Hunt Paula, Worrall David R, Wilkinson Frank, Batchelor Stephen N
Department of Chemistry, University of Technology Loughborough, Leicestershire, UK LE11 3TU.
Photochem Photobiol Sci. 2003 May;2(5):518-23. doi: 10.1039/b212051h.
Rate constants are measured for the addition reactions of 1-hydroxy-1-cyclohexyl (1HC) and 2-hydroxy-2-propyl (2HP) radicals to 7 alkenes and for the 1-electron reduction of 16 organic dyes by 1HC, and a subset of 5 of these dyes by 2HP. This was done to determine to what extent the many reported rate constants for reactions of 2-hydroxy-2-propyl radicals (2HP) may be used to predict the rates of reactions of other tertiary alpha-hydroxy-alkyl radicals, and to give a better understanding of the factors that control dye reduction. The dyes were chosen to represent a wide range of dye types (azo, anthraquinone, phthalocyanine, triaryl-methane, indocyanine and azine dyes). Radicals were produced by laser flash photolysis of the corresponding tertiary alpha-hydroxyketone giving carbonyl and tertiary alpha-hydroxy-alkyl radicals. Control experiments with a bis-acylphosphine oxide were carried out which clearly demonstrated that the carbonyl radicals did not interfere with the kinetics. On average the addition and reduction rate constants for 1HC are only 20% lower than for 2HP. Larger decreases are observed for sterically congested alkenes due to the increased steric bulk of 1HC. The rate constants for 1-electron reduction of the dyes are in the range 4 x 10(7) to 6 x 10(9) mol-1 1 s-1 and may be predicted, reasonably well using the Marcus equation with a reorganisation energy, lambda = 182 kJ mol-1.
测定了1-羟基-1-环己基（1HC）和2-羟基-2-丙基（2HP）自由基与7种烯烃加成反应的速率常数，以及1HC对16种有机染料的单电子还原速率常数，还有2HP对其中5种染料的单电子还原速率常数。这样做是为了确定众多已报道的2-羟基-2-丙基自由基（2HP）反应的速率常数能在多大程度上用于预测其他叔α-羟基烷基自由基的反应速率，并更好地理解控制染料还原的因素。所选择的染料代表了广泛的染料类型（偶氮、蒽醌、酞菁、三芳基甲烷、吲哚菁和嗪染料）。通过对相应的叔α-羟基酮进行激光闪光光解产生羰基和叔α-羟基烷基自由基。用双酰基氧化膦进行了对照实验，结果清楚地表明羰基自由基不干扰动力学。平均而言，1HC的加成和还原速率常数仅比2HP低20%。对于空间位阻较大的烯烃，由于1HC的空间位阻增加，观察到更大的下降。染料的单电子还原速率常数在4×10⁷至6×10⁹mol⁻¹·s⁻¹范围内，使用重组能λ = 182 kJ mol⁻¹的马库斯方程可以较好地预测。
