the stability of the OHat the active site is increased even more by long-range hydrogen-bonding interactions between the active (nucleophilic) H20 and components of what has been termed the "water chain" in the active site, indicating again the importance of residues around but not in the active site.Quantum and molecular mechanical calculations have thus provided insights into rate-enhancing factors in enzyme catalysis. Long-range electrostatic and hydrogen-bonding effects are crucial. It will be of interest to investigate these in a systematic manner to discover where the major portion of electrostatic stabilization energy in charge separation processes is centered. It will be further desirable to develop methods to calculate

The thermal N, exchange of a number of "N-labelled diazo compounds was studied in solution. The compounds involved were 3-diazo-1 -methylindolin-2-one (l), 9-diazofluorene (2), 5-diazo-1,3-cyclopentadiene-1,2,3,4-tetracarbonitrile (3), 2-diazo-2H-imidazole-4,S-dicarbonitriIe (4), 4-diazocyclohexa-2,5-dienone (5), and the conjugate acids of 4 and 5, namely 4,s-dicyano-IH-imidazole-2-diazonium ion (6) and 4-hydroxybenzenediazonium ion (7). Only 1, 4, 6, and 7 exchange their diazo group with 'external' molecular N,. The results are explained on the hypothesis that only organic species which have an empty [T orbital and which are effective in I( electron back-donation are able to react with N,. Thus, reaction with carbenes is likely to occur only if the carbene is in the 'A, singlet state and if its electrophilicity is high.

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