Vibrational spectra aromatic hydrocarbons

For some aromatic hydrocarbons such as naphthalene, anthracene and pery-lene, the absorption and fluorescence spectra exhibit vibrational bands. The energy spacing between the vibrational levels and the Franck-Condon factors (see Chapter 2) that determine the relative intensities of the vibronic bands are similar in So and Si so that the emission spectrum often appears to be symmetrical to the absorption spectrum ( mirror image rule), as illustrated in Figure B3.1. 

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-... 

Fig. 65 Magnification of two regions in the DRIFT spectra of Kapton presented in Fig. 62. a Region of C-H stretching vibrations at 3080 cm-1 aromatic hydrocarbons at 2950 cm 1 aliphatic hydrocarbons. The baseline applied for the peak area calculation is shown as a dotted line in the spectrum for 31,500 pulses, b Region of conjugated double bonds and triple bonds at 2270 cm 1 the -N=C=0 stretching vibration at 2255 cm 1 the -C=C- stretching vibration and at 2230 cm-1 the -C=N stretching vibration. Around 2350 cm 1 the typical doublet of gas-phase C02 is present. REPRINTED WITH PERMISSION OF [Ref. 135], COPYRIGHT (2000) American Chemical Society...

Another way of detecting interstellar species is provided by vibrational emission spectra (Figure 3). Let us mention here the so-called Unidentified Infra-Red (UIR) bands, which have not been unambiguously assigned yet. The striking resemblance of the UIR from the Orion bar with the Raman spectrum of an auto soot clearly seems to indicate that the carriers are carbon compounds. Very certainly, it will not be possible to make a one-to-one correspondence between the observed bands and some given species. Very certainly, also, the carriers of these bands are hydrogenated carbonaceous species. Whether these species are Polycylic Aromatic Hydrocarbons [24,25,26] (thereafter PAHs), coals [27,28,29], or amorphous carbon [30] is still a matter of debates and controversies that we shall not discuss further here. The interested reader can refer to a recent series published in the Faraday Discussions (1998). 

Without interactions with potential host molecules and in diluted solutions to avoid excimeric formations, pyrene presents in solution an intense and anisotropic fluorescence, as well as a high fluorescence quantum yield [34-37], Direct evidence of ground-state interactions of pyrene with potential host molecules is provided by the emission spectra. The vibrational structure of the emission spectrum of pyrene is constituted by five fine peaks, named I, I2, h, I4, and I5 (Fig. 13.2) [38]. An increase of the intensity of peak Ii is observed in polar solvents, while I, is solvent insensitive. Thus, the evolution of the ratio of intensities /1//3 gives information on the evolution of the polarity of the environment close to molecular pyrene, and consequently on the encapsulation of this guest in a host molecular or supramolecular object [39]. This sensitivity of pyrene, and of peri-fused polycyclic aromatic hydrocarbon molecules in general, to the polarity of the environment is a photophysic property that is extensively used to study host-guest interactions [40]. 

Figure 3.25 shows a simplified example of the absorption spectrum of an aromatic molecule such as the rigid, planar, cyclic hydrocarbons (e.g. benzene, naphthalene, etc.). The first absorption band shows a clear progression of vibrational sub-levels, but the higher absorption bands are broad and structureless this results from the very strong vibrational coupling between Si and S2, S2 and S3, etc. 

In Figure 29, the spectrum of an oil-sand sample shows the fundamental C-H peaks at 3.5 xm. From the two peaks in this region, one could determine the aromatic-aliphatic ratio of the hydrocarbons present in the sample. The fundamental water vibration is at approximately 3 xm (this peak would be substantially larger in a conventional emulsion sample), and the fundamental vibrations due to clays are at approximately 2.8 xm. The shape of the clay peaks indicates that kaolinite and a small amount of swelling clays such as bentonite are present in this sample. 

H-Nitrido-bis(triphenylphosphorus)(l +) ji-carbonyl-decarbonyl-ji-hy-drido-triosmate(l -), [(PPh3)2N][Os3(n-H)(n-CO)(CO)io] is obtained as a light red powder. The solid is air stable but solutions decompose in 5 min when exposed to the atmosphere. It is soluble in THF, dichlorome-thane, acetonitrile, methanol, and diethyl ether, and insoluble in hydrocarbon solvents. The IR spectrum of the compound contains four CO stretching vibrations for terminal carbonyls and one bridging carbonyl stretch (CH2CI2, cm- ) 2038 (w), 2021 (s), 1996 (s), 1951 (ms), and 1667 (w), respectively. The H NMR spectrum (80 MHz, chloroform-d, 8 in ppm downfield from TMS, ambient) shows a broad multiplet at 7.5, due to the protons attached to the aromatic rings in the cation, and a sharp metal hydride signal at -13.8. 

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... 

The vibrational spectra of arenium ion salts and respective hydrocarbons show great differences at 600-1600 cm" as well. This is not surprising since the addition of an aromatic molmile changes its atom symmetry and, hence, the vibrations active in the IR and the Raman spectra. Thus, when passing from benzene to the benzenium ion the symmetry d xeases from to C2,. In the IR spectrum of benzenium ion salts an intensive absorption band at 1595 cm > is appeared which is due to the skeleton C—C bond vibrations inactive in the IR spectrum of benzene but manifesting themselves in that of its derivatives. Other examples can be found in... 

Some typical results are shown in Table 1. Polymers are soluble in organic solvents such as benzene, toluene, THF and chlorinated hydrocarbons. The IR spectrum of these polymers displays characteristic absorption bands at 1640 cm (C=C stretching), 1600 and 1500 cm"l (phenyl ring vibrations), 1235 cm" (phenyl ether stretching) and 1015 cm (aliphatic ether stretching). The NMR spectrum of these polyethers recorded in CDCl at room temperature exhibits multiplets between 7.20 and 6.67 ppm (aromatic protons) and peaks at 6.02 ppm (-CH=CH-), at 4.50 ppm (-CH -0) and 1.60 ppm (-CH ). A small peak is observed at 4.05 ppm which can be attributed to the protons of the chloromethyl end groups. This peak is absent in the spectrum of sample 6 (Table 1). 

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