Time-resolved UV/VIS spectroscopy

Time-resolved UV-vis spectroscopy is the most common technique and has been widely used, but other time-resolved spectroscopies such as IR and EPR have become available as well. 

Protonated polymethylbenzenes281 and the chlorohexamethylbenzenium cation,282 intermediates in aromatic electrophilic substitutions known as Wheland intermediates, have been isolated as crystalline salts, allowing investigators to obtain their X-ray crystal structure. Nitrosoarenium a complexes of various arenes were directly observed by transient absorption spectroscopy.283 Kochi presented a method combining appropriate instrumental techniques (X-ray crystallography, NMR, time-resolved UV-vis spectroscopy) for the observation, identification, and structural characterization of reactive intermediates fa and n complexes) in electrophilic aromatic substitution.284... 

The effect of N-acetyl substitution in methionine on the nature of transients formed after one-electron oxidation was studied as a function of pH and NAM concentration. The observed absorption bands with X = 290 nm, 360 nm, and 490 nm were respectively assigned to a-(alkylthio)alkyl, hydroxysulfuranyl and dimeric radical cations with intermolecular three-electron bond between sulfur atoms. N-acetylmethionine amide (NAMA) (Chart 7) represents a simple chemical model for the methionine residue incorporated in a peptide. Pulse radiolysis studies coupled to time-resolved UV-Vis spectroscopy and conductivity detection of N-acetyl methionine amide delivered the first experimental evidence that a sulfur radical cation can associate with the oxygen of an amide function vide infra). ... 

Direct evidence for the formation of the oxyl radical was indeed obtained in a pulse radiolysis study of the OH-induced decay of 4-methoxycumylalcohol radical cation where, accordingly the 4-methoxycumyloxyl radical was observed by time-resolved UV-Vis spectroscopy on its way to form 4-methoxyacetophenone (Scheme 58) [215],... 

The technique has been described in detail elsewhere. [26] In short, a pulse of high energy electrons induces a series of chemical reactions that can be monitored, e.g., using time resolved UV-vis spectroscopy. The reaction of interest is usually induced by a reaction between a radical formed from radiolysis of the solvent (usually water) and a solute molecule. The primary radiolysis products in aqueous solution are HO, e q", H, HjOj, H2 and The major radical species, HO and e q, are formed in equimolar concentrations, 0.28 ol/J each, on electron or y-irradiation.[27] As can be seen in reaction 2, the hydroxyl radical does not yield a benzene radical cation instantly upon reaction with a substituted benzene. For this reason, secondary oxidants, such as S04, Brj and N3, are usually used to generate benzene radical cations. To determine one-electron reduction potentials of radical cations, the redox equilibrium between the radical cation of interest and a redox couple with a known one-electron reduction potential is studied. The equilibrium constant can be derived from the rate constants of the electron-transfer reaction and the back reaction and/or the equilibrium concentrations of the two redox couples (reaction 6).[28]... 

The rate constants for the reactions of OH with a series of organic nitroalkanes, nitrites and nitrates and of NO with a series of peroxy radicals were measured at 298 K and a total pressure of 1 atm. The rate constants were obtained using the absolute technique of pulse radiolysis combined with time-resolved UV-VIS spectroscopy. The results are discussed in terms of reactivity trends and the atmospheric chemistry. 

However, TRIR has also been applied to more classical coordination compounds. Ford and co-workers have used a combination of ns-TRIR and time-resolved UV/vis spectroscopy to investigate the mechanism of hydrocarbon C—H bond activation with the rhodium complex, trans-RhCl(CO)(PR3)2 (R = Ph, />-tolyl, or Me). Upon photoexdtation, each of these species was found to undergo CO dissociation to form the transient solvated complex, tra 5-RhCl(Sol)(PR3)2 (Sol = solvent). The solvated complexes reacted with added CO to regenerate the parent complex, and also underwent competitive unimolecular C H activation to form the Rh products of hydrocarbon oxidative addition. These were identified from the step scan FTIR spectra, which showed a positive shift in /(CO) relative to the parent complex, which is consistent with oxidation of the metal center. 

The direct verification of Mulliken s charge-transfer formulation has become feasible with the recent development of picosecond and subpicosecond spectroscopic techniques [177-182]. The observation of the oxidized donor and/or reduced acceptor (by time-resolved UV-vis spectroscopy) on a timescale fast enough to obviate diffusional quenching following local excitation confirms the initial photochemical event as... 

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved UV-VIS absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter almost 60 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... 

Ledo A, Martinez F, Lopez-Quintela MA, Rivas J (2007) Synthesis of Ag clusters in microemulsions a time-resolved UV-vis and fluorescence spectroscopy study. Phys B Condens Matter 398 273-277... 

Time-resolved UV/vis absorption spectroscopy has been initiated by Norrish and Porter who developed flash photolysis in the late 1940s, opening the way to the detection of transient chemical species with time resolution of a few microseconds [30, 31]. The present state of art transient absorption techniques allow detection of chemical intermediates with less than 10 fs resolution. The techniques used depend on the explored time scale but the principle, which is illustrated in Fig. 7.14, is the same. 

In our recent work, [40] we addressed the synthesis of ZnO by using a facile and reproducible wet-synthesis route based on the controlled hydrolysis and condensation of zinc acetylacetonate as precursor in 4 different solvents (1,2 propanediol, water, ethanol and glycerol) as dispersing media. Irrespective of the nature of the solvent. X-ray Diffraction (XRD) analysis shows the formation of crystalline hexagonal ZnO. Indeed, different particles sizes and very different morphologies were obtained. The composition of the obtained ZnO was determined by elemental analysis. X-ray Photoelectron Spectroscopy (XPS), FT-IR and Thermogravimetric Analysis (TGA) analysis, whereas time resolved UV-Vis and XAFS... 

Figure 19.22. Schematic representation of a set-up developed in-house for simultaneous SSITKA experiments and catalyst characterization by in situ time- and spatially-resolved UV/Vis spectroscopy.

Iglesias-Juez A, Beale AM, Maaijen K, et al A combined in sim time-resolved UV-Vis, Raman and high-energy resolution X-ray absorption spectroscopy study on the deactivation behavior of Pt and Pt-Sn propane dehydrogenation catalysts under industrial reaction conditions, J Catal 276(2) 268—279, 2010. 

Church and co-workers (77) have obtained time-resolved IR spectra of both Mn(CO)5 and Mn2(CO)9 by flash photolysis of Mn2(CO)I0 in solution. The spectra (Fig. 11) were in close agreement with the spectra of matrix isolated Mn(CO)5 (22) and Mn2(CO)9 (5,106). There was a bridging vc 0 band for Mn2(CO)9 showing that it has a CO-bridged structure in solution as well as in the matrix. Structural information of this type could not have been obtained from uv-vis spectroscopy. Similarly, the IR spectra indicated that Mn(CO)5 had the same C4v structure in solution (77) as in the matrix (22). In solution (77), the yield of Mn2(CO)9 was approximately equal to that of Mn(CO)5. Bearing in mind that one molecule of Mn2(CO),0 produces two molecules of Mn(CO)5 [Eq. (14)], CO loss from Mn2(CO)10 [Eq. (15)], must be the major process at these photolysis wavelengths (37,77). 

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... 

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. 

The currently accepted spectroscopic assignments were obtained by a combination of multiple techniques. Leyva et applied matrix absorption and emission spectroscopy along with flash photolysis techniques. Chapman and LeRoux obtained the matrix IR spectrum of cyclic ketenimine K and Hayes and Sheridan obtained the matrix IR and UV-Vis spectrum of triplet phenylnitrene and cyclic ketenimine K. Schuster and co-workers applied time resolved IR and UV-Vis spectroscopy and demonstrated the formation of cyclic ketenimine K in solution, the species that absorbs strongly at 340 nm. 

Grills DC, Huang KW, Muckerman JT, Fujita E. Kinetic studies of the photoinduced formation of transition metal-dinitrogen complexes using time-resolved infrared and UV-vis spectroscopy. Coord Chem Rev 2006 250 1681-95. 

The UV/vis spectroscopy [53, 62] and time-resolved Circular Dichroism (TRCD) [63] studies discovered lumi-R and meta-Ra states as the intermediate states between the Pr and Pfr forms. The experimental absorption peak maxima of lumi-R (1.80 eV) and meta-Ra (1.87 eV) states are very close to that of Pr form (1.86eV) [62]. The Cl5=Cl6 rotation is so far accepted as the primary step of the photo-isomerization... 

The absolute rate constants have been refined by time-resolved laser flash photolysis in which the change in concentration of the stannyl radical, or a reaction product, or an added probe is monitored by UV/vis spectroscopy.60 Rate constants for the reactions covering the literature up to 1981 are listed in Landoldt-Bomstein (1983, II13c, pp 323-336). A selection of current values is given in Table 20-2. 

---