Hydration structure in dilute hydrofluoric acid

We have performed the multistate empirical valence bond (MS-EVB) molecular dynamics simulations of a dilute hydrofluoric acid solution at ambient temperature to study the hydration structure associated with its weak acidity. The developed MS-EVB model showed reasonable agreement with experimental and previous ab initio molecular dynamics and reference interaction site model self-consistent field simulations for the free energy and structural properties. The local tetrahedral and translational order parameters around the fluorine atom significantly increase in the transition and product states of the HF dissociation reaction. This indicates that the angular and translational rearrangements of the hydrogen-bond topology are necessary especially around the fluorine atom. At the transition state of the proton transfer, the tetrahedral order parameters are very large, whereas the translational order parameters are not. This suggests that for the proton transfer to occur the large angular rearrangements of the hydrogen-bond topology are more necessary than the translational ones.     

Spectrophotometric determination of silicon in ultrapure, dilute hydrofluoric acid solutions

A new analytical procedure is described for the spectrophotometric determination of silicon in ultrapure, dilute hydrofluoric acid (HF) solutions. The method is a variation of the classical molybdenum blue method, but adds optimised quantities of boric acid to eliminate any HF interference in the colorimetric process. Its sensitivity and detection limit have been evaluated as, respectively, (180 ± 9) · 10^− 5/(μg/L Si) and 7.6 μg/L Si, and its reproducibility has been confirmed. The method has also been successfully applied and validated for the determination of the etch rate of single crystalline silicon surfaces in ultrapure 2% v/v HF solutions.     

Dynamics of proton transfer in bacteriorhodopsin

Proton transfer in bacteriorhodopsin from the cytoplasm to the extracellular side is initiated from protonated asp96 in the cytoplasmic region toward the deprotonated Schiff base. This occurs in the transition from the photocycle late M state to the N state. To investigate this proton-transfer process, a quantum mechanics/molecular mechanics (QM/MM) model is constructed from the bacteriorhodopsin E204Q mutant crystal structure. Three residues, asp96, asp85, and thr89, as well as most of the retinal chromophore and the Schiff base link of lys216 are treated quantum mechanically and connected to the remaining classical protein through linker atom hydrogens. Structural transformation in the M state results in the formation of a water channel between the Schiff base and asp96. Since a part of this channel is lined with hydrophobic residues, there has been a question on the mechanism of proton transfer in a hydrophobic channel. Ab initio dynamics using the CHARMM/GAMESS methodology is used to simulate the transfer of the proton through a partially hydrophobic channel. Once sufficient water molecules are added to the channel to allow the formation of a single chain of waters from asp96 to the Schiff base, the transfer occurs as a fast (less than a picosecond) concerted event irrespective of the protonation state of asp85. Dynamic transfer of the proton from asp96 to the nearest water initiates the organization of a strongly bonded water chain conducive to the transfer of the proton to the Schiff base nitrogen.     

Coherence transfer processes in vibrational relaxation of polyatomic molecules in condensed media

The vibrational relaxation of a polyatomic molecule in a condensed host is studied by a consideration of two molecular vibrations. Relaxation processes, intermode coupling terms and vibrational frequency fluctuation contributions are retained. Population decay (T₁), dephasing (T₂), and coherence transfer rates are evaluated through second order in the limit where the host bath dynamics are rapid compared to these molecular timescales. The rates are expressed in terms of temperature and frequency dependent bath correlation functions. For the special case of a three level system (the ground state and ones where one of the two vibrational modes is excited) the important effects of anharmonicity are incorporated. It is shown that certain coherence transfer terms involve zero frequency bath correlation functions, so they should be larger than the high frequency ones which obey modified energy gap laws. A discussion is presented of the types of interactions which may contribute to these coherence transfer processes.     

Photo-induced relaxation and proton transfer in some hydroxy naphthoic acids in polymers

Photophysical and photochemical properties of 3-hydroxy-2-naphthoic acid [3,2-HNA] and 1-hydroxy-2naphthoic acid [1,2-HNA] in different aprotic, protic, and ion exchange (Nafion) polymers have been described in this article. In both molecules, intramolecular hydrogen bond (IMHB) exists between OH and COOH functional groups. Both 3,2-HNA and 1,2-HNA form different emitting species in different polymeric media. Fluorescence characteristic of 3,2-HNA is found to depend on its concentration, nature of the microenvironment, and wavelength of excitation, while 1,2-HNA is less susceptible to these changes. 3,2-HNA exhibits dual fluorescence band (normal and large Stokes shifted) in aprotic and only a single large Stokes shifted fluorescence in protic polymers, while 1,2-HNA shows a single fluorescence band along with weak phosphorescence emission in these polymers. Both excited-state inter and intramolecular proton transfer (ESPT) take place in 3,2-HNA in aprotic and protic polymers, resulting in large Stokes shifted emission band. A competition between ESIPT and excimer formation is observed by the appearance of rise time on increasing the concentration of 3,2-HNA in protic polymer. In Nafion film, 3,2-HNA is present as a cationic as well as neutral species. The presence of extra protons in Nafion film facilitates excited-state intramolecular proton transfer (ESIPT) in the neutral species of 3,2-HNA and gives large Stokes shifted emission (10 500 cm(-1)). No such effect is observed in 1,2-HNA doped in Nafion film. It is observed that, depending on the position of the IMHB ring, the electronic spectra get modified and the strength of IMHB is affected by the micro-environment of the polymer which alters the photophysics of these molecules.     

Dynamics of pistachio oils by proton nuclear magnetic resonance relaxation dispersion

A number of pistachio oils were selected in order to test the efficacy of nuclear magnetic resonance relaxation dispersion (NMRD) technique in the evaluation of differences among oils (1) obtained from seeds subjected to different thermal desiccation processes, (2) retrieved from seeds belonging to the same cultivar grown in different geographical areas and (3) produced by using seed cultivars sampled in the same geographical region. NMRD measures relaxation rate values which are related to the dynamics of the chemical components of complex food systems. Results not only allowed to relate kinematic viscosity to relaxometry parameters but also were successful in the differentiation among the aforementioned oils. In fact, from the one hand, the larger the kinematic viscosity, the faster the rotational motions appeared as compared to the translational ones. On the other hand, relaxation rate curves (NMRD) varied according to the oxidative stresses and chemical composition of each sample. The present study showed for the first time that NMRD is a very promising technique for quick evaluations of pistachio oil quality without the need for time-consuming chemical manipulations.     

Observation of excited-state proton transfer in green fluorescent protein using ultrafast vibrational spectroscopy

The photodynamics of wtGFP have been studied by ultrafast time-resolved infrared spectroscopy (TIR). In addition to the expected bleaching and transient infrared absorption of bands associated with the chromophore, we observe the dynamics of the proton relay reaction in the protein. Protonation of a protein carboxylate group occurs on the tens of picoseconds time scale following photoexcitation. Comparison with data for mutant GFPs, in which excited-state proton transfer has been disabled, supports the assignment of the carboxylate to the side chain of E222, a component of the hydrogen bonding network that links the two ends of the chromophore. The TIR data show that the rate-limiting step in the proton relay is deprotonation of the chromophore.     

On the existence of H2F+ in dilute hydrofluoric acid solutions

The existence of H₂F⁺, which has been long postulated in dilute aqueous HF solutions, was investigated by fluoride ion-selective electrode potentiometry. Even under conditions conducive to its formation precise measurements yielded only equivocal evidence for its occurrence. It is thought that H₂F⁺ will exist only in very concentrated HF solutions with low water activities.     

Dynamics of competitive reactions: endothermic proton transfer and exothermic substitution

Dynamics of an endothermic proton-transfer reaction, F(-) with dimethyl sulfoxide, and an endothermic proton-transfer reaction with a competing exothermic substitution (S(N)2) channel, F(-) with borane-methyl sulfide complex, were investigated using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) and kinetic modeling. The two proton-transfer reactions have slightly positive and a small negative overall free energy changes, respectively. Energy-dependent rate constants were measured as a function of F(-) ion translational energy, and the resulting kinetics were modeled with the RRKM (Rice-Ramsperger-Kassel-Marcus) theory. The observed rate constants for the proton-transfer reactions of F(-) with dimethyl sulfoxide and with borane-methyl sulfide complex are identical, with a value of 0.17 x 10(-9) cm(3) molecule(-1) s(-1); for the S(N)2 reaction, k = 0.90 x 10(-9) cm(3) molecule(-1) s(-1) at 350 K. Both proton-transfer reactions have positive entropy changes in the forward direction and show positive energy dependences. The competing S(N)2 reaction exhibits negative energy dependence and becomes less important at higher energies. The changes of the observed rate constants agree with RRKM theory predictions for a few kcal/mol of additional kinetic energy. The dynamic change of the branching ratio for the competing proton transfer and the substitution reactions results from the competition between the microscopic rate constants associated with each channel.     

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.     

Intermolecular vibrational relaxation in liquids

The influence of intermolecular vibrational relaxation on dipole moment correlation functions, as obtained from IR band shapes, is discussed. It is explicitly shown that vibrational relaxation due to intermolecular interactions depends on the reorientational behaviour of the molecules in the liquid.Therefore, an a priori separation of the dipole moment correlation function into independent reorientational and vibrational factors is not generally possible. The implications for various procedures used to “correct” Raman and IR band shapes for vibrational relaxation are discussed.The expression derived for the intermolecular vibrational relaxation is used to calculate theoretically the effect of transition dipole-transition dipole coupling on dipole moment correlation functions.Experimental data obtained from isotopic dilution measurements support the interpretation of the isotopic dilution effect in terms of the transition dipole-transition dipole coupling.     

Quantum and dynamical effects of proton donor-acceptor vibrational motion in nonadiabatic proton-coupled electron transfer reactions

This paper presents a general theoretical formulation for proton-coupled electron transfer (PCET) reactions. The solute is represented by a multistate valence bond model, and the active electrons and transferring proton(s) are treated quantum mechanically. This formulation enables the classical or quantum mechanical treatment of the proton donor-acceptor vibrational mode, as well as the dynamical treatment of the proton donor-acceptor mode and the solvent. Nonadiabatic rate expressions are presented for PCET reactions in a number of well-defined limits for both dielectric continuum and molecular representations of the environment. The dynamical rate expressions account for correlations between the fluctuations of the proton donor-acceptor distance and the nonadiabatic PCET coupling. The quantities in the rate expressions can be calculated with a dielectric continuum model or a molecular dynamics simulation of the full system. The significance of the quantum and dynamical effects of the proton donor-acceptor mode is illustrated with applications to model PCET systems.     

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

The infrared emission of CO trapped in solid Ne and Ar is observed at low temperature. The first vibrational level of ¹²C¹⁶O is excited by a Q-switched frequency doubled CO₂ laser. The emission spectrum consists of several lines arising from upper vibrational levels of ¹²C¹⁶O and also of ¹³C¹⁶O and ¹²C¹⁸O which are present in natural abundance. An interpretation is proposed which is based on the assumption that long range dipole—dipole interaction is the main physical process involved in these experiments. Resonance energy transfer produces an energy migration among ¹²C¹⁶O molecules without any change in vibrational populations. Phonon assisted energy transfer takes place between vibrational levels of the various isotopic species present in the solution. In order to satisfy the resonance condition a phonon is emitted or absorbed whose energy compensates for the energy mismatch between the transitions in each interacting molecule due to isotopic effect and or vibrational anharmonicity. The range of this process is greatly extended by energy migration. At the low phonon bath temperature phonon emission is much more probable than phonon absorption. So a strong excitation of upper vibrational levels with in some cases population inversions is observed.Molecular impurities act as efficient quenching centers even at very low concentration. When highly purified samples are used, the fluorescence decay time is found to be 20.6 ms in Ne and 14.5 ms in Ar and does not significantly depend upon concentration and temperature. It is concluded that radiationless relaxation is unimportant.     

Ground- and excited-state intramolecular proton transfer in 3,5-dibromosalicylic acid

The existence of both ground- and excited-state proton transfer equilibrium between the enolic and keto tautomers of 3,5-dibromosalicylic acid has been investigated in aqueous solution using electronic absorption and luminescence emission spectroscopies, together with ab initio and semiempirical MO/CI calculations. The compound provides an example of a room temperature phosphorescence triplet-state emission from a phototautomer (ketonic) form. The interference from radiationless paths, which may in principle avoid the observation of the phosphorescence signal, was overcome by using a protective micellar medium and the presence of an external heavy atom.     

Reorientational and vibrational relaxation in liquid trichloracetonitrile

The first- and second-order reorientational correlation functions were determined from i.r. and Raman bandshapes of ν_s(CN)in liquid trichloracetonitrile and discussed in terms of the J-diffusion model. A consistent fit for G_1r(t) and G_2r(t) was possible for short times only (0–1.5 ps). The vibrational correlation functions, determined from Raman studies of the ν_s(CN) and ν_s(CC) bandshapes, are discussed in terms of recent theories of vibrational dephasing.     

Dielectric relaxation of nitroalkanes in dilute solutions

The relative permittivity at frequencies of 10kHz and 10GHz and the refractive indices have been measured for dilute solutions of (a) nitroethane, (b) 1-nitropropane, (c) 2-nitropropane, and (d) 1-nitrobutane in cyclohexane at 10°, 20°, 30° and 40°C. Materials (b) and (c) together with (e)2-nitro 1-propanol and (f) 2-nitro-l-butanol, have also been studied in different nonpolar solvents. The electric dipole moments, the dielectric relaxation times, and the molar activation energy parameters have been calculated. The results for nitroalcohols show the existence of internal hydrogen bonding between the hydroxyl group and an oxygen atom of the nitro group. The plots of log τ versus log η for materials (b) and (c) are linear for the solvents n-heptane, cyclohexane and decalin. The possibility of interaction of these materials with p-xylene, carbon tetrachloride and p-dioxane are discussed. Activation enthalpy (ΔH_ε) of these materials is observed to be higher in dilute solutions than in pure liquids, and also ΔH_ε for nitrobutane is significantly lower than for the other materials.     

Acidity constant of hydrofluoric acid

A detailed experimental investigation of the association (acidity) constant of hydrofluoric acid has been made at 25°C in sodium perchlorate media at ionic strengths between 0.01 and 0.5M. Results were obtained by potentiometry using a fluoride ion-selective electrode in conjunction with three types of reference electrode, all involving cells with liquid junction. After careful consideration of possible errors in the present results and a critical study of the literature, a zero ionic strength pK _a ^o (HF) value of 3.18±0.02 is recommended until existing contradictions can be resolved.