Time-resolved ultraviolet-visible spectroscopy

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved ultraviolet-visible (UV-Vis) absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter [1] over 50 years ago. Although a great deal of information about the reactivity of organic photochemical intermediates (e.g., excited states, radicals, carbenes, and nitrenes) in solution at ambient temperatures has been amassed with this technique, only limited structural information can be extracted from such investigations because absorption bands are usually quite broad and featureless. Questions of bonding, charge distribution, and solvation (in addition to those of dynamics) are more readily addressed with time-resolved vibrational spectroscopy. 

As stated earlier, polymer deposition occurs by precipitation of the oligomers when their chain length attains a value of critical solubility [38]. The growth of PT and poly(bithiophene) has been studied by time-resolved ultraviolet visible (UV-vis) spectroscopy. Comparison of the spectra of the reaction solution with those of authentic neutral thiophene oligomers has led to the conclusion that oligomers formed by 7-12 monomer units are formed in solution during electropolymeriza-... 

Enzyme reaction intermediates can be characterized, in sub-second timescale, using the so-called pulsed flow method [35]. It employs a direct on-line interface between a rapid-mixing device and a ESI-MS system. It circumvents chemical quenching. By way of this strategy, it was possible to detect the intermediate of a reaction catalyzed by 5-enolpyruvoyl-shikimate-3-phosphate synthase [35]. The time-resolved ESI-MS method was also implemented in measurements of pre-steady-state kinetics of an enzymatic reaction involving Bacillus circulans xylanase [36]. The pre-steady-state kinetic parameters for the formation of the covalent intermediate in the mutant xylanase were determined. The MS results were in agreement with those obtained by stopped-flow ultraviolet-visible spectroscopy. In a later work, hydrolysis of p-nitrophenyl acetate by chymotrypsin was used as a model system [27]. The chymotrypsin-catalyzed hydrolysis follows the mechanism [27] ... 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Most of the early gas lasers emitted in the visible region. Continuous-wave (CW) lasers such as Ar+ (351.1-514.5 nm), Kr+ (337.4-676.4 nm), and He-Ne (632.8 nm) are now commonly used for Raman spectroscopy. More recently, pulsed lasers such as Nd YAG, diode, and excimer lasers have been used for time-resolved and ultraviolet (UV) resonance Raman spectroscopy. 

Time-resolved spectroscopy (stopped-flow ultraviolet-visible (UV-vis) spectroscopy at -90° C, proprionitrile or acetonitrile, [O2] S> [complex]) has been used to characterize intermediates and evaluate the mechanism of the peroxo complex formation (see Fig. 16) (196). Based on the similarity of the spectral features with known superoxo copper(lI) and peroxo-dicop-per(ll) complexes (262, 268, 281) the mechanism shown in Scheme 17 was proposed, and the spectra of the superoxo copper(II) and peroxo-dicop-per(II) complexes were determined (see Table XI). For steric reasons and in... 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

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