Electronic spectra absorption

Electronic Absorption Spectra. The absorption spectra of the complexes formed by exchange from ferric nitrilotriacetate were determined in 0.1 M phosphate, pH 7- t, with a Beckman Model 25 recording spectrophotometer over the range kOO-700 nm. Agrobactin and parabactin gave wine colored iron complexes with a broad adsorption band centered at about 500 nm. At pH 7., the 

ACS Symposium Series American Chemical Society Washington, IXJ, 1980. 

Since the spectra of ferric agrobactin and ferric parabactin do show minor differences, a number of experiments were tried in which an attempt was made to mimic these differences via examination of the 1 3 ferric complexes of model salicyl and 2,3-dihy-droxyphenyloxazolines and the 1 1 1 ferric complexes of the model oxazolines with Tait s Compound II , -bis-(2,3-dihyd oxyben- 

Ferric tris-salicyloxazollne precipitated upon its formation by the addition of three equivalents of alkali. The neutral complex was dissolved in ethanol to give an orange colored solution with = 5 at the maximum, U65 nm. A specimen of ferric tris-2,3-dll droxyphenyloxazoline had a maximum at 520 nm with of 2.8 at pH 7.5 at pH 10, where the complex was fully formed, the amjj was k,6 and the meiximum had shifted to U95 nm. These data, and those obtained by admixture of the oxazolines with Tait s Compound II, were not particularly illuminating as regards the mode of iron complexatlon in the natural products. Examination of the complexes in the ultraviolet might have proved instructive, but this aspect was not pursued. 

Circular Dichroism. The siderophores studied herein contain one bidentate ligand mounted on an optically active substituent, namely, threonine or its oxazoline. Th is the particular type of coordination isomer considered by Corey and Bailar (17) could result in a chelate chromophore which is capable of rotation of plane polarized li t. 

Details about electronic absorption spectra will be given and discussed later in conjunction with the luminescense properties. Here some general features of the electronic spectra are discussed. 

The class of complexes considered in the present review, represents species which can be considered to lie at the borderline between classical Werner-type coordination compounds and organometallic molecules. In the former, the distinction of four types of excitation in the electronic spectra has been very useful [1]  

Metal-centered (MC) excitations, which are generally due to d-d transitions. The oscillator strength for such complexes is generally low, because of parity selection rules, giving rise to absorptions with low molar absorption coefficients. Because of their low intensity, these absorptions, which are always present in complexes with partly filled d-shells, are often buried below bands due to fully allowed transitions. 

Metal-to-ligand charge-transfer excitations (MLCT), which are observed in species with metals in relatively low oxidation states with ligands that have low lying empty orbitals (in general 7t -orbitals). 

Ligand-centered transitions (LC) occurring also in the free ligand. In the complex they are perturbed to a lesser or a greater degree, but not completely modified. 

Spectroscopy in the ultraviolet and the visible region was one of the first physical methods applied to the studies of aromatic compounds in acid systems. By this method the first indications were obtained for arenium ions. In particular, V. Gold and F. Tye showed the solutions of anthracene and 1,1-diphenylethylene in concentrated sulphuric acid to have absorption maxima very close in position and intensity 422 and 431 nm, respectively). This similarity meant that the hydrocarbons are protonated to form, respectively, the 9-H-anthracenium ion and the di-phenylmethyl cation having similar 7t-electron systems  

Electronic spectroscopy, however, should be handled with care since at low concentrations absorbing substances may be transformed in secondary photochemical and oxidizing reactions. For instance, complexes of toluene, 1- and 2-methylnaphtha-lenes with HCl and AICI3 and those of hexamethylbenzene with DF and BF3 

Cells used in recording the spectra of solutions in liquid hydrogen fluoride and chloride at room temperature and under high pressure are described in cells 

Curiously, the electronic spectra of the mesitylenium ion in the HF—BFj system and in the gas phase = 355 and 250 nm practically do not differ in the position of the absorption maxima. 

A further observation from these results is that some of the Ki values are much larger than those encountered in solubility studies. Correlations with log have been shown. 5.5.3.S Electronic absorption spectra 

The energy of an electronic transition is calculated from the familiar equation 

The phenomenologieal theory has been applied by Skwierezynski to the Ef values of the Dimroth-Reiehardt betaine, a quantity sensitive to the polarity of the medium.The approaeh is analogous to the earlier development. We need only eonsider the solvation ef-feet. The solute is already in solution at extremely low eoneentration, so solute-solute inter-aetions need not be aeeounted for. The solvent eavity does not alter its size or shape during an eleetronie transition (the Franek-Condon prineiple), so the general medium effeet does not eome into play. We write Ef of the mixed solvent as a weighted average of eontributions from the three states  

The procedme is to fit F to X2. As before, a 1-parameter version can be obtained by setting 

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The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

Structure of Small and Large Rings Table 7 Electronic Absorption Spectra of Small Heterocyclic Systems... 

Comparison of electronic absorption spectra of BPR and PR in solution with electronic absolution spectra of BPR and PR on NH -S and en-S surfaces has been made. In contrast to aqueous solutions, where shift of the absolution maximum of dyes is occurred when pH is changed, significant shift of maxima is not observed on aminosilica gels surface during pH changing. Both reagents BPR and PR have an absolution maximum in the field of 17900-18100 cm at different pH values. However use of BPR is more preferable than that of PR as qualitative soi ption of BPR occurs at wider pH range. 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

The ultraviolet absorption spectra of carboline derivatives have been repeatedly recorded. Since the basic jpyr-N in the carbohnes and in 3,4-dihydro-jS-carbolines is part of a conjugated system, protonation affects the electronic absorption spectra significantly. It is unfortunate therefore that the spectra of the protonated, as well as those of the unprotonated, species have not been reported in all instances. Protonation leads to a bathochromic shift of 20-30 mp,. This is illustrated by the absorption of j3-carboline, 1-methyl-jS-carboline, 7-methoxy-l-methyl-jS-carbohne, and the salts of these compounds. 

A. Electronic (Ultraviolet and Visible) Absorption Spectra The electronic absorption spectra of heterocyclic molecules have their origins in the transitions of electrons between different molecular orbitals. In general, the more these orbitals are spread out in space, the closer together are their energy levels and the longer the wavelengths... 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

McWeeny, R., Proc. Phys. Soc. A70, 593, (i) Electronic absorption spectra of heterocyclic systems. I. A comparison of theoretical methods pyridine and pyrazine. ... 

Fig. 8. Dichroism of electronic absorption spectra of oriented and nonoriented PAN films after IR irradiation, (a) Dependence of optical density ( >) on the wavelength for various values of (angle between film orientation axis and light polarization plane).

Fig. 18. Electronic absorption spectra of a poly(schiff base) obtained by polycondensation of 4,4 -diacetyldiphenyl sulfide and p-phenylenediamine (1) and of its complex with iodine (2)...

J.N. May cock et al, Electronic Absorption Spectra of Metallic Azides, Perchlorates, Nitrates and their Related Explosive Properties , SpectrochimicaActa 23A (1957), 2849—53... 

Electronic absorption spectra of a few typical transition-metal complexes are shown in Fig. 2-1. The following features are to be noted. 

Very few studies on complexes of palladium and platinum with 1,1-dithiolato ligands have been reported recently. The electronic absorption spectra of the Et2dtc complexes of Pd and Pt, as well as Zn, Cu, Fe, Co, and Mn, have been measured in MeCN, EtOH, and -heptane, with diffuse-reflectance spectra also being determined (401). In 1969 and... 

Resonance Raman (RR) spectroscopy provides information about the vibrational characteristics of a chromophore, for example, a metal center, within the complex environment of a protein. In RR spectra, those vibrational transitions are observed selectively that are coupled to electronic transitions. In iron sulfur proteins, this technique has been used to resolve the complex electronic absorption spectra and to identify both vibrational and electronic transitions. 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Fig. 6. Electronic absorption spectra of donor-functionalized expanded radialenes 76-78 in CHCI3 at 298 K...

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

Figure 1.7 Typical zero-order and corresponding second-derivative electronic absorption spectra of ethanol-reconstituted lipid/chloroform extracts of autoxidized model polyunsaturated fatty-acid compounds and inflammatory synovial fluid obtained after (1) reduction with NaBH4 and (2) dehydration with alcoholic H2S04- (a) Methyl linoleate subsequent to autoxidation in air at ambient temperature for a period of 72 h (—), or exposure to a Fenton reaction system containing EDTA (5.75 x 10 mol/dm ), H2O2 (1.14 X 10 mol/dm ) and Fe(ll) (5.75 x IO mol/dm ) as an aqueous suspension (—) (b) as (a) but with methyl linolenate (c) untreated rheumatoid knee-joint synovial fluid.

To assess performance of the selected DFT techniques in predicting electronic absorption spectra of quinones, the authors computed excitation energies of... 

The tautomeric product distribution has been a prerequisite for a further investigation aimed at predicting absorption properties of the transient semiquinones and quinones generated by pulse radiolytic oxidation of 2-4. The simulation of electronic absorption spectra has been computed using the TD-DFTapproach both in vacuum and in aqueous solution, using the large 6-311 + +G(2d,2p) basis set.19... 

Atomic spectra are much simpler than the corresponding molecular spectra, because there are no vibrational and rotational states. Moreover, spectral transitions in absorption or emission are not possible between all the numerous energy levels of an atom, but only according to selection rules. As a result, emission spectra are rather simple, with up to a few hundred lines. For example, absorption and emission spectra for sodium consist of some 40 peaks for elements with several outer electrons, absorption spectra may be much more complex and consist of hundreds of peaks. 

Figure 8. Electronic absorption spectra of precursor PBTB thin film (1) and conjugated derivatives (2-13) at different times during in-situ elimination reaction at 23°C.

Figure 9. Electronic absorption spectra of a conjugated derivative of FTTB.

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