Molecular beam contour diagrams

Figure 4.11 Molecular beam contour diagram showing backward scattering...

Again molecular beams give conclusive evidence as to whether a collision complex is involved. For such reactions the molecular beam contour diagram will be symmetric. The interactions keep the two reactants in close contact for long enough for them to forget the directions in which they would have parted had they been able to do so before rotation. They thus part in random directions (Figure 5.15). 

Figure 5.15 A molecular beam contour diagram showing a collision complex...

What are the energy requirements for reaction Explain the energy disposal in the products. Predict the molecular beam contour diagram. 

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. 

The back reaction will have the exact reverse characteristics. The activated complex will lie in the exit valley, and reaction will be enhanced by high vibrational energy. There will be high translational energy in the products, the cross section will be small, and the molecular beam contour diagram will show predominantly backward scattering, typical of a rebound mechanism. 

Total cross sections, translational energy distributions and velocity contour diagrams for many of the products for the H+ + D2 and D+ + HD reactions have been obtained from the crossed ion beam-molecular beam measurements and the TSH calculations making these reactions probably the best understood in the field. This combined experimental and theoretical investigation provided a unique opportunity for a detailed comparison of an a priori theory of chemical reactions with experiments. These comparisons are very encouraging since the TSH theory involves no empirical information or adjustable parameters. 

---