Absorption spectra ultraviolet/visible

The conjugated -electrons of a benzene ring have characteristic absorptions in the ultraviolet-visible spectrum that include the following ... 

Molecular absorption spectroscopy deals with measurement of the ultraviolet-visible spectrum of electromagnetic radiation transmitted or reflected by a sample as a function of the wavelength. Ordinarily, the intensity of the energy transmitted is compared to that transmitted by some other system that serves as a standard. 

As discussed above, there are three pertinent situations with respect to the absorption spectrum of the retinoids in the visual range. For most of the retinoids, there is no significant visible spectrum absorption. The dominant mode of molecular absorption is due to bulk excitation and this absorption is most prominent in the ultraviolet. The relevant visual band absorption cross section is zero. 

The ultraviolet-visible spectrum for indapamide was obtained with a 1-cm cell using a solution with a concentration of 8 microgrants/mL with methanol as a solvent (12). The spectrum (Figure 6) exhibits characteristic absorption bands with maxima at 242 nm (A 12 = 630), 278 nm (A 12 =98) and 286 nm (A 12 = 100). Aqueous solutions of indapamide (17) display an increase in absorptivity at approximately 242 nm with an increase in pH. 

For poly(9,9-di-n-octylfluorene) (PFO), which is deemed a model polymer in PFs, Janietz and coworkers reported its HOMO (equivalent to ionization potential, Ip) and LUMO (equivalent to electron affinity, Ea) levels as 5.8 and 2.12 eV below the vacuum level, respectively, as determined from cyclic voltammetry (CV) measurements of PFO thin solid film [11] as shown in Fig. la. However, the HOMO and LUMO levels provide a larger band gap of 3.68 eV than the value of 2.95 eV determined from the onset position in the ultraviolet-visible (UV) absorption spectrum of PFO film. If the band gap is taken as 2.95 eV and the HOMO level as 5.8 eV, the LUMO level of PFO... 

Injection of ethene at 150°C or above produces a new infrared band at about 1500 cm (Figure 6), which is in the frequency range characteristic of partially unsaturated C-C bonds. Alkanes are detected in the gas phase above zeolites heated in the presence of ethene, and an intense absorption band appears in the ultraviolet-visible spectrum of ZSM-5 on heating in ethene to 100°C at 280 nm which has been attributed to an allyl carbenium cation (refs. 

The ultraviolet-visible spectrum is generally recorded as a plot of absorbance versus wavelength. It is customary to then replot the data with either e or log e plotted on the ordinate and wavelength plotted on the abscissa. Figure 7.4, the spectrum of benzoic acid, is typical of the manner in which spectra are displayed. However, very few electronic spectra are reproduced in the scientific literature most are described by indications of the wavelength maxima and absorptivities of the principal absorption peaks. For benzoic acid, a typical description might be... 

Another route to depopulation of the upper level is spontaneous emission, which is described by an analogous spontaneous emission coefficient This mechanism is the basis of atomic emission spectrometry in the ultraviolet-visible spectrum particularly. In the MMW region with its much lower frequency transitions it is not a significant contributor, as will be demonstrated below. Modifying Equation 1.2 to bring in the populations N of the upper and lower states one can write for the absorption and emission processes... 

Ultraviolet/visible absorption spectrum for bromothymol blue. 

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

The derivatization process (5) is accomplished in aqueous media at basic pH (pH 7-10) in a matter of approximately 15 min to yield a 2-cyanobenz[f]isoindole (CBI), which is stable for 10 to 12 hr in solution. As shown in Figure 1, the absorption characteristics of the CBI adducts are also readily accessible for assay by standard fluorescence or ultraviolet detection. In addition to the absorption between 200 and 300 nm, there are two maxima in the visible spectrum at approximately 420 and 440 nm accessible for fluorescence or ultraviolet detection. A probable mechanism (5,11) for the CBI formation is illustrated in Scheme 1. 

We always recommend that structural changes occurring after the saponification procedure be verified. Since the hydroxyl groups have no influence on the chromophore, the wavelength of the maximum absorption, shape, and intensity of the ultraviolet-visible (UV-Vis) spectrum would be identical for unsaponified and saponified samples. 

The ultraviolet absorption spectrum of miconazole nitrate in methanol (0.0104%) shown in Fig. 3 was recorded using a Shimadzu Ultraviolet-visible spectrophotometer 1601 PC. The compound exhibited three maxima at 264, 272, and 280 nm. Clarke reported the following Methanol 264 and 272 nm = 17a), 282 nm [2]. 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

If unsaturated linkages like -C = C-, -N = N-C-O, -N = O etc. (also called chromophores) are present in the molecule, less energy is required for the excitation of n electrons and so the absorption bands occur in the near ultraviolet region and some times in the visible spectrum. 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

DNA is often present in amounts too small to be detected by direct spectroscopy. In this case, the fluorescent dye EtBr can be used to amplify the absorption. EtBr binds to the DNA molecule by intercalating between adjacent base pairs. It absorbs ultraviolet light at 300 nm and emits light at 590 nm in the red/orange region of the visible spectrum. The method can be used to determine the amount of DNA in a test-tube by comparing the EtBr-mediated fluorescence of the sample with that of standards of known amounts of DNA. 

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