IR spectroscopy time-resolved

Within physical chemistry, the long-lasting interest in IR spectroscopy lies in structural and dynamical characterization. Fligh resolution vibration-rotation spectroscopy in the gas phase reveals bond lengths, bond angles, molecular symmetry and force constants. Time-resolved IR spectroscopy characterizes reaction kinetics, vibrational lifetimes and relaxation processes. 

Furthermore, details of the isomerization of 1-aIkenes into 2-aUcenes were examined by deuteration experiments [177] and by using time-resolved IR spectroscopy in... 

Samjeske G, OsawaM. 2005. Current oscillations during formic acid oxidation on a Pt electrode Insight into the mechanism by time-resolved IR spectroscopy. Angew Chem 44 5694-5698. 

Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy... 

Until recently, fast time-resolved IR spectroscopy has been a technique fraught with difficulty. Generally it has been easier to use low temperature techniques, particularly matrix isolation (2,4), to prolong the lifetime of the fragments so that conventional spectrometers can be used. In the last 5 years, however, there have been major advances in fast IR spectroscopy. It is now posssible to detect metal carbonyl intermediates at room temperature in both solution and gas phase reactions. In Section II of this article, we explain the principles of these new IR techniques and describe the apparatus involved in some detail. In Section III we give a self-contained summary of the organometallic chemistry that has already been unravelled by time-resolved IR spectroscopy. 

This section must necessarily be regarded as a preliminary review. The first paper (60) on fast time-resolved IR spectroscopy of an organometallic species was published in mid-1982. By the end of 1984, a further 10 papers had been published, accepted, or submitted to journals. In order to present... 

Less work has been done on the time-resolved IR spectroscopy of Cr(CO)6 in the gas phase (69,91,92). Broadly, the results were similar to those with Fe(CO)5. Ultraviolet photolysis (249 nm) provides evidence for several CO loss products (92) tentatively assigned as Cr(CO)5 and Cr(CO)4. 

Poliakoff, M. Weitz, E. Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy, in Stone, F.G.A. R. West, Eds Advances in Organometallic Chemistry, 1986, 25, 277. 

Using a temperature-programmed surface reaction (TPSR) technique, Li et al. (154) showed that this complete oxidation of methane took place on the NiO catalyst during the CHfOi reaction. Weng et al. (145) used in situ microprobe Raman and in situ time-resolved IR spectroscopies to obtain a relationship between the state of the catalyst and the reaction mechanism. These authors showed that RuC>2 in the Ru/SiC>2 catalyst formed easily at 873 K in the presence of a CH4/02/Ar (2/1/45, molar) mixture and that the dominant pathway to synthesis gas was by the sequence of total oxidation of CH4 followed by reforming of the unconverted CH4 by C02 and H20. Thus, these results indicate that the oxidation of methane takes place principally by the combustion mechanism on the oxidized form of this catalyst. 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

In comparison, photolysis of 83 in protic solvents such as methanol, ethanol, and water yields 84 as expected, but 84 forms mainly 87 rather than 85. Furthermore, in these solvents, the transient absorption (Amax 425 nm) due to 84 decays not with a second-order rate law but by biexponential decay. For example, the decay of transient absorption of 84 (A ax 420 nm) in water at pH 7 had rate constants of 2 x 10 and 3 x lO s Subsequent to the decay of 84, a transient absorption was formed with Amax 330 nm and a weak absorption band at 740 nm. However, this transient was formed much slower than 84 decayed. The absorption at 330 nm was described as a biexponential growth with rate constants of 584 and 21 s h The authors assigned this absorption to 88. Since 84 and 88 do not form and decay at the same rate, the authors theorized that 84 decays into 87, which then furnishes 88. Even though intermediate 87 does not absorb in the near UV, the authors characterized it with time-resolved IR spectroscopy. The authors demonstrated that, in hexane and a strongly acidic or basic aqueous solution, the photorelease from 83 goes through the formation of 87, whereas in near neutral aqueous solution, formation of 85 predominates. The authors concluded that the dehydration of intermediates 85 and... 

Matrix IR data, like UV-vis, play an important role in the interpretation of the transient spectroscopic results obtained in the time-resolved IR spectroscopy described below (Sections 4.2 and 4.3). 

Time-Resolved IR Spectroscopy. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of micro-and even nanoseconds. Since IR spectra provide structural information in more detail than UV, this technique will be more powerful than TRUV-vis if one can find a carbene that can be detected and studied by this technique. To date, however, only one carbene has been studied by using TRIR. The matrix IR study shows that the planar triplet and twisted singlet states of 2-naphthyl(methoxycartbonyl) carbenes (NMC, 17) show distinctly different IR bands (see Section 3.1.4). Both NMC and NMC are detected by TRIR in solution and their kinetics have been studied. Such experiments provide clear cut data for the reaction kinetics as well as energetics of both states (see Sections 4.2 and 4.3... 

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the 0-nitrene, but detected the presence of PhCH2N=0 formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also 250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 10 M s. This value strongly suggests that the 6>-nitrene, in contrast to the Al-nitrenes has a triplet ground state. 

The presence of the charge-separated state involving electron transfer from Mg(II)TPP to Re(bpy) was shown by time-resolved IR spectroscopy . ... 

Decomposition of 2,6-difluorophenyl azide by LFP (266 nm) generated a singlet nitrene which was detected by time-resolved IR spectroscopy (1404 cm-1).75 The nitrene could only be detected between 243 and 283 K. At 298 K, the nitrene decay products, a ketenimine (1576 cm-1) and a triplet nitrene (1444 cm-1), were observed. The IR assignments were consistent with DFT calculations and previous UV-visible detection results. 

The reaction of benzoylnitrene with lithium bromide was studied to determine whether a nitreneoid [PhCON(Li)Br] or a Hoffman intermediate (PhCON-Br) would be formed.79 Time-resolved IR spectroscopy combined with DFT modelling indicated that the Hoffman intermediate is formed. 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

In this chapter we will first discuss coherent anti-Stokes Raman scattering (CARS) of simple liquids and binary mixtures for the determination of vibrational dephasing and correlation times. The time constants represent detailed information on the intermolecular interactions in the liquid phase. In the second section we consider strongly associated liquids and summarize the results of time-resolved IR spectroscopy (see, e.g., Ref. 17) on the dynamics of monomeric and associated alcohols as well as isotopic water mixtures. 

II. TIME-RESOLVED IR SPECTROSCOPY OF STRONGLY ASSOCIATED LIQUIDS... 

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