Pseudo-first-order rate constant laser flash photolysis

Laser flash photolysis of 3-pyridyl diazomethyl ketone in acetonitrile containing n-butylamine generates 3-pyridyl ketene as a reactive intermediate, as shown in Scheme 1.5. Subsequent sequential IR measurements at 2125 cm-1 on the microsecond time scale allowed determination of the pseudo-first-order rate constant for its capture by the amine. From the rate law for the capture of the ketene,... 

The polymers show UV absorption at 285-368 nm, and laser flash photolysis gives chain scission with the formation of the corresponding transient stannyl radicals with absorption at 400 nm (R = Me). They react with oxygen with a pseudo-first order rate constant of ca. 105 s 1 70... 

A mathematical model for the measurement of pseudo first order rate constants in laser flash photolysis has been put forward. Another data treatment provides a method for determining quantum yields of reactions of the type A(+hv) = B(+hv,A)... 

Photoinsertion of dioxygen in tr 5-[Co(tet a)(N02)2]" and trans-[Co(tet ft)(N02)2]" in acetonitrile has been studied by laser flash photolysis.On photolysis at 355 nm, the complexes display an absorbance increase from 350-600 nm in air-equilibrated acetonitrile, with the maximum at 520 nm. The increase at 500 nm obeys first-order kinetics and the pseudo-first-order rate constants are 1.02x10" and 1.32xl0 s for the teta and tet b complexes, respectively. The absorbances also decay in 10 ms full sweep and the decay also obeys first-order kinetics. The rate constants are 2.1 x 10 and 1.2xl0 s for tet a and tet 6, respectively. It is known that only mononuclear superoxo complexes absorb around 500 nm and hence the transient growth is due to the formation of such a complex. 

PHa + Oj- Products. The rate constants k were measured at 300 K by flash photolysis (FP) of PH3/O2/N2 mixtures (0.35 3.5 to 17 250 to 270 Torr) and PH3/O2 mixtures (0.35 1.5 Torr), above the upper and below the lower explosion limit, followed by absorption of radiation near 455 nm (from a pulsed dye laser) by PH2. k = (1.2 0.3)x10" at a total pressure of -270 Torr (i.e., above the upper explosion limit) was derived from the dependence of the effective rate constant k ff=d ln([PH2]o/[PH2])/dt on the O2 partial pressure, k = (0.8 0.2) x10" at 1.85 Torr (i.e., below the lower explosion limit) was obtained from a computer calculation of keff with known rate constants for PH3 + H PH2 + H2 (see p. 233) and 2PH2+M P2H4+M (see below). The agreement of both k values points to a pressure-independent reaction PH2 + 02 products. The step PH2 + 02 P02 + H2 is probably responsible for the observed rapid decay of PH2 [3, 4]. A later measurement of the removal of rotationally thermalized PH2 (X Bi, v = 0) in the presence of O2 by laser-induced fluorescence (LIF) under pseudo-first-order conditions gave k = 2.7x10 . The removal of vibrationally excited PH2 by O2 was also investigated [5]. 

Laser flash photolysis is often used to measure the binding rate of CO to heme proteins, such as myoglobin (Mb), because CO dissociates from the bound state relatively easily on absorption of energy from an intense and short pulse of light. The reaction is usually run under pseudo-first-order conditions. For a reaction in which [Mb]o = 10 mmol dm" , [CO] = 400 mmol dm" , and the rate constant is 5.8 X 10 dm mol" s" , plot a curve of [Mb] against time. The observed reaction is Mb + CO —> MbCO. 

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