Mechanisms Of Energy Flow Research Articles

?Statistical-mechanical? understanding of chemical reaction mechanism in solution: Energy fluctuations and heat capacities for isomerization of formamidine in aqueous solution

To explore the energy flow-mechanism of a solution-chemical reaction on the basis of an energy-fluctuation analysis and the time evolution of various kinds of energies [J. Phys. Chem. 48, 12506 (1994)], a chemical reaction molecular dynamics simulation was carried out in the microcanonical ensemble for the proton-transfer reaction of formamidine in an aqueous solution. The energy ΔE required to surmount the reaction barrier was found to be supplied mainly from the potential energy of the solvent water rather than from the solvent kinetic energy. The ratio of the reactive energy flow from the solvent potential vs. the kinetic energy, ΔV/ΔK, was 2.34 and was found to be in good agreement with the value of 1.96 predicted from the classical constant-volume heat capacity of water, C, via the Lebowitz–Percus–Verlet relation [Phys. Rev. 153, 250 (1967)]. This finding confirmed the results of Wilson et al. [J. Am. Chem. Soc. 113, 74 (1991)]; namely, that the ratio should be determined only by the heat capacity of the solvent with no relation to the kinds of solute molecules, and in aqueous solution, the coordinate fluctuation plays a more important role in the reaction occurrence than in the momentum fluctuation. Furthermore, on the assumption that the solute internal distribution is assumed to accomplish instantaneously thermal equilibrium with the surrounding solvent and to be characterized as an instantaneous canonical ensemble, the instantaneous partial molar constant-volume heat capacity of solutes, C(t), is defined at first by a simple extension of the relationship between the equilibrium heat capacity C and the ratio of the kinetic- and potential-energy fluctuations of the solutes. On the average, C(t) has a larger value than that evaluated within the harmonic approximation in the gas phase, i.e., 199.5 J K−1mol−1. The incompatibility was brought about by the intervention of the solute–solvent interaction. In addition, an exceptionally large value of C(t) was observed just 0.06 ps after (or before) the barrier crossing time and can be explained by the smaller fluctuation in the instantaneous kinetic energy. It was also observed that, during the relaxing (or surmounting) process of the reaction, C(t) becomes relatively larger than those in the transition and equilibrium periods, originating from the simple fact that the relatively larger potential fluctuation makes it easier to dissipate (or supply) the reactive energy from (or to) the reactants, as translated by a larger C(t). © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 133–145, 1998

Energy Flows in a Quasi-Isobaric Fusion Reactor: Part III, Radio-Frequency Heating

The important mechanisms of energy flow in a quasi-isobaric magnetic fusion device have been studied in a three-part paper. In Part I, the spatial profiles of plasma parameters that yield acceptable values of Q{sub dt} and plasma dimensions, were determined. These profiles were determined by balancing the dominant terms in the differential energy equations, i.e., conduction, bremsstrahlung, and collisional energy exchange, against each other. One class of equilibria was identified for a more detailed study. In Part II, the contributions of inelastic processes, radiation transport and alpha-particle heating were studied. These terms, in combination with the dominant terms studied earlier, yield the spatial profile of external heating that is required to balance the energy equations everywhere in the plasma. In Part III, the results of ray-tracing calculations for waves in the lower hybrid range are reported. These calculations show that it is possible to produce such a deposition profile for both electrons and ions, if the launch structure can couple the required k spectrum through the high-density edge plasma. 12 refs., 10 figs., 1 tab.

Energy Flows in a Quasi-Isobaric Fusion Reactor: Part II, Inelastic, Radiation, and Alpha-Particle Terms

The important mechanisms of energy flow in a quasi-isobaric magnetic fusion device are studied. In Part I of this paper, the spatial profiles of plasma parameters that yield acceptable values of Q[sub dt] and plasma dimensions are determined. These profiles are determined by balancing the dominant terms in the differential energy equations, i.e., conduction, bremsstrahlung, and collisional energy exchange, against each other. One class of equilibria was identified for a more detailed study. In Part II, the contributions of inelastic processes, radiation transport, and alpha-particle slowing down to the differential energy balances for electrons and ions are examined. Bremsstrahlung loss is found to be the dominant term for electrons. Inelastic processes involving hydrogen are important for ions in the fusion [open quotes]core.[close quotes] Impurity radiation can be important even with a low impurity content. Energy deposition by alpha particles is significant in the high-density edge, while cyclotron radiation transport plays some role in regions with large density gradients. 14 refs., 9 figs.

Intramolecular vibrational relaxation from CH stretching modes in dimethylnitramine

Intramolecular vibrational energy transfer in dimethylnitramine has been studied using classical trajectories. Model harmonic and anharmonic valence force-field potential-energy surfaces with force constants adjusted to give approximately the known normal-mode frequencies were used in the study. Both the rate and mechanism for energy flow from high CH stretch and methyl group excited states were investigated. The results show that the energy transfer is rapid (within approximately 0.25 ps) and irreversible. The energy from an excited CH stretch is initially transferred to the methyl bending motion and finally to the nitro and ‘‘second’’ methyl groups. The dominant energy transfer path involves flow from the stretch to the bend within the excited group followed by ‘‘methyl-to-methyl’’ transfer.

Studies of the intramolecular dynamics of model polyatomic molecules

The intramolecular dynamics of several model systems are investigated by using classical trajectories. Power spectra of auto- and cross-correlation functions obtained from the classical trajectories are used to classify the type of motion for excited local vibrational modes and to investigate the mechanisms of energy flow at various excitation energies. Coherency spectra are used to investigate interactions between two vibrational modes. Various models of four systems, HC3, HNNH, HCCH, and H2O2, are examined. The emphasis is on hydrogen peroxide. The motion is quasiperiodic at low energies of excitation. The onset of chaotic motion occurs at approximately 89% of the energy required for dissociation of the OH bond in H2O2. For acetylene the classical motion becomes chaotic only for energies very near the dissociation limit of the CH bond. For HC3 and HNNH, chaotic motion occurs at energies much less than that required for dissociation of the CH or NH bond. In general, the chaotic limit is lowered and the rate of energy flow is enhanced by the presence of bending degrees-of-freedom. The amount of energy transferred from a local mode is found to be invariant to the number of degrees-of-freedom in the chaotic regime while it is strongly dependent upon the number of degrees-of-freedom in the quasiperiodic regime. However, the initial rate of energy transfer in the chaotic regime is substantially affected by the number of degrees-of-freedom.

Dynamics of triplet-state formation and decay of gaseous propynal

state was assumed to involve mostly collisionally induced intersystem crossing (CIISC)'-3,5,9 because phosphorescence decay was ob- served only at high propynal pressures. In order to elucidate the roles each decay channel plays in the energy flow mechanism of excited propynal, we have measured both time-resolved single vibronic level (TRSVL) fluorescence spectral2 and time-resolved phosphorescence (TRP) spectral3 from low-pressure propynal. The experiments provide triplet-state quantum yields which comment on the efficiencies for collisionally induced singlet and triplet quenching of propynal. The collision partners used, propynal and a series of foreign gases, were chosen in order to clarify the relative efficiency of each partner in the collision-induced decay routes. The present study falls within the scope of previous gas-phase studies of radiationless transitions. Most distinct are the inves- tigations of carbonyl compounds such as biacetyl,14 gly~xal,]~~*~~ and methylgly~xal.'~,'~ In these previous studies, the pressure region in which collision-induced processes effectively compete with collisionless processes was examined. Theoretical interpre- tations of these data, which will be used also in the present study of propynal, involve CIISC'5,17-'9 leading to production of, among other species, a thermally equilibrated triplet state manifested by 'cold phosphorescence.