Method time-resolved infrared

Despite the considerable amount of information that has been garnered from more traditional methods of study it is clearly desirable to be able to generate, spectroscopically characterize and follow the reaction kinetics of coordinatively unsaturated species in real time. Since desired timescales for reaction will typically be in the microsecond to sub-microsecond range, a system with a rapid time response will be required. Transient absorption systems employing a visible or UV probe which meet this criterion have been developed and have provided valuable information for metal carbonyl systems [14,15,27]. However, since metal carbonyls are extremely photolabile and their UV-visible absorption spectra are not very structure sensitive, the preferred choice for a spectroscopic probe is time resolved infrared spectroscopy. Unfortunately, infrared detectors are enormously less sensitive and significantly slower... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

Another method found to be useful for the direct observation of alkane metal complexes is fast time-resolved infrared (TRIR) spectroscopy.- - - Upon UV irradiation of a metal-carbonyl complex, such as Cp Rh(CO)2 in liquid rare gases, an intermediate is formed, which, in the presence of an alkane, forms a weakly bound alkane complex [Eq. (6.93)]. Finally, the latter complex rapidly transforms through the transition state 112 into the alkyl hydride [Eq. (6.94)]. 

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). 

As mentioned above, the most informative method to study biochemical reactions would be time-resolved infrared difference spectroscopy. However, because the spectral changes are very small, all techniques require signal averaging over many reaction cycles. This limits application of the techniques to thermally reversible photoreactions. If such systems are in addition stable enough, the photoreaction can be triggered by thousands of flashes. 

The detection of short-lived transient species is often achieved by flash photolysis where an extremely short flash of UV/Vis radiation from a laser generates a high concentration of transient species, and a second probe beam monitors any changes that occur after the flash. Traditionally, UVA is spectroscopy has been used as a detection method. However, time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR detection, also has a long history. There are several different approaches to fast IR spectroscopy and the method of choice depends upon the timescale of the reaction. Measurements on the nanosecond to millisecond timescale are obtained using point-by-point techniques or by step-scan FTIR. In the point-by-point approach, a continuous wave IR laser (GO or diode) or globar is used as the IR source, which is tuned to one particular IR frequency (Figure 3). ... 

Another method, which allows the structural characterization and elucidation of the reactivity of transient species using infrared spectroscopy, is to observe them in real time, using fast time-resolved infrared (TRIR) spectroscopy. In this section we shall focus on the application of fast (submillisecond) and ultrafast (subnanosecond) TRIR spectroscopy to coordination compounds, and describe experiments that cannot be performed using conventional infrared spectrometers. 

Two spectroscopic methods that have been used in bioinorganic chemistry but not discussed in the text are time-resolved infrared spectroscopy and time-resolved resonance Raman spectroscopy. Speculate on the types of investigation that might be made with these methods. 

Other integral methods include in situ spectroscopic techniques, such as time-resolved infrared spectroscopy, which measures concentration or conversion as well as provides information on chemical identities of components in the reactor. In contrast to the chemical sampling method, the infi ared (IR) spectrum of the reacting system may be collected at a much faster pace than that normally possible for the chemical sampling method. Consequently, rate data may be derived with good accuracy by differentiating the IR intensity data with respect to time. 

In this book, most chapters deal with FT-IR spectrometry and its applications to various methods of infrared spectroscopic measurements. Only terahertz spectrometry in Chapter 19 and a large part of time-resolved infrared spectrometry in Chapter 20 are laser-based measurements. This shows how widely FT spectrometry is used at present in the measurements of vibrational spectfa. 

For microsecond to nanosecond time-resolved infrared absorption measurements, three types of spectroscopic methods have been developed (i) a method using an infrared laser, (ii) a method using a dispersive spectrometer, and (iii) a method using an FT-IR spectrometer. The time resolution of each of these is limited to the fastest time-response capability of the detector used. 

Many photochemical and photophysical phenomena occur on a time scale shorter than a nanosecond. In order to follow such fast phenomena by infrared spectroscopy, picosecond to femtosecond time-resolved infrared measurements are required. Since time resolving in this time range cannot be performed by utilizing the fast-response capability of a detector and the time-resolving power of an electronic circuit (gate circuit, etc.), the following optical methods are mainly used (i) a method based on the upconversion (optical gating) process, and (ii) a method which detects pulsed infrared radiation itself. At present, the latter method is commonly used for picosecond to femtosecond time-resolved measurements. 

Infrared pulses used for ultrafast time-resolved infrared spectroscopic measurements have been generated mainly by the following three methods ... 

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

Since about the end of the 1990s, generation of ultrashort pulses has become easier due to the progress of laser technology, and, as a consequence, a measuring method based on femtosecond Ti sapphire regenerative amplifier with a kilohertz repetition rate has become the mainstream of fast time-resolved infrared absorption measurements. 

In practice, an electromagnetic pulse with an infinitely short width does not exist, but ultrashort laser pulses are now used for various spectroscopic measurements. Terahertz spectrometry described in Chapter 19 is based on femtosecond laser pulses. In Chapter 20, time-resolved infrared spectroscopic methods using picosecond to femtosecond laser pulses are described. Such ultrashort laser pulses have large spectral widths in the frequency domain. Let us discuss the relation between the pulse width in the time domain and its spectral width expressed in either frequency or wavenumber. 

Selected entries from Methods in Enzymology [vol, page(s)] Biomolecular vibrational spectroscopy, 246, 377 Raman spectroscopy of DNA and proteins, 246, 389 resonance Raman spectroscopy of metalloproteins, 246, 416 structure and dynamics of transient species using time-resolved resonance Raman spectroscopy, 246, 460 infrared spectroscopy applied to biochemical and biological problems, 246, 501 resonance Raman spectroscopy of quinoproteins, 258, 132. 

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... 

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