Gas phase energy surface

Great interest in the decarbonylation process over the last few years has resulted from studies at the two extremes of the kinetic spectrum. On the one end, femtosecond studies in the gas phase have addressed the dynamics of the reaction within time scales that are barely enough for a few molecular vibrations and modest radical-radical separation but not enough for large-amplitude molecular motions. At the other end, studies in crystalline solids focus on reactions where excited molecules and reactive intermediates can only explore a very small fraction of the gas phase energy surface (Figure 48.1 the details of the photochemistry/photophysics are discussed in Section 48.3). When radical pairs and hiradicals... 

The above results give the asymptotic points of the potential surface in solution. Furthermore, with the use of the calculated solvation energies of the different fragments we can obtain from eq. (2.34) the asymptotic points for the gas-phase potential surface. This is done in Table 7.2. 

Figure 8.3 Proposed reaction mechanism for methanol synthesis on Pd and comparison with gas-phase mechanism surface intermediates are speculative and associated energies are estimates...

Figure 2. I2 in acetonitrile. Nonequilibrium free energy surface. Contours in kcal mol"1, with the gas phase energy of the separated / and I species as a reference. The line cutting across the contours represents the ESP.

DR. BRAUMAN First of all, the difference between whether one uses AG or AE depends on whether one is using a gas phase potential surface or not. Marcus specifically points out that one should use E and not G for this type of reaction. In fact, in this case it doesn t really matter which one uses. Using free energy will yield the same result because the entropies basically don t change. So that wouldn t make any difference. 

The rate expressions Rj — Rj(T,ck,6m x) typically contain functional dependencies on reaction conditions (temperature, gas-phase and surface concentrations of reactants and products) as well as on adaptive parameters x (i.e., selected pre-exponential factors k0j, activation energies Ej, inhibition constants K, effective storage capacities i//ec and adsorption capacities T03 1 and Q). Such rate parameters are estimated by multiresponse non-linear regression according to the integral method of kinetic analysis based on classical least-squares principles (Froment and Bischoff, 1979). The objective function to be minimized in the weighted least squares method is... 

In more detail, our approach can be briefly summarized as follows gas-phase reactions, surface structures, and gas-surface reactions are treated at an ab initio level, using either cluster or periodic (plane-wave) calculations for surface structures, when appropriate. The results of these calculations are used to calculate reaction rate constants within the transition state (TS) or Rice-Ramsperger-Kassel-Marcus (RRKM) theory for bimolecular gas-phase reactions or unimolecular and surface reactions, respectively. The structure and energy characteristics of various surface groups can also be extracted from the results of ab initio calculations. Based on these results, a chemical mechanism can be constructed for both gas-phase reactions and surface growth. The film growth process is modeled within the kinetic Monte Carlo (KMC) approach, which provides an effective separation of fast and slow processes on an atomistic scale. The results of Monte Carlo (MC) simulations can be used in kinetic modeling based on formal chemical kinetics. 

Fig. 1. Schematic energy profiles for the gas-phase versus surface reaction [R, react-ant(s) TS, transition state P, product(s)]. The relevant activation barriers and AE S usually are dramatically different because of different chemisorption energies of the reactant Qh and transition state ( TS.

In the early experiments of thermal CVD using methane -i- hydrogen gas mixtures, a temperature of about 1000°C and a pressure less than 0.1 atm meant that the formation of graphite was predominant.An activation energy is required to produce carbon and/or hydrogen radicals so that the gas phase and surface reactions leading to predominantly diamond deposition can proceed at a significant rate." ... 

The treatment by secondary plasma reactor utilizes chemically reactive species created in glow discharge without influences of electron and ion bombardments and luminous gas phase. In-glow LPCAT treatment, on the other hand, utilizes luminous gas phase without the influence of ion and electron bombardment, and chemically reactive species are created on PTFE by energy transfer from the luminous gas phase. Thus, surface treatment by secondary plasma works only with gases that produce relatively long-lived chemically reactive species. Most secondary plasma treatments appear to be surface modifications by air or oxygen. 

Sanchez Marcos and Pappalardo have recently performed considerably more elaborate studies of the solvent influence on barriers and conformation equilibria of ni-troenamines . The solute cavity was modeled after the shape of the molecule, and the solvent was treated both as a continuum " and as a supermolecule with 15 methanol molecules per solute molecule. The solute-solvent interaction energy was obtained from the potential of a suitably defined surface charge density and the electron distribution in the solute from AMI calculations. The agreement between experimental and calculated free-energy barriers was excellent, whereas the Z-E equilibria were less well reproduced with AMI calculations. This discrepancy was diminished when conformation gas-phase energies from ab initio calculations were used. 

Gas phase energy balance is represented as follows. Gas temperature is increased by heat transfer between gas and catalyst surface. 

In the first place, molecules adsorbed on a solid surface are in steady communication with a relatively unrestricted energy supply, so that limitations on the rate of communication of activation energy will not play the part which they may play in the reactions of the gas phase. Moreover, surface interactions have not the transiency of bimolecular collisons, and two molecules adsorbed on neighbouring sites have a better chance to attain the phase favourable to reaction than they have in the brief moment of a gas-phase encounter. 

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