Rapid-scan spectra

The most controversial issue is the number and exact stoichiometries of the iron(III)-sulfito complexes formed under different experimental conditions. Earlier, van Eldik and co-workers reported the formation of a series of [Fe(SO ) ]3-2" (n = to 3) complexes and the [Fe(S03)(0H)] complex (89,91,92). The stability constants of these species were determined by evaluating time resolved rapid-scan spectra obtained from the sub-second to several minutes time domain. The cis-trans isomerization of the complexes was also considered, under feasible circumstances. In contrast, Betterton interpreted his results assuming the formation and linkage isomerization of a single complex, [Fe(SC>3)]+ (93). In agreement with the latter results, Conklin and Hoffmann also found evidence only for the formation of a mono-complex (94). However, their results were criticized on the basis that the experiments were made in 1.0 M formic acid/formate buffer where iron(III) existed mainly as formato complex(es). Although these reactions could interfere with the formation of the sulfito complex, they were not considered in the evaluation of the results (95). Finally, van Eldik and co-workers re-examined the complex-formation reactions and presented additional data in support of... 

Figure 1. A series of rapid scan spectra obtained by the OLIS RSM-1000 Rapid-Scanning Monochromator (Courtesy of Olis Instruments, Inc., www.olisweb.com).

Spectra or rapid-scan spectra are obtained by the stopped-flow method establishing also a suitable wavelength at which kinetic measurements could be performed. 

The influence of variables such as pH, concentration, buffer, solvents, and different reactants on the observed rate constants or rapid-scan spectra is examined. 

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.]...

STOPPED-FLOW MEASUREMENTS IN THE INFRARED SIMULATED RAPID SCAN SPECTRA SOME OBSERVATIONS ON THE REACTIONS OF METAL CARBONYL COMPOUNDS WITH HALOGEN. 

Both cells were coupled with a Tracor Northern 1710 optical spectrometer multichannel analyzer to obtain time resolved spectra. One hundred rapid scan spectra (2.5 ms) were taken and averaged to give a final spectrum recorded as log Ro/R where Ro is the electrolyte/solvent without the sample or film and R is the electrolyte/solvent system with the sample or film. The chemical changes occurring in the bulk as well as in the films were monitored spectroelectrochemically from 300-900 nm at a potential scan rate from 2-5 mV/s or at a constant potential. 

Figure 9. a) Rapid scan spectrum b) cross correlation spectrum. 

In many spectrometers, some parameters (e.g., slit width, time constant, and amplification) cannot be chosen freely. Hence, their effects on measurement must be known. These settings depend on the type oi spectrum being observed (e.g., a spiectrum for routine analysis a rapid scan spectrum, or high-resolution spectrum). 

Slow scanning (i) of the mass spectrum over a GC peak for substance A gives spectrum (a), but rapid scanning (ii) gives spectrum (b), which is much closer to the true spectrum (c). 

Generally, measurement of absorption spectra of the colored form of spirobenzopyran is very difficult using normal spectrophotometry, as the colored form is thermally unstable. The absorption spectra of the colored form of 6,8-dinitro-BIPS 7, which is exceptionally stable in DMSO even at 23°C, are shown in Figure 1.4. Generally, it is possible to obtain a reasonable absorption spectrum of the colored form by the use of a rapid scanning spectrophotometer. 

Both Porter s original flash photolysis apparatus and Pimentel s rapid scan spectrometer recorded the whole spectral region in a time which was short compared to the decay of the transient species. Kinetic information was obtained by repeatedly firing the photolytic flash lamp and making each spectroscopic measurement at a different time delay after each flash. The decay rate could then be extracted from this series of delayed spectra. Such a process clearly has limitations, particularly for IR measurements, where the decay must be slow compared to the scan rate of the spectrum. 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

Figure 8.3 Schematic diagram of a quadrupole mass spectrometer. It consists of two pairs of parallel metal rods carrying a DC plus an oscillating voltage, in such a way that only a particular mass-to-charge ratio will pass down the center of the rods for a given setting. The mass spectrum can be rapidly scanned by varying the potentials on the rods. Adapted from Beynon and Brenton (1982), Figs. 4.6 and 4.7, by permission of University of Wales Press.

Rapid-scanning single-beam instruments exist. The absorption spectrum of the blank is first obtained, followed by that of the sample. The sample scan is then adjusted to its proper measurement by using the spectrum of the blank. 

Do not have to continually replace sample with blank when obtaining a molecular absorption spectrum 2) errors due to fight source and detector fluctuations are minimized and 3) accurate rapid scanning of wavelengths is possible. 

Similarly, the spectrum of a mixture of Fe(tpps)H20 and Fe(tpps)(OH) can be measured by rapid scan/stopped-flow at various pH s within a few milliseconds after generation (Fig. 3.9). In this short time, dimerization is unimportant so that the spectrum of Fe(tpps)OH can be measured and the pAi of Fe(tpps)H20 estimated. 

Fig, 1 Time series of spectra taken immediately after bleaching the purple membrane with white light with a rapid scanning photometer. The lower spectrum represents a filter spectrum for calibration. (Courtesy of Eur. J. Biochem.)... 

Conventional spectrophotometers scan through a spectrum one wavelength at a time. Newer instruments record the entire spectrum at once in a fraction of a second. One application of rapid scanning is chromatography, in which the full spectrum of a compound is recorded in seconds as it emerges from the chromatography column. 

Fourier transform NMR spectroscopy, on the other hand, permits rapid scanning of the sample so that the NMR spectrum can be obtained within a few seconds. FT-NMR experiments are performed by subjecting the sample to a very intense, broad-band, Hl pulse that causes all of the examined nuclei to undergo transitions. As the excited nuclei relax to their equilibrium state, their relaxation-decay pattern is recorded. A Fourier transform is performed upon this relaxation-decay pattern to provide the NMR spectra. The relaxation-decay pattern, which is in the time domain, is transformed into the typical NMR spectrum, the frequency domain. The time required to apply the Hl pulse, allow the nuclei to return to equilibrium, and have the computer perform the Fourier transforms on the relaxation-decay pattern often is only a few seconds. Thus, compared to a CW NMR experiment, the time can be reduced by a factor of 1000-fold or more by using the FT-NMR technique. 

The d-d absorption of the copper complex differs in each step of the catalysis because of the change in the coordination structure of the copper complex and in the oxidation state of copper. The change in the visible spectrum when phenol was added to the solution of the copper catalyst was observed by means of rapid-scanning spectroscopy [68], The absorbance at the d-d transition changes from that change the rate constants for each elementary step have been determined [69], From the comparison of the rate constants, the electron transfer process has been determined to be the rate-determining step in the catalytic cycle. 

Transient absorption spectroscopy, wherein one measures the electronic absorption spectrum of a molecule in an excited state, is still in its infancy, but the growing availability of ultra-high-speed, rapid-scan spectrometers augurs well for this area of spectroscopy. Thus one may, in the future, routinely probe excited state absorption spectra as well as ground state absorption spectra. The former can be expected to be as valuable in obtaining information about the excited state as is the latter for the ground state. 

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