Exoergic channel

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 ... 

For exoergic channels, there is often no accessible avoided crossing, in which case the trajectory assumptions underlying the LZS theory are violated. The nonadiabatic coupling region may extend over a considerable range of internuclear distance, and semiclassical methods using exact classical trajectories represent the minimal necessary improvement over LZS. 

Another common feature of relaxation in benzene and R2 aniline is that a very frequent first step in the collision-induced vibrational energy exchange is endoergic up-pumping of the excited molecule, even when exoergic channels are available. The ubiquity and overall importance of this first endoergic step must be explained by any plausible mechanism for the processes observed. 

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. 

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... 

It should be stressed that in case of the ethynyl-acetylene reaction, a molecular hydrogen loss channel synthesizing the 1,3-butadienyl radical is open as well. Since the reactions of cyano and ethynyl radicals have no entrance barrier, are exoergic, and aU transition states involved are lower than the energy of the separated reactants, these reaction classes are extremely important to form nitriles and complex unsaturated hydrocarbons in low-temperature environments. On the other hand, the corresponding phenyl radical reactions are—due to the presence of an entrance barrier—closed in those environments. However, the elevated temperature in combustion systems helps to overcome these barriers, thus making phenyl radical reactions important pathways to form aromatic molecules in combustion flames. 

A long-range electron transfer is possible in this reaction, as in alkali metal atom reactions. However, the resulting electron-transfer complex Ba NO does not correlate to the ground-state products BaO which has the structure Ba +0. Moreover, the NOJ ion is stable and its dissociation into NO 4- 0 is endoergic. Hence the Ba "NOj complex may survive for many rotational periods despite the availability of a very exoergic reaction channel. This is expected to dissociate after the transfer of the second valence electron of barium, which is probably hindered by an energy barrier. 

Multiple chemiluminescent channels are observed in the reaction of CI2 with barium dimers. They correspond to all the electronically excited products BaCl, BaCl2 and Ba that are energetically accessible [293]. In contrast, only the most exoergic chemiluminescence channel corresponding to the formation of BaO is observed in the reaction of O2 with Ba2. 

The first photochemical study of this reaction was carried out in 1969 by Oldershaw and Porter [104], who photolyzed static N2O/HI samples at different wavelengths, and used final product analyses to deduce reaction probability versus photolysis wavelength. This provided clear evidence of a substantial entrance channel barrier (i.e., 4400cm ) for the highly exoergic reaction (4a), which was later confirmed and quantified by Marshall et al. [40,41], who carried out experimental rate constant versus temperature measurements as well as ab initio calculations of the stationary points on the potential surface. Oldershaw and Porter were also able to discern the appearance of reaction (4c) with an apparent threshold of 13,500 1400cm, in accord with the thermochemistry, as well as our observations, as discussed below. 

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