Exoergic

These tliree effects, HCC, RE and FCC, are the main exoergic collisional process that take place in an MOT. They are the dominant loss mechanisms which usually limit the maximum attainable density and number in MOTs. They are not, however, the only type of collision in the trap. 

Eig. 2. Electron-transfer reaction rate, vs exoergicity of reaction the dashed line is according to simple Marcus theory the soUd line and data poiats are... 

Fig. 3. One-dimensional barrier along the coordinate of an exoergic reaction. Qi(E), Q i(E), QiiE), Q liE) are the turning points, coo and CO initial well and upside-down barrier frequencies, Vo the barrier height, — AE the reaction heat. Classically accessible regions are 1, 3, tunneling region 2.

The previous treatment applies to exoergic chemical reactions (with positive energy difference between the minima of the initial and final states AE = — > 0)- For endoergic reactions... 

The reorganization of the nuclear configuration in exoergic electron-transfer reactions is usually considered in the same framework. A typical diagram of the terms is depicted in fig. 12. 

A multidimensional PES for the reaction (6.45a) has been calculated by Wight et al. [1993] with the aid of the atom-atom potential method combined with the semiempirical London-Eyring-Polanyi-Sato method (see, e.g., Eyring et al. [1983]). Because of high exoergicity, the PES... 

Free radicals generally undergo one-electron transfer processes in homogeneous solution. Two-electron transfer processes, in which two radicals participate, are often highly exoergic. Typical examples are... 

Analogous to the reaction of ()(1 D) + H2, the interaction of the divalent S(4D) atom with 112 molecule leads to the reaction complex of I l2S on the ground PES through the insertion mechanism, in contrast to the 121.6-nm photolysis of H2S on the excited PES. The reaction products are formed via a subsequent complex decomposition to SI l(X2l I) + H. The well-depth of reaction complex H2S, 118 kcal/mol is greater than I l20, 90 kcal/mol as referenced to their product channels. The exoergicity for S + H2, however, is 6-7 kcal/mol, substantially smaller than that for O + H2, 43kcal/mol. 

Because of the deep potential well and small exoergicity, conventional wisdom will then predict a long-lived complex being involved in the title reaction and the statistical behavior might be borne out.1,22 23... 

The reaction 0(3P) C2H4 also plays a key role, besides in the combustion of ethylene itself,54,72,73 in the overall mechanism for hydrocarbon combustion.54,57,74,75 There are five exoergic channels ... 

The energetics of the Eley-Rideal reaction (A E —230 kJ mol-1) are well established.42 Here, the highly exoergic reaction forming gas-phase HC1 was probed by time-of-flight velocity measurements,39,41 scattering angular distributions,39,41 and state-selective laser spectroscopy.39-41... 

Despite the large exoergicity, less than about 100 kJ mol-1 appears as HC1 translation, rotation or vibration. On average, 60kJmol-1 appears as HC1 translation, 30 kJ mol-1 appears as HC1 vibration (peaks at v = 1) and 10 k.l mol 1 as rotation.39 It was argued in Ref. 39 that despite possessing similar energetics, the Eley-Rideal reaction is qualitatively different than the gas-phase reaction ... 

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

Interestingly, the efficiency for producing hot electrons scales with the reaction exoergicity.64 A recent theoretical treatment of such devices has shown that the efficiency with which H-atom adsorption on Cu(lll)... 

Fig. 11. Schottky diode device used for measurement of chemicurrents. Highly exoergic surface reactions like adsorption of an atom to the surface produce excited electrons and holes. Some of these electrons are able to surmount the Schottky barrier and arrive at the semiconductor conduction band. This results in a detectable chemicurrent. (From Ref. 64.)...

The reaction enthalpy switches from being exothermic to being endothermic between n = 3 and n - 4. In hydrated clusters, only reactions leading to partial replacement of the water molecules maintain thermodynamic exoergicity ... 

A simple phase space model can be used to compute the CO product vibrational energy distribution as a function of the available energy,12-14 Eav. The maximum energy which can be partitioned among the products degrees of freedom is the reaction exoergicity, Ex = hv-DH°[(CO)5W-CO]. For a 351 nm photolysis,... 

Ex 35 Kcal/mole.15 We find that the CO product vibrational distribution calculated using the phase space model with Eav = 35-40 Kcal/mole is in good agreement with our experimental results (Figure 2). Thus, the measured CO vibrational distribution indicates that vibrational energy disposal to the photolysis products is determined at a point on the potential surface where the full reaction exoergicity is available. This suggests that the 351 nm excitation of W(CO)g results in the sequence of events, (2)-(4), where the asterisk denotes vibrational excitation. 

The condition for a spontaneous reaction is that the change in the Gibbs free energy must be negative (dG < 0) - a so-called exoergic reaction. 

Thermodynamically the quantitative treatment of both active and passive processes requires them to be downhill or exoergic. The description of chemical potential as a function of mole fraction follows the same form as before for a neutral species (Section 8.2) ... 

Bacteria that live in the dark must derive the free energy to move electrons around in chemistry and a series of reactions based on the iron pyrite surfaces have been shown to be exoergic (Figure 9.13). The cationic pyrite surface serves as the catalyst for the binding of negatively charged species, perhaps tagged with... 

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