Single-product channels, multiple reaction

EXPLORING MULTIPLE REACTION PATHS TO A SINGLE PRODUCT CHANNEL... 

It is challenging experimentally to study two pathways leading to a single product channel for the simple reason that the products in either case are structurally identical. Nevertheless, there are several methods, each applicable to certain classes of reactions, that can distinguish the presence of multiple pathways. 

The study of multiple pathways leading to a single product channel provides a stringent test of our understanding of the potential energy surface and the calculations that use it to predict reaction outcomes. Although there are not many examples to date of pathway competitions, the increasing prominence of such systems, coupled with advances in experiment and theory that facilitate their study, promises a rich future in this normally hidden facet of reaction mechanisms. 

Figure 12 (from the chapter Exploring Multiple Reaction Paths to a Single Product Channel ). Two-dimensional cut through the potential surface for fragmentation of the transition state [OH CH3 ] complex as a function of the CF bond length and the FCO angle. All other coordinates are optimized at each point of this PES. Pathway 1 is the direct dissociation, while pathway 2 leads to the hydrogen-bonded [CH3OH F ] structure. The letter symbols correspond to configurations shown in Fig. 11. Reprinted from [63] with permission from the American Association for the Advancement of Science.

Figure 16 (from the chapter Exploring Multiple Reaction Paths to a Single Product Channel ). Projections onto 2D surfaces of trajectories (in green) of CH3O — H2 -H HCO. The left column is a projection onto the surface of Fig. 15. The right column is a projection onto the surface of Fig. 14. The black contour represents the saddle point energy for the H+ H2CO H2 + HCO reaction. Blue contours are lower in energy red contours are higher. Reprinted with permission from [67]. Copyright 2001 American Chemical Society.

Exploring Multiple Reaction Paths to a Single Product Channel... 

The reactions of Ca, Sr, and Ba with alcohols [43-45] are also different in that only the insertion channel is seen for both ground- and excited-metal atoms. The reaction intermediate is HMOR in all cases and the H atom leaves to give the MOR product. Very recently, the production of a small amount of CaOH was reported in the reaction of Ca with alcohols [50,51]. The bulky R group suppresses the production of the energetically favored MO + HR products. Under both single collision and multiple collision conditions the most important dynamical event is the insertion of an alkaline earth metal atom in an H—OR bond. 

From the viewpoint of modeling, the ultimate goal of the kinetic analysis of pressure-dependent reaction systems is to provide reliable time-independent rate expressions k(T, p) which can be incorporated into large kinetic models. The functional forms of these rate expressions can be rather complicated for multi-channel multiple wells systems, since—as we saw from the examples—the competition of product channels leads to strongly non-Arrhenius behavior. On the other hand, pressure-dependent rate constants for single-well single-channel reaction systems are comparably easy to describe. Therefore, we will divide this discussion into two sections going from simple fall-off systems to complex systems. 

T-HjCHD, Dj). An ion-beam target-gas study showed that the N4 +D2 >N2D + N2 + D channel dominates other available reaction channels in addition to N2D, only a comparatively small amount (<5%) of N2 from collision-induced dissociation was detected [5]. Drift tube studies showed significantly more (13%) [2] or only N4H (100%) [6] to be formed in N4+H2 reactive collisions. The difference in product distributions is attributed [5] to the single [5] and multiple collision [2, 6] conditions that were present. Thermal rate constants at 300 K for the N2H and N2D product channels in the reactions of N4 with H2... 

While the previous example is basically a single-well reaction (the isomerization and therefore reactions of the second well play only a very minor role), this final example deals with a multi-well multiple channel reaction. The reaction between vinyl radicals + O2 was studied by several groups [102-104] and we will present here an analysis close to that provided by Bozzelli and Dean [102]. A schematic PES is shown in Fig. 16. It is characterized by the existence of several product and isomerization channels with barriers clearly below the entrance channel. If one considers only the energetic properties, the most favorable products would be CH2O + HCO and OHCCOH (glyoxal) + H. However, these products can only be formed after two isomerization steps with relatively low 4-factors. 

The distribution of fluids to the multiple channels turns out to be complex, as every single molecule needs to experience the same conditions (i.e., temperature and residence time) to obtain optimal reactor performance, efficiency, and safety. Maldistribution in multichannel reactors results in loss of product selectivity and yield, as demonstrated for consecutive reactions [86]. The flow nonuniformities generally occur due to two reasons a poor reactor design and manufacturing... 

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