Hydrocarbon structures electronic spectra

Contents Theory of Electrons in Polar Fluids. Metal-Ammonia Solutions The Dilute Region. Metal Solutions in Amines and Ethers. Ultrafast Optical Processes. Metal-Ammonia Solutions Transition Range. The Electronic Structures of Disordered Materials. Concentrated M-NH3 Solutions A Review. Strange Magnetic Behavior and Phase Relations of Metal-Ammonia Compounds. Metallic Vapors. Mobility Studies of Excess Electrons in Nonpolar Hydrocarbons. Optical Absorption Spectrum of the Solvated Electron in Ethers and Binary Liquid Systems. Subject Index. Color Plates. 

Of course, a close stmctural relationship between radical cations and parent molecules is not likely to hold generally, but it is a fair approximation for alternant hydrocarbons. Deviations have been noted some stilbene radical cations have higher-lying excited states without precedent in the PE spectrum of the parent for radical cations of cross-conjugated systems (e.g., 1) already the first excited state is without such precedent. These states have been called non-Koop-manns states. Alkenes also feature major differences between parent and radical cation electronic structures. 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

This SF6-acetylene can also be obtained in —50% yield by dehydrobro-mination of SF5CH=CHBr (135). The gas-phase electron diffraction structure of SF5C=CH is reported (136). In the H NMR spectrum, the acetylenic proton resonates at a more shielded position and appears as a pentet with JF H = 3 Hz (134). This suggests that it couples significantly only with the four equatorial S-F atoms and not the axial S-F atom. Of particular interest are comparative 19F NMR spectral studies of F5SC=CH and other saturated hydrocarbons/fluorocarbons containing the SF5 group (137). 

In the bulk, the low concentration of ground-state pairs excludes their observation by absorption. The formation of the excited-state complex, termed exciplex, is a collisional process electronic excitation of either the acceptor or the donor leads to the formation of a locally excited state (for instance, in hydrocarbon molecules, it is a nn state). During the lifetime of this state, a collision with the other partner (which is in the ground state) leads to the formation of the exciplex. This mechanism is compatible with the fact that the absorption and fluorescence excitation spectra of the system are identical with those obtained by superimposing the spectra of the individual components. At the same time, the fluorescence emission spectrum changes drastically—a broad band, red shifted with respect to the bare molecule s emission spectrum, appears. It is usually devoid of vibrational structure, and is shifted to longer wavelengths as the solvent polarity increases [1],... 

Because mass-spectral fragmentation patterns are usually complex, it s often difficult to assign definite structures to fragment ions. Most hydrocarbons fragment in many ways, as the mass spectrum of hexane shown in, Figure 12.4 demonstrates. The hexane spectrum shows a moderately abundant molecular ion at m/z = 86 and fragment ions at m/z - 71, 57,43, and 29. Since all the carbon-carbon bonds of hexane are electronically similar, all break to a similar extent, giving rise to the observed ions. 

Abstract X-ray spectroscopy provides a number of experimental techniques that give an atom-specific projection of the electronic structure. When applied to surface adsorbates in combination with theoretical density functional spectrum simulations, it becomes an extremely powerful tool to analyze in detail the surface chemical bond. This is of great relevance to heterogeneous catalysis as discussed in depth for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated n-systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone-pair interactions, and (v) saturated hydrocarbons (physisorption). 

It seems reasonable that rapid rotation around the sixfold axis could occur more easily than any other reorientation (30). If the anisotropic hyperfine coupling for the protons is of the same form as that observed for other 7r-electron hydrocarbon radicals (20), then the anisotropy would be averaged out by a rapid rotation around this axis. However, the small asymmetry observed indicates an incomplete averaging. The additional structure of the two outermost lines on the high field side may be an anisotropy effect. Alternatively, an underlying spectrum—e.g., from cyclohexadienyl—may also distort the line shape. 

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