The UV-visible spectra

The calculated vertical transitions (AjE) are consistent with expectations based on resonance and PMO arguments. For example, rc-donors such as NH2, OH, SH, etc. increase AE substantially and are thus predicted to induce a blue shift in the absorption maximum. This is because the resonance stabilization of the ground-state singlet (empty 3p-orbital on Si) as in 118b is larger than in the excited singlet (with a half-filled 3p-orbital on Si). Electronegative substituents increase AE (compare R = F and Cl in Table 34), while 

In this section we will review the computational studies on reactions of silylenes, mainly addition and insertion reactions. Some reactions, in particular the following isomeriz-ations of silylenes to the corresponding multiply bonded species, were discussed above (a) to silaethylene in Section V.A.l.a.v, (b) to substituted silenes in Section V.A.l.b.iv, (c) to disilenes in Sections V.A.2.d and f, (d) to silanimines in Section Y.A.3, (e) to silanephos-phimines in Section V.A.4, (f) to silanones in Section V.A.5, (g) to silanethiones in Section V.A.6, (h) to silynes in Section V.B.l, (i) to disilynes in Section V.B.2, (j) to aromatic compounds in Section VI.A and to antiaromatic compounds in Section VI.D. 

The dimerization of two singlet methylenes to form ground-state ethylene (equation 38, X = Y = C) is symmetry-forbidden along the least-motion path2823 and therefore proceeds along a non-least motion path2826. Two ground-state triplet methylenes, on the 

FIGURE 46. Least and non-least-motion approaches for the d erization of H2X (X = C, Si) 

A recent study291, which employed more elaborate theoretical methods, concluded that the actual barrier for reaction 39 is much smaller than believed earlier285-290. At 

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Comparison of the photoelectron spectra and electronic structures of M-NS and M-NO complexes, e.g., [CpCr(CO)2(NX)] (X = S, O), indicates that NS is a better a-donor and a stronger r-acceptor ligand than NO. This conclusion is supported by " N and Mo NMR data, and by the UV-visible spectra of molybdenum complexes. 

A somewhat more detailed study of vanadium atoms and dimers has also appeared 108). Figure 1 shows the UV-visible spectra of V and V2 as a function of vanadium concentration. Figure 2 shows a tjqiical, metal-concentration plot illustrating the aforementioned kinetic anal-... 

Fig. 5. Correlation of the UV-visible spectra of Co atoms and Coj molecules isolated in Ar, Kr, and Xe matrices under identical conditions of temperature and gas and metal deposition rates (49, 154).

The UV-visible spectra of Ni and Nia have also been identified in argon matrices (93) Ni absorbed at 377, 529, and 4l0 nm, with vi-bronic structure on the first two bands, and with spacing of—330 cm , and Nis absorbed at 420 and 480 nm, the latter band showing vibrational spacing of -200 cm" . Higher-nuclearity clusters were observed, but not characterized. After prolonged warm-up of these matrices, nickel colloid was formed (93). 

Fig. 16. The UV-visible spectra of Ag,jj /Kr mixtures (Ag/Kr = l/10 )at 10-12K (A) After a 30-min irradiation centered at the atomic resonance absorption lines. (B ) The outcome of a 10-min, 423-nm Agj irradiation, showing major decay of the bands associated with Ag, (indicated by arrows) and the appearance of two new bands near 450 nm. (C) The result of a 5-min, 25K bulk thermal annealing period, showing regeneration of the original Ag3 spectrum eind loss of the new band near 445 nm USD.

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

As already briefly mentioned in the introduction, some metals exhibit so-called plasmon resonances in the UV-visible spectra, attributed to the interaction of electromagnetic waves (visible light) and the confined electron gas, if a critical size on the nanoscale is reached. The process is sketched in a simplified manner in Figure 8. 

Spectra at p (=20) wavelengths. Because of the Lambert-Beer law, all measured spectra are linear combinations of the two pure spectra. Together they form a 15x20 data matrix. For example the UV-visible spectra of mixtures of two polycyclic aromatic hydrocarbons (PAH) given in Fig. 34.2 are linear combinations of the pure spectra shown in Fig. 34.3. These mixture spectra define a data matrix X, which can be written as the product of a 15x2 concentration matrix C with the 2x20 matrix of the pure spectra ... 

Upon complexation, shifts in the UV-visible spectra of cupric chloride are manifested as a shoulder at approximately 370 nm, and a shift in the visible absorption from 865 to 850 nm. The method of continuous variation7 (Job s Method) was employed using the new, 370 nm, absorption. The results indicate one monomer residue (pyridine... 

The film is reduced to the desired doping level prior to its removal and transfer to the UV-vis cell and the UV-visible spectra at different doping levels are obtained on different films. 

Figure 2 compares the UV-visible spectra of AW-Ph-HMM and CW-Ph-HMM (curves a and b, respectively), after contact with iodine vapours and outgassing at room temperature. As it concerns the material with crystal-like walls, no peaks related to I2 molecules are observed. Since surface specific areas of the two materials are comparable, this finding suggests a lower availability of phenyl rings for the crystal-like walls material, where, following the model reported in [7], the aromatic moieties lie perpendicularly with respect to the pore surface. 

Figure 15 shows the modification in the UV-visible spectra of TS-1, initially in vacuo, upon interaction with H20 (752). Evidence of the interaction of NH3, a stronger base, is also shown. The LMCT band (mentioned in Section II.A.3) undergoes a red shift of the edge as a result of the increase of the coordination sphere about Ti4+ ions. In Ti02, in which Ti is surrounded octahedrally by six O atoms in its first coordination sphere, the Ti4+02- — Ti3+0- LMCT is also red shifted to lower wavenumbers (32,000 cm-1). A stronger perturbation is obtained upon dosing of NH3, but the line shape of the UV-visible curve is... 

Fig. 17. Evolution of the UV-visible spectra of a TS-1 catalyst brought in contact with an aqueous solution of H202 as a function of time 1 min, 4, and 8 h (curves 1, 2, and 3, respectively). Curve 4 shows the effect of H20 dosage on the catalyst sample after acquisition of spectrum 3 [Reproduced from Zecchina et al. (153) with kind permission of Kluwer Academic Publishers].

Reduction by pulse radiolysis of Mn04 under acidic conditions has allowed a study of the UV-visible spectra of the unstable ion 03Mn (0H) and the determination of the pAa of this ion, 7.4 0.1." Abrasive stripping voltammetry has been used to characterize solid barium and... 

These three types of clusters all involve one electron orbital. They provide a basis for the description of the d-d (i. e. ligand field) transitions and the ligand to metal charge transfer transitions which are responsible for most of the UV-visible spectra and opti-... 

Chemical oxidation of the TTF groups in compounds 34 and 35 has been achieved by reaction with an excess of iodine in dichloromethane solution, leading to new low-energy absorptions in the UV/visible spectra which are diagnostic of TTF cation radicals the broad absorption at = 830 nm for the iodide salt of 35 suggests the formation of aggregated TTF species. A charge transfer complex formed by 35 and tetracyano-p-quinodimethane (TCNQ) has been isolated as an insoluble black powder. The stoichiometry is (35), (TCNQ)3 (i.e. 8 TTF units 3... 

Evaluation of the UV-visible spectra has been used to demonstrate the formation of charge transfer complexes between the acridizinium (benzo[6]quinolizinium) ion and polycyclic aromatic hydrocarbons (78ZC33). Similar measurements have been employed to demonstrate the existence of an interaction between DNA and coralyne, a dibenzo[a,g]-quinolizinium salt (76JMC1261). 

There are several highly useful sources of data on the absorption spectra and photochemistry of atmospheric species. NASA publishes on a regular basis a summary of kinetics and photochemical data directed to stratospheric chemistry (DeMore et al., 1997). However, much of the data is also relevant to the troposphere. This document can be obtained from the Jet Propulsion Laboratory in Pasadena, California. Alternatively, the data are available through the Internet (see Appendix IV). IUPAC also publishes regularly in The Journal of Physical Chemical Reference Data a summary directed more toward tropospheric chemistry (Atkinson et al., 1997a, 1997b). Finally, Nolle et al. (1999) have made available a CD-ROM containing the UV-visible spectra of species of atmospheric interest. 

UV/visible spectroscopy is undoubtedly the most important and widely used technique for identifying and quantifying Q-state effects for MCs in LB films. Correlations between particle size and features in the absorption spectrum have been used frequently. Examples of such a curves used for CdS are shown in Figure 3.5.1. UV/visible spectroscopy has indicated Q-state effects for other MCs in LB films, including CdSe (18,30,31,75), CdTe (30,31), CuSv (9), HgS (23,45), PbS (35,36,39,44,69-71), MS (M = Pt, Pd) and PtS2 (10), and ZnS (39,53). The UV/visible spectra of CdX (X = S, Se, Te) made by exposure of Cd2+/diynoic FA films to H2X are given in Figure 3.5.6 (31). The absorption onsets of the Cd... 

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