Topological and thermodynamic investigations of molecular interactions in binary mixtures: Molar excess volumes and molar excess enthalpies

Molar excess volumes and molar excess enthalpies of butyl acetate (i) with cyclohexane or benzene or toluene or o-, m- or p-xylene (j) binary mixtures have been measured dilatometrically and calorimetrically over the entire composition range at 308.15 K. The observed data have also been analyzed in terms of graph theoretical approach. The analysis of V^E data by graph theoretical approach suggests that butyl acetate in pure state exists as associated entity and (i + j) mixtures are characterized by the presence of (i–j) molecular entity. It has further been observed that V^E and H^E values calculated by this approach agree well with the corresponding experimental values. The presence of molecular entity is further confirmed by IR study of (i + j) mixture.     

Determination of hydrogen ion activities in various ternary water/methanol/dioxane solvent systems

The practical pH values for solutions in ternary water/methanol/dioxane solvents measured by a pH meter standardized with aqueous buffer solutions do not lie on the conventional scale of hydrogen ion activity referred to the standard state in the corresponding medium (pa^*_H). These values can be converted to pa^*_H by the introduction of a correction term δ = $\text{E}\text{?}$_j ? log(_mγ_H) (where $\text{E}\text{?}$_j is a term incorporating the liquid junction potential, which depends on the solvent composition and _mγ_H is the medium effect on hydrogen ion). Values of δ were determined for various ternary solvents at 25°C and were found to be constant in each medium independent of the solute composition.     

Buffers for the Physiological pH Range: Studies of 4-(N-Morpholino)butanesulfonic Acid (MOBS) from 5 to 55 °C

In this study, we report the pH values of two buffer solutions without chloride ion and eight buffer solutions with NaCl with an ionic strength I=0.16 mol?kg^?1. Electromotive force (emf) techniques have been used to get the cell potentials at 12 temperatures from 5 to 55?°C, including 37?°C. An extended form of the Bates-Guggenheim convention is used in the entire ionic strength range, 0.04 to 0.16?mol?kg^?1. The residual liquid junction potentials (??E _j ) of the buffer solutions of MOBS have been estimated from previous measurements with a flowing junction cell. These values of ??E _j have been used for correction in order to ascertain the operational pH values of four buffer solutions of MOBS at 25 and 37?°C. These solutions are recommended as pH standards for physiological application in the pH range 7.4 to 7.7.     

State-to-state dynamics of the hydrogen exchange reaction

Nascent product quantum state distributions have been measured for reactive, H + D₂ → HD + D, and inelastic, H + D₂ → D₂? + H, collisions of H with D₂, at collision energies of 0.98 eV, 1.1 eV, and 1.3 eV. These distributions are extracted from coherent anti-Stokes Raman scattering (CARS) spectra of HD and D₂ (recorded under single-collision conditions) following laser photodissociation of HI to generate translationally hot H atoms in a D₂/HI gas mixture. Variation of the photolysis wavelength allows selection of the H atom translational energy. These rotational and vibrational state distributions are compared with those obtained from quasiclassical trajectory calculations on an ab initio potential energy surface. The agreement between the calculated and measured distributions is extremely good. TheHD and D₂ product quantum state distributions are well represented by simple linear surprisal functions, with large positive vibrational and rotational surprisal.     

Molar excess enthalpies of ternary mixtures of non-electrolytes

Molar excess enthalpies, H^E_ijk(T₁, x_i, x_j), for methylenebromide (i)+pyridine (j)+β-picoline (k); pyridine (i)+β-picoline (j)+cyclohexane (k); benzene (i)+toluene (j)+1,2-dichloroethane (k); benzene (i)+o-xylene (j)+1,2-dichloroethane (k); and benzene (i)+p-xylene (j)+1,2-dichloroethane (k) mixtures have been measured calorimetrically as a function of temperature and composition. The data have been analysed in terms of the Sanchez and Lacombe theory and using an approach employing the “graph theoretical” concept of connectivity parameters to characterize its pure components. It has been observed that the H^E_ijk (T, x_i, x_j) data calculated from the “graph theoretical” approach using ³ξ values based on δ^v considerations (that take into consideration the valency of individual atoms of the molecular graph constituent components) best reproduces the corresponding experimental H^E_ijk data.     

Transfer of state multipoles in excited A 1Σ+u7Li2 following rotationally inelastic collisions with He: Experiment and theory

Circularly polarized dye laser radiation is used to prepare rotational levels j = 1 to j = 20 of the A ¹Σ⁺_u excited state of ⁷Li₂ with well defined values of the state multipoles K = 0, 1 and 2. Inelastic collisions with helium atoms populate other j levels and we have measured the circular polarisation ratio of emission, C, from these levels. C is plotted versus final j′ for each value of Δj from +2 to +18 and a family of curves is obtained which may be used as a critical test of current theories. The results are interpreted in terms of cross sections σ^K for transfer of the state multipoles under isotropic collision conditions. The observation of substantial polarisation following inelastic collision indicates that the σ^K are dominated by certain restricted scattering channels. Relative magnitudes of the multipole cross sections are calculated using the “l-dominant”, “restricted Δm_j channels” nd the Born approximation. These cross sections are then used to calculate C.     

Resonance-enhanced multiphoton ionization of molecular hydrogen via the E,F1Σg+ state: Photoelectron energy and angular distributions

Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H₂ X¹Σ_g⁺ (ν = 0,J) + 2hv → E,F¹Σ_g⁺(ν_E,J_E = J) + hν → H₂⁺X²Σ_g⁺ (ν⁺) + e^?, for ν_E = 0, 1, or 2 and for J_E = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν⁺ = ν_E, the detailed H₂⁺ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the J_E value of the intermediate state and the ν⁺ value of the ion.     

Pressure and isotope effects on the proton jump of the hydroxide ion at 25°C

The limiting molar conductances Λ⁰ of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 25°C as a function of pressure to disclose the difference in the proton-jump mechanism between an OH^? (OD^?) and a H₃O⁺ (D₃O⁺) ion. The excess conductance of the OD^? ion in D₂O λ _E ^O (OD ^-), as estimated by the equation $$\lambda _E^O (OD^ - ) = \Lambda ^O (KOD/D_2 O) - \Lambda ^O (KCl/D_2 O)$$ increases a little with pressure as well as the excess conductance of the OH^? ion in H₂O $$\lambda _E^O (OH^ - ) = \Lambda ^O (KOH/H_2 O) - \Lambda ^O (KCl/H_2 O)$$ However, their rates of increase with pressure are much smaller than those of the excess deuteron and proton conductances, λ _E ^O (D ⁺) and λ _E ^O (H ⁺). With respect to the isotope effect on the excess conductance, λ _E ^O (OH ^-)/λ _E ^O (D ⁺) decreases with presure as in the case of λ _E ^O (H ⁺)/λ _E ^O (D ⁺), but the value of λ _E ^O (OH ^-)/λ _E ^O (OD ^-) itself is much larger than that of λ _E ^O (H ⁺)/λ _E ^O (D ⁺) at each pressure. These results are ascribed to the difference in the pre-rotation of water molecules, which is brought about by the difference in the intial orientation of the rotating water molecule adjacent to the OH^? (OD^?) or the H₃O⁺ (D₃O⁺) ion.     

The born approximation as a simple diagnostic method for direct molecular collisions with applications to Cl + HI and F + H2 reactions

Born approximation computations are presented and discussed for the Cl + HI → I + HCl and F + H₂ → H + HF reactions and their isotopic analogues. Most aspects of the role of reagent energy or the energy disposal in the products previously deduced from experiment or trajectory computations can be accounted for the Born approximation. The procedure used here neglects the interaction between non-bonded atoms. It does thereby provide a very simple computational scheme which requires as input only the spectroscopic constants of the reactants and products. In addition it offers simple qualitative interpretations of the trends in the results. The overall satisfactory agreement between the present results and past studies lends credibility to the basic propensity rule provided by the Born approximation: The most probable transitions are those that minimize the momentum transfer to the nuclei. The principle is discussed with special reference to exothermic (E′_T ? E_T) and endothermic transitions.The computations for Cl + HI indicate a decline of the reaction cross section with increasing kinetic energy and a strong enhancement by HI rotational energy. The surprisal analysis confirms the absence of vibrational population inversion for endothermic transitions. For the F + H₂ (and isotopic variants) reactions, the product-rotational state distribution extends nearly to the energy cut-off. The vibrational state distribution is somewhat different for para- and normal H₂ and, in general, the collision outcome is very sensitive to the initial rotational state of H₂ particularly at low translational energies. The HF/DF branching ratio is F + HD collisions is increasing with increase of the HD rotational state. The vibrational surprisal is essentially isotopically invariant.     

Some spectroscopic aspects of electron transfer in ruthenium(II) polypyridyl complexes

The metal-to-ligand charge transfer (MLCT) absorption and emission properties of several ruthenium(II)-bipyridine am(m)ine complexes are compared. The Gaussian deconvolution of the spectra indicates that: (a) the emission MLCT bandwidths are smaller than the absorption bandwidths for the first components of the apparent vibronic progressions; (b) the emission bands decrease in energy and width when a polypyridyl is replaced by an am(m)ine. The observations can be interpreted in terms of a two state model and the perturbation theory-based treatment of the attenuation of the effective reorganizational energy, λ_r =^~ λ_r ^o(1- 4α² _DA), where λ_r ^o is the reorganizational energy corresponding to no mixing between the two electron transfer states and α_DA = (H_DA/E_DA) is the mixing coefficient. Both the solvent and molecular contributions to λ_r are attenuated. The MLCT excited state lifetimes also decrease with am(m)ine substitution, and the non-radiative decay rate constant at 77 K is roughly proportional to the number of am(m)ine moieties coordinated to the ruthenium center.     

Close-coupling elastic and inelastic integral and differential cross sections for Li+ + H2 collisions

The quantum mechanical close-coupling formalism is applied to the study of elastic and rotationally inelastic Li⁺ + H₂ collisions making use of the Kutzelnigg-Staemmler-Hoheisel potential energy surface. Integral and differential cross sections for j = 0 → 0 and j = 0 → 2 are obtained in the collision energy range 0.2 to 0.9 eV and for j = 1 → 1 and j = 1 → 3 at 0.6 eV. A rainbow structure is observed in both the elastic and inelastic angular distributions and a quenching of the fast oscillations is found in the cross sections for j = 1 initially compared to the case j = 0 initially. At 0.6 eV. the calculated quantum mechanical angular distributions are compared to those from a classical trajectory calculation using the same surface and to the experimental ones. The dynamics of rotational excitation in the Li⁺ + H₂ system is contrasted to rotational excitation in systems for which the atom-diatom interaction is predominantly repulsive.     

Photoelectron spectra and angular distribution in reson three-photon ionization of atomic iron: J dependence

Kenetic energies and angular distributions of photoelectrons emitted by three-photon ionization fo atomic iron through two-photon resonant state e⁷D_J and e⁵ D_J, were measured, by using visible lasers. J-resolved photoelectron bands attributab to the a⁶D_J and a⁴F_j ionic states are reported.     

Quantal close-coupling calculation of the cross sections for the j 1 m 1 → j 2 m 2 transitions in the 4p 2P and 3d 2D excited states of Ca+ induced by collisions with helium

Cross sections for the j ₁ m ₁ → j ₂ m ₂ transitions in the resonance 4p ²P and metastable 3d ²D states of the singly charged calcium ion induced by collisions with the ground-state He atom have been calculated using the quantal close-coupling method. The calculations are based on the earlier obtained Ca⁺-He pseudopotential SCF potential energy curves. The calculated cross sections are discussed in the energy range from threshold to 1.5 eV. Satisfactory agreement with other theoretical results has been found for the 4p ²P state. However, relatively large discrepancy between theory and available experimental data still exists for both the Ca⁺ states.     

Primary photoprocesses in retinol

Pulsed laser photolysis techniques, with nanosecond time-resolution, are applied to solutions of retinol (vitamin A) and retinyl acetate, under varying conditions of solvent polarity and oxygen content. The observed absorbance and conductivity changes are identified as due to: (a) the excited fluorescent state, λ_max = 435 nm; (b) the lowest excited triplet state, λ_max = 405 nm; (c) the retinylic cation (λ_max = 590 nm), formed via: ROH→^hv R⁺ + OH^?. The results constitute the first experimental evidence for ionic photodissociation and for spontaneous, as well as oxygen-induced, intersystem crossing in retinol. Both processes may provide routes for isomerization, being thus relevant to the visual process.     

Molar excess volumes of ternary mixtures of nonelectrolytes

Molar excess volumes V^E_ijk of methylenebromide i + pyridine j + β-picoline (k, cyclohexane (i) + pyridine (j) + β-picoline(K), benzene(i)+toluene(j)+1,2-dichloroethane(k), benzene(i) + 0-xylene(j) + 1,2-dichloroethane(k) and benzene(i) + p-xylene(j) + 1,2-dichloroethane(k) mixtures have been determined dilatometrically at 298.15 K. The data have been examined in terms of Sanchez and Lacombe theory and the graph-theoretical approach, and it is found that they are described well by the latter. Self- and cross-volume interaction coefficients V_jk, V_jjk and V_jkk, etc., have also been evaluated and the values utilised to study molecular interactions between the jth and kth molecular species in the presence of the ith in these i + j + k mixtures.     

Equivalence and conservation rules for the total integral cross section in molecular collisions

The rule of conservation of the total integral cross section o_Tj) is described in a general way using the opacity function. An equivalence rule which states that at the same initial kinetic energy, the total integral cross section from the initial state j is equal to that from the initial state j, i.e., o_Tj) is also presented. Both rules are demonstrated with numerical examples in the He + H₂ collision system and counter-examples are also presented. Possible uses of the rules in the inversion of the intermolecular surface and as an experimental tool are also discussed.     

Thermodynamics of solid-state connected ion-sensitive membrane electrodes: The silver-Silver chloride system: Part II. Standard potentials (ε0′+εj) between 5 and 90°C of the 2nd kind silver-Silver chloride reference electrode with 3.5 M and sat'd KCl measured by means of membrane electrodes

Standard potentials (ε_M^0′+ε_j)_T of solid-state connected silver-silver chloride membrane electrodes (“chloride ion-sensitive electrodes”) in saturated and in 3.5 M (25°C) aqueous KCl solution (Ag|AgCl|KCl(sat'd or 3.5 M)||…) were measured between 5 and 90°C. The cells with transference comprised a Pt,H₂ electrode half-cell with NBS (DIN) phosphate buffer solution D. Identical thermodynamic behaviour of the membrane electrodes and respective 2nd kind silver-silver chloride electrodes and the small differences ΔE⁰ of their standard potentials E_M⁰ and E⁰, both reported in Part I of this work [1], allowed the application of the membrane electrode data to determine also standard potentials (ε^0′+ε_j)_T of the respective silver-silver chloride reference electrode of the second kind with “fixed potential” (Ag|AgCl(sat'd), KCl(sat'd or 3.5 M)6…). The results are discussed and compared with literature data.     

Distorted-wave calculations for the three dimensional chemical reaction H + H2(v ⩽ 2, j = 0) → H2(v′ ⩽ 2, j′, mj) + H

Calculations of the vibrational—rotational product state population distributions and differential cross sections for the chemical reaction H + H₂(v ? 2, j = 0) → H₂(v′ ? 2, j′, m_j) + H have been carried out on the Porter—Karplus potential energy surface. The vibrationally-adiabatic-distorted-wave (VADW) method has been used. The relative rotational product distributions, differential cross sections and the helicity m_j, dependences of these quantities for the v = 0 reaction agree well with accurate close-coupling results. The absolute integral cross sections are considerably smaller than the accurate quantum values, however. The calculations for the v = 1 reaction agree with the findings of previous sudden quantum, limited close-coupling and quasiclassical theoretical studies and experiments that product H₂(v′ = 1) is more likely to be produced than H₂(v′ = 0). For the reaction with v = 2, it is found that at high translational energies product H₂(v′ = 2) is favoured over H₂(v′ = 1) or H₂(v′ = 0). The VADW differential cross sections for the v = 1 and v = 2 reactions have a similar shape to those of the v = 0 reaction, with backward peaking when summed over all m_j states. The relative rotational distributions for the v = 2, j = 0 → v′ = 2, j and v = 1, j = 0 → v′ = 1, j reactions are also similar to those obtained for the v = 0, j = 0 → v′ = 0, j reaction, with low rotational excitation.     

Magnetic circular dichroism spectra of triphenylene and coronene

The absorption spectra of triphenylene (I) and coronene (II) are assigned by using the MCD spectra and the results of the SCF screened potential π-MO CI calculations. The calculated Faraday A/D values for the ¹E and ¹E_1u excited states of I and II are in excellent agreement with the experimental values, in contrast to the calculated Faraday B value for the ¹E_1u state of II.     

A DWBA study of angular distributions for the state-to-state reactive scattering process of F + H2 → HF + H

Based on our earlier three-dimensional DWBA theory, we discuss angular distributions and the roles of various angular momenta. The theory predicts a relatively small number of partial waves, backward scattering, and broadening of angular distributions with increased energy, for the reaction F+ H₂ (v_a = 0, j_a = 0) → HF (v_b = 2 j_b = 0) + H