Aromatic hydrocarbons absorption spectrum

Lavalette, D. Polarized excitation spectrum of the triplet-triplet absorption of aromatic hydrocarbons. Chem. Phys. Letters 3, 67 (A969). 

Unlike methane and the other alkanes, aromatic hydrocarbons have absorptions in the UV part of the spectrum, and thus may be detected through UV spectrometry using silica fibers. This scheme is useful for "aromatic" water pollutants such as toluenes and xylenes with their absorption bands between 250 and 300 nm. Similarly, nitrate anion can be monitored (albeit with low sensitivity) in water via its UV absorption at 250 nm. 

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. 

Direct evidence for photoassociation of aromatic hydrocarbons in solution is afforded by the appearance of a structureless emission band, at longer wavelengths than the molecular fluorescence spectrum, as the solute concentration is increased the molecular fluorescence undergoes a corresponding reduction in intensity as shown in Figure 1. The absence of permanent chemical change is confirmed by the invariance of the absorption spectrum under these conditions and the restoration of the molecular emission spectrum on dilution. 

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

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-... 

Tris[bis(trimethylsilyl)amido]uranium is an extremely air- and moisture-sensitive, red-purple solid, which can be stored for months in the absence of air and moisture without noticeable signs of decomposition. It is soluble in both aliphatic and aromatic hydrocarbons. The NMR spectrum in benzene- (250 MHz, 20°C) shows a single broad resonance at d — 11.5.11 The compound sublimes readily at 80°C under good vacuum (10 6 torr). The checkers report a melting point of 137-140°C. The IR spectrum, recorded as a Nujol mull between KBr plates, has absorptions at 1248(s), 1170(w), 990(s), 860(s), 828(s), 764(m), 676(m), 654(m), and 598(m) cm1. Other physicochemical properties are described in the literature.4... 

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. 

The narrow fluorine resonance of "Ci BF " is quite in contrast to the fluorine absorption found for the product of Illinois 6 coal with BF3. At room temperature, we observe a 0.25 mT (=2.5 G) wide, dipolar-broadened, spectrum not indicative of translation freedom. In contrast to the weakly bound complexes of BF3 with aromatic hydrocarbons, we anticipate BF3 to react strongly with oxygen functionality in the coal, through hydration with water, hydrolysis with acids (13), and ether complex formation (14), to give fluorine absorption lines which are in the rigid lattice condition. 

Substitution in aromatic hydrocarbon naturally shifts the wavelength of fluorescence in agreement with the effect of the same substitution on the absorption spectrum. Alkyl substitution has little effect. Chlorine and bromine weaken the florescence and iodine completely inhibits it. The simplest aromatic heterocyclics, pyridine, pyrrole, furan and thiophene do not show fluorescence. 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

The triplet-triplet absorption spectrum for C o is shown in Figure 1. The extinction coefficient at 480 nm (<-r - 6s ), 2.4 X 10 M" cm", was estimated by the method of Bensasson and Land by comparison with the T-T absorption of acridine. The triplet lifetime under our experimental conditions is 40 4 ps. The triplet state of Cjo is efficiently quenched by O2 in air-saturated C5H4, the lifetime is 330 25 ns. This yields a quenching rate constant by oxygen of kJ02) = 2 X 10 M" s", which is typical for aromatic hydrocarbons. 

It is generally assumed that closed-shell molecules do not interact strongly with each other. However, as early as 1909, it was observed that new intense absorption bands were observed when I2 was dissolved in an aromatic hydrocarbon. By the mid-twentieth century the concept of nonbonded charge transfer complexes was postulated to explain the intense new absorption spectrum that arose when a closed-shell donor was added to a closed-shell acceptor. The theory of such complexes was formulated in terms of the electron affinity of the acceptor and the ionization potential of the donor and led to the development of new techniques for the determination of properties of such complexes. 

It is known that a new component appears in the emission spectrum of many aromatic hydrocarbons with increasing concentration [147]. This new fluorescent component is ascribed to excimers which are complexes of an electronically excited molecule with an identical molecule in the ground state. In these complexes molecules are parallel at a distance of about 0.3 nm. Excimers only exist in the excited state after de-excitation the two partners repel each other. Therefore no corresponding change is observed in the absorption spectrum. The kinetic scheme just presented must be complemented by the following steps ... 

Identification of the Fluorescent Species. Figure 2 compares the fluorescence excitation spectra of the polymers with the absorption spectrum of a simple ,/3-unsaturated carbonyl compound (pent-3-ene-2-one) (13). The three spectra are very similar. Figure 2 shows also that the fluorescence from the polymers in the region 300-400 nm cannot be caused by the presence of polynuclear aromatic hydrocarbons such as naphthalene as postulated earlier by Carlsson and Wiles (13). Furthermore, as shown below, the excitation spectrum also differs significantly from that of a fully saturated aldehyde or ketone. 

Both the Forster and the Dexter energy transfer mechanisms require spectral overlap of the donor emission spectrum and the acceptor absorption spectrum. However, energy transfer is known to occur even in the absence of spectral overlap, resulting in effective quenching of excited states. As an example, we can cite the quenching of the fluorescence of aromatic hydrocarbons by dienes, a process which involves thermal deactivation of an excited state encounter complex, or exciplex, between D and A (Eq. (3.7)) ... 

Monoclinic plates from alcohol, d2 1.179 mp 100 bp 340. Sublimes in high vacuum. Absorption spectrum Clar, Lomberdi, Ber. 65, 1412 (1932) Mayneord, Roe, Proc. Roy, Soc. London A 152, 317 (1935), Practically insol in water sol in organic solvents, especially In aromatic hydrocarbons. One gram dissolves in 60 ml cold, 10 ml boiling 95% alcohol, 25 ml abs alcohol, 2.4 ml toluene or carbon tetrachloride, 2 ml benzene, 1 ml carbon disulfide, 3.3 ml anhydr ether. Soluble in glacial acetic acid. Solus exhibit a... 

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