Spacing Factors and Structural Correspondence

For about 30 years a discussion has been going on about the mechanism of reaction (1.8). Three viewpoints having come to light (1) the Bonhoifer-Farkas mechanism suggesting a complete break of bonds  

D2 at H), giving rise to a chain reaction. Recent kinetic measurements show that the second mechanism is the most probable one 61), at least at low pressure and at temperatures higher than 300° C. In conformity with the multiplet theory [pattern (1.2)], deformed molecules H2 and D2 must react here which is closest of all to the second mechanism 62). 

Deuterium exchange of ammonia is expressed by an index analogous to (1.8), where N stands instead of one H. Because of the pyramidal form of NH3, the other two atoms of H, according to the multiplet theory, must consecutively come into contact with the surface and make possible their exchange with deuterium the molecule must be turned (1.9). 

Extra-index substituents may be different. Thus, we have investigated the deuterium exchange of cyclohexane on Pt and on Cr203 64). According to the multiplet theory the ring participates in reaction here only with one C atom (1.10). For a detailed survey on deuterium exchange of hydrocarbons see Kemball 65) and Burwell 66). 

A further complication of the index (1.7) will be the index of hydro-genolysis in which not one but two atoms differ from H. Thus, for the 

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