The Journal of Physical Chemistry 最新文献

Mass-analyzed threshold ionization spectroscopy (MATI) was used for a detailed investigation of the dissociation process of a fluorobenzene·Ar complex. The threshold ion spectra were recorded for the fluorobenzene monomer and the fluorobenzene·Ar complex by exciting the chromophore via different vibrational states of the S₁. The ion state spectra of the (fluorobenzene·Ar)⁺ cation exhibit significant progressions of the van der Waals bending mode b_x, which indicates a significant structural change of the complex upon ionization. The disappearance of the threshold ion signal for a certain excess energy in the complex (fluorobenzene·Ar)⁺ and the simultaneous appearance of the missing bands in the spectra of the fragment ion give an upper limit for the dissociation energy of the complex in the cationic state. In addition, one feature observed in the fragment ion spectrum of the complex recorded via exciting the S₁6b ¹ state reveals the relatively fast predissociation (<4 ns) of the complex, if the vibrational energy in the S₁ state exceeds the binding energy D₁ (D₁ < 302 cm^-1).

Two methods of physically justified improvement of the STO and GTO basis set quality are suggested for ab initio calculations of molecular magnetic properties. They are based on the analysis of analytical expressions for the first-order correction (response) functions to the unperturbed basis AO's. The response functions have been obtained by solution of the inhomogeneous Schr?dinger equation for the model problem “a one-electron atom in an external uniform field”, by the closed representation of the Green's function. It has been shown that unlike the London orbitals for magnetic field the Green's function method enables us to get the general solutions of the inhomogeneous Schr?dinger equation. The methods elaborated have been applied in test calculations of magnetic susceptibilities and nuclear shielding constants of the first- and second-row hydrides and diatomics in STO-NG, split-valence CGTO basis sets, and extended ones constructed on their base. Analysis of results obtained has allowed us to determine the field of applicability for the suggested methods of basis set construction adapted for the magnetic properties calculations.

B? ²A‘(3p) ← X? ²A‘‘ spectra of the isotopically substituted hydroxymethyl radicals (CH₂OH, CH₂OD, CD₂OH, CD₂OD) were observed between 39?700 and 43?000 cm^-1 by 2+1, 2+2, and 1+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Analyses of the vibrational hot bands in these spectra show that the ν₈ torsion modes and ν₉ CH₂-wag mode are strongly coupled and governed by nonharmonic potential energy functions; for example, for ¹²CH₂¹⁶OH(X? ²A‘‘) we obtain 2ν₈ = 846 ± 6 cm^-1(1σ), 1ν₉ = 234 ± 5 cm^-1, and 2ν₉ = 615 ± 6 cm^-1. Using MP2/6-311G(2df,2p) ab initio calculations, we constructed the two-dimensional potential energy surfaces that govern the ν₈ torsion modes and ν₉ CH₂-wag in the X? ²A‘‘ radical and the X? ¹A‘ cation core of the B? ²A‘(3p) Rydberg state. Energy levels calculated with these potential energy surfaces account for the REMPI bands originating from the ν₈ hindered rotor and the ν₉ CH₂-wag modes. The experimental and ab initio results lead to improved heat capacities and entropies (S°_298.15(CH₂OH) = 244.170 ± 0.018 J (mol K)^-1). Ab initio CBS-QCI/APNO calculations predict that Δ_fH°_298.15(CH₂OH) = ?18.4 ± 1.3 kJ mol^-1. Re-evaluation of photoionization data yields IP_a(CH₂OH) = 7.562 ± 0.004 eV. Re-evaluated photoionization appearance data, kinetic equilibrium data, and shock tube data indicate that Δ_fH°₀(CH₂OH⁺) = 718.1 ± 1.8 kJ mol^-1, Δ_fH°_298.15(CH₂OH⁺) = 716.4 ± 1.8 kJ mol^-1, Δ_fH°₀(CH₂OH) = ?11.5 ± 1.3 kJ mol^-1, and Δ_fH°_298.15(CH₂OH) = ?17.8 ± 1.3 kJ mol^-1. We report the proton affinity, PA₀(CH₂O) = 705.2 ± 1.9 kJ mol^-1. Thermochemical tables based upon these values are presented for CH₂OH and CH₂OH⁺.

Ultrasonic absorption coefficients and sound velocities of aqueous solutions of symmetric tetraalkylammonium bromides have been measured at 25 °C as a function of frequency ν (300 kHz ≤ ν ≤ 5 GHz) and molal concentration m of salt (0 ≤ m ≤ 6 mol/kg). The hydrophobic chains of the cations (C_nH_2n+1)₄N⁺ have been varied from n = 1 to n = 5. The absorption spectra for solutions of Me₄NBr (n = 1) did not show contributions in excess to the classical absorption, while those for solutions of larger hydrophobic cations revealed two relaxation regions. One of these regions can be represented by a Debye-type relaxation process with a relaxation time τ_D (τ_D ≈ 20 ns) which is almost independent of the solute concentration and the length of the cation alkyl groups. The process is attributed to an intramolecular mechanism of rotational isomerization. The other relaxation region reflects a relaxation time distribution. Its principal relaxation time τ_max adopts values between 15 and 230 ps. This relaxation appears to be due to a microheterogeneous structure of the salt solutions. It can be well represented by the Romanov?Solov'ev model of concentration fluctuations if this model is extended to also consider effects of correlations. The values for the correlation length are found to nearly agree with the particle radius that can be calculated from the mutual diffusion coefficient and the shear viscosity of the solutions according to the Stokes?Einstein relation. A noticeable result is the finding that the extended Romanov?Solov'ev model meets with the unusual concentration dependence in the relaxation amplitude. The volume viscosity data derived from the classical part of the sound absorption and data for the isentropic compressibility as resulting from the sound velocity are also discussed in terms of structural properties of the organic salt solutions.

Time-resolved fluorescence anisotropy and lifetime measurements on the dye 3,3‘-diethyloxadicarbocyanine iodide in phospholipid/cholate mixtures have been carried out. The different populations of the dye were resolved by virtue of the difference in their microenvironmental emission behavior. Rotational dynamics of the dye in cholate/phospholipid mixtures show that gradual removal of cholate from the medium leads to a large increase in rotational correlation time corresponding to formation of large vesicles. The lamellar?micellar transition takes place near the critical micellar concentration (CMC) of cholate. The mechanism of stepwise formation of lipid vesicles on removal of cholate could readily be reconciled with the mechanism of membrane protein reconstitution into lipid vesicles by the cholate dialysis method.

The pairwise descreening approximation provides a rapid computational algorithm for the evaluation of solute shape effects on electrostatic contributions to solvation energies. In this article we show that solvation models based on this algorithm are useful for predicting free energies of solvation across a wide range of solute functionalities, and we present six new general parametrizations of aqueous free energies of solvation based on this approach. The first new model is based on SM2-type atomic surface tensions, the AM1 model for the solute, and Mulliken charges. The next two new models are based on SM5-type surface tensions, either the AM1 or the PM3 model for the solute, and Mulliken charges. The final three models are based on SM5-type atomic surface tensions and are parametrized using the AM1 or the PM3 model for the solute and CM1 charges. The parametrizations are based on experimental data for a set of 219 neutral solute molecules containing a wide range of organic functional groups and the atom types H, C, N, O, F, P, S, Cl, Br, and I and on data for 42 ions containing the same elements. The average errors relative to experiment are slightly better than previous methods, butmore significantlythe computational cost is reduced for large molecules, and the methods are well suited to using analytic derivatives.

The transient conductivity resulting from nanosecond pulsed ionization of alkoxy-substituted phenylene?vinylene/ethylidene copolymers, “dMOM-PPV(n)”, with n the fractional vinylene content, has been studied using the pulse radiolysis time-resolved microwave conductivity (PR-TRMC) technique. Minimum values of the sum of the charge carrier mobilities within the bulk solids, ∑μ_min, have been estimated from the end-of-pulse conductivity. For the freshly precipitated materials at room temperature, ∑μ_min decreases gradually with decreasing n from 1.8 × 10^-7 m²/(V s) for n = 1 (full conjugation) to 0.4 × 10^-7 m²/(V s) for n = 0.57. After annealing dMOM-PPV(1) at 100 and 150 °C, ∑μ_min at room temperature increased to 3.2 × 10^-7 and 8.0 × 10^-7 m²/(V s), respectively. No significant effect of high-temperature annealing was found for n ≤ 0.87. On cooling dMOM-PPV(1) from 150 to ?50 °C, ∑μ_min decreased initially with an activation energy of approximately 0.07 eV but approached a plateau at the lowest temperatures. The after-pulse decay of the conductivity was disperse in all cases. First half-lives of several microseconds were found for n = 1. The decay kinetics were independent of the dose in the pulse. Large accumulated radiation doses (up to 1.2 MJ/kg) did not effect the end-of-pulse conductivity but did increase the decay rate. This effect could be reversed by high-temperature annealing.

2-Naphthol (NOH) in its ground state forms a 1:1 complex with β-cyclodextrin (β-CD) both in the absence and presence of linear alcohols. Association constants, K_app, were measured using a steady-state fluorescence method. K_app decreases linearly with an increasing number of carbon atoms in the chain of the alcohol, n_C, up to n_C = 5. We attribute this to a competition between NOH and alcohol for the β-CD cavity. Fluorescence studies confirm the redistribution of NOH from the CD environment to the aqueous phase when alcohols are present. NOH fluorescence is quenched by iodide in all the systems studied. At 2 mM β-CD, alcohols increase the Stern?Volmer constant above the value found in the absence of alcohols. These results suggest that alcohols occupy space within the β-CD cavity with the result that the aqueous NOH concentration is increased. This was further investigated by dynamic fluorescence measurements on the system β-CD:NOH:pentanol. Global biexponential analysis of fluorescence decay data shows that the Stern?Volmer constants correlate inversely with the fraction of NOH complexed by β-CD. By global compartmental analysis of the fluorescence decays, values for the excited-state association and dissociation rate constants were determined. The dissociation rate constant increases from approximately 500 s^-1 in the absence of pentanol to about 14?000 s^-1 at a pentanol concentration of 0.1 M. The association rate constant increases from 2.5 × 10⁹ to 5.8 × 10⁹ M^-1 s^-1 upon addition of pentanol. The more pronounced increase of the dissociation rate constant leads to an exclusion of complexed NOH into the aqueous bulk phase. As the complexed NOH is shielded against iodide quenching, this explains the increase of the Stern?Volmer constant when an alcohol is added to the aqueous β-CD:NOH system.

Structure-dependent electrochromism was investigated with vapor-deposited thin films of zinc and vanadyl naphthalocyanines (ZnNc and VONc). The amorphous ZnNc film deposited on an indium tin oxide (ITO) coated glass electrode exhibited an irreversible electrochromic oxidation in 0.1 M KCl. This reaction gave rise to swelling of the film surface due to expansion of grains caused by incorporation of charge-compensating anions between the aggregated molecules. Epitaxially oriented films were prepared by deposition onto the (001) cleavage surface of NaCl and then transferred onto the ITO electrode. The epitaxial ZnNc film, in which the planar molecules are piled up in columns taking on the face-to-face, eclipsed stacking, exhibited both electrochromic reduction and oxidation. This electrochromic activity was attributed to a reversible incorporation of counterions through hollow channels between the molecular columns, which was accompanied with rearrangement in the molecular stacking. By contrast, no electrochromic reaction occurred for the epitaxial VONc film, where the alternately slipped dimeric structure did not facilitate a penetration of counterions into the stacked molecules.

Precise kinetic studies of photocatalytic reactions in solid catalyst water suspensions require the accurate description of the radiation fieldlight distributioninside the reactor. Solution of the radiative transport equation (RTE) inside the reaction is one of the best ways of accessing to such information. For solving this equation, a minimum of two parameters (the absorption and scattering coefficients) and one scattering spatial distribution function (the phase function) are needed. These attributes are directly associated with the optical behavior of the reacting system and are not independent of catalysts more conventional properties. A complete report on the physical and optical characteristics of titanium dioxide particulate suspensions in water is presented. Results were obtained for six different commercially available powders. The investigated parameters were (i) size of elementary particles, (ii) size of particle aggregates in water suspensions, (iii) specific surface area, (iv) spectral extinction coefficient, (v) spectral absorption coefficient, and (vi) spectral scattering coefficient. The last three were obtained as a function of wavelength in the range 275?405 nm. All measurements were made following a standardized protocol for the preparation of the solid suspensions. Scattering and absorption effects could be deconvoluted from the extinction coefficient by applying a very simple radiation transport model to the analysis of the experimental data. Experimental information was obtained by means of specially designed spectrophotometric measurements made with conventional cells, combined with results obtained with an integrating sphere accessory operated in the transmission mode. These propertiesparticularly the optical onesare required to solve the RTE and (i) to calculate precise values of photocatalytic reaction quantum yields and (ii) to fully characterize radiation energy absorption effects in the kinetics of photocatalytic reactions. Moreover, these data are indispensable for devising scaleup procedures in photocatalytic reactor design.