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.     

A physiochemical study of excited state intramolecular proton transfer process-Luminescent chemosensor by spectroscopic investigation supported by ab initio calculations

A group of novel Schiff base derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy Schiff base (SB4) has been studied using emission spectroscopy and it was detected that the two distinct ground state isomers of I and II are responsible for the emission. The comparison of the emission wavelength in hydrocarbon solvent strongly supports that trans enol form predominates over the cis enol form for Schiff base (SB4). With increasing base concentration of the solutions of hydroxy substituted Schiff bases (SB4 and SB5), two isobestic points are found which confirm the equilibrium among the trans enol form, anion and the cis enol form. The fluorescence of (SB4) quenched markedly with the gradual addition of Cu(2+) but the fluorescence properties of (SB5) was influenced by other metal ions. Therefore Schiff base (SB5) can be used as a new fluorescence sensor to detect the quantity of Cu(2+) ion in any sample solution depending on the relative intensity change. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the Schiff base derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.     

Ab initio molecular dynamics study of glycine intramolecular proton transfer in water

We use ab initio molecular-dynamics simulations to quantify structural and thermodynamic properties of a model proton transfer reaction that converts a neutral glycine molecule, stable in the gas phase, to the zwitterion that predominates in aqueous solution. We compute the potential of mean force associated with the direct intramolecular proton transfer event in glycine. Structural analyses show that the average hydration number (N(w)) of glycine is not constant along the reaction coordinate, but rather progresses from N(w) = 5 in the neutral molecule to N(w) = 8 for the zwitterion. We report the free-energy difference between the neutral and charged glycine molecules, and the free-energy barrier to proton transfer. Finally, we identify the approximations inherent in our method and estimate the corresponding corrections to our reported thermodynamic predictions.     

An ab initio study of the proton transfer and tautomerization processes in hydroxycarbene

We have investigated the hydrogen-bonded complexes formed by hydroxycarbene in trans configuration at MP2 and CCSD computational levels. In addition, these complexes have been used as starting point in the potential tautomerization of hydroxycarbene to produce formaldehyde. The presence of molecules that can be involved in the tautomerization significantly reduces its barrier. The electron density of the different structures obtained has been analyzed with the Atoms in Molecules methodology.     

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: an ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.     

Cooperativity in intramolecular bifurcated hydrogen bonds: an ab initio study

Molecular orbital and density functional theory calculations are performed on some di- and tetrasubstituted derivatives of anthraquinone, dihydrophenazine, and acridone to investigate cooperativity in a pair of bifurcated hydrogen bonds occurring in the same molecule. The various structures were selected as convenient model systems for three-center hydrogen bonding of both H...A...H and A...H...A types. In the former type, the C=O moieties in anthraquinone and acridone act as bifurcated hydrogen bond acceptors, and substituted OH groups act as hydrogen bond donors. In the latter type, the N-H moieties in dihydrophenazine, acridones act as bifurcated hydrogen bond donors, and the carbonyl oxygens of substituted CHO groups act as hydrogen bond acceptors. Different indicators of cooperativity reveal that two intramolecular bifurcated hydrogen bonds simultaneously present in the same molecule significantly reinforce each other.     

An ab initio calculation on proton transfer in the benzoic acid dimer

We have made an ab initio calculation of the barriers for proton transfer in the hydrogen-bonded dimers of benzoic acid and acetic acid. Geometrical optimization values which are closer to the experiment one.     

A comparative ab initio study of ethylene,acetylene and benzene

Ground state properties have been calculated by use of a medium-sized Gaussian basis set and comparison with other bases has been made. Contraction to double-zeta of a comparatively small basis is found to be superior to a large set of primitive Gaussians contracted to minimal basis. Molecular optimization is not important for double-zeta bases. Inclusion of a balanced set of polarization functions is essential in all cases studied. Population analysis gives a certain insight in molecular properties but contour maps are found to be significantly superior. This is demonstrated on bonding properties of corresponding orbitals within the series. In case of benzene Slater's energyband plot is shown to be useful for classifying bonding properties.     

Water mediated proton transfer in a mesostructured aluminosilicate framework: an ab initio molecular dynamics study

The proton transfer process mediated by water molecules adsorbed in an aluminosilicate framework has been studied using ab initio molecular dynamics simulations. This investigation has been carried out using a quasi-one-dimensional model simulating the mesoporous aluminosilicate channel structures. The effects of both the water loading and temperature of the system have been considered. At low coverage (one water molecule per acid site), the hydroxonium ion (H(3)O)(+) is found to be a transition state, in agreement with earlier studies on zeolites. At a higher water coverage (two water molecules per acid site), the (H(5)O(2))(+) species and the hydrogen bonded "neutral complex" structure are both found to be stable complexes at finite temperatures. The vibrational frequency spectrum is simulated by performing a Fourier transform of the velocity autocorrelation function (VAF), and the peak positions in the VAF are compared with IR measurements and zero-temperature calculations.     

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.     

A computational study of intramolecular proton transfer in gaseous protonated glycine

The minimum energy paths for intramolecular proton transfer between the amino nitrogen and carbonyl oxygen atoms in gaseous protonated glycine were estimated at the Hartree-Fock (HF) and second-order M?ller-Plesset Perturbation (MP2) levels of theory. Potential energy profiles and their associated reactant, transition state, and product species calculated at the MP2/6-31G* level were shown to differ significantly from those obtained at the HF/6-31G* level. Effects of electron correlation and basis functions on the calculated geometries and energies of relevant species were examined at the HF, MP2, MP4, CCSD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G**, 6-311+G**, 6-31+G(2d,2p), 6-311+G(3df,2p), cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. The HF and MP2 optimized levels with the 6-31G*, 6-31G**, 6-31+G**, and 6-311+G** bases were used to calculate the thermodynamic and kinetic properties of the proton transfer reaction at 298.15 K and 1 atm, which include enthalpy, entropy, Gibbs free energy, equilibrium constant, potential energy barriers, tunneling transmission coefficients, and rate constants. Results indicate that the proton in a carbonyl O-protonated glycine undergoes a rapid migration to the amino nitrogen atom, while the reverse process is extremely unfavorable. The objective of this work is to develop practical theoretical procedures for studying proton transfer reactions in amino acids and peptides and to assemble physical data from these model calculations for future references.     

Theoretical study of intramolecular proton transfer of perylenequinone

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An ab initio molecular orbital study of intramolecular H-bonding: 1,3 propanediol

An ab initio molecular orbital calculation has been carried out for three different conformations of 1,3 propanediol, one of which permits intramolecular H-bond studied by ab initio quantum mechanical methods. The ΔE for H-bonding formation is compated to be 0.9 kcal/mole and the charge redistributions and molecular orbital energy changes are compared to those found in intermolecular H-bonds.     

An ab initio comparative structural study of alkenylphosphonic acid derivatives

A comparative study of the electronic structure and conformational properties of alkenylphosphonic acid derivatives with different substituents has been carried out by means of ab initio quantum mechanical methods. The ab initio calculations have been performed using different basis sets. A strongly polarized partial triple bond for the phosphoryl group has been found. A very weak π conjugation has been detected in the C=C/P=O system. Intramolecular hydrogen bonds have been found in 2-cis-carboxyvinylphosphonic acid.     

A comparative ab initio study of methyl formate, methyl thiolformate and methyl thionoformate

Ab initio SCF-MO calculations have been carried out for HCOOCH₃, HC(=O)SCH₃ and HC(=S)OCH₃. Relative stabilities of s-trans/s-cis conformers are reported and discussed in terms of specific intramolecular interactions. The energy difference between the s-trans and the s-cis form increases in the order methyl thiolformate < methyl thionoformate < methyl formate. The major stabilizing factors of the s-cis forms are the bond dipolar interaction and the mesomeric delocalization through the five member ring involving both the X=C---Y---C (X, Y = O, S) skeleton and the out-of-plane hydrogen atoms. These effects are used to explain the trends mentioned. The non-planarity previously proposed for the thionoester is reinvestigated. Our calculations show that this molecule is planar. Molecular atomic charges, dipole moments and ionisation potentials are determined and compared with available experimental values.     

Double-proton transfer in adenine-thymine and guanine-cytosine base pairs. A post-Hartree-Fock ab initio study

The results of a comprehensive study on the double-proton transfer in Adenine-Thymine (AT) and Guanine-Cytosine (GC) base pairs at room temperature in gas phase and with the inclusion of environmental effects are obtained. The double-proton-transfer process has been investigated in the AT and GC base pairs at the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory. It has been predicted that the hydrogen-bonded bases possess nonplanar geometries due to sp3 hybridization of nitrogen atoms and because of the soft intermolecular vibrations in the molecular complexes. An analysis of the energetic parameters of the local minima suggests that rare AT base pair conformation is not populated due to the shallowness of this minimum, which completely disappears from the Gibbs free energy surface. The stabilization of canonic or rare forms of the DNA bases by water molecules and metal cations has been predicted by calculating the optimal configuration of charges (using differential product/transition state stabilization approach) followed by calculations of the interactions between the base pair and a water/sodium cation.     

A comparative ab initio study of some molecular systems based on hydrazine and formamide

A series of ab initio calculations has been performed on formamide and its hydrogen bonded dimer, s-diformylhydrazine, s-dimethylhydrazine and hydrazine. All systems were assumed planar. The CO, CN and NN bond lengths were optimized in all cases, in order to study the changes of these bonds with variations in the environment. Gross atomic populations and force constants were also considered, and in some calculations the effect of basis enlargements was studied. Significant changes were found in the formamide unit upon dimerization via hydrogen bonds or a NN bond. And, also, a systematic variation was found for the NN bond by going from hydrazine via dimethylhydrazine to diformylhydrazine. The results of this study indicate that the effect of substitution within the framework used is well simulated by ab initio calculations.     

Excited state intramolecular proton transfer in 5-hydroxy flavone: A DFT study

Potential energy surfaces (PES) for the ground and excited state intramolecular proton transfer (ESIPT) processes in 5-hydroxy-flavone (5HF) were studied using DFT-B3LYP/6-31G(d) and TD-DFT/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer (GSIPT) in 5HF. Excited states PES calculations support the existence of ESIPT process in 5HF. ESIPT in 5HF has been explained in terms of HOMO, LUMO electron density of the enol and keto tautomer of 5HF. PES scan by phenyl group rotation suggests that the twisted form, i.e., phenyl group rotated by 18.7° out of benzo-γ-pyrone ring plane is the most stable conformer of 5HF.