to impact, have high energetic parameters and thus classified into the category of High-energy-density materials (HEDMs). The high nitrogen content compounds gives out nitrogen gas (N2) as major detonation product, which will burn cleanly, producing less carbon monoxide and paving the way for new generation of environmentally compatible energetic materials.

Effect of nitrogen substitution and π-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation

Effects of nitrogen substitution and π-conjugation on the strength of intramolecular hydrogen bond (H-bond), photophysical properties, and excited-state intramolecular proton transfer (ESIPT) reactions of MS derivatives have been systematically investigated using DFT and TD-DFT methods at the B3LYP/TZVP level. The H-bond strength of all studied compounds becomes stronger in the S1 than in the S0. This result is confirmed by vibrational IR spectra and topological analysis. The H-bond strength of nitrogen substituted compounds is lower than those of compounds with the π-extended conjugation. For photophysical properties, the HOMO-LUMO energy gaps in normal and tautomer forms of nitrogen substituted compounds are insignificantly increased, whereas those of compounds with the π-extended conjugation are notably decreased. The slight widening of the energy gaps results in the slight blue-shifts of the absorption and emission peaks of nitrogen substituted compounds compared to those of MS, whereas the significant narrowing of the energy gaps makes the incredible red-shifts of both electronic spectra of compounds with the π-extended conjugation. Thus, remarkably large Stokes shifts of about 1000 cm−1 are observed for compounds with the π-extended conjugation (M3HN, M2HQC, and M3HQC). In addition, potential energy curves along the PT reaction indicate that the ESIPT of all studied compounds should occurred due to low PT barriers in the exothermic reaction. Dynamic simulations reveal that the ESIPT reactions of studied compounds are observed in an ultrafast time scale (38–130 fs). Most of them have remarkably high PT probabilities (0.84–1.00) except M3HP (0.56), which are in good agreement with the results of H-bond strength. Therefore, the photophysical properties of studied compounds can be significantly tuned by controlling the effect of π-conjugation.

Elucidating Solvent Effects on Strong Intramolecular Hydrogen Bond: DFT-MD Study of Dibenzoylmethane in Methanol Solution.

The dynamic aspect of solvation plays a crucial role in determining properties of strong intramolecular hydrogen bonds since solvent fluctuations modify instantaneous hydrogen-bonded proton transfer barriers. Previous studies pointed out that solvent-solute interactions in the first solvation shell govern the position of the proton but the ability of the electric field due to other solvent molecules to localize the proton remains an important issue. In this work, we examine the structure of the O-H⋅⋅⋅O intramolecular hydrogen bond of dibenzoylmethane in methanol solution by employing density functional theory-based molecular dynamics and quantum chemical calculations. Our computations showed that homogeneous electric fields with intensities corresponding to those found in polar solvents are able to considerably alter the proton transfer barrier height in the gas phase. In methanol solution, the proton position is correlated with the difference in electrostatic potentials on the oxygen atoms of dibenzoylmethane even when dibenzoylmethane-methanol hydrogen bonding is lacking. On a timescale of our simulation, the hydrogen bonding and solvent electrostatics tend to localize the proton on different oxygen atoms. These findings provide an insight into the importance of the solvent electric field on the structure of a strong intramolecular hydrogen bond.

Synthesis, spectroscopic studies and DFT/TD-DFT/PCM calculations of molecular structure, spectroscopic characterization and NBO of charge transfer complex between 5-amino-1,3-dimethylpyrazole (5-ADMP) with chloranilic acid (CLA) in different solvents

New charge transfer complex between the n- and π-donor 5-amino-1,3-dimethyl pyrazole (5-ADMP) with the π-acceptor chloranilic acid was synthesized and characterized experimentally and theoretically. The experimental work was carried out in solution and solid state. The selected solvents were chloroform (CHL), methanol (MeOH), ethanol (EtOH) and acetonitrile (AN). upon mixing the donor with acceptor a change in color takes place immediately to pink one with absorption maxima at λmax 530, 531, 530 and 522 nm in CHL, EtOH, MeOH, and AN, respectively. The complex was stable for more than 2 h. The stability constant was calculated using Bensie-Hildebrand equation where it recorded the highest value in chloroform and the least one in methanol. Thermodynamic parameters were calculated where the enthalpy of formation recorded high value in chloroform. The solid complex was prepared where elemental analysis confirmed its composition at 1:1 in concordant with that in solution. The infrared spectra of the formed complex confirmed the existence of proton transfer hydrogen bond beside charge transfer interaction. A theoretical computations using DFT at the basis set B3LYP/6-31 G(d,p) was applied to support the experimental results. Optimization energy and geometrical parameters confirmed the high stability of the complex due to the presence of CT beside PT in its chemical structure. Reactivity parameters strongly recommended that 5-ADMP is good electron donor and CLA is good electron acceptor. MEP maps confirmed the presence of proton transfer hydrogen bond where OH of CLA is the electrophile and the pyridinic-like ring nitrogen of 5-ADMP is the nucleophile. The origin of electronic spectra was computed by TD-DFT using PCM as solvation model. Two absorption maxima were recorded in the calculated spectra attributing to π-π⁎ and n-π⁎ transitions in all media. Natural population analysis was applied to estimate the natural atomic charges and natural bond orbital (NBO). Both confirmed the existence of strong proton transfer hydrogen bonding beside charge transfer in the produced complex.

A path integral molecular dynamics study on intermolecular hydrogen bond of acetic acid-arsenic acid anion and acetic acid-phosphoric acid anion clusters.

We apply ab initio path integral molecular dynamics simulation employing ωB97XD as the quantum chemical calculation method to acetic acid-arsenic acid anion and acetic acid-phosphoric acid anion clusters to investigate the difference of the hydrogen bond structure and its fluctuation such as proton transfer. We found that the nuclear quantum effect enhanced the fluctuation of the hydrogen bond structure and proton transfer, which shows treatment of the nuclear quantum effect was essential to investigate these systems. The hydrogen bond in acetic acid-arsenic acid anion cluster showed characters related to low-barrier hydrogen bonds, while acetic acid-phosphoric acid anion cluster did not. We found non-negligible distinction between these two systems, which could not be found in conventional calculations. We suggest that the difference in amount of atomic charge of the atoms consisting the hydrogen bond is the origin of the difference between acetic acid-arsenic acid and acetic acid-phosphoric acid anion cluster. © 2018 Wiley Periodicals, Inc.

Theoretical exploration of gas-phase conformers of proton-bound non-covalent heterodimers of guanine and cytosine rare tautomers: structures and energies

Due to complexity arising from the presence of various rare tautomers and multiple protonation sites, little is known of the proton-bound heterodimers of guanine (G) and cytosine (C) that may exist in the gas phase. We extensively explored structures and energies of the heterodimers by density functional theory using the B3LYP functional, wherein rare tautomers of G and C that were predicted to be energetically accessible were considered in possible heterodimer structures. This extensive search found more than 90 stable structures for the proton-bound heterodimers, where various types of base-pairings were observed. For the six lowest energy structures that lie within 3 kcal/mol in energy from the most stable structure, ionic hydrogen bonding was found to be involved in base-pairing. The non-covalent complexation energies of the heterodimers were found to be quite strong at 40–50 kcal/mol. Some pairs of the heterodimers were connected by simple proton transfer in the ionic hydrogen bond as well. In addition, a Hoogsteen base pair was predicted to be the most stable conformer in the gas phase with a more than 90% population. Solvent field effects also suggested that the Hoogsteen base pair is one of the most stable conformers in an aqueous environment. Thus, we suggest that the Hoogsteen base pair is the most predominant form of the proton-bound heterodimers of G and C that may exist in the gas phase.

Synthesis and optical and electrochemical properties of julolidine-structured pyrido[3,4-b]indole dye.

The julolidine-structured pyrido[3,4-b]indole dye ET-1 has been newly designed and developed as a small D-A fluorescent dye. ET-1 showed bathochromic shifts of the fluorescence band upon changing from aprotic solvents to protic solvents, as well as positive fluorescence solvatochromism. Moreover, it was found that ET-1 can form a 1 : 1 Py(N)-B complex with boron trifluoride and a hydrogen-bonded proton transfer (Py(N)-H) complex with trifluoroacetic acid, which exhibit photoabsorption and fluorescence bands at a longer wavelength region than the pristine ET-1. Based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical (cyclic voltammetry) measurements, Lippert-Mataga plots, 1H NMR spectral measurement and density functional theory (DFT) calculation, this work indicated that the Py(N)-B complex or the Py(N)-H complex is effectively formed and stable in solution. This is due to the strong Py(N)-B interaction or Py(N)-hydrogen-bond, which can be attributed to the enhanced basicity or the accumulated electron density on the nitrogen atom of the pyridine ring caused by the introduction of a julolidine (quinolizidine) moiety as a strong electron-donating group. We propose that the D-A-type dye ET-1 based on the julolidine-structured pyrido[3,4-b]indole possesses the ability to act as a calorimetric and fluorescent sensor for Brønsted and Lewis acids.

Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid.

The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) becomes ferroelectric upon cooling through the loss of inversion symmetry. Further cooling results in the development of an incommensurate ferroelectric phase, followed by a lock-in transition towards a twofold superstructure. Here we present the incommensurately modulated crystal structure of Phz-H2ca at T = 139 K with a symmetry given by the superspace group P2(1)(½ σ(2) ½)0 and σ(2) = 0.5139. The modulation mainly affects the positions of the protons within half of the intermolecular hydrogen bonds that are responsible for the spontaneous polarization in all three low-temperature phases. Evidence for proton transfer in part of the hydrogen bonds is obtained from the correlated dependence on the phase of the modulation of the lengths of bonds involved in resonance stabilization of the acidic anion, and much smaller variations of bond lengths of atoms not involved in the resonance mechanism. Incommensurability is explained as competition between proton transfer favored for single hydrogen bonds on the basis of pKa values and avoiding unfavorable Coulomb repulsion within the lattice of the resulting ionic molecules.

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)(-) bonded system.

We present a joint experimental and quantum chemical study on the influence of solvent dynamics on the protonation equilibrium in a strongly hydrogen bonded phenol-acetate complex in CD2Cl2. Particular attention is given to the correlation of the proton position distribution with the internal conformation of the complex itself and with fluctuations of the aprotic solvent. Specifically, we have focused on a complex formed by 4-nitrophenol and tetraalkylammonium-acetate in CD2Cl2. Experimentally we have used combined low-temperature (1)H and (13)C NMR and UV-vis spectroscopy and showed that a very strong OHO hydrogen bond is formed with proton tautomerism (PhOH···(-)OAc and PhO(-)···HOAc forms, both strongly hydrogen bonded). Computationally, we have employed ab initio molecular dynamics (70 and 71 solvent molecules, with and without the presence of a counter-cation, respectively). We demonstrate that the relative motion of the counter-cation and the "free" carbonyl group of the acid plays the major role in the OHO bond geometry and causes proton "jumps", i.e. interconversion of PhOH···(-)OAc and PhO(-)···HOAc tautomers. Weak H-bonds between CH(CD) groups of the solvent and the oxygen atom of carbonyl stabilize the PhOH···(-)OAc type of structures. Breaking of CH···O bonds shifts the equilibrium towards PhO(-)···HOAc form.

Spectroscopic characterization of hydrogen-bonded proton transfer complex between 4-aminopyridine with 2,6-dichloro-4-nitrophenol in different solvents and solid state

Proton transfer reaction between the proton donor 2,6-dichloro-4-nitrophenol (DCNP) with the proton acceptor 4-aminopyridine (4APy) has been investigated spectrophotometrically in different solvents included the aprotic solvent acetonitrile (MeCN), the protic one methanol (MeOH) and a mixture consists of 50% acetonitrile+50% dichloroethane (ANDC). The proton transfer complex is produced instantaneously with deep yellow color and absorption maxima in the range 395–425nm. The composition of the complex was characterized spectrophotometrically to be 1:1 in all solvent proving that the solvent has no effect on the complex stoichiometry. The proton transfer formation constant has been estimated by using Benesi–Hildebrand equation where the highest value was recorded in the mixture ANDC. This proofs the high stability of the complex in less polar solvent as a result of the high stability of the complex ground state. The solid complex has been synthesized and characterized by elemental analysis to be 1:2 [(proton donor) (proton acceptor)2]. The obtained solid complex was analyzed by infrared spectroscopy where two broad band’s at 3436 and 2500cm−1 characterized for asymmetric NHN+ hydrogen bond were identified. Molecular modeling utilizing GAMESS computations as a package of ChemBio3D Ultra12 program was carried out where asymmetric NHN+ was explored with NN bond distance 2.77Å. The computations showed a difference in molecular geometry of the complex compared with reactants especially bond lengths, bond angles and distances of close contact.