Room temperature difference spectra

At room temperature, the Mossbauer spectrum of amorphous Fe203 shows a broadened doublet (see Fig. 18.17) [72] with the typical values of the Mossbauer hyperfine parameters listed in Table 18.1. However, the approach howto fit the room-temperature spectrum of amorphous Fe203 differs. The room-temperature spectral profile is fitted either by a broadened doublet employing non-Lorentzian lines (under an assumption of an electric field gradient distribution) or by two doublets originating from the nonequivalent nanoparticle surface and core iron atoms [72]. The large value of A q parameter implies a large deviation of the octahedral environment of the probed Fe nucleus from a spherical symmetry. 

A wide variety of different mechanisms may participate in the PT process and influence the interpretadon of a spectrum. At room temperature, PL emission is thermally broadened. As the temperature is lowered, features tend to become sharper, and PL is often stronger due to fewer nonradiadve channels. Low temperatures are typically used to study phosphorescence in organic materials or to identify particular impurides in semiconductors. 

FIGURE 21.9 Typical visible absorption spectra of cytochromes, (a) Cytochrome c, reduced spectrum (b) cytochrome c, oxidized spectrum (c) the difference spectrum (a) minus (b) (d) beef heart mitochondrial particles room temperature difference (reduced minus oxidized) spectrum (e) beef heart submitochondrial particles same as (d) but at 77 K. a- and /3- bauds are labeled, and in (d) and (e) the bauds for cytochromes a, h and c are indicated. 

B. Method for determining the number of —COOH groups in a molecule The TMS derivative of an acid can be converted to the methyl ester using anhydrous methanolic HQ. 1 2 3 Obtain a mass spectrum of the TMS derivative of the acid, and then evaporate the TMS reaction mixture with clean, dry nitrogen. Add 250 pd of anhydrous methanolic HC1 (Pierce cat. no. 33050) and heat at 60° for 20 min. Many TMS derivatives of acids are converted to methyl esters at room temperature after 20 min. If the sample is rerun as the methyl ester, the number of carboxyl groups can be determined by the mass differences before and after making the methyl ester from the TMS derivative. 

Fig. 1.7. Pressure-induced spectrum of H2 in He [46] at room temperature. The curves correspond to different densities of He between 950 and 1465 amagat.

Since then, the vibrational spectrum of Ss has been the subject of several studies (Raman [79, 95-100], infrared [101, 102]). However, because of the large number of vibrations in the crystal it is obvious that a full assignment would only be successful if an oriented single-crystal is studied at different polarizations in order to deconvolute the crystal components with respect to their symmetry. Polarized Raman spectra of samples at about 300 K have been reported by Ozin [103] and by Arthur and Mackenzie [104]. In Figs. 2 and 3 examples of polarized Raman and FTIR spectra of a-Ss at room temperature are shown. If the sample is exposed to low temperatures the band-widths can enormously be reduced (from several wavenumbers down to less than 0.1-1 cm ) permitting further improvements in the assignment. 

The features due to adsorbed water and carbonates observed on the boehmite and y-alumina deserve further attention as they differ from results published by previous investigators. Figure 4 shows a series of difference spectra for adsorption on y-alumina. Spectra were taken after drying the y-alumina at 350 C, cooling to room temperature and carrying out room temperature adsorption. The spectra are the difference of the sample before and after adsorption. Spectrum 4e is the spectrum for the as received alumina differenced with the dried alumina. The positive band at 3400 cm" is due to adsorbed water, and the small negative feature at 3740 cm" is due to isolated hydroxyls on the dried surface. Besides the three... 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

The Cr5+ ion has only one unpaired electron hence, no zero-field splitting is expected. Indeed, a well-resolved spectrum has been observed for the ion on alumina 147, 148), silica gel 149-151), silica-alumina 152-154), and magnesium oxide 155). The line may be resolved into parallel and perpendicular g values. As van Reijen and Cossee (151) have shown, the values of g range from 1.970 to 1.975 whereas, the values of g range from 1.898 to 2.002, depending on the treatment of a Cr/SiCh sample. These authors have suggested that Cr5+ in two different symmetries is present one has a long relaxation time and can be observed at room temperature, but the other has a very short relaxation time and can be observed only at very low temperatures (—253°). 

Fig 49 FTIR difference spectrum recorded 80 min (trace a) and 320 min (trace b) after loading of TS- l/TiOOH molecular sieve with 6.5 mbar of propene at room temperature [Reprinted from Lin and Frei (133) with permission. Copyright (2002) American Chemical Society],... 

These two examples illustrate how Mossbauer spectroscopy reveals the identity of iron phases in a catalyst after different treatments. The examples are typical for many applications of the technique in catalysis. A catalyst is reduced, carburized, sulfided, or passivated, and, after cooling down, its Mossbauer spectrum is taken at room temperature. However, a complete characterization of phases in a catalyst... 

Mossbauer Spectroscopy. Figure 1 shows room temperature Mossbauer emission spectra of two of the unsupported Co-Mo catalysts which we have studied in the present investigation. It is observed that the MES spectra of the two catalysts are quite different. For the catalyst with the low Co/Mo ratio (0.0625), a quadrupole doublet with an isomer shift of 6=0.33 mm/s and a quadrupole splitting of AE =1.12 mm/s are observed (spectrum a). These parameters are very similar to those observed previously for the Co-Mo-S phase in other catalysts (6-9). Furthermore, the spectrum of an unsupported catalyst with Co/Mo = 0.15 is found to be essentially identical to spectrum (a). The MES spectrum (b) of the catalyst with Co/Mo =... 

In order to characterize electron acceptor (basic type) properties of the samples, tetracyano ethylene compound, known to be easily ionizable in TCNE radical anion, was introduced at room temperature in the samples outgassed at different temperatures up to 800°C. No ESR signal was observed. As steric hindrance could preclude the experiment, smaller molecules as SO and p-dinitro benzene were also introduced. Then too, no ESR spectrum could be detected although the ESR technique is extraordinarly sensitive. It may thus be concluded that the ZSM-5 and ZSM-11 materials did not exhibit electron donor (basic) properties as detectable by ESR. 

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