Absorption band shift

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

The absorption band shifts to lower energy from top to bottom in the table. Roughly speaking the degree of covalency increases also in this sequence. Therefore it may be thought that we are dealing with one and the same transition. This, however, is unlikely. The literature contains many different, and sometimes not firmly based, assignments. 

Optical Properties. The double-decker complexes of porphyrazines have characteristic electronic absorption spectra (Table V). The intense Soret bands of the double-decker complexes are blue shifted with respect to the single pz ligand as a consequence of the strong n-n interactions. Another characteristic of sandwich compounds is the additional appearance of absorption bands shifted to the red (termed Q ) and to the blue (termed Q") of the normal g-band region. These new transitions are thought to result from orbitals delocalized over the two macrocyclic ligands (33, 82). 

Fig. 12 (a) Image of PMAA-protected fluorescent silver clusters prepared with increasing initial ratio Ag+ MAA from 0.5 1 to 12 1 and equal irradiation time, (b) Absorption spectra of the same samples as in (a), (c) Variation of absorption maxima of some of the samples in (a) with molar ratio. Black arrows indicate how the absorption band shifts to the blue with the addition of extra polymer to a fluorescent cluster solution explaining the transfer effect of silver clusters among PMAA chains [20]... 

TA1 >LE I Dimerization induced absorption band shifts (cm ) for some porphyrins... 

In the absorption spectra of nanoparticles of CdSe and other semiconductors, not only can the shift in wavelength be observed, but there are also bands corresponding to absorption to discrete energy levels in the conduction band. For example, 11.5 nm diameter particles of CdSe have an absorption spectrum that shows an almost featureless edge, but particles of diameter 1.2 nm show features resembling molecular absorption bands shifted about 200 nm to shorter wavelengths, as depicted in Figure 11.8. 

FIGURE 11.8 The absorption spectrum of 11.5 nm diameter particles of CdSe has an almost featureless edge, but particles of diameter 1.2 nm show features resembling molecular absorption bands shifted about 200 nm to shorter wavelengths. 

In KBr (solid state) the OH for all three isomers is H-bonded. In CCI, the H-bonds of meta and para isomers, which are intermolecular, are broken. Their ir OH absorption bands shift to higher frequencies (3325-3520cm" ). There is no change in the absorption of the ortho isomer (3200cm ), since intramolecular H-bonds are not broken upon dilution by solvent. 

Relationship of absorption positions and intensity to structures. While quantum mechanical calculations permit prediction of the correct number and approximate positions of absorption bands, they are imprecise. For this reason, electronic spectroscopy also relies upon a combination of empirical rules and atlases of spectra that can be used for comparison purposes.74 76 The following may help to orient the student. The position of an absorption band shifts bathochromically (to longer wavelength, lower energy) when the number of conjugated double bonds increases. Thus, butadiene absorbs at 46,100 cm 1 (217 nm) vs the 61,500 cm 1 of ethylene. As the number of double bonds increases further, the bathochro-mic shifts become progressively smaller (but remain more nearly constant in terms of wavelength than wave number). For lycopene (Fig. 23-10) with 11... 

Charge transfer and metal d-d bands are probably buried in the n- n bands. The n- n absorption bands shift bathochromically with electron density increase or expansion of the n system. The effect of 7,8-dehydrogenation is larger than that of 2,3-unsaturation. 

When polysilane bears two quite dissimilar alkyl groups, the absorption band shifts gradually to longer wavelength as the temperature is lowered. This is the case for (Vj-I IexSiMc),. shown in Figure 5.9(a). 

However, spectroscopic investigations have revealed an even more direct way of measuring a quantity closely related to the electronegativities. Most electron-transfer bands are caused by transitions from filled MO s predominantly localized on the ligands X, say of MXS, to one of the two sub-shells mainly concentrated on M. It was observed that these absorption bands shift to lower wave numbers when the electronegativity of X decreases. This shift is so regular in all the MX6 studied that it is possible n>13 to define optical electronegativities, xopt, of... 

In the [2]catenate, the circumrotation of the terpyridine-containing macrocyclic component can be reversibly controlled 21,22 (Figure 5), by altering the redox state of the metal. The absorption spectrum of a red-brown solution of [5 Cu] BF4 in MeCN shows a band, centered on 437 nm, characteristic of a Cu+ ion tetracoordinated to two phenanthroline ligands. Upon oxidative electrolysis, or upon addition of Br2, Cu+ is oxidized (step 1 in Figure 5) to Cu2+ and the solution turns deep green. The absorption spectrum shows a band, centered on 670 nm, typical of a Cu2+ ion tetracoordinated to two phenanthroline ligands. However, this absorption band shifts to... 

Saturated groups such as the methyl group exert a small secondary effect which is, in general, bathochromic (some exceptions in methylazulenes). This effect is caused by the electron-donating properties of the alkyl groups, which are less electrophilic than hydrogen (see also p. 264). In the series methane, ethane, hexane the absorption band shifts from 1250 A to 1350 A and to 1530 A. This shift is exclusively due to differences in the resonance possibilities of the molecule in the very highly excited upper level where one electron is nearly ionized. 

In efforts to explore the significance of the nephelauxetic effect, the possible relationship between the nephelauxetic absorption band shift and the structure of the complex has been investigated. The dependence of the nephelauxetic effect upon the coordination numbers in the complexes was recognized by Jorgensen and coworkers [49]. The nephelauxetic... 

Within a series of 9-ethyl-3,6-bis(arylazo)carbazoles featuring nonlinear optical A-Jt-D-7t-A type chromophotes, absorption bands shifting with the degree of conjugation can be observed in DMF at 225-350nm (due to the carbazole moiety) and at 350-700 nm (due to the 7t-7t transition of the azo conjugated unit) <2006DP(71)109>. 

The absorption spectrum of the solvated electron depends not only on the nature of the solvent but also on parameters that modify the structure and properties of the solvent, such as pressure and temperature. The optical absorption band shifts to higher energies (shorter wavelengths) with increasing pressure up to 2000 bar the shift is larger in primary alcohols than in water and it correlates with the increase in liquid density rather than with the rise in dielectric constant. A rise in the temperature induces a red shift of the solvated electron absorption spectrum. Thus, the absorption maximum in water is located around 692 nm at 274 K and 810 nm at 380... 

The absorption spectra of solutions containing Bt2 are solvent-dependent. When elemental bromine is dissolved in nonpolar solvents such as hexane, a single absorption band in the visible spectrum is observed near 500 nm. When Br2 is dissolv in methanol, however, this absorption band shifts and a new band is formed. 

Sq Si absorption band, shifting of the probe wavelength affects probe transmission just as does shifting of the crystalline absorption spectrum. In Figure 15, the major contribution is that due to coherent scattering. Quantitative determination of the phonon-induced spectral shift is impossible, but an approximate upper limit can be set. Its value ( 1 cm" for a 10" -A phonon amplitude) indicates that the initial slope of the Sj potential with respect to intermolecular separation is rather gradual and that the slope must increase as displacement increases [98]. Thus, in addition to information about excimer formation dynamics, partial (and at this point, qualitative) information about the reactive potential surface has been elucidated. 

Closer study of the absorption band shifted by stress showed a shape deformation which could be interpreted as a non-uniform stress distribution i.e. some bonds are more highly stressed than others and consequently suffer a higher-than-average frequency shift. Such bonds were designated over-stressed bonds and their distribution with respect to stress was deduced. An example is shown in Fig. 5 for... 

---