Chapter 15. Direct Photolysis

Summary

Transformations of organic compounds that are a consequence of that compound's own light absorption are called direct photolysis. The rates of these light-initiated reactions are determined by two factors: (a) the compound's rate of light absorption and (b) the quantum yield for reaction of the excited state. Structure features that enable specific organic compounds to absorb available sunlight (>300 nm) generally involve the presence of pi and nonbonded electrons in an extended resonating system. Recognizing such susceptible compounds, the rate of light absorption in an environmental system of interest can be calculated with knowledge of the variation of sunlight intensities as a function of wavelength, season, geographical location, water depth, and presence of competing chromophores, and the wavelength-dependent absorptivity of the compound of interest. Empirical observations are needed to quantify quantum yields for situations of interest. Finally, distributions between dissolved and sorbed species have been shown to influence overall rates of photolysis in both surface waters and soils.