Hydrocarbon structures vibrational spectra

ELS Studies of Acetylene Chemisorption on Rh(lll). The vibrational spectrum of the (2x2) hydrocarbon surface structure formed from the chemisorption of C2H2 on Rh(lll) between 210 and 270 K is shown in Figure 5a. The peak positions and their... 

Alkene (olefinic) structures introduce several new modes of vibration into a hydrocarbon molecule a C=C stretching vibration, C—H stretching vibrations in which the carbon atom is present in the alkene linkage, and in-plane and out-of-plane bending of the alkene C—H bond. The spectrum of Figure 3.10 is that of a typical terminal alkene. 

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-... 

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],... 

Fluorescence spectroscopy is not considered a major structural or qualitative analysis tool, because molecules with subtle structural differences often have similar fluorescence spectra. Also, fluorescence bands in solution are relatively broad at room temperature. However, fluorescence has proved to be a valuable tool in oil spill identification. The source of an oil spill can often be identified by comparing the fluorescence emission spectrum of the spill sample with that of a suspected source. The vibrational structure of polycyclic hydrocarbons present in the oil makes this type of identification possible. 

Spectra of proteins and nucleic acids. Most proteins have a strong light absorption band at 280 nm (35,700 cm ) which arises from the aromatic amino acids tryptophan, tyrosine, and phenylalanine (Fig. 3-14). The spectrum of phenylalanine resembles that of toluene (Fig. 23-7)whose 0-0 band comes at 37.32 x 10 cm. The vibrational structure of phenylalanine can be seen readily in the spectra of many proteins (e.g., see Fig. 23-llA). The spectrum of tyrosine is also similar (Fig. 3-13), but the 0-0 peak is shifted to a lower energy of 35,500 cm (in water). Progressions with spacings of 1200 and 800 cm are prominent. The low-energy band of tryptophan consists of two overlapping transitions and The Lb transition has well-resolved vibrational subbands, whereas those of the La transition are more diffuse. Tryptophan derivatives in hydrocarbon solvents show 0-0 bands for both of these transitions at approximately... 

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