Spectral electronic absorption

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

As already mentioned, the macrocyclic ring can be further enhanced if the propylene moiety is substituted by a butylene group. Visible spectral data of a series of related compounds [Cu(L BF2)D]X 132 and the structural study of the derivative with D = H2O show that the seven-membered chelate ring influences the structure and other properties like electronic absorption of the copper(II) complex [193]. 

Fig. 9. Calculated relative energies (in kK) of the most important MO s (a) and spectral excitation energies derived from the electronic absorption spectrum (b) of ClFe(Et2

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. 

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. 

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.,... 

Figure 2. Electronic absorption spectral changes during 366-nm irradiation of an isooctane solution of [ReH5(PMe2Ph)s] under an Ht atmosphere.

Regarding the study of these complexes by various physical techniques, only IR spectroscopy has been widely used so far. Only a few X-ray structural, electronic absorption, and fluoresence emission spectral data are available. Other methods such as ESR (especially of Gd(III) complexes), NQR, and Mossbauer (especially of Eu-151) have not been seriously applied for the study of these complexes in the solid state. In solution, only conductance studies have attracted attention NMR, dipole moment, and electronic spectral studies are few in number. The lack of physical data limits our understanding of the structure and bonding in these complexes. In future, when more interest is evinced in applying various physical techniques to study these complexes, one may hope to come across more interesting and useful revelations. 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

Electronic absorption and fluorescence excitation and emission spectra of phenazines were determined in several solvents of various polarities <1995SAA603>, and the effect of the solvent upon the spectral characteristics was studied. 

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. 

Of these four properties, spectral shifts are the most sensitive to environmental changes and also the most readily measured. As a result the majority of investigations into electronic absorption spectral changes resulting from surface adsorption have been confined to measurements of spectral shifts. While the shift of the 0-0 bands is the most meaningful measurement to make, these 0-0 bands are not always discernible, especially when the molecules are adsorbed on polar surfaces, so it has become common practice simply to measure the shift of the absorption maximum. In most cases this measurement would correspond to the shift of the 0-0 band, in others, however, adsorption processes can produce unequal displacement of the ground and excited state potential curves, resulting in a different vibronic band shape. 

Electronic Absorption and Epr Spectral Data for Four-Coordinate Nickel(I)... 

Electronic Absorption Spectral Data for Five-Coordinate Nickel(I) Complexes with Macrocyclic Ligands... 

Figure 1. Electronic absorption spectral changes accompanying 366-nm photolysis of a 2.3 X 10 2M degassed CH2CI2 solution of [IrClH2(PPh J (27)...

Figure 3. Electronic absorption spectral changes resulting from 366-nm irradiation of a I0 3M CH2CI2 solution of [RuClH(CO)-(PPh)37 (35)...

Irradiation of a degassed benzene solution of [RuH2(CO)(PPh3)3] results in the electronic absorption spectral changes shown in Figure 4. As irradiation proceeds, the solutions change from colorless to yellow, and a shoulder appears... 

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