Aromatic absorption spectra

Figure 4 Aromatic absorption spectra of tryptophan, tyrosine and phenyiaianine. Note the different concentrations required to ensure absorbance maxima of 1 absorbance unit.

This characteristic is used to analyze aromatics in gas oil cuts an example of a UV absorption spectrum is shown in Figure 3.5. 

The infrared spectrum of the l-methoxy-l,4-cyclohexadiene shows the absence of strong aromatic absorption at 1600 cm.the ultraviolet spectrum shows absence of absorption at 270 nm., indicating absence of the conjugated isomer. 

Whereas the intermediate existence of polarons has been unequivocally proved by ESR measurements and optical absorption data, up to now, the existent of bipolarons has been only indirectly deduced from the absence of the ESR signal and the disappearance of the visible polaron bands from the optical absorption spectrum On the other hand, spinfree — diionic-charge — states in aromatics, whose optical properties bear a remarkably resemblence to the predictions of the bipolaron model, have long been known Further evidence of bipolarons is the fact that doped... 

Weber G, Shinitzky M (1970) Failure of energy transfer between identical aromatic molecules on excitation at the red edge of the absorption spectrum. Proc Natl Acad Sci USA 65 823-830... 

The pyrene-like aromatic chromophore of BaPDE is characterized by a prominent and characteristic absorption spectrum in the A 310-360 nm spectral region, and a fluorescence emission in the X 370-460 nm range. These properties are sensitive to the local microenvironment of the pyrenyl chromophore, and spectroscopic techniques are thus useful in studies of the structures of the DNA adducts and in monitoring the reaction pathways of BaPDE and its hydrolysis products in DNA solutions. 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

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. 

Weber G. and Shinitsky M. (1970) Failure of Energy Transfer between Identical Aromatic Molecules on Excitation at the Long Wave Edge of the Absorption Spectrum, Proc. Nat. Acad. Sci. USA 65, 823-830. 

The method was first applied by Rothman, Case, and Kearns to the determination of the Ti- -So absorption spectrum of 1-bromonaphthalene. Sixteen photochemically active aromatic ketones cind aldehydes have been investigated by Kearns and Case Transitions from So to two triplet states were located and assigned as n,n ) and... 

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

The red shift of the absorption spectrum is an unambiguous proof that the bases are being attacked since the absorption spectrum of DNA arises from its aromatic bases. 

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. 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

With an interplanar separation of 3.73 A, 4,4 -paracyclophane is the lowest member of the series to exhibit an alkylbenzene absorption spectrum and the broad structureless fluorescence spectrum of this molecule with a peak intensity at 3400 A is by definition an excimer band further separation of the aromatic rings in 4,5 and 6,6 -paracyclophanes restores the fluorescence spectrum to that of the alkylbenzenes. These observations by Rice et al.115 illustrate the critical nature of the interplanar separation in determining the extent of interaction between -electron systems in the ground and excited configurations. 

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. 

In the model compounds, this red shift has been ascribed to a combination of silicon backbone with the x-orbitals of the aromatic substituent coupled with a decrease in the LUMO energy due to x -(a, d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. 

The XH NMR spectrum of complex 2 in D20 shows three resonances in the aromatic region. The singlet at 8.94, and two doublets centered at 7.93 and 7.73 ppm (Table I), with respect to TMS, are assigned to C-3H, C-6H, and C-5H, respectively (Fig. 1). Table II presents the absorption and emission spectral properties of complex 2.8 The absorption spectrum of this complex... 

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