Spectroscopic studies on proton transfer equilibria: 2,4-dinitrophenol with several electron donors

The proton transfer equilibria of 2,4-dintrophenol with triethylamine, 1,4-diazabicyclo(2,2,2)octane, piperazine, N,N-dimethyl piperazine, N-methylacetamide, N,N-dimethylacetamide, N,N-dimethyl formamide and dimethyl sulfoxide have been studied in cyclohexane/carbon tetrachloride and benzene solvents by electronic absorption spectroscopy. Thermodynamic parameters for the proton transfer equilibria have been determined. It is shown that the extent of interaction between 2,4-dinitrophenol and the aprotic solvent determines the position of equilibrium.     

Picosecond proton transfer studies in water-alcohols solutions

Picosecond proton ejection rates from 8-hydroxy-1,3,6-pyrene trisulfonate and 2-naphthol-6-sulfonate in water-alcohol solutions have been found to decrease rapidly with increasing alcohol concentrations. It was found that the rate of proton transfer decreases more rapidly in water propanol solutions than in water ethanol solutions. This phenomenon can be interpreted in terms of water breaking structure by the organic solvent.     

Semiempirical evaluation for proton affinities of phosphorus compounds

We here report the AM 1 and PM 3 evaluation of proton affinities for phosphorus compounds. The substituent effects, the preferred site of protonation, and the geometry changes produced upon protonation are discussed in terms of the present semiempirical calculations that are compared with experimental data and previous ab initio results. © 1993 John Wiley & Sons, Inc.     

Semiempirical molecular orbital studies of phthalocyanines

A semi-empirical MO method, the PEEL method, has been applied in an investigation of the electronic structure and excited states of copper phthalocyanine. The absorption spectrum of CuPc is discussed and assignments are made. New bands ofn- ^*,d-d and CT types are predicted. The calculated values of ESR parameters accord very well with experimental values, indicating that the method gives a correct electronic distribution.

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Zusammenfassung Eine semiempirische MO-Methode, die PEEL-Methode, wurde bei der Untersuchung der Elektronenstruktur und der angeregten Zustände von Kupferphthalocyaninen angewendet. Das Absorptionsspektrum des CuPc wird interpretiert. Neue Banden vomn- ^*-,d-d- und CT-Typ werden vorhergesagt. Die berechneten Werte der ESR-Parameter stimmen gut mit den experimentellen Werten überein, wodurch gezeigt wird, daß die Methode eine korrekte Elektronenverteilung ergibt.

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Résumé Une méthode d'OM semi-empirique, la méthode PEEL, est appliquée à l'étude de la structure électronique et des états excités de la phtalocyanine du cuivre. Le spectre d'absorption de CuPc est discuté et des attributions sont effectuées. De nouvelles bandes de typen- ^*,d-d et CT sont prédites. Les valeurs calculées des paramètres ESR sont en bon accord avec les valeurs expérimentales, indiquant que la méthode donne une distribution électronique correcte.

     

Spectroscopic studies on proton transfer and hydrogen bonding in p-nitrophenol-donor systems

The interaction of p-nitrophenol with several electron-donors has been studied in aprotic and protic solvents by electronic absorption spectroscopy. The equilibrium data for different kinds of equilibria in 1,2-dichloroethane, n-butanol and acetonitrile have been obtained and discussed. The strong hydrogen bonding interaction between p-nitrophenol and a variety of electron donors has been investigated in heptane by electronic absorption spectroscopy. The interesting correlations obtained in terms of Mulliken's charge transfer model have been examined and extended in many cases.     

Semiempirical molecular orbital studies of phthalocyanines

Theπ andσ lone pair electron system of the phthalocyanine molecule has been studied by a semiempirical SCF-MO method. Electronic transitions of bothπ-π ^* andn-π ^* types have been considered. The excited states have been calculated by means of the method of superposition of configurations where all singly excited states are included. Assignments for the electronic spectrum of phthalocyanine could be made in good agreement with experiment. The position of the lowest electronically allowedn-π ^* transition is predicted to be found in the region of the strong Soret band.     

Comprehensive studies on an overall proton transfer cycle of the ortho-green fluorescent protein chromophore

Initiated by excited-state intramolecular proton transfer (ESIPT) reaction, an overall reaction cycle of 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the green fluorescent protein (GFP), has been investigated. In contrast to the native GFP core, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI), which requires hydrogen-bonding relay to accomplish proton transfer in vivo, o-HBDI possesses a seven-membered-ring intramolecular hydrogen bond and thus provides an ideal system for mimicking an intrinsic proton-transfer reaction. Upon excitation, ESIPT takes place in o-HBDI, resulting in a ～600 nm proton-transfer tautomer emission. The o-HBDI tautomer emission, resolved by fluorescence upconversion, is comprised of an instantaneous rise to a few hundred femtosecond oscillation in the early relaxation stage. Frequency analysis derived from ultrashort pulse gives two low-frequency vibrations at 115 and 236 cm(-1), corresponding to skeletal deformation motions associated with the hydrogen bond. The results further conclude that ESIPT in o-HBDI is essentially triggered by low-frequency motions and may be barrierless along the reaction coordinate. Femtosecond UV/vis transient absorption spectra also provide supplementary evidence for the structural evolution during the reaction. In CH(3)CN, an instant rise of a 530 nm transient is resolved, which then undergoes 7.8 ps decay, accompanied by the growth of a rather long-lived 580 nm transient species. It is thus concluded that following ESIPT the cis-proton transfer isomer undergoes cis-trans-isomerization. The results of viscosity-dependent dynamics are in favor of the one-bond-flip mechanism, which is in contrast to the volume-conserving isomerization behavior for cis-stilbene and p-HBDI. Further confirmation is given by the picosecond-femtosecond transient IR absorption spectra, where several new and long-lived IR bands in the range of 1400-1500 cm(-1) are assigned to the phenyl in-plane breathing motions of the trans-proton transfer tautomer. Monitored by the nanosecond transient absorption, the 580 nm transient undergoes a ～7.7 μs decay constant, accompanied by the growth of a new ～500 nm band. The latter is assigned to a deprotonated tautomer species, which then undergoes the ground-state reverse proton recombination to the original o-HBDI in ～50 μs, achieving an overall, reversible proton transfer cycle. This assignment is unambiguously supported by pump-probe laser induced fluorescence studies. On these standpoints, a comparison of photophysical properties among o-HBDI, p-HBDI, and wild-type GFP is discussed in detail.     

Semiempirical studies on the structure and bonding of fluorosulfuranes and aminofluorosulfuranes

The structures of sulfur tetrafluoride, SF₄, dimethylaminosulfur trifluoride, Me₂NSF₃, and bis(dimethylamino)sulfur difluoride, (Me₂N)₂SF₂ have been investigated using the PM3 semiempirical method. Full geometry optimizations do not agree with experimentally determined geometries. Constraining the F_ax-S-F_ax angle significantly improves the agreement.     

Time-resolved studies of water dynamics and proton transfer at the alumina-air interface

The present study aims to understand the dynamical properties of water and OH groups layered on an alumina surface mainly by means of femtosecond IR-pump IR-probe transient absorption spectroscopy. The experimental results obtained demonstrate the existence of several kinds of O-H vibrators on the surface of alumina membranes, distinguishing them by their behavior on the femtosecond time scale and by the anisotropy of their spectral response. In the high-frequency region (>3400 cm-1), the absorption is due to well-packed aluminol groups and to physisorbed water patches on the surface. When pumping at 3200 cm-1, physisorbed water hydrogen-bonded to AlOH2+ groups is observed. The anisotropy measurements demonstrate the existence of an efficient energy-transfer mechanism among the water molecules characterized by a time constant of 400 +/- 100 fs. The persisting anisotropy at long times, especially in the case of AlOH groups and of the structured physisorbed water layer on top of them, proves the anisotropic structuring induced by the surface. The excitation at 3000 cm-1 enables the detection of a photon-induced proton-transfer reaction. The proton back-transfer reaction time constant is 350 +/- 50 fs. From anisotropy measurements, we estimate the proton hopping time to be 900 +/- 100 fs in a locally extended water network lying on the surface.     

Semiempirical studies of core-electron binding energy shifts

The inner-core binding-energy shifts (BEs) of boron and carbon atoms in various chemical environments were studied by the semiempirical Self-Consistent Charge Molecular Orbital (SCC MO) method. The calculations are based on the initial ground state electrostatic potential model. The main feature of our approach is the empirical treatment of the coefficient relating BEs with the orbital populations of the host atom and the Madelung energy term. These adjustable parameters absorb a large portion of relaxation energy. The so obtained results are in good agreement with experimental data. They are better than earlier CNDO/2 results obtained by using either ground state or relaxation potential models. Present results indicate that semiempirical methods like SCC MO are able to account for changes in BE(1s) with a fair accuracy although the inner-shell electrons are not explicitly considered in the actual calculations.     

Semiempirical quantum mechanical studies on some hydrogen bonded systems

The CNDO/2 method is used to study the hydrogen bond strength in several representative systems. Enthalpies of hydrogen bond formation are generally predicted fairly well although some exceptions are noted. The proton-proton acceptor distances are predicted to be about 10 percent too small in all cases where comparison with experiment can be made.

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Zusammenfassung Mit Hilfe der CNDO/2-Methode wird in verschiedenen repräsentativen Systemen die Stärke von Wasserstoffbrücken untersucht. Die Enthalpien werden bis auf wenige Ausnahmen recht gut vorausgesagt. In allen Fällen, in denen experimentelle Unterlagen vorhanden sind, werden die Abstände der Proton-Proton-Akzeptoren urn etwa 10% zu klein vorausgesagt.

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Résumé La méthode CNDO/2 est utilisée pour étudier le force de la liaison hydrogène dans quelques systèmes représentatifs. Les enthalpies de formation de la liaison hydrogène sont en général convenablement prédites à quelques exceptions près. La distance entre le proton et l'accepteur de proton est prédite avec une erreur par défaut d'environ 10% dans les cas où le résultat expérimental est connu.

     

Solvent effects on the fluorescence quenching of tryptophan by amides via electron transfer. Experimental and computational studies

Hybrid quantum mechanical/molecular mechanics (QM-MM) calculations [Callis and Liu, J. Phys. Chem. B 2004, 108, 4248-4259] make a strong case that the large variation in tryptophan (Trp) fluorescence yields in proteins is explained by ring-to-backbone amide electron transfer, as predicted decades ago. Quenching occurs in systems when the charge transfer (CT) state is brought below the fluorescing state (1L(a)) as a result of strong local electric fields. To further test this hypothesis, we have measured the fluorescence quantum yield in solvents of different polarity for the following systems: N-acetyl-L-tryptophanamide (NATA), an analogue for Trp in a protein; N-acetyl-L-tryptophan ethyl ester (NATE), wherein the Trp amide is replaced by an ester group, lowering the CT state energy; and 3-methylindole (3MI), a control wherein this quenching mechanism cannot take place. Experimental yields in water are 0.31, 0.13, and 0.057 for 3MI, NATA, and NATE, respectively, whereas, in the nonpolar aprotic solvent dioxane, all three have quantum yields near 0.35, indicating the absence of electron transfer. In alkyl alcohols the quantum yield for NATA and NATE is between that found for water and that found for dioxane, and it is surprisingly independent of chain length (varying from methanol to decanol), revealing that microscopic H-bonding, and not the bulk dielectric constant, dictates the electron transfer rate. QM-MM calculations indicate that, when averaged over the six rotamers, the greatly increased quenching found in water relative to dioxane can be attributed mainly to the larger fluctuations of the energy gap in water. These experiments and calculations are in complete accord with quenching by a solvent stabilized charge transfer from ring to amide state in proteins.     

Conjugation and proton exchange equilibria I. Significant role of proton exchange and homoconjugation equilibria in protonic hetero systems

The significant role of the proton exchange and homoconjugation (formation of AHA^– and A₁HA ₁ ^– complexes) equilibria in protonic hetero systems (HA+A ₁ ^– ) has been shown from analysis of published data and from my own simple experiments. It is concluded that there is a need to reconsider some basic hydrogen bond problems. One of these problems-the existence of heteroconjugates in polar solvents — has been verified and illustrated with suitable experimental evidence (IR and UV-VIS spectra). The most accepted experimental evidence for the double minimum potential in the hydrogen bridge has been questioned as being in disagreement with the equilibrium law. A simple relation between the heteroconjugation constant, the parent homoconjugation constants and the proton exchange constant is also postulated. This relation fulfills important theoretical requirements, however, it is not applicable in extreme cases.To the memory of Bogdan Przybyszewski.     

Semiempirical molecular-orbital studies on the problem of nonclassical resonance in the homoallylic cation

The quantum-mechanical methods (EHT and CNDO /2) have been applied to the homoallylic cation. Both methods successfully predict that the ion need not be represented as a nonclassical structure.     

Thermal studies on N-substituted maleic acid amides

Eight N-substituted maleic acid monoamide derivatives were studied by DSC, TG (DTG) and IR techniques. The thermal studies revealed that the compounds containing a free carboxyl group start to decompose before melting, and the decomposition continues in the melt phase as the temperature is elevated. This was explained by the presence of dimers involving strong intermolecular hydrogen-bonds. This assumption concerning the structure was supported by the results of the IR spectroscopic studies.

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Zusammenfassung Mittels DSC, TG (DTG) und IR-Techniken wurden acht N-substituierte Maleinsäuremonoamidderivate untersucht. Die thermischen Untersuchungen zeigen, daß die Zersetzung von Verbindungen mit freier Carboxylgruppe schon vor dem Schmelzen beginnt und der Zersetzungsprozeß dann in der Schmelze bei Anheben der Temperatur fortgesetzt wird. Dies wurde mit der Existenz von Dimeren mit starken intermolekularen Wasserstoffbrückenbindungen erklärt. Diese Annahme zur Struktur wurde durch die Ergebnisse der IR-spektroskopischen Untersuchungen bekräftigt.

     

Stereochemical studies: XVII—the proton coupling constants and conformational equilibria of trans-1,2-disubstituted cyclohexenes

The vicinal ³J_aa and ³J_ee spin-spin coupling constants of a number of deuterated trans-1,2-disubstituted cyclohexenes and the ΔH and ΔS values of the conformational equilibria of these compounds have been determined by computer optimisation of the ³J(HH) = f(T) function. Compounds with —CF₃ and CCl₃ substituents were shown to have an enhanced proportion of the diaxial conformer.     

A potential function for computer simulation studies of proton transfer in acetylacetone

A potential energy model is developed to study the intramolecular proton transfer in the enol form of acetylacetone. It makes use of the empirical valence bond approach developed by Warshel to combine standard molecular mechanics potentials for the reactant and product states to reproduce the interconversion between these two states. Most parameters have been fitted to reproduce the key features of an ab initio potential surface obtained from 4-31G^* Hartree-Fock calculations. The partial charges have been fitted to reproduce the electrostatic potential surface of 6-31G^* Hartree-Fock wave functions, subject to total charge and symmetry constraints, using a fitting procedure based on generalized inverses. The resulting potential energy function reproduces the features most important for proton transfer simulations, while being several orders of magnitude faster in evaluation time than ab initio energy calculations. © 1997 by John Wiley & Sons, Inc.     

Spectroscopic studies on the proton transfer and hydrogen bonding interactions between p-nitrophenol and aliphatic tertiary amines

The interaction of p-nitrophenol with aliphatic tertiary amines, namely triethylamine, tributyl-amine and trioctylamine, has been examined in a variety of solvents by means of electronic absorption spectroscopy. The acid—base interaction in these systems leads to various types of complexes depending upon the dielectric constant of the medium. The equilibrium data for different kinds of equilibria have been evaluated and discussed. The pK-equilibrium constant plots are found to be linear.