Regions near ultra-violet

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

The band spectrum of chlorine in the visible and near ultra-violet is well known from the work of Kuhn8 and others. Absorption from at least the first five vibrational levels of the normal molecule is observable. One can say from which particular vibrational levels the absorption of chlorine in the above regions at ordinary temperatures originates, and the energy of these levels is known. This is sufficient to determine the temperature coefficient of such absorption. Indeed it is partly by a process the reverse of this that the allocation of absorption to the various vibrational levels is accomplished. And so from the positions of the four... 

Example 10.2-3 Since it is the nearest-neighbor atoms in a complex that determine the local symmetry and the vibronic interactions, irans-d i c h I orob i s( cthylenediamine)cobalt(III) (Figure 10.3(a)) may be regarded as having D4h symmetry for the purpose of an analysis of its absorption spectrum in the visible/near-ultra-violet region (Ballhausen and Moffitt (1956)). The fundamental vibrational transitions therefore involve the 21 — 6 = 15 normal modes of symmetry 2Ate, B,. B2g, Eg, 2A2m, BltJ, 3Em. 

Since each positive 2 corresponds to two states (in the sense of mutually orthogonal many-electron wave-functions) the transfer of one electron from either ( = 1/2) or (co = 3/2) among the filled M.O. of Eq. (4) to the empty 5f shell corresponds to 28 states. Hence, the blue and near ultra-violet region containing the weak absorption bands of the uranyl ion may correspond to 28, 56 or 84 excited states, allowing a considerable... 

It may be generalized at this point that the absorption bands of almost all organic molecules normally found in the near ultra-violet and visible regions arise from either tt -> tt or n -> 77 transitions. The 77 77 transitions may... 

Ultra-violet and visible spectrophotometry can be effectively used for the control of purification and specification of purity of compounds. If a compound is transparent in the near ultra-violet and the visible regions, the purification is continued until the absorbancy is reduced to a minimum (e < 1). Traces of impurities present in pure transparent organic compounds can be readily detected and estimated, provided the impurities themselves have fairly intense, absorption bands. Before a liquid is used as a spectroscopic solvent, it should be tested for spectrophotometric purity. For example, commercial absolute alcohol usually contains benzene as impurity. The absence of benzene in the Alcohol should be confirmed spectrophoto-metrically by using sufficiently large cells (4 or 10 cm cells), before using the alcohol as a solvent. The presence of carbon disulphide in carbon tetrachloride may be detected by the presence of the disulphide absorption tend at 318 mytt. The detection of the characteristic benzenoid absorption in the spectra of many organic compounds (e.g. diethyl ether, cyclohexene) showed that the bands attributed to these compounds earlier were only due to the contamination by benzene1. 

Electronic absorption spectroscopy did not provide any direct evidence for the characterization of penicillins, since they show little absorption in the. near ultra-violet or visible regions which can be associated with chromo-phoric groups. However, ultra-violet absorption spectra were useful in solving the structures of some of the degradation products7. On reaction with alcohols, penicillins give a-esters of penidlloates, IV, which are quantitatively converted to penamaldates, V, in the presence of alcoholic mercuric OO... 

In the early papers on the ultra-violet absorption spectra of proteins, the absorption in the 250-280 m/x region had been attributed to the peptide linkage, —CO—NH1 . But it is now established beyond doubt that amides absorb in the far ultra-violet region (see Chapter 3). Thus the absorption of proteins in the near ultra-violet region can only be due to the component aromatic amino acids . 

Identification of organic compounds by their absorption spectra has become a routine procedure for the past several years. It is a standard practice now , to record either the infra-red or the ultra-violet spectrum while proposing a structure for a new compound or while reporting its physical properties. Electronic absorption spectroscopy has been used as confirmatory evidence for the identity of a previously known substance, just as any other physical property (e.g., melting point, refractive index). Many examples may be cited where a particular structure of a compound was selected from several possibilities on the basis of its ultra-violet or visible spectrum. The high intensity of many of the absorption bands in the near ultra-violet and visible regions not only permits the identification with minute quantities of material, but also serves as an aid in the control of purification of substances. In this book, an attempt has been made to present the basic concepts of electronic spectroscopy and to survey its analytical and structural applications in the different branches of chemistry. 

PHYSICAL PROPERTIES finely divided, white particulates dispersed in air odorless exposure may occur when magnesium is burned, thermally cut, or welded upon very slightly soluble in pure water solubility increased by carbon dioxide soluble in dilute acids and ammonium salt solutions not soluble in alcohol highly reflective in visible region and near ultra-violet region MP (2800°C, 5072 F) BP (3600°C, 6512°F) DN (3.65-3.75 gW solid) SG (3.58 at 25°C) CP (37.2 J/K-mol crystal at 25°C) VD (no information found) VP (0 mmHg approximately). 

Like the porphyrins, phthalocyanines absorb in the near-ultraviolet and visible region, but the intensities of the absorptions are entirely different. It is the visible absorption bands that are more intense than the near-ultra-violet bands, not the other way round, as with porphyrins. The reasons for this are perturbations to the phthalocyanine ir-system caused by, (a) the nitrogen atoms in the meso-positions (they are more electronegative than carbon atoms so that they tend to attract -ir-electron density towards themselves) and, (b) the fused benzene rings on the pyrrole 3-positions, which extend the -iT-system (they increase the size of the electron "box"). 

The NIR approach has also been used for direct prediction of chemical constituents in marine sediment. Balsam Deaton (1996) determined the carbonate, organic carbon, and opal content in marine sediments from spectral data. Besides near-infrared spectral data, reflectance spectra from the near ultra-violet and the visible region were also included in their study,... 

Purification of anthracene. Dissolve 0-3 g. of crude anthracene (usually yellowish in colour) in 160-200 ml. of hexane, and pass the solution through a column of activated alumina (1 5-2 X 8-10 cm.). Develop the chromatogram with 100 ml. of hexane. Examine the column in the hght of an ultra-violet lamp. A narrow, deep blue fluorescent zone (due to carbazole, m.p. 238°) will be seen near the top of the column. Immediately below this there is a yellow, non-fluorescent zone, due to naphthacene (m.p. 337°). The anthracene forms a broad, blue-violet fluorescent zone in the lower part of the column. Continue the development with hexane until fluorescent material commences to pass into the filtrate. Reject the first runnings which contain soluble impurities and yield a paraffin-hke substance upon evaporation. Now elute the column with hexane-benzene (1 1) until the yellow zone reaches the bottom region of the column. Upon concentration of the filtrate, pure anthracene, m.p. 215-216°, which is fluorescent in dayhght, is obtained. The experiment may be repeated several times in order to obtain a moderate quantity of material. 

Passing now to the ultra-violet, where apparently in Padoa and Buti-roni s work the group of lines in mercury at 3660 A were the chief source, we are in a region in which the absorption is probably nearly complete... 

The absorption spectroscopy has been widely used for monitoring the rate of chemical reactions. During the reaction, if there is either appearance of colour in a colourless solution or disappearance of colour in a coloured solution or a species which absorbed at a specific wavelength is formed, the spectroscopic technique can be used. Instruments like colorimeters and spectrophotometers are available to cover the visible, near infrared and ultra violet region of the spectrum (200-1000 nm). The absorption spectroscopy is governed by well-known Beer-Lambert s Law according to which ... 

Burning stars, such as those originating from Roman candles or shells (Figure 8.5), emit radiation in the ultra-violet, the visible, near-infrared and mid-infrared regions of the electromagnetic spectrum, as displayed in Figure 8.6. 

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