Complex Reaction Mechanism Chapter 37 Complex Reaction Mechanism Engel & Reid

Figure 37.1

Figure 37.2

Figure 37.3

Catalysis 37.4 Catalysis

37.4 Catalysis Two assumptions : [SC] is small, [SC]2 can be neglected At early stage of the reaction [P] can be neglected

Case 1: [C]0<<[S]0 37.4 Catalysis Case 2: [C]0>>[S]0

Figure 37.4a

Figure 37.4b

Michaelis-Menten Enzyme Kinetics Figure 37.6

37.4 Catalysis Example Problem 37.1 DeVoe and Kistiakowsky [J. American Chemical Society 83 (1961), 274] studied the kinetics of CO2 hydration catalyzed by the enzyme carbonic anhydrase: CO2 + H2O HCO3-1 In this traction, CO2 is converted to bicarbonate ion. Bicarbonate is transported in the bloodstream and converted back to CO2, in the lungs, a reaction that is also catalyzed by carbonic anhydrase. The following initial reaction rates for the hydration reaction were obtained for an initial enzyme concentration 0f 2.3 nM and temperature of 0.5 oC: Determine Km and k2 for the enzyme at this temperature. Rate (M s-1) [CO2] (mM) 2.7810-5 1.25 5.0010-5 2.5 8.3310-5 5.0 1.6710-4 20.0

Figure 37.7

Figure 37.8a

Figure 37.8b

Figure 37.9

Figure 37.10

Figure 37.11

Figure 37.12

Figure 37.13

37.8 Photochemistry 37.8.1 Photophysical Processes Figure 37.14 A Joblonski diagram depicting various photo-physical processes, where S0 is the ground electronic singlet state, S1 is he first excited singlet state, and T1is the first excited triplet state. Radiative processes are indicated by the straight lines. The nonradiative processes of intersystem crossing (ISC), internal conversion (IC), and vibrational relaxation (VR) are indicated by the wavy lines.

37.8 Photochemistry Figure 37.15 Kinetics description of photo-physical processes. Rate constants are indicated for absorption (ka), fluorescence (kf), internal conversion (kic), intersystem crossing from S1 to T1 (ksisc), and phosphorescence (kp)

Table 37.1

37.8.2 Fluorescence and Fluorescence quenching Steady-State approximation Absorption Internal Conversion Quenching Fluorescence Intersystem crossing

Fluorescence life-time, tf Fluorescence Intensity, If

Fluorescence and Fluorescence Quenching 37.8 Photochemistry Fluorescence and Fluorescence Quenching Stern-Volmer plots Figure 37.16 A Stern-Volmer plot. Intensity of fluorescence as a function of quencher concentration is plotted relative to the intensity in the absence of quencher. The slope of the line provides a measure of the quenching rate constant relative to the rate constant for fluorescence.

When kf >> kic and kf >> ksisc 37.8.3 Measurement of tf When kf >> kic and kf >> ksisc

Example Problem 37.4 Thomaz and Stevens (in Molecular Lumiescence, Lim, 1969)studied the fluorescence quenching of pyrene in solution. Using the following information, determine kf and kq for pyrene in the presence of the quencher Br6C6. Example Problem 37.4 [Br6C6] (M) tf (s) 0.0005 2.66×10-7 0.001 1.87×10-7 0.002 1.17×10-7 0.003 8.50×10-8 0.005 5.51×10-8

slope = 3.00×109 s-1 = kq intercept = 1.98×106 s-1 = kf

P37.31) For phenanthrene, the measured lifetime of the triplet state (tP) is 3.3 s, the fluorescence quantum yield is 0.12, and the phosphorescence quantum yield is 0.13 in an alcohol-ether class at 77 K. Assume that no quenching and no internal conversion from the singlet state occurs. Determine kp, kTisc, and kSisc/kr.

Figure 37.17

Figure 37.18

Example Problem 37.1

Example Problem 37.2-1

Example Problem 37.2-2