Volume 111, Issue 24, 22 December 1999
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


Lateral instabilities of cubic autocatalytic reaction fronts in a constant electric field
View Description Hide DescriptionThe region of instability for planar reaction fronts of cubic autocatalysis between ionic species under constant electric field has been determined accurately. The ratio of diffusion coefficients at the onset of instability is substantially varied by the componentdependent drift and directly proportional to the concentration of the autocatalyst behind the front as This opens the possibility to use electric field as a control parameter for reactionfront instabilities. The dispersion relation calculated from the linear stability analysis of the full system is in good agreement with the initial evolution of the Fourier modes associated with the slightly perturbed planar reaction front obtained by the direct integration of the governing equations in two spatial dimensions.

Rotational tunneling of ammonia in
View Description Hide DescriptionThe rotational dynamics of in the ammoniated fulleride are studied by the inelastic neutron scattering technique. Welldefined excitations appear in the lowenergy spectra at 1.5 K as a broad band centered at ∼350 μeV with shoulders at ∼240 and 590 μeV. The temperature dependence of their intensities, energies, and widths is consistent with their assignment to rotational tunneling transitions of the molecule. The estimated barrier of the threefold hindrance potential to rotation of the H atoms about the K–N axis is ∼5.7 meV. The existence of minority hindering sites with both larger (∼8 meV) and smaller (∼2 meV) torsional barriers shows that orientational disorder effects survive in even at low temperatures.

Trapping sites of hydrogen atoms in solid HD and An electron spin echo study
View Description Hide DescriptionTrapping sites of H and D atoms in solid HD and have been determined using electron spin echo (ESE) spectroscopy. It was found that all the H and D atoms are trapped in substitutional sites and that the H atoms push back surrounding HD molecules to produce local lattice distortion around the atoms, whereas the D atoms do not. It is expected that the local lattice distortion is produced by zeropoint motion of the H atoms whose amplitude is larger than that of host HD molecules and that the isotope effect is due to difference in the amplitude between the H and D atoms. The lattice distortion around the H atoms may induce the increase in rate constant for the tunnelingreaction with the increase in temperature in solid HD reported in Chem. Phys. Lett. 261, 463 (1996).

Molecular dynamics of isoamyl bromide by dielectric spectroscopy, and the effects of a nonpolar solvent, 2methylpentane, on the spectral features
View Description Hide DescriptionTo gain insight into the effects of the weakening of the electrostatic interactions on molecular dynamics when polar molecules are dissolved in a nonpolar solvent, the dielectric polarization and relaxation behaviors of isoamylbromide and its 50 mol % solution in 2methylpentane have been studied in detail over the frequency range, 1 mHz–1 MHz, and a temperature range approaching their liquid to glass transition. Features of the (i) αrelaxation spectrum, (ii) the Johari–Goldstein relaxation process in the liquid state at low temperatures, with an asymmetric spectral shape, and (iii) the temperature dependence of the relaxation dynamics have been determined and the effects of weakening of the electrostatic interaction on these features examined. The highfrequency wing of the loss spectrum of the αrelaxation is proportional to The dynamics of its αrelaxation follows the Arrhenius equation initially at high temperatures and thereafter the Vogel–Fulcher–Tamman equation. Alternative equations for the change in the relaxation rate have been discussed. A decrease in the dipole–dipole interaction and reduction in the internal field in a solution with a nonpolar solvent leads to a remarkable change in the shape of the relaxation spectra at high frequencies such that the dielectric loss for the αrelaxation becomes proportional to with α, β<1. The relaxation spectra of isoamyl bromide dissolved in 2methylpentane follows the H–N function and therefore behaves similar to a polymer, whereas for pure isoamyl bromide follows the Davidson–Cole behavior.

Conductance of symmetric electrolyte solutions: Formulation of the dressedion transport theory (DITT)
View Description Hide DescriptionThe concentration dependence of the electrical conductance of several symmetrical electrolytes is studied using the dressedion transport theory (DITT) formalism. This theoretical scheme is constructed from the Fuoss–Onsager theory using the equilibrium dressedion theory (DIT) distribution functions for electrolyte and colloid media. The conductance equation is constructed from previous results for the relaxation of the ionic atmosphere [Varela et al., J. Chem. Phys. 110, 4483 (1999)], now completed with the calculation of the concentrationdependent electrophoretic velocity of ions on the basis of a DIT in a hydrodynamic point of view. Onsager’s limiting law for the electrophoretic velocity based on the Debye–Hückel (DH) equilibrium distribution function is reformulated in terms of the DIT renormalized quantities, effective charges and effective screening length. The electrophoretic correction to the velocity calculated in the framework of this DITT is shown to exhibit a nontrivial dependence on the renormalized dielectric constant of the medium The concentration dependence of both the deviations of the renormalized dielectric constant from the classical limit and the electrophoretic correction to the mobility is analyzed using the modifiedmean spherical approximation (MMSA) value of the DIT linear response function The behavior of and therefore of the electrophoretic correction to the ionic (or colloidal)mobilities is studied for both the randomphase approximation (RPA) and the MMSA and they are related to ionic association through the dimensionless coupling parameter, ζ, made up from the ratio between the Bjerrum length and the mean ionic radius. The expression for the conductance of the electrolyte solutions obtained in this new theoretical approach is cast in the form of a generalized Onsager’s limiting law with a concentrationdependent slope which is predicted in terms of the formation of DIT renormalized quasiparticles in the solution. Conductance predictions of several theoretical expressions for the renormalized magnitudes of electrolyte solutions are compared to direct experimental data of KCl, and are in very good agreement with the theoretical predictions of the meanfield DITT, which confirms the convenience of understanding transport processes in terms of effective dynamical entities with renormalized ionic charges interacting by means of a renormalized screened potential.

On the structure and thermodynamics of solvated monoatomic ions using a hybrid solvation model
View Description Hide DescriptionThe hydration free energies relative to that of the proton are calculated for a representative set of monatomic ions These include cationic forms of the alkali earth elements Li, Na, and K, and anionic forms of the halogens F, Cl, and Br. In the current model the relative ion hydration free energy is defined as where the solvated ions are represented by ion–water clusters coupled to a dielectric continuum using a selfconsistent reaction field cycle. An investigation of the behavior of as the number of explicit waters of hydration is increased reveals convergence by This convergence indicates that the free energy change for the addition of water to a solvated proton–water complex is the same as the free energy change associated with the addition of water to a solvated –water complex. This is true as long as there are four explicitly solvating waters associated with the ion. This convergence is independent of the type of monatomic ion studied and it occurs before the first hydration shell of the ions (typically ⩾6) is satisfied. Structural analysis of the ion–water clusters reveals that the waters within the cluster are more likely to form hydrogen bonds with themselves when clustering around anions than when clustering around cations. This suggests that for small ion–water clusters, anions are more likely to be externally solvated than cations.

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. I. Spectral theory
View Description Hide DescriptionA unified nonMarkovian theory for the vibrorotational Raman and infrared spectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the time autocorrelation functions determining the collective effects of the solvent molecules on the vibrorotational dynamic of the diatomic. The spectrum is obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal part accounting for the redistribution of intensity due to interbranch and intrabranch mixing effects. This theory generalizes previous theoretical frames based on a secular contribution modified by an (approximate) interference term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra.

Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. II. Rotational spectroscopy of HCl in liquid
View Description Hide DescriptionThe nonMarkovian spectral theory presented in the preceding paper is applied to the calculation of the pure rotational spectroscopy of HCl in liquid and its comparison with available experimental farinfrared and anisotropicSbranch results. A quantitative and qualitative study of memory and interference effects on the overall infrared and anisotropicRaman spectra has been carried out. The relation of these effects to the time scales of the dephasing processes and the strong cancellation effects between interbranch and intrabranch mixing are especially stressed. The statistical parameters required for the theoretical calculation of the profiles have been obtained from a microscopic extended cell model for the liquid.

Resonance Raman study of the Aband shorttime photodissociation dynamics of axial and equatorial conformers of iodocyclopentane
View Description Hide DescriptionWe have obtained resonance Raman spectra of iodocyclopentane in cyclohexane solution at three excitation wavelengths resonant with the Aband absorption. The Aband resonance Raman spectral bands can be assigned to fundamentals, overtones, and combination bands of seven axial conformer and eight equatorial conformer Franck–Condon active modes. The resonance Raman and absorption cross sections were simultaneously simulated using wave packet calculations and a simple model. The best fit parameters of the simulations and the normal mode descriptions were used to determine the Aband shorttime photodissociationdynamics of the axial and equatorial conformers of iodocyclopentane. The axial and equatorial conformers exhibit noticeably different shorttime photodissociationdynamics that suggest that the C–I bond cleavage process is conformation dependent. The axial conformer shorttime photodissociationdynamics have larger changes in the carbon–carbon stretch and three carbon atom bending motions as well as the torsional motion about the α and β carbon atom bond. The CCI bending motions for the axial and equatorial conformers of iodocyclopentane as well as previously reported results for the equatorial conformer of iodocyclohexane are significantly smaller than CCI bending motions found for most noncyclic iodoalkanes examined so far. This suggests that the cyclic backbone restricts the initial motion of the C–I bond cleavage along the CCI bend in iodocycloalkanes compared to the noncyclic iodoalkanes.

Infrared spectra and pseudopotential calculations for NUO, and NThO in solid neon
View Description Hide DescriptionThe title cation and molecules have been prepared by reactions of laserablated metal cations and atoms with NO during condensation in excess neon at 4 K. Infrared fundamentals for the NUO and NThO molecules blue shift 1.6%–2.9% on going from argon to neon matrices and are calculated from 5.8% to 0.0% too high using density functional theory,GAUSSIAN 98, and pseudopotentials on the actinide metal. The isolated cation, formed in previous gasphase ion–molecule reactions, is characterized by new 1118.6 and 969.8 cm^{−1} neon matrix absorptions. Two normal modes (isotopic frequencies) are accurately modeled by the calculations for NUO, and NThO. The isolated cation observed here provides a vibrational model for its important isoelectronic analog, which has only been characterized in condensed phases where partial neutralization of the dication readily occurs.

Collision model for activated rate processes: turnover behavior of the rate constant
View Description Hide DescriptionA theory of reaction rates is developed on the basis of the Bhatnagar–Gross–Krook model, which assumes instantaneous Maxwellization of the particle velocity at each collision. This model may be regarded as an alternative to the Kramers model for reactiondynamics in the condensed phase. The main results are two expressions for the rate constant for single and doublewell potentials. These cover the entire range of collision frequency. These expressions predict a turnover of the rate constant as a function of the collision frequency, analogous to the Kramers–Mel’nikov–Meshkov solution for the rate constant in the Kramers model. In contrast to the prediction for the Kramers model, the maximal value of the rate constant is noticeably below the TST estimate even for so high a barrier as This is a consequence of two facts: (1) The rate constant grows slowly from zero at small collision frequencies. (2) In addition, the rate of growth increases weakly with the barrier height, as Simulated results indicate good agreement with the theory.
