The rates of binding of carbon monoxide and imidazoles to ferrous hemes are reported. By flash photolysis of appropriate heme(CO)(Im) mixtures it is possible to determine all ten rate constants relating the five species heme, heme(CO), heme(Im), heme(Im)2, and heme(CO)(Im). Under all conditions, some of the return from heme(Im) to heme(CO)(Im) proceeds via loss of imidazole, followed by addition of CO and subsequent addition of imidazole, the "base-elimination" pathway. Both benzene and aqueous detergent (cetyltrimethylammonium bromide) have been used as solvents; the similarities and differences are discussed. The rates of the heme-CO-imidazole mixtures are compared with those found in heme proteins. The sterically hindered 2-methylimidazole serves as a model for T-state hemoglobin, while the unhindered imidazole or 1-methylimidazole serves as a model for R-state hemoglobin.

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Decomposition of meso-and d/-3,4-Diethyl-3,4-dimethyldiazetine (a 1,2-Diaza-l -cyclobutene)1 Sir:Study of the thermal decomposition of diazenes (azo compounds) has provided much information on the nature of diradicals and on the question of concerted vs. stepwise decomposition paths.2 The four-membered-ring diazene-the 1,2-

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