Ultraviolet absorption maxima, for

Details of the ultraviolet absorption maxima for simple silenes, silaaro-matics, and for some relatively stable silenes are known and have been summarized.6 The simplest silenes absorb in the region 245-260 nm, with unknown extinction coefficients but as the substituents become increasingly complex, the Xmax values of the silenes increase until, with the silene (Me3Si)2Si=C(OSiMe3)Ad, the absorption occurs at 340 nm5 with an extinction coefficient of about 7400, consistent with a tt-it transition. A few further studies of interest are summarized below. 

Bis(4-imino-2-pentanonato)nickel(II) crystallizes from a benzene-petroleum ether mixture as dark red needles or as fine red-orange needles. The two forms have identical melting points. The compound is very soluble in chloroform, but less soluble in benzene, pyridine, and carbon tetrachloride, and very insoluble in water. The compound crystallizes from pyridine without adduct formation. The compound is diamagnetic and apparently has the trans configuration. Partial resolution in optically active fractions has been achieved by means of a chromatographic technique. Molecular weight determinations indicate that the compound is monomeric in chloroform and benzene solution. The visible absorption spectrum of this compound in chloroform is characterized by a band centered at 552 m/i (e = 43). The ultraviolet absorption maxima for solutions in 1 1 benzene-petroleum ether occur at 298, 348, and 364 m x (e = 4150, 4760, and 4460, respectively). ... 

Table 9.18 Ultraviolet absorption maxima for some chromophoric groups (from Kates, 1972)...

The prediction of the values of such shifts has been considered in closely related iV-heteroaromatio systems. Ultraviolet absorption maxima have been determined for pyrido[2,3-d]pyrimidines, i i . ti-44, 56,110-112 pyrido[3,2-d]pyrimidines/ pyrido[3,4-d]... 

Carbonyl group frequencies for quinuclidin-2-ones are on the average 80 cm-1 higher than for common lactams, and integral intensities of the same absorptions are nearly half those of quinuclidin-2-ones. Ultraviolet (UV) absorption maxima for quinuclidin-2-ones are midway between those for amides and ketones. On account of the lack of amide mesomerism quinuclidin-2-ones gain in reactivity. This is shown by the rather high rates of hydrolysis of such compounds... 

When polysilanes absorb energy in the ultraviolet region, electrons are promoted from the a valence band to the a conduction band.50 Absorption maxima for polysilanes fall between about 290 and 410 nm typical ultraviolet spectra for some polysilanes in solution are shown in Figure 5.5. Because this o-o transition is permitted, the electronic absorptions are intense, with extinction coefficients e between 5,000 and 10,000 per Si-Si bond. Consistent with the splitting portrayed in Figure 5.4, as the number of silicon atoms increases, the energy of the electronic transition decreases. The absorption wavelength therefore increases and eventually reaches a limit when... 

For data on other compounds see the index of Ultraviolet Absorption Maxima in Part 3. 

The wavelengths of main peaks are listed for acid, alkaline, and neutral solution in the index of Ultraviolet Absorption Maxima (p. 1128). Infra-Red Absorption. The wavenumbers of the six major peaks in the range 2000 to 650 an (5 to 15 Lim), in descending order of ampHtude, are recorded in the monographs in Pm 2. In many cases ftie infi-a-red spectrum is also reproduced. When selecting the six principal peaks, those which are in ftie region where Nujol absorbs (1490 to 1320 cmr, 6.7 to 7.6 um) have been omitted. Corrections for cahbration errors have been applied where these are known. The six principal peaks, in ascending order of ftie main peak, are listed in the index of Infra-red Peaks (p.1141). 

The formation of these charged complexes in solution has been used as the basis for an electrophoretic separation of the Schardinger dextrins. Beckmann and Forster also found that complex formation with a-dextrin enhances approximately 2J- -fold the ultraviolet absorption maxima in iodine-iodide solutions at 290 and 350 m/i. It is probable that the colored complexes of iodine with methyl ethers and with the tosyl and mesyl esters of the Schardinger dextrins are also inclusion compounds of the same general type. 

The ultraviolet absorption spectra for lomefloxacin were obtained on a Hewlett Packard 8451A Diode Array Spectrophotometer as a function of pH. The lomefloxacin solutions were prepared in 0.15 M acetate buffer, 0.05 M phosphate buffer, and 0.15 M borate buffer ( i = 0.15 M with NaCl) at pH 5,7, and 9, respectively. These spectra are given in Figure 13. The molar absorptivities (a) and absorption maxima as a function of pH are summarized in Table 3. 

A solvent for ultraviolet/visible spectroscopy must be transparent in the region of the spectrum where the solute absorbs and should dissolve a sufficient quantity of the sample to give a well-defined analyte spectrum. In addition, we must consider possible interactions of the solvent with the absorbing species. For example, polar solvents, such as water, alcohols, esters, and ketones, tend to obliterate vibration spectra and should thus be avoided to preserve spectral detail. Nonpolar solvents, such as cyclohexane, often provide spectra that more closely approach that of a gas (compare, for example, the three spectra in Figure 24-14). In addition, the polarity of the solvent often influences the position of absorption maxima. For qualitative analysis, it is therefore important to compare analyte spectra with spectra of known compounds measured in the same solvent. 

Detection systems for GC are chosen for their sensitivity and selectivity for a particular class of VOCs. Detectors for GC include FID, the BCD, the photoionization detector (PID), the pulsed discharge detector (PDD), and the reduction gas detector (RGD). A variety of mass spectrometers can also be interfaced with a GC for confirmation of molecular structure and quantitation. Singlewavelength ultraviolet-visible detectors (190 to 600 nm) and diode array detectors are used to detect carbonyls as their 2,4-dinitrophenylhydrazone derivatives. The absorption maxima for aliphatic carbonyls, aromatic carbonyls, and dicarbonyls are near 360 nm, 385 to 390 nm, and 415 to 430 nm, respectively. Formic, acetic, and pyruvic acid are detected by ion conductivity. 

This difference in behaviour of the shift of ultra-violet absorption on melting is borne out by other characteristics of the ultraviolet absorption bands. For melts that appear to contain association complexes, these bands tend to be narrower and may have absorption maxima of intensity comparable with that in the crystals. This contrasts with melts of ions with inert gas structures, but conforms with expectations for association complexes in which the packing, though of lower symmetry, is somewhat tighter than in the crystals. 

The electronic data in different solvents indicated a solvatochromic shift for all compounds and a red shift of the absorption maxima for the n/t/o-species with regard to the closo derivatives. These neutral and anionic carboranyl-functionalized aryl ether derivatives represent a new family of high-boron-content luminescent compounds that show strong blue emission under ultraviolet radiation in... 

Ultraviolet absorption spectra of tetrahy dro-1,3-oxazines do not show any maximum. Only after the introduction of a chromophoric group do bands appear. Thus 5-nitro derivatives show a strong maximum near 270 m/A, which is typical for a nitro group, and another one near 200 m(x which is probably also produced by the nitro group.In the instance of 5-nitro-5-hydroxymethyl derivatives, the absorption is much weakened this was explained by Urbanski in terms of a hydrogen bond between the hydroxyl and the nitro group. Other chromophores, such as C=0, C=NH, C—C, also cause the appearance of absorption maxima in the range 210-265 m/A and near 360... 

Gaudette and Coatney [115] reported that primaquine phosphate was unstable when subjected to dry heat of 100 °C in the presence of sodium chloride for 24 h, when boiled in water for 24 h and when heated for 24 h at 100 or 200 °C in melted hydrogenated vegetable oil. These findings exclude the use of primaquine phosphate as a salt additive in cooking. Primaquine phosphate was isolated from the test preparations at alkaline pH by extraction into ethylene chloride, after which primaquine phosphate was returned to an aqueous phase by shaking with 0.1 N sulfuric acid the concentration of primaquine phosphate was then determined spectrophotometrically. The ultraviolet absorption curve of primaquine phosphate has maxima at 224, 266, 282, and 300 nm, and minima at 216, 250, 276, and 310 nm. A solution containing 10 yl/mL has an optical density of 0.375 at 282 nm optical densities were proportional to concentrations. 

The difficulties in the use of fluorescence for quantitative measurement of hydrocarbons are much like those for the ultraviolet absorption methods. Each compound has its own excitation and emission maxima, with the fluorescence quantum yields varying sometimes by an order of magnitude. Thus the amount of hydrocarbon reported by an analysis will depend upon the emission and excitation wavelengths chosen, and upon the compound selected as the standard. 

Hennig [40] has applied ultraviolet spectroscopy to the determination of aromatic constituents of residual fuel oil in hexane extracts of marine sediment samples. Examination of the ultraviolet spectra of samples of an oil pollutant from a beach and crude oil, at various concentrations, revealed strong absorption maxima at approximately 228nm and 256nm. The ratio of the peak heights at these wavelengths is constant for a particular oil, and is independent of concentration. These permit quantitative analysis of sediment samples many months after an oil spill. 

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