Infrared experiment

The earliest molecular beam infrared experiments on Van der Waals complexes used photodissociation spectroscopy a molecular beam is irradiated witli a tunable infrared laser and tire molecular beam intensity is measured as a function of... 

The vibrational motions of the chemically bound constituents of matter have fre-quencies in the infrared regime. The oscillations induced by certain vibrational modes provide a means for matter to couple with an impinging beam of infrared electromagnetic radiation and to exchange energy with it when the frequencies are in resonance. In the infrared experiment, the intensity of a beam of infrared radiation is measured before (Iq) and after (7) it interacts with the sample as a function of light frequency, w[. A plot of I/Iq versus frequency is the infrared spectrum. The identities, surrounding environments, and concentrations of the chemical bonds that are present can be determined. 

The goal of the basic infrared experiment is to determine changes in the intensity of a beam of infrared radiation as a function of wavelength or frequency (2.5-50 im or 4000—200 cm respectively) after it interacts with the sample. The centerpiece of most equipment configurations is the infrared spectrophotometer. Its function is to disperse the light from a broadband infrared source and to measure its intensity at each frequency. The ratio of the intensity before and after the light interacts with the sample is determined. The plot of this ratio versus frequency is the infrared spectrum. 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

The amount of papers dealing with TPSR technique used to complete studies based on infrared experiments is increasing [164-166] and this methodology will probably be further developed. 

Two of the three catalysts, viz, Ir-8 and Ir-45, have also been used in the infrared experiments. Ir-600 is an iridium-black catalyst. Figure 15 shows the cyclohexane isotopic distributions for the iridium catalysts. 

Stavola etal. (1989b) have recently carried out stress-dependent infrared experiments on Be—H in GaAs. They conclude that the reorientation of this complex is thermally activated, with an activation energy of 0.35 eV, and that its structure is similar to that of B—H in Si. 

One study (DeLeo et al., 1988 Fowler et al., 1989) has found that neutral H at the B site in Si has a tendency to preferentially bind to one of the two Si neighbors, leading to an asymmetric configuration, with Si—H distances of 1.48 A and 1.77 A respectively. This tendency was interpreted in terms of a pseudo-Jahn-Teller distortion. However, the potential barrier that leads to the asymmetric position is so low (< 0.2 eV) that it can readily be surmounted by zero-point motion of the proton. Experimental observation of such an asymmetry is therefore unlikely, except maybe through an isotope shift measurement in an infrared experiment (DeLeo et al., 1988). None of the other theoretical approaches has produced this type of asymmetry. 

Conversly, the Fe3(C0)12 NaY adduct is active for syngas conversion. A non-decomposed sample exhibits a significant activity at 230°C whereas the catalytic efficiency for the decar-bonylated one already appears at 200°C. Infrared experiments show an increase in the stability of the Fe3(C0)- 2 units upon thermal treatment under CO atmosphere so that total carbon monoxide evolution only takes place at 230°C thus suggesting that the catalyst is certainly not Fe3(C0)- 2. This cluster has to be transformed into higher nuclearity species which bind less strongly with carbon monoxide upon CO re-adsorption (1 7). 

Solutions at concentration Cq = 0.5j 1.2 and 1.6 10 g cm were prepared under vigorous stirring at 150°C and then cooled down at 23 C. It was checked by NMR and infrared experiments that no alteration nor chemical modification had occured (1+). After quenching, the intensity scattered by the solutions was recorded as a function of time. After approximately 36 h, no variation was detected for the sample Cq = 0.5 10 g cm 3 while a slow linear increase of scattered intensity with time was detected for the two other samples. In this study, the solutions were allowed to stay at room temperature for three days and then diluted at the concentration C = 10 g cm . ... 

The overall response to the reaction variables is very similar in the carbonylation and reductive carbonylation reactions. This may indicate similar catalysts and reaction mechanisms. In the carbonylation reaction Co(CO) " was identified by its characteristic CO stretching frequency ( v(CO) r 1890 cm" as the dominant species present in high pressure infrared experiments carried out at 170 °C and 5000 psig. Similar results were obtained in the reductive carbonylation of methanol. It is known that Co(CO) " rapidly reacts with CH I to yield CH C(0)Co(C0) (J9) however, in the carbonylation and reductive carbonylation reactions acyl-cobalt complexes are not observed by infrared under catalytic conditions. This indicates that once formed, the acyl complex rapidly reacts as outlined by Equations 7 and 8. 

The experimental results on the various samples may be summarized as follows (i) The excited electronic state absorption decays rapidly on the time-scale of pico- or subpicoseconds, (ii) Longer lasting absorption changes are observed in spectral ranges with chromophore absorption for the cyclic and bicyclic peptides, (iii) Infrared experiments have clearly shown that absorption transients in the pico- to nanosecond range occur. They monitor directly structural change of the peptide backbone. 

Infrared spectroscopy is not as inherently informative with regard to metal interactions in highly symmetrical metal-metal bound dimers as is Raman spectroscopy, since the totally symmetric metal-metal stretch is a forbidden absorption in the infrared experiment. Oldham and Ketteringham have prepared mixed-halide dimers of the type Re2ClxBr 2xto lower the symmetry and hence introduce some infrared allowedness into the Re-Re stretching mode (206). Indeed, the appearance of a medium-intensity band at 274 cm 1 in the infrared spectrum of the mixed-halo species was considered to be the result of absorption by the metal—metal stretching vibration, which was also observed in the Raman spectrum at 274 cm ". ... 

Since its launch in 1991 the Upper Atmospheric Research Satellite (UARS) has circled the Earth in a low earth non sun-synchronous orbit. The UARS flew three infrared experiments. In addition to ISAMS (described above) the Cryogenic Limb Array Etalon Spectrometer (CLAES) and the Halogen Occultation Experiment (HALOE) make infrared measurements designed to yield information about stratospheric and tropospheric trace constituents. 

It has been suggested that an adsorbed cyanato complex (NCO) exists on supported Ru catalysts under conditions of relatively high pressure, on the basis of transmission infrared experiments (67,68). However, no evidence for formation of such an intermediate, which is expected to have vibrational frequencies at 1150-1480 cm-1 due to the C-0 stretch, and at 2150- 2280 cm-1 due to the N-C stretch (46), was observed in the co-adsorption experiments on the Ru(001) surface. 

Recent work by Selwood (9), based on changes in the magnetization of nickel during chemisorption of ethylene, indicates that ethylene is associatively adsorbed on bare nickel. He suggests that the discrepancy between this result and the dissociative chemisorption indicated by the infrared experiments is due to factors such as the relative activity of the sample surfaces and temperature effects caused by the heat of chemisorption. Low-temperature infrared experiments in which ethylene is studied at —78° C. are expected to provide evidence on the importance of the above factors in determining the course of ethylene chemisorption. 

The infrared results do not necessarily mean that the complex has no role in the Fischer-Tropsch synthesis. They do rule out, however, the possibility of any appreciable quantities of this complex having formed under the specific conditions used for the infrared experiments. 

All of the infrared experiments were performed on a Digilab FTS-40 Fourier transform infrared (FT-IR) spectrometer equipped with a narrow-band liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector. The spectrometer was operated at a nominal resolution of 4 cm-1 using a mirror velocity of 1.28 cm/s. The data collected using the gas chromatography (GC) IR software were measured at 8 cm-1 resolution. Protein assays for all the experiments were measured on a Beckman DU-70 UV-visible spectrophotometer. 

There are three types of infrared experiment that can be conducted ... 

With IR light sources like this one, a technology is available which, in terms of day-to-day reliability and long-term and short-term stability, is entirely comparable with Ti sapphire regenerative amplifiers. As shown in this article, it was possible to perform femtosecond experiments on all kinds of condensed phase phenomena involving vibrational transitions (such as energy relaxation, dephasing, spectral diffusion, coupled systems) with essentially the same facility and accuracy as can be achieved in visible and near-infrared experiments. 

The infrared spectra of chemisorbed molecules provide relatively clear and direct evidence concerning the structure of these molecules. Most of the problems to which the infrared techniques have been applied have been stimulated by an interest in heterogeneous catalysis. Since chemisorption is vital to catalysis and since the structure of chemisorbed molecules can be determined by infrared, it is reasonable to ask what has been learned about catalytic activity from these spectra. The number of cases where even a tenuous relationship between the spectra and activity is seen is not large. However, the infrared experiments were not designed specifically to seek such relationships. Despite this, interesting observations concerning catalytic activity have been made and will be described here to illustrate the type of reasoning involved rather than to claim well-defined relationships. 

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