Electron absorption bands

RRS has also introduced the concept of a Raman excitation profile (REPy for thefth mode) [46, 4lZ, 48, 49, 50 and M]. An REP. is obtained by measuring the resonance Raman scattering strength of thefth mode as a fiinction of the excitation frequency [, 53]. Flow does the scattering intensity for a given (thefth) Raman active vibration vary with excitation frequency within an electronic absorption band In turn, this has led to transfomi theories that try to predict... 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.9 Electronic Absorption Bands for Representative Chromophores... 

The charge-tranter concept of Mulliken was introduced to account for a type of molecular complex formation in which a new electronic absorption band, attributable to neither of the isolated interactants, is observed. The iodine (solute)— benzene (solvent) system studied by Benesi and Hildebrand shows such behavior. Let D represent an interactant capable of functioning as an electron donor and A an interactant that can serve as an electron acceptor. The ground state of the 1 1 complex of D and A is described by the wave function i [Pg.394]

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

Fulvalene and heptafulvalene are predicted, in agreement with the result obtained using the symmetry rule, to suffer a symmetry reduction 2v in their lowest excited states. The longest wave-length electronic absorption bands of these molecules are expected to be relatively broad. This seems to be what is observed . On the other hand, the lowest excited state of sesquifulvalene is predicted not to undergo symmetry reduction, which again supports the prediction based on the symmetry rule. 

FIGURE 6. (a) Values of for the solvated electron absorption bands plotted against the mole fraction DMSO for DMSO/H O mixtures, (b) Photon energy of the absorption band maxima for the solvated electron in DMSO/HjO mixtures plotted against the bulk static dielectric constant (25 °C) of the mixture. Non-linear axes showing dielectric constant and mole fraction for (a) and (b) respectively are given as top abscissae. Reproduced by permission of the authors from Reference 30. 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

The exposure of a colorless solution of anisole to iodine monochloride (IC1) in dichloromethane leads to the appearance of a new electronic absorption band (lmax = 350 nm) arising from the formation of a donor/acceptor complex,225 i.e.,... 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

Although chemisorption is not essential, when it does occur there may be further enhancement of the Raman signal, since the formation of new chemical bonds and the consequent perturbation of adsorbate electronic energy levels can lead to a surface-induced RR effect. The combination of surface and resonance enhancement (SERRS) can occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonance, yielding an overall enhancement as large as 10lo-1012. 

With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic colour changes are manifested in the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (IP) in the order benzene 9.23 eV, naphthalene 8.12eV, and anthracene 7.55 eV, much in the same manner as that observed with the tropylium acceptor (Takahashi et al.,... 

A few typical examples having electronic absorption bands for various representive chromophores are provided in the following Table 21 1 ... 

Resonance-enhanced Raman scattering occurs when the energy of the Incident radiation, hvg. Is close to or within an electronic absorption band of the sample (7,8). In this case, vlbronlc coupling with the electronically excited state Increases the probability of observing Raman scattering (hv-gg) from vibrational transitions In the electronic ground state (Figure lb). The Intensity of such resonance-enhanced vibrational transitions can be described In simplified terms as ... 

Ru=0 stretch. The position of this band is insensitive to the nature of the macrocyclic amine ligand, in accord with the formulation that the electronic transition involves the nonbonding d orbitals (Table 5, Figure 4). Similar electronic absorption bands have also been found for trans-... 

The dissolution of sulfur in ammonia has been known for more than 100 years [17]. The identification of the chemical species in these solutions was a matter of confusion until the identification of S4N and 83 , by Chivers and Lau [18] and Bernard et al. [19], using Raman spectroscopy. When considering the species formed in the dissolution process, it is quite remarkable that this dissolution is reversible sulfur is recovered after evaporation of ammonia. These solutions are strongly colored (blue), mainly due to the electronic absorption band of S4N at 580 nm. It must be mentioned that this dissolution is moderately fast at room temperature (but much slower than the dissolution of alkali metals) and that the rate is much slower when temperature decreases. It should also be mentioned that concentrated solutions of sulfur in liquid ammonia can be used as the solution at the positive electrode of a secondary battery. The solution at the negative electrode can be a solution of alkali metal in liquid ammonia [20], the electrodes being... 

This section discusses some simple chemical reactions which convert a chiral nonracemic compound containing no electron absorption band in an easily accessible spectral range into a derivative with absorption in either the visible or quartz ultraviolet region. This is a useful operation if a reliable correlation exists between absolute configuration (conformation) and chiroptical properties. A collection of useful chromophoric systems is found in reference 167. 

The absolute configuration of chiral nonracemic compounds can be established by forming a complex which can bind to the axial position of [Rh2(02CCF3)4] and thus induce circular dichroism within its electronic absorption bands. This method works well for chiral secondary alcohols and monoolefins. For these compounds tentative rules were proposed, which correlate the configuration of the starting materials with the sign of certain Cotton effects of their complexes178. 

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