Rare gases reactivity

A simple physical example to illustrate dispersion interactions are the dimers of rare-gas atoms such as He, Ne, and Ar. These atoms are well known for their lack of chemical reactivity, but the fact that these gases can be liquefied at sufficiently low temperatures makes it clear that attractive interactions between rare-gas atoms exist. Zhao and Truhlar examined the performance of a large number of meta-GGA functionals for describing He-He, He-Ne, He-Ar, and Ne-Ar dimers with localized basis set calculations.15... 

In this Chapter, we consider the theory of collision-induced absorption by rare gas mixtures. We look at various theoretical efforts and compare theoretical predictions and computations with measured spectra and other experimental facts. The theory of induced absorption is based on quantum mechanics, but in certain cases, the use of classical physics may be justified, or indeed be the only viable choice. The emphasis will be on the computation of induced absorption by non-reactive, small atomic systems in the infrared. Diatomic and triatomic systems show most of the features of collisional absorption without requiring complex theory for their treatment. The theory of induced absorption of small clusters involving molecules will be considered in Chapter 6. 

We count the N2 + O2—>N + N+ 0 + 0 channel as a reactive one because all the trajectories we examined yielded a four atom final state only when it was preceded by the formation of NO, however briefly. In addition, this channei has a dynamical energy threshold significantly higher than the endoergicity. The total yield is shown vs. the impact velocity for N2 + O2 molecules embedded in a cluster of 125 rare gas atoms, One can replot this figure in terms of a reduced variable, see Fig. 13. 

On the other hand, heavier rare gases are not acting as effectively in removing energy from the internally hot, nascent, molecular products. The yield of stable molecular products is favored in lighter rare gases (cf. Fig. 14) where the reactive yield is plotted for Ne and Ar rare gas clusters on a reduced energy scale. 

Fig. 17. Top panels The force (in reduced units) along the bond of the CI2 molecule, applied by the different rare gas atoms 150 fs after surface Impact at v = 5 km s vs. the (arbitrary) serial number of the laxe gas atoms, for the two trajectories shown in Fig. 15, Bottom panels The position of the CI2 molecule and the laxe gas atoms that applied a significant force along the CI2 bond, see top panels, for the reactive and nonreactive trajectories shown in Fig. 15. Note how small changes in the position of the atoms (Cl-Ar distance and Cl-Cl-Ar angle) cause a big change in the magnitude of the force applied by the rare gas atoms.

The role of the cluster in making reactants that failed to react on the first try, try again, is enhanced if it is heavier because it is better able to reverse the velocity of the center-of-mass of the reactive atoms, see Fig. 18. Again there is the correlation that heavier rare gases favor overall reactivity. On the other hand, a heavier rare gas is more likely to induce dissociation of an internally hot nascent product molecule. 

Due to their long range attraction the reactants cluster together and the rare gas atoms surround the reactive molecules. The three roles of the cluster are as in the previous cases, to make sure that the reactions occur through the bulk of the cluster and not only in the layer nearest to the surface, to activate the reactants and to stabilize the products. 

The reason we employ two rather distinct methods of inquiry is that neither, by itself, is free of open methodological issues. The method of molecular dynamics has been extensively applied, inter alia, to cluster impact. However, there are two problems. One is that the results are only as reliable as the potential energy function that is used as input. For a problem containing many open shell reactive atoms, one does not have well tested semiempirical approximations for the potential. We used the many body potential which we used for the reactive system in our earlier studies on rare gas clusters containing several N2/O2 molecules (see Sec. 3.4). The other limitation of the MD simulation is that it fails to incorporate the possibility of electronic excitation. This will be discussed fmther below. The second method that we used is, in many ways, complementary to MD. It does not require the potential as an input and it can readily allow for electronically excited as well as for charged products. It seeks to compute that distribution of products which is of maximal entropy subject to the constraints on the system (conservation of chemical elements, charge and... 

Von Bunau and Kuhnert studied the y-radiolysis of cyclopropane both in direct radiolysis and in Kr- and Xe-sensitized radiolysis. The sensitized radiolysis was studied earlier by Smith and coworkers who showed that the addition of a rare gas increases the percent of cyclopropane consumed for constant irradiation time. The sensitization increases with increasing pressure of the rare gas for constant pressure of cyclopropane. The degree of rare gas sensitization was found, as expected, to be in the order of its energy absorption characteristics (electron density), i.e. Xe > Kr > Ar. Von Bunau and Kuhnert studied the effect of electron scavenger (SFe) and radical scavenger (NO) on the yield of the various products in direct radiolysis and in the sensitized radiolysis. They explained the different effects of the additives in the three systems by assuming the formation of two reactive species, an excited cyclopropane molecule and an excited cyclopropane ion, whose yields are different in the three systems as can be seen in Table 3. 

The metal monohydroxides CaOH and SrOH are the simplest monovalent polyatomic derivatives of the alkaline earths. Both CaOH and SrOH have a surprisingly long history in view of their high chemical reactivity. While CaOH and SrOH can only be stored when isolated in rare gas matrices [14], substantial steady-state concentrations exist in a variety of energetic environments. 

Fluorescence from electronically excited fragments formed in collisions of rotor accelerated beams of metastable rare-gas atoms with Brj and BrCN has been measured and used to probe the mechanisms of the processes involved. Polarization of fluorescence from KBr B) and Brj, produced in the Kr( P2,o) + 2 system shows that rotational alignment perpendicular to the relative velocity vectors of the colliding partners is retained in both inelastic and reactive scattering processes. No such polarization is observed in the fluorescence of CN(5 Z ) formed in reaction (18), and this can be explained by a two-stage harpooning... 

The most intensely studied species are the group 6 pentacarbonyls, M(CO)5(H2), which have been observed in rare-gas matrices, in liquid Xe solutions at - 70°C (a very useful medium), in alkane solvents, and in the gas phase. Perhaps the most novel preparation is photolysis of the hexacarbonyls impregnated in polyethylene disks under H2 or N2 pressures to give M(CO)6 n(L)n, where n = 1,2 for L = H2 and 1-4 for N2.175 Reactivity follows the order Mo > Cr > W, and H2can displace coordinated N2 in the polyethylene systems. In all media, vibrational spectroscopy provides evidence for H2 rather than dihydride binding, and the H-H, H-D, and... 

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