Low temperature spectroscop

For over 15 years we have conducted research utilizing metal atoms in low temperature spectroscopic and synthetic studies at Rice University.8 Our synthetic work was started in the late 1960s with the work of Krishnan, on lithium atom reactions with carbon monoxide, extended by Meier- in his studies of lithium atom reactions with water and ammonia and expanded over the next several years to include metal atom interactions with HF, H2O, H3N, H4C, and their hundreds of organic analogs—RF, R2O, ROH, R3N,. . . H3N, R4C, R3CH, etc. A most exciting aspect of... 

Compounds I and II are quite stable at low temf>erature and therefore can serve as pure reactants to study the mechanisms of peroxide oxidase processes (Douzou, 197la,b). When compound II reacted with indole 3-acetate, this compound was immediately regenerated without the appearance of any other intermediary compound. Moreover, indole 3-acetate in large excess induced the conversion of compound III into compound II. A study of reaction mechanisms of indole 3-acetate degradation by various peroxidases was recently carried out by Ricard and Job (1974) using low-temperature spectroscopic techniques. They obtained new data that made it possible to propose electronic mechanisms of reactions less speculative than those dependent upon data obtained under normal conditions of temperature. 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

For measurements at temperatures other than ambient, cells with double walls, which can be thermostatted, are also available commercially. If measurements are required at temperatures between ca. —5°C and room temperature, the sample compartment of the spectrometer can be flushed with dry air or nitrogen to reduce condensation on the cell windows. Below ca. — 5 °C the windows can be covered with a thin polythene film, but measurements below —25 °C are very troublesome. The problems associated with low temperature spectroscopic measurements were solved by enclosing the cell in an air-tight box fitted with glass windows (Dadley and Evans, 1967). The box was so designed that it fitted into the spectrophotometer and the air inside the box was dried with phosphoric oxide which, it is claimed, stopped condensation even at temperatures as low as — 60 °C glass windows could be used because only absorptions above 380 nm were of interest. 

The 2,5-dibromo addition products (analogous to 1 and 3) were formed in the ratio 1 3 (67BCJ130). A more recent low-temperature spectroscopic study largely confirmed this result, but also detected around 20% of trans-2,3,-dibromo-2,3-dihydrofuran (analogous to 4) (75CC875). Again it is unclear whether these adducts, which demonstrate competition between... 

The solution structure of halomethylzinc derivatives has been the subject of several debates. Among those, the possible Schlenk equilibrium between IZnCH2l and Zn(CH2l)2/ Znl2 has been invoked to account for the stereochemical outcome of some cyclopropanation reactions, but little has been established unequivocally. Recently, in-depth low temperature spectroscopic studies of iodomethylzinc complexes have clearly established that the equilibrium between these species lies heavily on the side of IZnCH2l (equation 11) . ... 

Ricard, J. and Job, D., 1974, Reaction mechanisms of indole-3-acetate degradation by peroxidases. A stopped-flow and low-temperature spectroscopic study, Eur. J. Biochem. 44 359-374. 

Our low-temperature spectroscopic technique may be, as we have already indicated, very sensitive to surface-state modification. However, it provides only an overall and indirect observation of the crystal-surface reconstruction. For this reason, the conclusions we are proposing here need further confirmation by calculations and by complementary experimental techniques. 

The absorption spectrum of Bk(III) in a lanthanum chloride host matrix at 77 K was first obtained in 1964 (102). A prediction of the energy level structure of Bk(III) was made by others the same year (103). Extensive, low-temperature spectroscopic studies of BkCl3 showed the absence of transitions to excited J = 0 and J = 1 states (104, 105). This provided good evidence for a fi = 0 ground level for Bk(III), consistent with that of Tb(III)-LaCl3 (106). Experimental and theoretical studies of the crystal field parameters of Bk(III) in a LaCl3 host lattice have also been reported (107). 

The low temperature spectroscopic observation of the FeIV ferryl complex (134) [P = TPP, B = py, pip or jV-Me-im] formed via reaction (31) has been followed by the demonstration by the same group of its role in oxygen atom transfer reactions. Thus, it was found446 that PFe11 catalyzes... 

Iron manganese complex (159,160). The isolated products are the a-keto-acyl manganese complex 158 [Eq. (136)] and the rhenium acyl complex 160 [Eq. (137)]. Low temperature spectroscopic studies revealed the initial... 

Hydroxyindoxazenes undergo analogous rearrangements to benzoxa-zolin-2-ones.67 However, sensitization studies support a rearrangement from the triplet state and these results, coupled with low-temperature spectroscopic data, which fail to detect any aziridone or o-hydroxyaryl isocyanate intermediates, lead the authors to propose a radical mechanism (Scheme 10)... 

A great deal more is known for binuclear systems, which are better for comparison to O2 adducts within the trinuclear T2/T3 multicopper oxidase active site (see Figure 11). Mononuclear ligand-Cu(I) or dicopper(I) complexes with a binucleating ligand react with O2 to yield peroxo or hydroperoxo-dicopper(II) complexes. These products have been characterized, often via low-temperature spectroscopic studies (see Chapter 8.15). Kitajima s -peroxo (side-on peroxo) dicopper(II) adduct ... 

The optical absorption spectra of the stable Fe, Fe, and Fe NO species were determined, and stopped-flow rapid scan, flash photolysis, and low-temperature spectroscopic methods have been applied to study the reactions of Equations (7)-(9). The species P450nor(Fe Njqo) formed in reaction (7) was characterized by stopped-flow as having a diminished Soret absorbance at 434 nm and a single broad band at 558 nm. Formation of this species was complete within the dead time of the stopped-flow apparatus, even at 10 °C and [NO] of 5 pM, allowing a limit for k, the rate constant for formation of this species, to be estimated as larger than 10 s ... 

Probably the main utilization of low-temperature spectroscopic studies in biophysics has been to trap short-lived intermediate chemical states of a protein. Typically the chromophore alone is the object of study, and the protein is viewed basically as a structure which simply holds the chromophore in place or may contribute some critical amino acids in close proximity to the chromophore. The protein is not believed to play an overtly dynamic role, a dynamic role being one where the reaction process is steered by time-dependent protein conformational changes. Temperature is used to slow down or arrest the reaction kinetics occurring at the chromophore and perhaps to discover additional chemical states which have too short a lifetime to be observed at room temperature. Such kinds of kinetic arrest studies have been successfully used in studies of heme... 

In summary, there seems to be no fluorimetric alternative to excel low-temperature techniques in analytical information throughput if chemical separations are to be avoided. Massive use in the environmental field has so far been restricted by the need to use fairly sophisticated devices such as high-resolution monochromators, cryostats, and, perhaps, dye lasers. On the other hand, a number of commercially available spectrofluorimeters provide inadequate spectral resolution for use in low-temperature spectroscopic techniques. However, given the high selectivity of such techniques, their use in the characterization of complex environmental samples will continue to grow in the foreseeable future. 

Bentsen JG, Wrighton MS (1987) Wavelength-dependent primary photoprocesses of Os3(CO)]2 in fluid solution and in rigid alkane glasses at low temperature spectroscopic detection, characterization, and reactivity of coordinatively unsaturated Os3(CO)n. J Am Chem Soc 109 4518 530... 

Further experiments for analyzing the electronic structure of intermediate states were not very successM, until Sixl et al. [23] and Bubeck et al. [24] published their first low-temperature spectroscopic experiments on partially photopolymerized TS crystals. Thereby, the monomer crystal is cooled to 4.2 K in the dark. Then it is irradiated with UV light (1 < 310 nm) for a short period. After this procedure a large number of different species show up in both, the optical absorption and the ESR spectrum. They persist at helium temperature. Subsequent annealing in the dark produces further intermediate reaction products which are also identified by their optical or ESR spectra. Finally, further irradiation with visible light, which is absorbed only by the intermediate reaction products, produces still more and different reaction products. In a comprehensive series of investigations [25-47] most of these intermediate products have been identified and classified. Moreover, the mechanisms of their production and their reaction kinetics have been analyzed. According to Sixl, there exist three different series of intermediate products ... 

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