UV-Vis characterization

Hu and coworker developed the silica, titania, ceria, and y-alumina supported vanadium catalyst, which shown unique reactivity for the direct amination of toluene with hydroxylamine hydrochloride, over 50 % total yield of toluidines was obtained on the y-alumina supported vanadium catalyst operated at optimal conditions [87]. They also investigated a sodium metavanadate catalyzed one-step amination of benzene to aniline with hydroxylamine [88], The reaction became more efficient in the presence of oxygen. A free-radical mechanism was proposed based on the results of EPR, NMR, and UV-Vis characterization. Moreover, the catalytic activity of a series of vanadium complexes with N,0- or 0,0-ligands for the liquid-phase direct amination of benzene with hydroxylamine to aniline has been investigated [89]. [VO(OAc>2] was proved to be the most active catalyst for the amination because of its relatively greater electrophilicity and smaller steric hindrance of ligand. 

Molecular absorption, particularly in the UV/Vis range, has been used for a variety of different characterization studies, including determining the stoichiometry of metal-ligand complexes and determining equilibrium constants. Both of these examples are examined in this section. 

Photochemical decomposition of riboflavin in neutral or acid solution gives lumichrome (3), 7,8-dimethyl all oxazine, which was synthesized and characterized by Karrer and his co-workers in 1934 (11). In alkaline solution, the irradiation product is lumiflavin (4), 7,8,10-trimethyhsoalloxazine its uv—vis absorption spectmm resembles that of riboflavin. It was prepared and characterized in 1933 (5). Another photodecomposition product of riboflavin is 7,8-dimethy1-10-foTmylmethy1isoa11oxazine (12). 

Either UV-VIS or IR spectroscopy can be combined with the technique of matrix isolation to detect and identify highly unstable intermediates. In this method, the intomediate is trapped in a solid inert matrix, usually one of the inert gases, at very low temperatures. Because each molecule is surrounded by inert gas atoms, there is no possiblity for intermolecular reactions and the rates of intramolecular reactions are slowed by the low temperature. Matrix isolation is a very useful method for characterizing intermediates in photochemical reactions. The method can also be used for gas-phase reactions which can be conducted in such a way that the intermediates can be rapidly condensed into the matrix. 

Figure 13.37 shows the UV spectrum of the conjugated diene cis,trans-, 3-cyc o-octadiene, measured in ethanol as the solvent. As is typical of most UV spectra, the absorption is rather broad and is often spoken of as a band rather than a peak. The wavelength at an absorption maximum is refened to as the X ax of the band. There is only one band in the UV spectrum of 1,3-cyclooctadiene its X ax is 230 ran. In addition to UV-VIS bands are characterized by their- absorbance (A), which is a measure of how much of the radiation that passes through the sfflnple is absorbed. To correct for concentration and path length effects, absorbance is converted to molar absorptivity (e) by dividing it by the concentration c in moles per liter and the path length I in centimeters. 

Abstract Molecular spectroscopy is one of the most important means to characterize the various species in solid, hquid and gaseous elemental sulfur. In this chapter the vibrational, UV-Vis and mass spectra of sulfur molecules with between 2 and 20 atoms are critically reviewed together with the spectra of liquid sulfur and of solid allotropes including polymeric and high-pressure phases. In particular, low temperature Raman spectroscopy is a suitable technique to identify single species in mixtures. In mass spectra cluster cations with up to 56 atoms have been observed but fragmentation processes cause serious difficulties. The UV-Vis spectra of S4 are reassigned. The modern XANES spectroscopy has just started to be applied to sulfur allotropes and other sulfur compounds. 

The most important analytical tool to characterize sulfane molecules is NMR spectroscopy followed by Raman spectroscopy while the UV-Vis spectra are rather uncharacteristic. 

If color reactions occur these serve to help characterize the substance. They can only ever act as a pointer to the presence of a substance, but never as proof even when accompanied by a separation process. Unequivocal identification requires a mosaic of many pieces of information (h/ f values, color reactions UV/VIS, IR, Raman, mass spectra etc). 

In this work, highly active epoxidation catalysts, which have hydrophobic surface of TS-1, were synthesized by the dry gel conversion (DGC) method. Ti-MCM-41 was synthesized first by a modifed method and the TS-l/MCM-41 catalysts were subsequently synthesized by the DGC method. The catalysts were characterized by the XRD, BET, FT-IR, and UV-VIS spectroscopy. TS-l/MCM-41 catalysts were applied to the epoxidation of 1-hexene and cyclohexene with aqueous H202to evaluate their activities for the epoxidation reaction. ... 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

Peaks are analyzed separately by their retention times, absorption, and fluorescence properties. RCCs show absorbance maxima near A.500 and 316 nm. For FCCs, UV-Vis specna show two prominent bands near 361 and 320 mn and a luminescence maximum at 436 mn and NCCs show UV-Vis spectra with absorbance maxima near 320 and 210 nm. Nevertheless, as none of these approaches is suitable for elucidating structures, it is necessary to apply additional MS and NMR analyses to fully characterize snuctural features. Electron spray ionization (ESI) and high-resolution EAB mass spectroscopy have been applied to elucidate the molecular formulae of colorless compounds. ... 

Similarly to the methods used to characterize natural chlorophylls, RP-HPLC has been chosen by several authors to identify the individual components in Cn chlorophyllin preparations and in foods. The same ODS columns, mobile phase and ion pairing or ion suppressing techniques coupled to online photodiode UV-Vis and/or fluorescence detectors have been used. ° ... 

If a vacant site is occupied by another Nb atom, such that it is a dimer, new catalysts may be designed. The Nb dimer catalyst(2) was prepared by reaction of [Nb(ri -C5H5)H-p-(T, Ti -CsH4)]2 with a Si02 at 313 K, followed by treatment with 02 at 773 K. A proposed structure(2) was characterized by EXAFS, x-ray absorption near-edge structure(XANES), FT-IR, UV-vis, and XPS, which shows Nb-Nb (coordination number 0.9) and Nb-Si(2.3)... 

To better understand the differences in their catalytic behavior, the catalysts were characterized by XRD and UV-vis DRS. Unfortunately, except for the peak at 77.6° 26 (311 diffraction), the other Au diffraction peaks overlapped with those of y-Al203. The size of the coherent domains of Au, listed in Table I, were estimated using the width of this diffraction peak and the Debye-Sherrer equation. They showed that catalysts of both groups A and C had small coherent domains, whereas those of group B had large domains. 

Nowadays there is a general consensus that the Ti(IV) atoms are incorporated as isolated centers into the framework and are substituting Si atoms in the tetrahedral positions forming [Ti04] units. The model of isomorphous substitution has been put forward on the basis of several independent characterization techniques, namely X-ray [21-23] or neutron [24-26] diffraction studies, IR (Raman) [52-57], UV-Vis [38,54,58], EXAFS, and XANES [52, 58-62] spectroscopies. 

---