Ultraviolet absorption transitions

Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

COMPARISON OF ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLE AND HS METHYL DERIVATIVES WITH CALCULATED VALUES OF TRANSITIONS BY DIFFERENT METHODS... 

TABLE 1-19. ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLIUM TOSYLATES IN EtOH (186) COMPARED WITH CALCULATED TRANSITION ENERGIES (187)... 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

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. 

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

The ultraviolet absorption spectrum of GeF2 has been measured by Hauge, Khanna and Margrave 102 The spectrum is fairly simple and is probably due to the perpendicular 1B1 X1 Aj transition. All progressions were explained in... 

Ultraviolet absorption spectra appear when outer electrons of atoms or molecules absorb radiant energy and undergo transition to higher energy levels. These transitions are quantised and depends on the compound under examination. 

Rydberg spectrum spect An ultraviolet absorption spectrum produced by transitions of atoms of a given element from the ground state to states in which a single electron occupies an orbital farther from the nucleus. rTd.borg. spek-trom ... 

The ultraviolet absorption spectrum of isoxsuprine HCl dissolved in 0.1 N HCl is shown in Figure 3, and was recorded using a Shimadzu UV-VIS model 1601 PC spectrophotometer. The transition characteristic of a hydroxy-substituted phenyl group was found to split into two maxima, with the first being noted at 274.7 nm (Al% 1cm = 66, and molar absorptivity 2230 L/mole cm), and the other at 268.9 nm (Al% 1 cm = 67, and molar absorptivity 2265 L/mole cm). The far more intense singlet->singlet transitions of the phenyl systems are evident in the rapidly rising absorption noted below 240 nm. 

Br(42Pi/2) being rapidly quenched by any Br2 present.76 While Cl(32iVt) would be expected from the photolysis of Cl2, the strongest absorption transition of the excited atom at 1351.7 A (Table IV) was obscured by the molecular spectrum of undissociated Cl2 and only an absorption transition of the ground state atom at 1335.7 A (Table IV) could be detected through a window in the vacuum ultraviolet molecular spectrum.29... 

Benzene. The ultraviolet absorption spectrum of benzene is characterized by the low intensity L band at 256 nm and the more intense La band at about 200 nm, both bands being 77 77 transitions. Changes in the absorption spec-... 

Sukhorukov et al. 99 measured the displacement of the near ultraviolet absorption bands of several cytosines upon transition from the neutral to the ionic form and evaluated the ionization constants in the excited state of the compounds. They concluded that on excitation there was no considerable shift in the tautomeric equilibrium of the forms 2 and 6 (in the excited state) = 1.55 x 104]. They also... 

In the vacuum ultraviolet absorption bands in the region 1280 to 1600 A correspond to the fourth positive system A1 n-X L+. The absorption cross sections of this system are given in Fig. V-7. Since the widths of the CO rotational lines are much smaller than the instrumental resolution ( 10 cm" 1), it is not possible to obtain the absorption cross section of each rotational line [see Section 1-8 for details]. Thus, the cross sections shown in Fig. V-7 are much less than the true cross sections. An estimate of the integrated absorption coefficient of the (0,0) band is 1.7 x 104cm-latm-1 (899). Various electronic states and transitions are given in Fig. V-8. 

The ground state of CF3I is X /l, of C3t, symmetry. The near ultraviolet absorption is continuous with a maximum at 2650 A in the region 2480 to 2815 A. Three transitions have been observed in the vacuum ultraviolet (16). D0d—CF3) = 2.31 0.05 eV. 

The near ultraviolet absorption spectrum lies in the 3000 to 4200 A region with many diffuse bands [Back ct al. (52)]. Back ct al. attribute the spectrum to transition forbidden by electric dipole. The absorption is weak with an absorption coefficient of 3.9 1 mol"1 cm-1 (base 10) at 3650 A. 

The observed ultraviolet absorption spectrum of l212 (for a display of the recorded spectrum see Section I.G of Wiberg s review article on the cyclopropyl group14) consists of three broad bands, of which the first (60,000 to 66,000 cm-1 7.44-8.18 eV) possesses a maximum at 63,000 cm 1 (7.8 eV oscillator strength/ 0.12), far to the red of most saturated absorbers and in the region of the n->n transitions of unsaturated molecules. This observation has... 

The tetrahedral ions, POtHSOr and CIO - show no near ultraviolet absorption whereas the ions, XO4", of the fourth-row transition elements, which have the same number of valence electrons, show characteristic visible and near ultraviolet absorption, two strong maxima with the corresponding peaks displaced toward shorter wavelengths with decreasing atomic number of the central atom. This trend is also observed for the fifth- and sixth-row transition element compounds. A satisfactory theory must account for these differences and regularities. 

The longest-wavelength ultraviolet absorption maxima of methanol and methoxymethane (dimethyl ether) are noted in Table 9-3. In each case the absorption maximum, which probably involves an n ——> cr transition, occurs about 184 nm, well below the cut-off of the commonly available spectrometers. 

The ultraviolet absorptions of simple saturated amines occur at rather short wavelengths (—220 nm) and are not particularly useful for identification. These are n —> cr transitions that correspond to excitation of an electron of the unshared pair on nitrogen to the antibonding [Pg.1105]

Fig. 17. Comparison of the transition temperatures for RNase-A (circles), RNase-S (triangles), and S-protein (squares) as determined by optical rotation (open symbols) and ultraviolet absorption difference spectroscopy (filled symbols). Reproduced from Sherwood and Potts (387).

For organic materials, ultraviolet absorption spectra are substantially determined by the presence of functional groups. Identical functional groups in different molecules may not absorb at precisely the same wavelength due to different structural environments which modify the local electric field. The magnitude of the molar extinction coefficient ( e ) for a particular absorption is directly proportional to the probability of occurrence of the particular electronic transition. Spectral features of some isolated chromophoric groups are presented in Table 2... 

The tautomerism of 4 (Figure 1) was also studied by UV-Vis (ultraviolet-visible) spectroscopy in polar aprotic solvents the effect of added water, darkness, and indirect sunlight were also evaluated. The experimental spectroscopic results are discussed in Section 13.14.3.1.1 (i). Theoretical calculations using ZINDO/S were performed to state the allowed absorption transitions <2005SAA875>. The five tautomeric structures of 4 as well as the calculated energies for each are depicted in Figure 2. 

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