Low-intensity absorptions

Ultraviolet. Benzene has a series of relatively low intensity absorption bands in the region of 230 to 270 nm. When there is a substituent on the ring with nonbonding electrons, such as an amino group, there is a pronounced increase in the intensity of these bands and a shift to longer wavelength. Aniline shows an absorption band at 230 nm (e = 8600) and a secondary band at 280 nm (e = 1430). Protonation of the amino groups reduces these effects and the spectmm resembles that of the unsubstituted benzene. 

The NMR study of steroidal epoxides (discussed in section II-F) parallels that of the analogous thiiranes. It is possible to relate the location and configuration of the thiirane group with the angular methyl and thiirane proton resonances. The proton NMR relationships for the intermediate thiocyanatohydrins have been included inageneral NMR study of steroids. Electronic spectra may be used in the analysis of steroidal thiiranes. Spectroscopic measurements have shown the existence of a low intensity absorption in the 240-260 m region. The regular patterns of rotatory contributions of thiiranes which are comparable with those of ketones prompted an accumulation of ORD and CD data for steroidal thiiranes. 

Conversely, it is believed that non aromatic polymers NC and PES, ablate much faster than aromatic ones, as shown by the k values of Table I, because single bonds are statistically broken with one photon only. Besides, the higher ablation speed cannot be accounted for by a presumed higher absorptivity, since it is suggested to be lower by low intensity absorption coefficients displayed in Table I. 

The spectrum of bis(acetylacetonato)oxovanadium(IV) (Fig. 7) shows two low-intensity absorption bands (e 10—100) in the 10—20 kK region, at about 14 (band I) and 17 kK (band II) 23). Low-temperature spectra reveal a splitting of band I into three bands 23) either electronic 20, 23) or vibronic (24) in origin. Another band (band III) is found at about 25 kK. Recently, beautiful vapour-phase spectra of several five-coordinate bis(j9-diketonato)oxovanadium(IV) have been recorded (25). 

Aliphatic ketones show broad, low-intensity absorption maxima in the vicinity of 280 nm which are a result of n - n transitions. By use of the Stark effect, Freeman and Klemperer estimated that the dipole moment in the n, n singlet state is reduced to 1.48 D from its ground state value of 2.34 D [253]. 

The PPP configuration analysis technique and the mo ecules-in-molecules/localized configuration interaction (MIM/LCI) method can provide an explanation of the fact that some cyclic sulfur diimides are colorless, while others are not. Thus, while 1,2,5-thiadiazoles 231 and 232 are colorless, naphtho[l,8- /][l 4,2,6]thiadiazine 229 is colored (low intensity absorption at 642 nm). The energy level of the color band appears to be largely determined by frontier orbital interactions. By considering the overlap between the two fragments, (i) the NSN group and (ii) the... 

Job and Littlehailes [127] have irradiated a 1 1 molar mixture of benzene and acrylonitrile under nitrogen at 0°C and obtained 7-cyanobicyclo[4.2.0] octa-2,4-diene. The reaction did not proceed in the absence of ultraviolet irradiation or in a Pyrex apparatus. The yield of photoadduct is little affected by the presence of air, which, according to the authors, strongly supports the intermediacy of a singlet excited complex. They were, however, unable to detect a UV absorption band of a ground-state complex. However, the yields of adduct were low and the possibility is considered that a low-intensity absorption band is hidden by the benzene spectrum. 

The absorption spectrum of phenylalanine is shown in Fig. 1. The low intensity absorption peak centered slightly below 2600 A corresponds to a forbidden x it transition. Vibrational fine structure is quite evident in this region. Aside from increased blurring of the fine structure which is to be expected on passing into successively more polar media, change of solvent effects little change in the general size, shape, and location of the absorption envelope. 

The relatively narrow, low intensity absorption bands of the rare-earths have been studied a great deal over the years, and most of the... 

Even though contributions from the various transitions are important only within a narrow spectral region, band overlap similar to that occurring in ordinary absorption spectra is possible in CD spectra as well The relative intensities of different transitions in the CD spectrum may, however, be quite distinct from those in the absorption spectrum. Accordingly, low-intensity absorption bands of chiral compounds, which are hidden by other bands, may well be detectable in the CD spectrum. As examples, the UV and CD spectra of an octahydrobenzoquinoxaline are shown in Figure 3.3. 

Trinitroalkanes deriving from tiitruform have a low intensity absorption band at ca, 280 nm and with hydrazine give a strong absorption at 350 mn. This tact can serve to spectropholometric analysis (145, 146]. 

Tetravalent neptunium forms complexes with picolinic acid even at pH l. In the spectrum of Np, the new low intensity absorption band at 967 nm appears in the presence of picolinic acid. From the spectrophotometrical data the formation of the chelate [Np(Pic)] with the stability constant of logy0i=6.5O O.Ol was determined [47],... 

The UV spectra of acetylated sugar isothiocyanates show a low-intensity absorption at 250-254 nm, which allows UV detection in the course of chromatographic separations.5070,78 The band is frequently overlapped when stronger chromophores (such as benzoyl groups) are present in the mole-... 

The term log (/q/T) is also known as the absorbance (or the optical density in older literature) and may be represented by A. The molar absorptivity (formerly known as the molar extinction coefficient) is a property of the molecule undergoing an electronic transition and is not a function of the variable parameters involved in preparing a solution. The size of the absorbing system and the probability that the electronic transition whl take place control the absorptivity, which ranges from 0 to 10 . Values above 10" are termed high-intensity absorptions, while values below 10 are low-intensity absorptions. Forbidden transitions (see Section 7.1) have absorptivities in the range from 0 to 1000. 

The molar absorptivity is a characteristic property of a compound and is not affected by its concentration or the length of the light path. Values range from zero to 10 M cm. Values above 10 M em correspond to high-intensity absorptions values below 10 to low-intensity absorptions. The molar absorptivity... 

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