Wavelength reaction

The photooxidation of mono-ketones appears to proceed at all wavelengths as if one were studying the oxidation of the radicals produced by photolysis. This appears to be true also for dialkyl peroxides, aldehydes, and azoalkanes. In the photooxidation of diacetyl there is evidence that at longer wavelengths reaction occurs between excited molecules and oxygen, whereas with ketene under similar conditions deactivation of the excited molecule appears to be the chief reaction. 

Fig. 4. Single-wavelength reaction time courses reconstructed from the rapid-scanning spectra presented in Fig. 3. The time slices are for (A) 324, (B) 350, (C) 398, and (D) 575 nm, respectively. Note that two time scales are shown in each panel 0-210 ms ( ), and 0-21 s (A). The final absorbance value (A) taken from the 19th spectrum at t = 47.5 s is shown at the extreme right margin of each panel. [Taken from Koerber et al. (3) with permission.]...

The vacuum-u.v. photolysis of NaO has been reported in several studies.366 For Aex > 185 nm, reaction (121) occurs with unit quantum yield, but at shorter wavelengths reactions (122) and (123) become of importance, the quantum yield... 

Chemical Limitations to Beer s Law Chemical deviations from Beer s law can occur when the absorbing species is involved in an equilibrium reaction. Consider, as an example, an analysis for the weak acid, HA. To construct a Beer s law calibration curve, several standards containing known total concentrations of HA, Cmt, are prepared and the absorbance of each is measured at the same wavelength. Since HA is a weak acid, it exists in equilibrium with its conjugate weak base, A ... 

Selectivity Selectivity is rarely a problem in molecular absorption spectrophotometry. In many cases it is possible to find a wavelength at which only the analyte absorbs or to use chemical reactions in a manner such that the analyte is the only species that absorbs at the chosen wavelength. When two or more species contribute to the measured absorbance, a multicomponent analysis is still possible, as shown in Example 10.6. 

The concentration of chromic acid can be determined from its reduction by alcohols under conditions when the kinetics are pseudo-first-order in analyte. One approach is to monitor the absorbance of the solution at a wavelength of 355 nm. A standard solution of 5.1 X lO " M chromic acid yields absorbances of 0.855 and 0.709 at, 100 s and 300 s, respectively, after the reaction s initiation. When a sample with an unknown amount of chromic acid is analyzed under... 

The reaction of H2O2 and H2SO4 generates a reddish brown solution whose absorbance is measured at a wavelength of 450 nm. A regression analysis on their data yielded the following uncoded equation for the response (Absorbance X 1000). 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

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