Stopped-flow IR spectroscopy

A kinetic smdy of the acylation of ethylenediamine with benzoyl chloride (110) in water-dioxane mixtures at pH 5-7 showed that the reaction involves mainly benzoylation of the monoprotonated form of ethylenediamine. Stopped-flow FT-IR spectroscopy has been used to study the amine-catalysed reactions of benzoyl chloride (110) with either butanol or phenol in dichloromethane at 0 °C. A large isotope effect was observed for butanol versus butanol-O-d, which is consistent with a general-base-catalysed mechanism. An overall reaction order of three and a negligible isotope effect for phenol versus phenol- /6 were observed and are consistent with either a base- or nucleophilic-catalysed mechanism. Mechanistic studies of the aminolysis of substituted phenylacetyl chlorides (111) in acetonitrile at —15 °C have revealed that reactions with anilines point to an associative iSN2 pathway. ... 

Kinetic study of the reaction of cyclopentyl-MgBr + 4-methylmercaptoacetophenone by using stopped-flow UV and IR spectroscopies also showed that the rate constant was in the range of 2-6 s in diethyl ether at 25 °C, and the rate was first order in RMgX at low concentration (ca 0.1 M) and 0th order at high concentrations (0.3-0.8 M). The results were interpreted by the mechanism which involved pre-equilibrium of complex formation and conversion of the complex to the product. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

EPR and FT-IR spectroscopy have been used to study the decomposition of 02F2 and 02F in the gas phase by flow and stopped-flow methods [68]. 

In general, stopped-flow methods provide a reasonably inexpensive means of determining a large number of fairly fast reactions. Stopped-flow mixing is usually coupled with real-time optical observation using absorbance (UV through IR see. Vibrational Spectroscopy), fluorescence emission, or circular dichroism (CD) spectroscopy. In addition, the stopped-flow technique has been implemented in conjunction with many other biophysical techniques, such as EPR, NMR (see Nuclear Magnetic Resonance (NMR) Spectroscopy... 

The same electrochemical flow cell has also been used in combination with IR spectroscopy in a stopped-flow system [68, 69]. The purpose was to study the electrogeneration of the Collman reagent Fe(CO)4 obtained from the two-electron reduction of Fe(CO)5. It was possible to monitor the consumption of Fe(CO)s and... 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

If an intermediate is not sufficiently stable to be isolated, it might nevertheless be formed in sufficient concentration to be detected spectroscopically. Techniques used for this purpose include UV—vis spectroscopy in stopped-flow kinetics experiments for relatively stable intermediates or IR spectroscopy in matrix isolation spectroscopy for more reactive species. For photochemical reactions, we can detect transient spectra of intermediates in the millisecond to microsecond ( conventional" flash spectroscopy) or nanosecond to picosecond or femtosecond (laser flash spectroscopy) time scale. In all cases we must be certain that the spectra observed are indeed indicative of the presence of the proposed intermediate and only the proposed intermediate. Theoretical calculations have been useful in determining the spectroscopic properties of a proposed intermediate, whether it is likely to be sufficiently stable for detection, and the t)q e of experiment most likely to detect it. In addition, kinetic studies may suggest optimum conditions for spectroscopic detection of an intermediate. ... 

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