Photolysis first order rate constant

Photolysis first-order rate constants for photosensitized reactions in water with various humic substances as sensitizers k = 0.17 h-1 with aquatic humus from Aucilla River, k = 0.12 Ir1 with Aldrich humic acid, k = 0.091 Ir1 with Fluka humic acid and k = 0.11 Ir1 with Contech fulvic acid in sunlight, corresponding to half-lives of 4 to 8 h (Zepp et al. 1981) photolysis t,/2 = > 50 yr at 15°C and a pH 5-9 (Torang et al. 2002). Oxidation rate constant k for gas-phase second order rate constants, koH for reaction with OH radical, kN03 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures and/or the Arrhenius expression see reference ... 

Subst- rate No. Initial concentration of the starting dioxin, P o (10" M) Maximum irradiation time, max (h) Average percentage disappearance of the starting dioxin after max (%) Photolysis first-order rate constant k P (10 sec Absolute half-life , % (h) Quantum yield for the photodegradation. 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

Photolysis of all brominated dibenzodioxins investigated occurs very fast in n-hexane. The rate of degradation of all compounds follows a good first-order kinetic scheme. In Table 4 the calculated first order rate constants k are summarised along with the quantum-yields. The corresponding results for three brominated dibenzofurans are also included. 

First Order Rate Constants for Decay of Transients Seen by Longer Wavelength (A rr > 390 nm) Flash Photolysis of Ru3(C0)]o in Cyclohexane Solutions with Various Added Ligands a... 

Net addition of CO to 3Fe(CO)3(H2) to form Fe(CO)4(H)2 was observed upon photolysis of Fe(CO)s in sc Ar in the presence of H2 (24). The intermediate species 3Fe(CO)3(H2) is involved as a minor product of the photolysis, and was proposed to arise from addition of H2 to Fe(CO)3 or Fe(C0)3Ar (24). Experimentally, this species was shown to decay in the presence of excess CO with a pseudo-first-order rate constant 0bs — 4.1( + 0.3) x 107s-1. The mechanism for this spin-forbidden process was however unclear from experiment alone, and computation was used to explore the various possibilities (24). 

The H20 exchange mechanism was studied by Hunt et al. (32) who reported that exchange between aqueous solvent and Fein(TPPS)(H20)2 occurs with a first-order rate constant (kex = 1.4xl07s-1 in water at 298 K) far exceeding the k0 s values determined at any [NO]. If the steady state approximation was applied with regard to the intermediate Fem(Por)(H20), the kohs for the exponential relaxation of the non-equilibrium mixture generated by the flash photolysis experiment would be,... 

Vary fast reactions, both in gaseous and liquid phases, can be studied by this method. In flash photolysis technique, a light flash of very high intensity and very short duration ( 10 6 sec) is produced in the neighborhood of the reaction vessel. This produces atoms, free radicals and excited species in the reaction system. These species undergo further reactions which can be followed by spectroscopic means. The method is also known as kinetic spectroscopy. The first order rate constant as large as 105 sec-1 and second order rate constants as large as 1011 mol dm sec-1 can be measured by this technique. 

Small differences in light sources have definite effects on photochemical processes. Some irradiation systems match the solar spectrum poorly, thus affecting the relative importance of the various photodissociation processes occurring in the atmosphere. Ambient variations in the nitrogen dioxide photolysis pseudo-first-order rate constant,... 

FIGURE 4.31 Calculated first-order rate constants for loss of CH, COCH, due to reaction with OH (i.e., /c[OH]) or photolysis (i.e., A p) as a function of altitude (adapted from Gierczak et at., 1998). 

First-Order Rate Constant for Quantification of Direct Photolysis Illustrative Example 15.3 Estimating the Photolysis Half-Life of a Weak Organic Acid in the Well-Mixed Epilimnion of a Lake Determination of Quantum Yields and Chemical Actinometry Advanced... 

First-Order Rate Constant for Quantification of Direct Photolysis... 

Similarly, the near-surface first-order rate constant k° for direct photolysis is given by ... 

Table 16.2 Chemical Structure, Hammett Constants, and Pseudo-First-Order Rate Constants for Suwanee River Fulvic Acid (SRFA) Sensitized Photolysis of a Series of Phenyl Urea Herbicides (PUHs)...

A pulsed-laser photolysis study of [Cr(en)3]3+ illustrates quite dramatically the enhancement in reactivity that can result upon populating a ligand field excited state.26 Thus a significant fraction of the primary photoproduct, [Cr(en)2(enH)(OH2)]4+, is formed within the 20 ns duration of the laser pulse and is thought to arise from reaction of the lowest excited quartet state, Q° (see Figure 6). This observation establishes that the pseudo-first-order rate constant for this excited state... 

In many cases, the crystal retains enzymatic activity. In some cases, the activity of the enzyme in the crystal is the same as that in solution. The methods used for initiating reactions for study by the Laue method are used to measure activity. For example, pH-jump the acylenzyme indolylacryloyl-chymotrypsin was crystallized at a pH at which it is stable. On changing the pH to increase the reactivity, the intermediate was found to hydrolyze with the same first-order rate constant as occurs in solution the reactions of crystalline ras p21 protein, glycogen phosphorylase, and chymotrypsin have been initiated by photolysis.52 Glyceraldehyde 3-phosphate dehydrogenase has also identical reaction rates in the crystal and solution under some conditions.53... 

The ratio -ln[yp(r)]/T = 1 describes first-order decay that is unaffected by mass transport. When yp is calculated by Eq. 6 the ratio will not equal 1, and will express the deviation between the case of the measured first-order rate constant with flow and diffusion and the ideal case of no flow and diffusion. Figure 6 shows a plot of -ln[yp(r)]/T vs. z for the case when reaction zone at t = 0. The parameters are those from an investigation of the reaction flash photolysis of CF2ClBr in the presence of 02 and NO, where the reaction of CF2C102 radicals with N02 was studied [41]. For reference, rd = 0.1024 corresponds to a total pressure of 1 torr. Figure 6 clearly shows that at low pressures the deviation from exponential decay occurs at shorter times, z = kt, than at higher pressures. This is due to the pressure dependence of the diffusion coefficient. 

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,... 

Kinetic measurements of the binding of NO to the porphyrin species were carried out in excess [NO] 3> [porphyrin], as a function of varying [NO], pH, temperature, and pressure. The first-order rate constants showed a linear dependence on [NO], allowing the determination of the kon and k,)tl values at pH 3 of 9.6 x 10 dm3/(mol s) and 51 s 1, respectively, in good agreement with values determined from earlier flash photolysis studies.327 Kinetics measurements over a range of pH permitted the... 

On sandy loam soil first-order rate constants for photolytic decline, k = 4.07 x 10 3 lr1 irradiated in moisture-maintained soil, k = 1.07 x 10-3 h 1 irradiated in air-dried soil, k = 1.52 x 10 3 h 1 in dark control moist soil and no degradation in dark control air-dried sandy loam soil from Madia, CA. t,/2 = 120 h for the first 96 h irradiation over all t,/2(calc) = 200 h from 96-168 h but in dark control system t,/2 = 460 h in moist sandy loam soil irradiated metabolism t,/2 = 650 h in dry sandy loam soils. Rate constants due to photolysis k = 2.55 x 10-3 h, and due to moisture k = 3.0 x 10 3 h in moist sandy loam soil (Graebing Chib 2004)... 

In hydrocarbon solvents, /3-diketones are predominantly (> 90%) enolized and these solutions have been subjected to flash photolysis, which causes photoisomerization to the diketo form9). Reversion of the diketone to the more stable cis enol was then followed by UV spectroscopy and pseudo first order rate constants at room temperature were in the range 14-68 x 10-3 s-1 (e.g. AA = 23 x 10-3 s 1) with half-lives of several hours. The same research also reveals an alternative transformation on irradiation. In this the cis enol form is converted by rotation about a C-C or C=C bond into one of the possible trans enol isomers. These may then go on to the diketo form but mainly they revert very rapidly to the cis enol with rate constants of 0.1 to 70 s 1 (e.g. AA = 0.27 s-l)9). 

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