Multiple collision conditions

Kolwas, M. and Szonert, J. (1986). Reorientation of Najj by He under multiple-collision conditions, Chem. Phys. Lett., 130, 498-503. 

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. 

In fundamental studies of the CA phenomenon it is important to distinguish between single collision and multiple collision conditions, since in the latter case any conclusions drawn about the CA mechanism will be blurred by the effect of superposition of several different colhsions on the observed outcome. However, this aspect is of little consequence for present purposes, far less important than optimization of both the 5deld and specificity of the observed fragmentations. Of course CA in a Paul trap is inherently a multiple collision phenomenon that involves climbing the internal energy ladder (McLuckey 1997). As discussed in Section 6.4.2a, use of relatively high pressures of heavier collision gas in a linear RF-only quadrupole collision cell can yield unexpected improvements in overall performance. 

Which experimental approach can best reveal the chemical dynamics of carbon-centered radicals Recall that since the macroscopic alteration of combustion flames, atmospheres of planets and their moons, as well as of the interstellar medium consists of multiple elementary reactions that are a series of bimolecular encounters, a detailed understanding of the mechanisms involved at the most fundamental microscopic level is crucial. These are experiments under single collision conditions, in which particles of one supersonic beam are made to collide only with particles of a second beam. The crossed molecular beam technique represents the most versatile approach in the elucidation of the energetics and... 

We briefly examined vibrational relaxation of l2 X,v > 20) induced by collisions with He at low temperatures. These measurements were performed by focusing state preparation and probe laser beams into the downstream region of a He free-jet expansion that had been seeded with l2-Transfer out of the initial levels u = 23 and 42 was examined at 5 K. For these conditions the probe laser spectra were quite simple and uncongested. A typical probe laser spectrum is shown in Fig. 5. The delay used to record this trace was long enough for multiple collisions to occur, so populations are seen in levels corresponding to Av = — 1 and —2 transitions. 

There have been several studies of the reaction dynamics of the ground and excited states of the alkaline earth atoms with various oxygen-containing molecules under single collision conditions. Although these studies are not directly applicable to the multiple collision regime in the Broida oven, they clarify the dynamics of a single encounter between a metal atom and an oxidant molecule. Oberlander and Parson [43] looked at the reactions of Ca and Sr with water, alcohols, and peroxides. Similar studies... 

The Fixed Bed Nuclear Reactor (FBNR) concept assumes the use pressurized water reactor (PWR) technology, but incorporates hi temperature gas cooled reactor (HTGR) type fuel and the concept of a suspended fixed bed core. Spherical fuel elements are fixed in the suspended core by the flow of water coolant. Any accident signal will cut off the power to the coolant pump causing a stop in the flow. This would make the fuel elements fall out of the reactor core, driven by gravity, and enter a passively cooled fuel chamber where they would reside in a subcritical condition. The Fixed Bed Nuclear Reactor (FBNR) is a simplified version of the fluidized bed nuclear reactor concept [XII-1 to XII-9]. In the FBNR, spherical fuel elements are in a fixed position in the core therefore, there is no concern about the consequences of multiple collisions between them, an issue that may be raised about the fluidized bed concept. Relatively little work has been done for the fixed bed nuclear reactor so far, but the experiences gained from the development of a fluidized bed reactor can facilitate the development of the FBNR. 

The dynamics of the hard-core, square-well problem is particularly simple for the initial conditions just considered. Sometimes multiple collisions occur as shown in Fig. 10.6a. The representative particle collides with wall 1 more than once, i.e., A hits B, then B hits C, and finally B and A collide again before atom and diatom separate. If, as in Fig. 10.6a, BC is initially vibrationless, multiple collisions can only occur when < 60° as ms decreases these become more common and are predominant if < 50°. Such repeated collisions significantly increase the energy transfer instead of (10.7) the inelasticity is... 

The need to actual measure kinetic data, rather than extrapolating data from lower temperatures, is that the theoretical models are insufficiently sophisticated to accmately describe the transition from subcritical conditions to supercritical conditions, as is evident from the data plotted in Fig. 88. Thus, a simple Arrhenius extrapolation of Elhot s subcritical data or by using a multiple collision model does not produce the data at all well in the... 

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... 

In principle, these approaches are very attractive because they probe multiple pathways in the critical regions where the pathways are separated, but in practice these are extremely challenging experiments to conduct, and the interpretation of results is often quite difficult. Furthermore, these experiments are difficult to apply to bimolecular collisions because of the difficulty of initiating the reaction with sufficient time resolution and control over initial conditions. 

In 1990, Schroder and Schwarz reported that gas-phase FeO" " directly converts methane to methanol under thermal conditions [21]. The reaction is efficient, occuring at 20% of the collision rate, and is quite selective, producing methanol 40% of the time (FeOH+ + CH3 is the other major product). More recent experiments have shown that NiO and PtO also convert methane to methanol with good efficiency and selectivity [134]. Reactions of gas-phase transition metal oxides with methane thus provide a simple model system for the direct conversion of methane to methanol. These systems capture the essential chemistry, but do not have complicating contributions from solvent molecules, ligands, or multiple metal sites that are present in condensed-phase systems. 

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