Proton Transfer Form Research Articles

Supramolecular adducts of mesocyclic diamines with various carboxylic acids: Charge-assisted hydrogen-bonding in molecular recognition

Aggregation of saturated mesocyclic diamine 1,4-diazacycloheptane (dach) or piperazine (pipz) and diversiform carboxylic acids with mono- or di-carboxyls yields a series of novel binary supramolecular adducts via two-point molecular recognition. All the supramolecular assemblies were obtained by solvent evaporation method from different media. X-ray single-crystal diffraction analyses reveal that these supramolecular moieties present 1D chain motif, 2D flat, corrugated sheet structures and 3D CdSO4, pillar-layered networks through carboxylate-amide N–H⋯O, as well as its proton transfer form N+–H⋯O−, carboxyl head to tail O–H⋯O, and extended hydrogen-bonding interactions. Their compositions and structures were also confirmed by Fourier transform infrared (FT-IR) spectroscopy. Thermal stability of these binary crystalline adducts has been investigated by thermogravimetric analysis (TGA), suggesting similar thermal stabilities.

A density functional theory‐time‐dependent density functional theory investigation of photo‐induced hydrogen bond and proton transfer for 2‐(3,5‐dichloro‐2,6‐dihydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid

AbstractGiven the paramount importance of excited‐state relaxation in the photochemical process, excited‐state hydrogen bonding interactions and excited‐state intramolecular proton transfer (ESIPT) are always hot topics. In this work, we theoretically explore the excited‐state dynamical behaviors for a novel 2‐(3,5‐dichloro‐2,6‐dihydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid (DDPBC) system. As two intramolecular hydrogen bonds (O1H2⋯N3 and O4H5⋯O6) exist in the DDPBC structure, we first check if the double proton transfer form cannot be formed in the S1 state. Then, we explore the changes of geometrical parameters involved in hydrogen bonds, based on which we confirm that the dual intramolecular hydrogen bonds are strengthened on photo‐excitation. The O1H2⋯N3 hydrogen bond particularly plays a more important role in excited state. When it comes to the photo‐induced excitation, we find charge transfer and electronic density redistribution around O1H2 and N3 atom moieties. We verify the ESIPT tendency arising from the O1H2⋯N3 hydrogen bond. In the analysis of the potential energy curves, along with O1H2⋯N3 and O4H5⋯O6, we demonstrate that the ESIPT reaction should occur along with O1H2⋯N3 rather than O4H5⋯O6. This work not only clarifies the specific ESIPT mechanism for DDPBC system but also paves the way for further novel applications based on DDPBC structure in the future.

A Theoretical Investigation on Intramolecular Hydrogen Bond: The ESIPT Mechanism of dmahf Sensor

The properties of intramolecular hydrogen bond of a new photochemical sensor 4'-N,N-dimethylamino-3-hydroxyflavone (dmahf) has been investigated in detail. Using Atoms-In-Molecule method, we have demonstrated that the intramolecular hydrogen bond was formed in the ground state (S-0 state). The calculated dominating bond lengths and angles involved in hydrogen bond demonstrates that the intramolecular hydrogen bond can be strengthened in the first excited state (S-1 state). In addition, the variation of hydrogen bond of dmahf has been also testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of excited state intramolecular proton transfer process. According to the calculated results of potential energy curves along O-H coordinate, the potential energy barrier of about 7.49 kcal/mol is discovered in the S-0 state. However, a lower potential energy barrier of 1.61 kcal/mol has been found in the S-1 state, which demonstrates that the proton transfer process is more likely to happen in the S-1 state than the S-0 state. In other words, the proton transfer reaction can be facilitated based on the photoexcitation effectively. In turn, through the process of radiative transition, the proton-transfer form dmahf-keto regresses to the ground state with the fluorescence of 578 nm.

Intermolecular Interactions in the Solid State of Ionic Secondary Mannich Bases

Two new secondary Mannich bases, 4-bromo-2-[(aminopropyl)methyl]-phenol (1) and 4-nitro-2-[(aminopropyl)methyl]-phenol (2), were synthesized. Crystal structures were determined at liquid nitrogen temperature. It was found that in both compounds the proton transfer forms exist in the solid state. In the case of 1, this was unexpected, because of the weak acidity of p-bromophenol being the parent component of this Mannich base. The reason for that was found to be the formation of the O–···H–N+ hydrogen bonded tetramer in the solid state. Two cyclic aggregates R42(8) and R22(12) describe the pattern of hydrogen bonded interactions in the crystals of both compounds. Additionally, C–H···π interactions stabilize the crystal structures. The hydrogen bonds in 1are slightly stronger (N···O distances 2.708 and 2.733 Å) than in 2 (2.721 and 2.765 Å, respectively) despite the fact that p-nitrophenol participating in 2 is a stronger acid. The influence of permittivity of surroundings and the hydrogen bonding pattern on the properties of intermolecular hydrogen bonds are discussed on the basis of B3LYP and MP2 calculations with basis sets 6-31+G(d,p) and 6-31++G(2d,2p). The coupling between hydrogen bonds in crystals was studied with the application of the IR spectra of isotopically diluted species. It was found that such a coupling is stronger for 2, forming weaker hydrogen bonds. Both the theory of IR spectra and quantum chemical calculations demonstrate that the source of the observed behavior is electronic participation in vibronic absorption.

Theoretical investigation of tautomeric equilibrium in ortho-hydroxy phenyl Schiff bases

This Letter presents a study of the tautomeric equilibrium in ortho-hydroxy phenyl Schiff bases. The influence of substitution and solvent (simulated by the self-consistent reaction field model, SCRF) on the energy barrier of the transition state and on proton transfer is investigated. Dependencies of the HOMA and HOSE aromaticity indices on the molecular, transition state, and proton transfer forms were obtained. The state of chelate chain and phenyl ring aromaticity depending on the tautomeric equilibrium is studied.

Understanding the photophysics of 4-nitro-1-hydroxy-2-naphthoic acid: A controlled excited state proton transfer

This paper reports on the excited state intramolecular proton transfer (ESIPT) in 4-nitro-1-hydroxy-2-naphthoic acid (NHNA) as compared to the excited state photophysics of 1-hydroxy-2-naphthoic acid (HNA). Normally, HNA does not show an ESIPT, although the other regioisomer of the molecule, (3, 2HNA), does show an ESIPT. The calculated excited state double-well potential, charge density, bond length and dipole moment evince that, in contrast to the parent compound HNA, NHNA is conducive to ESIPT. Emission bands were attributed to the neutral form (410 nm), the intermolecular proton transfer form (490 nm) and the intramolecular proton transfer (zwitterionic) form (560 nm). NHNA showed phosphorescence arising from the open conformer, whereas HNA showed no phosphorescence. Intramolecular hydrogen bonding in NHNA seems to hinder the formation of the open conformer in polar solvent. A complete picture of the photophysical processes in NHNA after photoexcitation was obtained from the time-resolved emission along with kinetic analysis.

Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O 2(ads) is predicted to be reduced to adsorbed superoxide, O 2 −(ads), which dissociates with a low activation barrier to O(ads) + O −(ads). Then a proton transfer form H 2O(ads) to O −(ads) takes place, forming OH(ads) + OH −(aq). The second electron transfer reacts O(ads) with H 2O(aq) to form a second OH(ads) + OH −(aq). The third and fourth electron transfers react the two OH(ads) with two H 2O(aq) to form two H 2O(ads) + two OH −(aq). All three different surface reduction reactions are predicted to have reversible potentials in the −0.24 V(SHE) to −0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38–0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O 2 −(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O 2 −(ads) is the first reduction intermediate, is discussed.

Direct photoexcitation of intramolecular proton transfer states in 3-hydroxyflavone

It is shown that the photoexcitation of 3-hydroxyflavone at the red edge of its absorption spectrum can directly excite luminescing intramolecular proton-transfer forms of molecules of this compound. This offers new possibilities for studying the fundamental properties of molecular objects with intramolecular proton transfer, which are currently considered as promising multiparametric molecular sensors.

Deuterium isotope effect on 13C chemical shifts of tetrabutylammonium salts of Schiff bases amino acids

Deuterium isotope effects on 13C chemical shift of tetrabutylammonium salts of Schiff bases, derivatives of amino acids (glycine, L-alanine, L-phenylalanine, L-valine, L-leucine, L-isoleucine and L-methionine) and various ortho-hydroxyaldehydes in CDCl3 have been measured. The results have shown that the tetrabutylammonium salts of the Schiff bases amino acids, being derivatives of 2-hydroxynaphthaldehyde and 3,5-dibromosalicylaldehyde, exist in the NH-form, while in the derivatives of salicylaldehyde and 5-bromosalicylaldehyde a proton transfer takes place. The interactions between COO- and NH groups stabilize the proton-transferred form through a bifurcated intramolecular hydrogen bond.

Structural modifications resulting from proton transfer in complexes of phenols with pyridine

AbstractThe structural consequences of proton transfer in complexes of phenol (1), 2,6‐dichlorophenol (2), 4‐nitrophenol (3) and 2,6‐dichloro‐4‐nitrophenol (4) with pyridine were analyzed on the basis of results of B3LYP/6–31G** calculations. Three methods of describing the progress of proton transfer are proposed: the O—H [d(OH)] and C—O [d(CO)] bond lengths and the fraction XPT of the proton transfer form, calculated from the values of the dipole moments. The d(OH) parameter reveals behaviour near to XPT and can be used as a universal measure of the degree of proton transfer. The d(CO) parameter gives nearly linear dependences for various structural parameters, but independent estimation of the specific effects of the substituents is necessary, as separate correlations for each complex are found. A role of resonance interaction in systems containing a p‐NO2 substituent is demonstrated. Copyright © 2005 John Wiley & Sons, Ltd.

Tape and layer structures in cocrystals of some di- and tricarboxylic acids with 4,4′-bipyridines and isonicotinamide. From binary to ternary cocrystals

Linear, zigzag tapes and flat, corrugated sheet structures are described in binary cocrystals 1–9 of some di- and tricarboxylic acids with 4,4′-bipyridine bases and isonicotinamide. Carboxylic acid⋯pyridine O–H⋯N, its proton transfer form N+–H⋯O−, carboxamide dimer N–H⋯O, and extended motifs are present in these crystal structures. Our results show modular expansion of carboxylic acid dimer in neutral cocrystals with pyridine type bases. Even as the structures of these aggregates can be understood from the molecular constituents in a straightforward manner, the nature of acid⋯pyridine synthon, i.e. neutral or ionic, is difficult to anticipate from their ΔpKa values. We modify the ΔpKa (pKa conjugate acid of pyridine base – pKa carboxylic acid) rule in COOH–pyridine cocrystals: ΔpKa 3.75 results in proton migration to the ionic interaction N+–H⋯O−. Proton transfer results in the assembly of a supramolecular 3-connected node of cyclohexane tricarboxylate and these anions form chair cyclohexane or parquet grid sheets in 10 and 11. The length of bipyridinium cation plays a role in the overall structure. A novel approach to three-component adducts is crystallization of 1,3cis,5cis-cyclohexane tricarboxylic acid with 4,4′-bipyridine and 4,4′-bipyridine-N-oxide bases of different strengths, which results in a ternary cocrystal 12 sustained via O–H⋯N and O–H⋯O− hydrogen bonds.

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.

Characterisation of the PT-form of o-hydroxy acylaromatic Schiff bases by NMR spectroscopy and DFT calculations

A series of o-hydroxy Schiff bases showing tautomerism to a varying degree are investigated together with compounds totally at the proton transfer form (enamines). Deuterium isotope effects on 13C chemical shifts are measured at different temperatures. Structural features determining the equilibrium constants are discussed. DFT calculations are done in order to obtain structures. Schiff bases derived from dehydracetic acid (3-acetyl-6-methyl-pyran-2,4-dione) as well as from 1,3,5-triacetyl-2,4,6-trihydroxybenzene are found to be fully at the PT-form. These forms are characterised and the use of these compounds as reference for the PT-form in general is discussed.

Intramolecular hydrogen bond in molecular and proton-transfer forms of Schiff bases

The force field and structural parameters modifications upon the formation of intramolecular hydrogen bond and proton transfer reaction in N-methyl-2-hydroxybenzylidene amine (HBZA) are determined on the basis of ab initio and DFT calculations. Reliability of the calculations is verified by comparing of the theoretical vibrational spectra with those experimentally determined in the gas phase. A model of resonance interactions is applied and the quantitative contribution of ortho-quinoid structure in the particular conformers is estimated. A comparison is also made to the systems without π-electron coupling (Mannich bases).

Hydrogen Bonding of Phenols or Their Radical Cations with Water or Ammonia: Substituent Effects and the Influence on Phenol Oxidation

We did UB3LYP/6-31++g** and ROMP2/6-311++g** calculations on the hydrogen bonding of para-substituted phenols and their radical cations with water and ammonia. It was found that the magnitudes of the proton affinities increase in the order water (165 kcal/mol) < ammonia (204 kcal/mol) ≈ phenoxyl radicals (193−235 kcal/mol) < phenolate anions (321−352 kcal/mol). The slopes of the proton affinities against the substituent σp constants are about 22 and 15 kcal/mol for phenoxyl radicals and phenolate anions. It was also found that the slopes of the binding energies against the substituent σp constants decrease in the order phenol−water complex (1.1 kcal/mol) < phenol−ammonia complex (1.4 kcal/mol) < phenol radical cation−water complex (4.1 kcal/mol) < phenol radical cation−ammonia complex (9.3 kcal/mol). The structure of the substituted phenol radical cation−ammonia complex was found to rely on the proton affinity of the corresponding phenoxyl radical. When the proton affinity is larger than 214 kcal/mol, the non-proton-transferred form is the only minimum on the potential energy surface. When the proton affinity is smaller than 210 kcal/mol, the proton-transferred form is the only minimum. The only complex for which both the proton-transferred and non-proton-transferred forms are minima was found for p-hydroxylphenol radical cation. On the other hand, all the phenol radical cation complexes with water have the non-proton-transferred form as the only minimum on the potential surface. Hydrogen bonding to ammonia was found to lower the adiabatic oxidation potentials of phenols by 0.5−1.2 eV. Hydrogen bonding to water was found to lower the adiabatic oxidation potentials of phenols by 0.4−0.6 eV. In general, a phenol substituted with a more electron-withdrawing group shows larger reduction in the adiabatic oxidation potential when complexed to water or ammonia.

A gradient-corrected density functional and MP2 study of phenol–ammonia and phenol–ammonia(+) hydrogen-bonded complexes

Abstract A gradient-corrected DFT and MP2 study of phenol–ammonia neutral and cationic dimers were carried out. Various combinations of exchange and correlation functionals were employed within the DFT approach: B3LYP, MPW1PW91, PBE1PBE, and the modified B3LYP(M) [J. Mol. Struct. 416 (1997) 1]. The BSSE-corrected PES of the neutral dimer was also explored at B3LYP, B3LYP(M) and MP2/6-31++G(2d,p) levels. A single minimum was located on the neutral complex PES corresponding to the hydrogen-bonded non-proton-transferred form, while the only minimum on cationic complex PES corresponds to a phenoxyl-ammonium proton-transferred form, regardless on the level of theory. Dissociation energies corrected for BSSE and including the relaxation energy terms are reported for both complexes. The calculated vertical IE values at all DFT levels of theory are in excellent agreement with the experimental data. While the anharmonic vibrational analysis based on all 1D DFT vibrational potentials overestimate the ν (OH) mode frequency shift upon complexation, the value obtained from the 1D MP2 potential for the BSSE-corrected MP2/6-31++G(2d,p) PES is in excellent agreement with the experimental data (457 vs. 469 cm −1 ). Various contributions to the overall anharmonic ν (OH) frequency shifts are considered, as well as the BSSE influence which was found to be very small. As revealed by the CFP 1D ν (OH) DFT and MP2 vibrational potentials, the electrostatic interaction alone can not account neither for the substantial red shift of this mode upon hydrogen bonding, and especially for the substantial intensity enhancement.

Size-Dependent Hydrogen Bonds of Cluster Ions between Phenol Cation Radicals and Water Molecules: A Molecular Orbital Study

The molecular structures and vibrational frequencies of [C6H5OH−(H2O)n]•+ (n = 1−4) are studied by employing ab initio molecular orbital methods. Since the hydrogen bond between phenol cation radicals and water molecules is much stronger than that of the neutral phenol−water system, the position of the proton of phenol cation radicals depends on the number of water molecules in the clusters [C6H5OH−(H2O)n]•+. Although the stable structure of [C6H5OH−(H2O)n]•+ varies depending on the method used in the calculation, the result obtained with the B3LYP density functional method gives good agreement with the experimental IR spectra. The proton-nontransferred form is found to be most stable for n = 1 and n = 2 clusters. In the cases of n ≥ 3, the most stable structures are the proton-transferred form. There are two types of structures obtained for n = 3 clusters, where the branched form is more stable than the chained form. The optimized structures for n = 3 and n = 4 clusters show that the H3O+ moiety prefers to interact with the phenoxy radical and two water molecules.

Intramolecular proton transfer in the triplet state of 1-(acylamino)anthraquinones

Intramolecular proton transfer in the triplet state of 1-(acylamino)anthraquinones has been studied by means of transient absorption spectroscopy. The substituent and solvent effects were observed and were discussed in conjunction with the energy-state diagram. The singlet-triplet energy-gap of the proton-transferred form is much less than that in the normal form.

Characterization of the Hydrogen-Bonded Cluster Ions [Phenol−(H2O)n]+ (n = 1−4), (Phenol)2+, and (Phenol−Methanol)+ As Studied by Trapped Ion Infrared Multiphoton Dissociation Spectroscopy of Their OH Stretching Vibrations

OH stretching vibrations of hydrogen-bonded cluster ions of phenol (PhOH), [PhOH−(H2O)n]+ (n = 1−4), (PhOH)2+, and (PhOH−methanol)+ have been observed with infrared photodissociation spectroscopy in combination with an ion-trapping technique. Cluster ions were efficiently generated by ionization of phenol followed by a jet expansion and were mass-selectively stored by the radio frequency ion trap method, which allows us to observe infrared multiphoton dissociation yield spectra of size-selected cluster ions. For [PhOH−(H2O)n]+, the OH stretching vibrations of the water moieties strongly suggested that the n ≥ 3 cluster ions exhibit the proton-transferred form, [PhO−H3O+(H2O)n-1], while the n = 1 and 2 ions are of the nontransferred form, [PhOH+−(H2O)n]. As for (PhOH)2+, the infrared spectra indicate that the dimer ion is characterized as the open form, in which the phenol ion acts as a proton donor and the neutral phenol as an acceptor through their single hydrogen bond. The similar open form is also foun...

Relative ground and excited state energies of CH3(CH = CH)5CH = NC4H9, its hydrogen-bonded and proton-transferred species, and charge partitioning and distribution in the protonated Schiff base of retinal.

CH3(CH = CH)5CH = NC4H9 (compound 1) is structurally related to the Schiff base of retinal, the prosthetic group in visual pigments. Dilute solutions of a weak acid (phenol) and 1 in a hydrocarbon solvent, when subjected to decreasing temperature, show striking changes in electronic absorption spectra. Initially only the spectrum of compound 1 is present, but as the temperature is lowered, the absorbance of 1 decreases, and the spectrum of the H-bonded form of 1 appears and increases. Continued temperature lowering then causes a decrease in absorption of the H-bonded form and an appearance and rise in absorption of the proton-transferred form of 1. Concentrations of the various species are measured as a function of temperature, and by standard procedures, the thermodynamic constants for both reaction steps are computed. Values of delta H0 are taken as relative energies among the three ground states, and the lambda max value of each species yields relative energies among excited states. By employing data from electronic absorption spectroscopy, nuclear magnetic resonance (NMR) and theoretical calculations for retinal Schiff base, charge partitioning between nitrogen and the polyene chain and charge distribution among the carbon atoms of the polyene chain are calculated.