Reaction mechanisms molecular beam reactions

The molecular beam and laser teclmiques described in this section, especially in combination with theoretical treatments using accurate PESs and a quantum mechanical description of the collisional event, have revealed considerable detail about the dynamics of chemical reactions. Several aspects of reactive scattering are currently drawing special attention. The measurement of vector correlations, for example as described in section B2.3.3.5. continue to be of particular interest, especially the interplay between the product angular distribution and rotational polarization. 

Kinetics on the level of individual molecules is often referred to as reaction dynamics. Subtle details are taken into account, such as the effect of the orientation of molecules in a collision that may result in a reaction, and the distribution of energy over a molecule s various degrees of freedom. This is the fundamental level of study needed if we want to link reactivity to quantum mechanics, which is really what rules the game at this fundamental level. This is the domain of molecular beam experiments, laser spectroscopy, ah initio theoretical chemistry and transition state theory. It is at this level that we can learn what determines whether a chemical reaction is feasible. 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

Perhaps the first clear observation of a reactive resonance in a collision experiment was recently made for the F + HD —> HF + D reaction.65-67 This reaction was one isotopomer of the F + H2 system studied in the landmark molecular beam experiments of Lee and co-workers in 1985.58 Unlike the F + H2 case, no anomalous forward peaking of the product states was reported, and results for F + HD were described as the most classical-like of the isotopes considered. Furthermore, a detailed quantum mechanical study68 of F + HD —> HF + D reaction on the accurate Stark-Werner (SW)-PES69 failed to locate resonance states. Therefore, it was surprising that the unmistakable resonance fingerprints emerged so clearly upon re-examination of this reaction. 

A close analogy to the localized surface interaction can be found in the field of chemical kinetics, namely, in the spectator stripping mechanism (5, 6) of the gas reactions, as evidenced by the recent crossed-molecular-beams experiments. Here the projectile seems to meet with only a part of the target molecule (that one to be transferred), while the rest of the target behaves as a spectator, in a sense not taking part in the reaction. 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

The detailed mechanism of the dissociation of formaldehyde (H2CO) has been investigated in molecular beam experiments. The reaction itself may appear quite simple... 

Another apparatus that is very useful in studies of the mechanism of catalytic surface reactions is shown in Fig. 17. This is used in a molecular-beam surface scattering experiment (22b) in which a well-collimated beam of the reactant gas or gas mixture is scattered from a crystal surface and the products that are desorbed after a single scattering at a given solid angle... 

Ultrahigh-vacuum (UHV) surface spectroscopy has been used with molecular beams of SiH4 and mass spectroscopy to elucidate the Si growth mechanism (67, 143). Joyce et al. (67) found that Si growth is preceded by an induction period when surface oxide was removed as SiO. The subsequent film growth proceeds by growth and coalescence of adjacent nuclei with no apparent formation of defects. Henderson and Helm (144) proposed a step-flow model in which adatoms from SiH4 surface reactions difluse to kink sites. 

Crossed molecular beams have been used to study nearly as wide a range of alkali metal atom reactions as has been examined by diffusion flames. An excellent review has been provided by Herschbach2. The multi-step mechanism displayed for chemiluminescence studies does not apply to the scattering experiments. Only the initial bimolecular reaction is important at the low pressures used. 

The calculation of the PE surface is basically quantum mechanical. Accurate surfaces are used to show how the topography of the surface affects the reaction unit as it changes configuration across the surface. Predictions can be made, and these can be tested by molecular beams, spectroscopic techniques and chemiluminescence. 

Reactions between light molecules have been extensively studied in the last two decades, generally by molecular beam techniques (see Chapter 4, Section 4.2), and these have allowed detailed testing of the predictions made from calculated potential energy surfaces. There are three typical mechanisms for gas phase reactions. 

Since the critical configuration is reached when P is far from M, then the reaction will have a large cross section. The velocity of the products will be low, and the products will initially be in excited vibrational levels. The molecular beam contour diagram will show predominantly forward scattering typical of a stripping mechanism. 

---