Proton Transfer Potential Research Articles

Relationship between strength of hydrogen bond and barrier to proton transfer

Barriers to proton transfer are computed using ab initio methods for a number of systems, charged and uncharged, that contain either intramolecular or intermolecular H-bonds. It is shown that the very strong H-bonds that occur in ionic H-bonds between electronegative atoms like O or F are characterized by proton transfer potentials that have a single symmetric minimum. The transfer barrier that separates the two minima in less strongly-bound complexes rises as the H-bond weakens, as a result of involvement of less electronegative atoms. The weakening is evident from the equilibrium H-bond lengths, as well as the calculated energetics. The strain introduced into intramolecular H-bonds can act to raise the proton transfer barrier. Nonetheless, the relationship between weaker H-bonds and lower barriers is apparent, even in excited electronic states.

The neglect of diatomic differential overlap (NDDO) fragment self-consistent field method for the treatment of very large covalent systems

We present a semiempirical NDDO procedure, called the fragment SCF (FSCF) method, to treat very large molecules. The covalent system is partitioned into a relatively small subsystem where substantial chemical changes take place and an environment that remains more-or-less unperturbed during the process. We expand the wavefunction on an atomic hybrid basis and perform an SCF procedure for the subsystem in the field of the iteratively determined electronic distribution of the environment. We wrote a program for the IBM RISC/560 computer and did several test calculations for a variety of large classical molecules. Protonation energies, proton transfer potential curves, rotational barriers, atomic net charges, and HOMO and LUMO energies, as computed by the exact version of the NDDO method, are fairly well reproduced by our approximation. Using the FSCF method, we calculated the molecular electrostatic potential on the van der Waals envelopes of the specificity pocket of trypsin and the lysine side chain of the bound substrate and visualised electrostatic complementarity. We developed a novel bulk phase Monte Carlo simulation technique and calculated the energy by the above approximation and applied the method to amorphous silicon (a-Si). Starting from a distorted tetrahedrally bonded random network model of a-Si with 216 atoms, we performed Monte Carlo simulations using the FSCF energy calculation. For the second and subsequent configurations, we exploited the feature of the Metropolis– Teller algorithm, namely, that, to generate a new configuration, we displace only a single atom. Thus the number of integrals to be calculated drastically decreases since only those have to be reevaluated that contain the coordinates of the displaced atom. After equilibration we obtained distribution functions being almost identical to the one corresponding to the distortion free tetrahedrally bonded network. The same technique was applied to liquid chlorosilanes. We found that SiCl bonds elongate by 6 to 16 pm while H–Si–Cl and Cl–Si–Cl angles change by 2–4° as compared to the gas phase. © 1994 John Wiley & Sons, Inc.

Ab initio potential energy functions for proton transfer in [H 3N … H … NH 3] + and [H 3N … H … OH 2] +

Based on ab initio MP2 calculations with the 6-31G ∗ basis set, as well as medium-size polarized basis sets, we updated analytic functions for proton transfer potentials in the proton bound ammonia-ammonia and ammonia-water dimers. These functions exhibit proper asymptotic behavior and reproduce very well the numerical data. In addition, an updated version of the Φ 4 potential for the ammonia dimer is also presented. The optimized potential energy functions can be used in quantum and quantum-classical molecular dynamics simulations.

Modeling proton transfer potentials in angularly deformed hydrogen bonds

Earlier work demonstrated that either a simple fourth-order polynomial or a pair of Morse functions could be fit with high accuracy to model proton transfers across H-bonds. The work is extended here to a systematic set of angular distortions in the H2O‥H+‥OH2 and H3N‥H+‥NH3 systems. So long as the deformation does not impose a left-right asymmetry into the system, either of these types of functions can reproduce ab initio transfer potentials well. But the Morse potentials are superior in that neither stretches nor bends of the H-bonds cause large variation in the parameters. For those modes of angular distortion which introduce asymmetry into the transfer potential, the ab initio data can be accommodated by permitting small variations in two of the parameters in the Morse functions. © 1993 John Wiley & Sons, Inc.

Ground and excited state intramolecular proton transfer in OCCNN ring

Molecular orbital calculations reveal that the keto tautomer of HOCHCHNHNCH2, unlike the enol, does not represent a minimum in the ground state proton transfer potential. Excitations to the first excited singlet or triplet state leads to a rapid and barrierless transfer to the keto which becomes strongly preferred. Similar conclusion apply to a larger system with a full aromatic ring appended.

Hydrogen bonding and proton transfers of the amide group

Ab initio methods are used to probe the proton-bound complex involung a water molecule and an amide, modeled by formamide or acetamide. A polarized basis set was applied in conjunction with MP2 treatment of electron correlation. This approach affords a good reproduction of experimental proton affinities of the species involved. The O atom of the amide is the preferred site of protonation or complexation with the water, with acetamide binding most strongly to the water. The proton-transfer potential of cach complex contains a single minimum corresponding to H 2 NHCOH + ...OH 2 , due to the more basic character of the amide oxygen

Calculation of barriers to proton transfer using variations of multiconfiguration self-consistent-field methods. II. Configuration interaction

Various means are tested of including additional electron correlation into multiconfiguration self-consistent-field (MCSCF) methods for computing proton transfer potentials in HF2−, H7N2+, H3O2−, and H5O2+. Configuration interaction allowing single excitations (CIS) and configuration interaction with single + double excitations (CISD) calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The CISD results are excellent, being fairly independent of choice of reference space although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied molecular orbitals prior to the MCSCF part of the calculation results in a further improvement since it is possible to use a smaller number of occupied orbitals and thereby allow more virtuals to be included. These results are compared to configuration interaction computations using the canonical orbitals and which are not preceded by MCSCF preparation of the wave function.

Calculation of barriers to proton transfer using multiconfiguration self-consistent-field methods. I. Effects of localization

The usefulness of multiconfiguration self-consistent-field (MCSCF) calculations in computing correlated proton transfer potentials is investigated for the systems HF2−, H7N2+, H3O2−, and H5O2+. In deciding whether to include particular molecular orbitals, it is important to consider the balance of electron density between the donor and acceptor groups and the interactions that are incorporated in the orbitals. Only orbitals which have the proper symmetry to interact with the transferring hydrogen need be included in the MCSCF active space. Reasonable transfer barriers are obtained when the orbitals are balanced and only interactions relevant to the transfer process are allowed in the MCSCF active space. Equivalent barriers are determined, but the criteria are more easily met, if the canonical molecular orbitals are first subjected to a localization. Only the two localized molecular orbitals that contain the F, N, or O interaction with the transferring hydrogen are needed, which reduces the difficulty of eliminating unproductive interactions. In addition, the localization allows additional virtual orbitals to be included without producing an undesirable correlation.

NDDO fragment self‐consistent field approximation for large electronic systems

AbstractA semi‐empirical NDDO method, generalized from a similar scheme at the CNDO/2 level developed previously, is presented to treat very large molecules. The extended molecular system is divided into a relatively small subsystem where substantial chemical changes take place and an environment remaining more‐or‐less unperturbed during the process. Expanding the wave function on an atomic hybrid basis an SCF procedure is performed for the subsystem in the field of the iteratively determined electronic distribution of the environment. A computer program has been written for the IBM RISC System/6000 530 computer and several test calculations were done for a variety of large classical molecules, like substituted aliphatic hydrocarbons, water oligomers, and a heptapeptide. Protonation energies, proton transfer potential curves, rotational barriers, atomic net charges, and HOMO and LUMO energies, as computed by the exact version of the NDDO method, are fairly well reproduced by our approximation if the subsystem is appropriately defined. © 1992 by John Wiley & Sons, Inc.

Analytic functions fit to proton transfer potentials

Proton transfer potentials are traced out in (H 2OH 2OH +OH 2OH 2) and (H 3NH 3NH +NH 3NH 3) by ab initio computations for a series of different H-bond lengths. Attempts are then made to fit these quantum mechanical results by various forms of analytic functions. Best results are achieved by a pair of Morse functions with correlation coefficients in excess of 0.997. The numerical values of the Morse parameters are fairly insensitive to H-bond length, allowing their use in more general situations. The Φ 4 function and its related fourth-order polynomial also fit well, but the parameters are much more sensitive to H-bond length. Gaussian-type functions or a Lippincott—Schroeder potential do not fit as well and a sinusoidal function gives rather poor agreement with the quantum mechanical results.

Calculation of barriers to proton transfer using a variety of electron correlation methods

The usefulness of various combinations of MCSCF and CI methods in computing correlated proton transfer potentials is investigated for the systems, HF, H7N, H3O, and H5O. MCSCF calculations can accurately determine proton transfer barriers, provided the correlation is limited to the proton transfer process. The proper correlated space can be obtained more easily if the canonical occupied MOs are first subjected to a localization. Various means are tested of including additional electron correlation into the MCSCF methods. CIS and CISD calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The MCSCF + CISD results are excellent, being fairly independent of choice of virtual Mos, although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied MOS prior to the MCSCF part of the calculation again results in a further improvement. These results are compared to CI computations using the canonical orbitals (and which are not preceded by MCSCF preparation of the wave function) and to Møller–Plesset results. © 1992 John Wiley & Sons, Inc.

Correlated proton transfer potentials. (HO-H-OH) − and (H 2O-H-OH 2) +

Potentials are computed for the transfer of a proton between two hydroxide anions in (HO-H-OH) − and between two neutral water molecules in (H 2O-H-OH 2) + using polarized basis sets of various sizes. SCF transfer barriers are lowered in both complexes by electron correlation. As progressively higher orders of Moller-Plesset perturbation theory are applied, the correction reverses sign and diminishes in magnitude. As a consequence, MP2 results nearly coincide with full fourth-order data. The highest level calculations indicate the transfer barrier of (HO-H-OH) − exceeds that of (H 2O-H-OH 2) + by 20–30%. The MP4 calculations agree well with coupled cluster CCD + ST results which include single, double, and triple substitutions to high order.

Molecular phosphorescence enhancement via tunneling through proton-transfer potentials

A set of four substituted aminosalicylates, whose respective structures permit normal fluorescence, twisted intramolecular charge-transfer fluorescence, and proton-transfer fluorescence, in various combinations, were studied at 77 K to observe the effect on intersystem crossing. It is shown that the molecular structures which are capable of intramolecular proton transfer exhibit greatly enhanced normal molecule phosphorescence. The excitation mechanism involves proton-tunneling S{sub 1}{yields}S{sub 1}{prime}(PT) after primary excitation, enhanced intersystem crossing from the tautomer excited state S{sub 1}{prime}(PT){yields}T{sub 1}{prime}(PT), followed by reverse proton-transfer tunneling via triplet potential T{sub 1}{prime}(PT){yields}T{sub 1}. The competitive singlet-state excitation processes are delineated by picosecond transient absorption spectrometry. The mechanism described requires the specific electronic state energy ordering S{sub O} < S{sub O}{prime} < T{sub 1} < T{sub 1}{prime} < S{sub 1}{prime} < S{sub 1} {hor ellipsis} where the primes represent the states of the tautomer form of the molecule.

Factors contributing to distortion energies of bent hydrogen bonds. Implications for proton-transfer potentials

ADVERTISEMENT RETURN TO ISSUEPREVArticleFactors contributing to distortion energies of bent hydrogen bonds. Implications for proton-transfer potentialsSlawomir M. Cybulski and Steve ScheinerCite this: J. Phys. Chem. 1989, 93, 17, 6565–6574Publication Date (Print):August 1, 1989Publication History Published online1 May 2002Published inissue 1 August 1989https://doi.org/10.1021/j100354a055RIGHTS & PERMISSIONSArticle Views40Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Relationship between the angular characteristics of a hydrogen bond and the energetics of proton transfer occurring within

For a number of different H-bonded complexes, the proton transfer potential is computed by ab initio methods for the optimal geometry as well as for various angular distortions. In all cases, it is found that a rocking of one subunit A so as to turn its dipole moment away from the other subunit B causes the equilibrium position of the proton to shift toward A, even if B has a higher proton affinity than A. This shift may be explained on the basis of a better charge-dipole attraction in the preferred AH··B configuration as compared to A··HB. This principle is straightforward for the N bases where the dipole moment is approximately coincident with the N lone pair. The presence of two O lone pairs, neither of which points along the direction of the molecular dipole moment, adds a second factor which results in a net magnification of the above rule.

Long-range interactions in model DNA systems

The role of the long-range interactions has been examined in model doubly stranded DNA systems within the semiempirical approach. The essential influence of long-range corrections to the proton transfer potential was found, in general to result in a notably more unsymmetrical potential energy curve. However, in the case of singlet excited ( n, π*) electronic states a much more symmetrical potential is predicted. It is concluded that highly polar sugar—phosphate species are of essential importance for interaction energy components (mainly electrostatic terms) and thus for related proton transfer processes.

Long-range interactions in model DNA systems

The role of the long-range interactions has been examined in model doubly stranded DNA systems within the semiempirical approach. The essential influence of long-range corrections to the proton transfer potential was found, in general to result in a notably more unsymmetrical potential energy curve. However, in the case of singlet excited (n, π*) electronic states a much more symmetrical potential is predicted. It is concluded that highly polar sugar—phosphate species are of essential importance for interaction energy components (mainly electrostatic terms) and thus for related proton transfer processes.

Effects of external ions on the dynamics of proton transfer across a hydrogen bond

ADVERTISEMENT RETURN TO ISSUEPREVArticleEffects of external ions on the dynamics of proton transfer across a hydrogen bondM. M. Szczesniak and Steve ScheinerCite this: J. Phys. Chem. 1985, 89, 9, 1835–1840Publication Date (Print):April 1, 1985Publication History Published online1 May 2002Published inissue 1 April 1985https://doi.org/10.1021/j100255a059RIGHTS & PERMISSIONSArticle Views50Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (722 KB) Get e-Alerts

Possible role of long-range interactions in the proton transfer of model DNA systems.

The possible role of the long-range interactions has been examined within the semiempirical approach for model doubly stranded DNA systems involving the screw symmetry operation. The interaction energy terms seem to be sensitive to the sequence of base pairs. The essential influence of long-range corrections to the proton transfer potential was found resulting in a remarkably more unsymmetrical energy curve. Only in the case of the singlet excited (n, pi*) electronic state of the A...T base pair, more symmetrical potential is predicted. It is concluded that highly polar sugar-phosphate species are of significant importance for the interaction energy components as well as for related proton transfer processes.

Proton transfers between first- and second-row atoms: (H2OHSH2)+ and (H3NHSH2)+

A b initio molecular orbital methods are used to study the transfer of the central proton along the hydrogen bonds in (H2OHSH2)+ and (H3NHSH2)+. Proton transfer potentials are generated using the 4-31G* basis set at the Hartree–Fock level for various values for the hydrogen bond length R(XS). Full geometry optimizations are carried out at each stage of proton transfer. The barrier to proton transfer increases as the hydrogen bond is lengthened. For a given bond length, the highest barriers are observed for transfer from N to S and the smallest for the reverse process. Intermediate between these two extremes are transfers between O and S for which the forward and reverse transfers lead to nearly identical barrier heights. Adiabatic transfers, in which the intermolecular separation is allowed to change as the transfer progresses, are studied as well. The barrier to adiabatic transfer from OH2 to SH2 is 2.6 kcal/mol; 1.9 for the reverse process. Similar relaxation of R(NS) leads to no stable (NH3)(SH3)+ structure and hence transfer from N to S is not expected. Application of larger basis sets and inclusion of correlation effects through second and third-order M≂ller–Plesset corrections support the reliability of the HF/4-31G* results.