Maximum absorption

After in situ neutralisation, the complexation behaviour of 4.44 was studied using UV-vis spectroscopy. The absorption maximum of this compound shifted from 294 nm in pure water to 310 nm in a 10 mM solution of copper(II)nitrate in water. Apparently, 4.44, in contrast to 4.42, does coordinate to copper(II)nitrate in water. 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

UV-VIS Simple ethers have their absorption maximum at about 185 nm and are trans parent to ultraviolet radiation above about 220 nm... 

The shape of the broad absorption curve in Figure 9.17 is typical of that of any dye suitable for a laser. It shows an absorption maximum to low wavelength of the Og band position, which is close to the absorption-fluorescence crossing point. The shape of the absorption curve results from a change of shape of the molecule, from Sq to 5i, in the... 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Electronic Properties. What distinguishes polysdanes from virtually ad. other polymers is their backbone CJ-conjugation. This leads to strong electronic absorption in the near-uv from a O —O transition. For most homo- and copolymers the absorption maximum (/-j ) hes between 300 and 400... 

The pale blue tris(2,2 -bipyridine)iron(3+) ion [18661-69-3] [Fe(bipy)2], can be obtained by oxidation of [Fe(bipy)2]. It cannot be prepared directiy from iron(III) salts. Addition of 2,2 -bipyridine to aqueous iron(III) chloride solutions precipitates the doubly hydroxy-bridged species [(bipy)2Fe(. t-OH)2Fe(bipy)2]Cl4 [74930-87-3]. [Fe(bipy)2] has an absorption maximum at 610 nm, an absorptivity of 330 (Mem), and a formation constant of 10. In mildly acidic to alkaline aqueous solutions the ion is reduced to the iron(II) complex. [Fe(bipy)2] is frequentiy used in studies of electron-transfer mechanisms. The triperchlorate salt [15388-50-8] is isolated most commonly. 

In analogy to the situation for bipyridine, the blue tris(l,10-phenanthroline)iron(3+) ion [1347949-7], [Fe(phen)2], must be obtained by oxidation of the corresponding iron(II) ion. [Fe(phen)2] has an absorption maximum at 590 nm, an absorptivity of 600 (Mem), and a formation constant of 10 . In solutions of pH > 4, this species is reduced to the iron(II) complex. The reduction is instantaneous in alkaline solution. At pH < 2, protons compete with iron(III) for the phenanthroline nitrogens and coordination is incomplete. [Fe(phen)2] is used most often in solution as an oxidant, but the trichloride [40273-22-1] and the triperchlorate monohydrate [20774-81-6] salts have been prepared. 

Unlike PMDs having a single chromophore, the absorption band of a bis-dye spHts into two components, so that one maximum is shifted bathochromicaHy and the other hypsochromicaHy with respect to the absorption maximum of the parent dye. The distance between bis-dye maxima depends on the magnitude of the chromophore interaction. Interaction of this kind has been discovered to be universal (45,46). 

There are two main kinds of dye aggregates, characterized by their typical spectral properties J-aggregates and H-aggregates. The absorption band maximum (f-band) of the J-aggregates is shifted bathochromicaHy with respect to that of an isolated molecule (M-band) the absorption maximum of the H-aggregates is shifted hypsochromicaHy (H-band). The dyes can also form dimers with a shorter absorption wavelength (D-band). 

In the example shown, the reduced form [49765-27-7] is blue with a visible absorption maximum at 610 nm. The rate of the reoxidation of the reduced form (cation radical, C24H22N" 2) is usually, but not always, strongly dependent on the presence of oxygen. 

A quantitative expression of these observations is shown in equation 1, where is the observed absorption maximum for the unsymmetrical carbocyanine and Xj is the arithmetic mean (isoenergetic wavelength) for the absorption maxima of the related symmetrical dyes. 

The absorption maximum of a disubstituted anthraquinone gready depends on the substituents and their positions (Table 2). The 1,4-disubstituted compound shows a remarkable bathochromic shift. The effects of P-substituents on 1,4-dianainoanthraquinones (14) are shown in Table 3. Larger bathochromic shifts are observed with increasing electron-withdrawing abiUty of P-substituents. 

Isothiazole has an absorption maximum in ethanol solution at 244 nm, with a molar absorptivity of 5200. This absorption occurs at a longer wavelength than with pyrazole or isoxazole, the displacement being due to the presence of the sulfur atom. A series of approximate additive wavelength shifts has been drawn up in Table 11 and this should enable prediction of UV maxima of isothiazoles with reasonable accuracy, even for multiply substituted compounds. The longest wavelength band results from a electronic... 

Absorption spectra of standard solutions of Cyt c was obtained at different concentration. Maximum of absolution was observed at wavelength 410 nm. It is known haemoglobin and other haems have absolution maximum at the same wavelength. For elaboration of selective method of Cyt c determination in semm of mice its reaction with phtalocyanine of copper was investigated. Absorption maximum of Cyt c with Cu phtalocyanine in H SO was observed at wavelength 710 nm. Dependence on optical density at 710 nm against concentration of Cyt c have linear character in range 0.162-10-"-6.49-10 mol/L. 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

This method of detection is at its most sensitive if the absorption maximum (A a,) of the sample molecule is exactly at the wavelength of the UV light employed for irradiation. The further lies from this the less radiation is absorbed and the lower the sensitivity of detection. If the compound does not absorb at the wavelength of radiation or if it possesses an absorption minimum just there then such components are not detected by this method. Figure 4C illustrates this with the sweeteners saccharin and dulcin as examples. 

Diphenylboric acid-2-aminoethyl ester reacts to form complexes with 3-hydroxy-flavones with bathochromic shift of their absorption maximum. 

Figure 13.37 shows the UV spectrum of the conjugated diene cis,trans-, 3-cyc o-octadiene, measured in ethanol as the solvent. As is typical of most UV spectra, the absorption is rather broad and is often spoken of as a band rather than a peak. The wavelength at an absorption maximum is refened to as the X ax of the band. There is only one band in the UV spectrum of 1,3-cyclooctadiene its X ax is 230 ran. In addition to UV-VIS bands are characterized by their- absorbance (A), which is a measure of how much of the radiation that passes through the sfflnple is absorbed. To correct for concentration and path length effects, absorbance is converted to molar absorptivity (e) by dividing it by the concentration c in moles per liter and the path length I in centimeters. 

LUMO (tc ) transition. The 1,3- and 1,5- isomers of the eight-membered R2P(NSN)2PR2 (R = Me, Ph) rings, 3.27 and 3.28, respectively, exhibit characteristically different colours. The 1,3-isomer 3.27 is dark orange with an absorption maximum at ca. 460 nm, whereas the 1,5 isomer has a very pale yellow colour. In the absence of a... 

In this system the product of the first reaction possesses an absorption maximum at 222 nm and the final product has k ax = 288 nm. The initial reactant is essentially nonabsorbing at these wavelengths. Hence, spectrophotometric observation at 222 and 288 nm allowed two simultaneous equations to be written, and thus Cb and Cc were determined as functions of time. From the known quantity c°, the concentration Ca was calculated with Eq. (3-28). The rate constant A , was then found from the plot of In Ca vs. time. An estimate of rate constant k was obtained from a plot of In Cb vs. time in the late stages of the reaction, and this value was refined by curvefitting the Cb and Cc data. Figure 3-6 shows the data and final curve fits. 

---