Bulk Experiments with Collisions

All products of these reactions could be identified by combined gas diromography and mass spectrometry. It turns out that the c rohloroethene//niRS-chloroethene ratio is pressure dependent, particularly for the (aa) elimination. The initial formation of the c/j-product is strongly favoured in the (oa) process. 

Ring-opening reactions have been investigated by chemical activation with H atoms for H + cydopentene (see also the theoretical discussion in ref. 209), and for H + vinylcyclopropane, Qrclopentene, or dohexene. The insertion reaction of 0( Z ) into C—H bonds leads to ch cally activated alkanols. An extensive comparison between reactions of chemically activated alkanols and their thermal decomposition on the basis of RRKM calculations has been published recently.  

Photon absorption provides another route for obtaining specific rate constants from highly excited molecules. Making some assumptions about collisional energy transfer, one may use the collision frequency as a clock in the measurement of the competition between photoreaction and collisional stabilization. Alternatively, one may make some model calculations for the specific rate constants for photoreaction and for collisional energy transfer. From these one may compute experimental quantities like photolysis quantum yields as a function of pressure and test them for consistency with experiments. This procedure has been used in a detailed study of NO2 photolysis at wavelengths between 313 and 416 nm (predissociation threshold at 397.9nm) and N2 pressures between 0 and 1000atm. The 

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Figure 14. OH(o = 0) rotational distributions obtained with room temperature CO2/HI samples under single-collision conditions using photolysis wavelengths of 227,239,251,263, and 280nm. The H + COj collisions are effectively monoenergetic, except for the superthermal widths characteristic of hot-atom bulk experiments, since the I channel does not contribute. Open circles are the data dark circles were computed using the indicated values of the linear rotational surprisal parameter, 0. ...

Despite the fact that relaxation of rotational energy in nitrogen has already been experimentally studied for nearly 30 years, a reliable value of the cross-section is still not well established. Experiments on absorption of ultrasonic sound give different values in the interval 7.7-12.2 A2 [242], As we have seen already, data obtained in supersonic jets are smaller by a factor two but should be rather carefully compared with bulk data as the velocity distribution in a jet differs from the Maxwellian one. In the contrast, the NMR estimation of a3 = 30 A2 in [81] brought the authors to the conclusion that o E = 40 A in the frame of classical /-diffusion. As the latter is purely nonadiabatic it is natural that the authors of [237] obtained a somewhat lower value by taking into account adiabaticity of collisions by non-zero parameter b in the fitting law. 

When the mean free path is small compared with pore diameter, the dominating experience of molecules is that of collision with other molecules in the gas phase. In that respect, the situation is much the same as that which exists in the bulk gas. The appropriate diffusion coefficient Dm may be obtained from published experimental values, or calculated from a theoretical expression. For molecular diffusion in a binary gas, the Chapman and Enskog equation may be used, as discussed by Bird, Stewart and Lightfoot(32). This takes the form ... 

Laser pulses of short duration have been used to prepare, in bulk samples, a large initial concentration of reactive atoms or radicals.98-100 More recently, by using a second probe laser pulse, delayed with respect to the initiation pulse, it is found possible to monitor reaction products formed after a single collision of the reactive species. The recent progress101-103 in experimental studies of the H + H2 exchange reaction is due to this technique. It is finally possible (Fig. 7) to report agreement between experiments and for this fundamental chemical reaction. 

First and foremost, note that interest is in the nature of intramolecular dynamics of molecules in isolation. That is, observations must be made over a time scale where the molecule does not collide with others in the reaction vessel. Modern techniques allow very low pressures under which such measurements can be made. Most desirable among these methods are beam techniques in which molecules are studied in a low-density beam produced, for example, by vaporizing molecules in an oven. Experiments prior to this T>eam age (circa 1960) often inferred information about intramolecular dynamics from bulk data, which contained effects due to collisions, with resultant loss in accuracy. 

Similarly, most experiments on intramolecular energy transfer fall into one of two categories with respect to preparation of the molecule. In the crudest, the system is prepared by a coarse technique where little detail about the initial molecular state is available. Such, experiments include preparation via collision—that is, where the molecule of interest, A, collides with, and absorbs energy from, another molecule B—and preparation via reaction, where the molecule A is the product of a precursor reaction. Although the bulk of early work on intramolecular dynamics was carried out with these techniques, far greater insight emerges from modern experiments in which the molecule A is prepared by the absorption of radiation. 

To control the impact parameter the collision must be started with a restricted range of angles between the relative velocity and the relative position. This kind of atomic resolution control of geometry is achievable, for example, by a variant of the experiment we discnssed in Section 1.2.5. There we prepared a reactive atom by photolysis of a precursor in the bulk, and so the precursor was randomly... 

Compared to the molecules themselves, the channels and pores through which the molecules diffuse are typically large, and the main interaction for a particular molecule is collision with other molecules, the driving force for bulk diffusion. In some cases for flow in very small pores and channels, however, the molecular collisions with the solid walls of the channel become a significant component of the overall number of collisions the molecule experiences, and the effective diffusion will deviate from that predicted with Fickian diffusion theory. Consider normal diffusion in a cylindrical pore (Figure 5.14). The molecular interactions with the channel wall are negligible compared to the collisions with other molecules. 

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