Modulation of intramolecular proton transfer by electronic excitation and environment: 2-Pyridone as a case study

The kinetics and thermodynamics of lactam/lactim tautomerization in 2-pyridone have been investigated, with special attention to direct and assisted proton transfer mechanisms in the ground and first excited electronic state. Specific interactions with a single water molecule strongly enhance the reaction rate and shift the equilibrium toward the lactam form. The effect of bulk solvent is comparatively negligible, although the lactam form is further stabilized. Electron excitation strongly destabilizes the saddle point for proton transfer and, especially, the lactim form with respect to the lactam species. As a consequence, the direct reaction barrier is increased, but the reverse barrier is lowered. Nonpotential energy effects are relatively small and do not modify the aforementioned general trends. © 1994 by John Wiley & Sons, Inc.     

DFT and MP2 study of isomery scheme in Formazan and intermolecular and intramolecular proton transfer between its tautomers

DFT and Moller Plesset (MP2) calculations were applied to study of isomery scheme in formazan. Formazan that can be presented by three tautomers and eight isomers has various applications in dyes, complexes, and biochemistry. The structures of its isomers and related transition states were optimized, and important molecular parameters, IR frequencies, and energetic results were extracted. The relative stabilities of formazan isomers in the gas phase were found to be as 1EZ >1ZZ >1EE >1ZE >2EE > 3 >2EZ >2ZZ >TS1 >TS3 >TS2. Thermodynamic data confirms that tautomer 1 is major tautomer, and all possible tautomerism interconversions have small rates at room temperature. Then, relative stabilities were calculated in different solvents (chloroform, tetrahydrofuran, acetone, and water). The relative stabilities and thermodynamic data in solvents are nearly similar to those in the gas phase, but the rate constants are slightly more than that in the gas phase. Moreover, relative stabilities of formazan isomers and intermolecular proton transfer in presence of one to three molecules of water have been studied. The results showed that activation barriers in water‐assisted tautomerisms are in general lower than those in the gas phase, but the relative energies of isomers do not change importantly by water clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Catalytic and bulk solvent effects on proton transfer: Formamide as a case study

Structural, thermodynamic, and kinetic aspects of the tautomerization of formamide through direct and solvent-assisted proton transfer have been investigated. Both specific and bulk effects of the solvent play a role in determining the overall result so that only a mixed discrete-continuum model is sufficiently reliable. Structural modifications induced by the solvent are significant, but have only a slight effect on thermodynamic and kinetic quantities. The same remarks apply to the vibrational shifts induced by the solvent. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1993–2000, 1997     

A comparative ab initio study of intramolecular proton transfer in model alpha-hydroxyalkoxides

A comparative ab initio study was performed on the intramolecular proton-transfer reaction that occurs in alpha-hydroxyethanoxy, alpha-hydroxyphenoxide, and alpha-hydroxyethenoxy anions. The intramolecular proton transfer occurs in a five-member atom arrangement, between two oxygen atoms separated by a carbon-carbon bond. The chosen systems serve as models for alpha-hydroxyalkoxide molecules where the carbon-carbon bond varies from a single bond (the glycolate anion or alpha-hydroxyethanoxide anion) to a part of an aromatic ring (the alpha-hydroxyphenoxide anion), and finally to a double bond (the alpha-hydroxyethenoxide anion). Particular attention was given to the evolution along the intrinsic reaction coordinate of such properties as energies, relevant structural parameters, Mulliken charges, dipole moments, and 1H-NMR chemical shifts to reveal the similarities and differences for the proton transfer in the model systems.     

Negative influence of pKa on activation energy barrier: A case study for double proton transfer reaction in inorganic acid dimers

Strength of acid can be determined by means of pK_a value. Attempts have been made to find a relationship between pK_a and activation energy barrier for a double proton transfer (DPT) reaction in inorganic acid dimers. Negative influence of pK_a is observed on activation energy (E_a) which is contrary to the general convention of pK_a. Four different levels of theories with two different basis sets have been used to calculate the activation energy barrier of the DPT reaction in inorganic acid dimers. A model based on first and second order polynomial has been created to find the relationship between activation energy for DPT reaction. © 2018 Wiley Periodicals, Inc.     

Proton-donating power of 100% perchloric acid: An MP2 study

The ab initio quantum chemical method MP2 with a 6-311++G(d, p) basis set is used to calculate the energy of gas-phase solvation of the H₂Cl₄⁺ cation and Cl₄⁻ anion by one perchloric acid molecule. The energy of additional solvation of the resulting complexes by liquid perchloric acid is estimated within the continuum model of solvation by the PCM method, with the acid modeled as a continuum with a large dielectric constant of ɛ = 115. The calculated data have provided an almost quantitative estimate for the energy of selfionization of 100% liquid perchloric acid. The similarly calculated energy of solvation of protons by 100% perchloric acid is 30.7 kcal/mol lower than the heat of hydration of protons in aqueous solution. This result explains the fact that anhydrous perchloric acid exhibits the properties of a superacid.     

Application of DFT and MP2 calculations on structural and water-assisted proton transfer in 3-amino-4-nitrofurazan

Density functional theory (DFT) and MP2 calculations have been employed to study of 3-amino-4-nitrofurazan molecule using the standard 6-311++G(d,p) basis set. The chemical properties of the 3-amino-4-nitrofurazan have been extensively studied. The geometries of molecules in the gas phase were optimized and compared with the crystallography of this substance. The results suggest that A form is the most stable form in the gas phase and it is the predominant tautomer in solution according to the DFT and MP2 calculations, respectively. In addition, variation of dipole moments in the gas phase, the specific solvent effects with addition of one molecule of water near the electrophilic centers of tautomers, the transition state of proton transfer assisted by a water molecule, the NBO charges of atoms and the potential energy surface were investigated. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are presented.     

A comparative AM1 and ab initio study of the intramolecular proton transfer in tautomeric organic compounds

The proton-transfer barriers along the intramolecular hydrogen bond in a series of substituted salicylaldehyde anils were calculated using the AM1 SCF semiempirical method. The reliability of this method for the calculation of proton-transfer barriers was analyzed by the comparison of AM1 barriers for a series of different tautomeric organic compounds with those calculated using ab initio SCF and second-order perturbation theory with extended basis sets. In general, the AM1 method systematically overestimates the barrier height. However, this error is approximately constant for given pairs of groups involved in the intramolecular proton transfer. © 1996 John Wiley & Sons, Inc.     

尿酸质子转移反应的理论研究I——分子内质子转移研究

使用量子化学中的Hartree-Fock方法和密度泛函理论中的B3LYP方法，分别在3－21G^*和6－31G（d)水平上，计算了尿酸分子从三羰基异构体向三羟基异构体的转化。结果表明，转化过程经历了单羟基和双羟基异构体2种中间物和3种过渡态时的分子内质子转移（IPT），转移中的H原邻近的N，O和C原子形成了具有四元环结构的过渡态。随着IPT的进行，N－H键逐渐被削弱和断裂，O－H键则逐渐生成。3个反应的活化能分别为190.3kJ/mol,181.4kJ/mol和249.9kJ/mol(B3LYP/6-31G(d))。较高的活化能表明在室温下，无催化剂的IPT难以进行。     

Kinetics of proton transfer in a green fluorescent protein: A laser-induced pH jump study

The sub-millisecond protonation dynamics of the chromophore in S65T mutant form of the green fluorescent protein (GFP) was tracked after a rapid pH jump following laser-induced proton release from the caged photolabile compoundo-nitrobenzaldehyde. Following a jump in pH from 8 to 5 (which is achieved within 2 μs), the fluorescence of S65T GFP decreased as a single exponential with a time constant of ∼90 μs. This decay is interpreted as the conversion of the deprotonated fluorescent GFP chromophore to a protonated non-fluorescent species. The protonation kinetics showed dependence on the bulk viscosity of the solvent, and therefore implicates bulk solvent-controlled protein dynamics in the protonation process. The protonation is proposed to be a sequential process involving two steps: (a) proton transfer from solvent to the chromophore, and (b) internal structural rearrangements to stabilize a protonated chromophore. The possible implications of these observations to protein dynamics in general is discussed     

Double proton transfer and charge transfer transitions in hydrogen-bonded systems: formic acid dimer

The barriers for double proton transfer in the ground and lowest Π-Π^* and Π-Π^* excited states of the formic acid dimer have been calculated within a modified INDO scheme. Analysis of the nature of the excited electronic states, with emphasis on charge-transfer transitions, has been performed. The results indicate a lower barrier in the excited Π-Π^* states than in the ground state.     

Excited-state proton transfer of 3-hydroxybenzoic acid and 4-hydroxybenzoic acid

The excited-state proton transfer of 3-hydroxybenzoic acid and 4-hydroxybenzoic acid was studied by time-resolved laser-induced fluorescence spectroscopy with ultra-short laser pulses. The excited-state reactions were identified in aqueous media as a function of the pH value. Apart from the well-known inversion of the ordinary dissociation properties of these compounds, new species were found which exist only in the excited-state resulting from a temporal and reversible annihilation of the aromatic bond system. These species and their reaction mechanisms were detected by their absorption and fluorescence spectra.     

A comparative ab initio study of the primary hydration and proton dissociation of various imide and sulfonic acid ionomers

We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF(3)SO(2)NHSO(2)CF(3) and CF(3)CF(2)SO(2)NHSO(2)CF(3)), two partially fluorinated sulfonyl imides (CH(3)SO(2)NHSO(2)CF(3) and C(6)H(5)SO(2)NHSO(2)CF(2)CF(3)), and two aromatic sulfonic acid based materials (CH(3)C(6)H(4)SO(3)H and CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G** level of density functional theory. Spontaneous proton dissociation was observed upon addition of three water molecules in each system, and the transition to a solvent-separated ion pair occurred when four water molecules were introduced. No considerable quantitative or qualitative differences in proton dissociation, hydrogen bond networks formed, or water binding energies were found between systems containing similar electron withdrawing groups. Each of the sulfonyl imide ionomers exhibited qualitatively similar results regarding proton dissociation and separation. The fully fluorinated sulfonyl imides, however, showed a greater propensity to exist in dissociated and ion-pair separated states at low degrees of hydration than the partially fluorinated sulfonyl imides. This effect is due to the additional electron withdrawing groups providing charge stabilization as the dissociated proton migrates away from the imide anion.     

Hydrogen transfer vs proton transfer in 7-hydroxy-quinoline.(NH3)3: a CASSCF/CASPT2 study

Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.     

Spectrophotometric determination of palladium: A comparative study of 2-mercaptobenzothiazole and 2-mercaptobenzimidazole as analytical reagents

As spectrophotometric reagents for palladium 2-mereaptobenzothiazole and 2-mereaptobenzothiazole behave more or less in the same way. Colour system formed by them at a pH region of 3.0 to 6.5 obey Beer's law at 380 mμ palladium concentration of 0.4 to 6.0 mμ per ml and the optimum range with the minium is from 1.6 to 4.0 μg of palladium per ml at 30.1% and 35.8% treansmission with 2-mereaptobenzothiazole and 2-mereaptobenzothiazole respectively. By applying job's method it was found that in solution the ratio of metal to reagent in the complexes is 1:2 and that the dissociation constants of the order of 10^-12.     

Extra hydrogen-bonding and water chain altering water-mediated ground-state multiple proton transfer in 2-aminopyridine: a theoretical study

Structural Chemistry - A systematic study on the effect of extra hydrogen bonding (H-bonding) and water chain on ground-state multiple proton transfer (GSMPT) in 2-aminopyridine (2AP) complexes...     

A density functional theory study of a concerted mechanism for proton exchange between amino acid side chains and water

 A concerted mechanism for proton exchange between water and the amino acid side chains of cysteine, serine, arginine and glutamic acid has been investigated with hybrid density functional theory. The models used include, besides the amino acid side chain, a number of water molecules ranging from one to five in some cases. The modeling of the amino acids without their backbones is shown to be an excellent approximation. Long-range polarization effects were incorporated through a dielectric cavity method allowing a better comparison to existing measurements for free amino acids in water. The barriers converge rather fast with the number of water molecules for all the present amino acids and the converged values are in reasonable agreement with experiments with discrepancies in the range 2–6 kcal/mol. The dielectric effects were found to be small for all systems except cysteine, where there is a lowering of the barrier by 3–5 kcal/mol. The transition states for these concerted pathways form rings in which the separated charges can be stabilized. Received: 25 October 1999 / Accepted: 5 April 2000 / Published online: 21 June 2000     

Modeling proton transfer in imidazole-like dimers: a density functional theory study

A detailed theoretical study of proton transfer reaction in protonated imidazole, 1,2,3-triazole, and tetrazole dimers, the basic components of polymeric membrane used in proton exchange membranes fuel cells, has been carried out. In particular, several approaches based on density functional theory have been considered and their results compared with those provided by post-HF methods. From a computational point of view, these molecules appear to be a very challenging playground also for robust and recent functionals. Indeed none of the considered approaches provide results in close agreement with the reference post-HF data and a combined BMK//B3LYP model seems the only approach able to reproduce both the energetic and the structural features. From a chemical point of view, two new mechanisms of proton transfer in tetrazole dimers have been investigated and found to be more favorable than that previously hypothesized in literature. At the same time, the theoretical results show a direct connection between the obtained proton transfer barrier and the charge localized on the transferred hydrogen.     

Hydrogen-bonding interactions in peptide nucleic acid and deoxyribonucleic acid: a comparative study

Peptide nucleic acid (PNA) is a synthetic analogue of deoxyribonucleic acid (DNA) capable of tightly binding to itself and DNA with high specificity. Using hybrid density functional methods, hydrogen-bond (H-bond) strengths have been evaluated for isolated Watson-Crick base pairs, PNA base pairs, and charged as well as neutral DNA base pairs. Heterogeneous base pairs of PNA with charged and neutral DNA have also been investigated. The competing effects of short-range H-bonding and long-range Coulombic repulsions in charged DNA base pairs have been analyzed. Polarizable continuum models have been employed to evaluate solvation effects on the binding energies.