Aromatic compounds spectroscopic properties

The spectroscopic properties of meso-ionic compounds have been discussed in detail elsewhere and the reviewers do not feel that it would be useful to include a comprehensive account here. Ultraviolet, infrared, and nuclear magnetic resonance spectra of meso-ionic heterocycles provide general support for the conjugative interaction that would be expected for aromatic heterocycles, " but detailed interpretation of their spectra is not justifiable. Mass spectrometry has been shown to be particularly useful for distinguishing between pairs of meso-ionic isomers... 

A. NOMENCLATURE AND SELECTED PHYSICAL AND SPECTROSCOPIC PROPERTIES OF POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) AND POLYCYCLIC AROMATIC COMPOUNDS (PACs)... 

Aromatic compounds. These compounds exhibit characteristic absorption in the ultraviolet-visible region of the spectrum, and although they are frequently easily recognised from their other spectroscopic properties, examination of their electronic spectra can often lead to the elucidation or confirmation of some of the detailed structural features. 

The methods of preparation which are considered in this chapter involve, principally, ring-forming reactions leading to a restricted and subjective selection of various typical and interesting heterocyclic systems. These cyclic systems may be classified as (a) saturated or partly unsaturated heterocyclic, or (b) heteroaromatic, according to their physical, chemical and spectroscopic properties and to the reactivity of the functional groups attached to the ring atoms. In this way such properties and reactivity are broadly related to those of alicyclic and aromatic compounds. 

The influence of solubilization on solution properties may vary from no appreciable effects to very marked ones. We may take solutions of CTAB as an illustrative example of the manifold of possibilities. Here solubilization of cyclohexane has a very small influence on a variety of rheological and spectroscopic properties while addition of aromatic compounds and long-chain alcohols may cause extensive changes. 

To understand the photoresponsive properties of azobenzene and its molecular family, it is necessary to discuss their spectroscopy and the mechanistic options of isomerization. A review of the spectroscopic properties of azo compounds appeared in 1973. The isomerization properties of azobenzene were reviewed for several periods. Wyman covered the literature up to 1954, Ross and Blanc up to 1969, and Rau up to 1988. The pre nt standalone review is restricted to the spectroscopy and isomerization of simple aromatic azo compounds. It is meant to serve as a basis for the detailed treatments of complex photoresponsive systems in the following chapters of this monograph. 

More work with other aromatic compounds will be necessary to verify this conclusion. However, the results for anthracene certainly suggest that the results obtained in the reaction are strongly dependent upon the aromatic substrate. The results obtained thus far also suggest that the reduction and subsequent alkylation of large polycyclic molecules, e.g., compounds such as pyrene, are not the most critical factors for the formation of soluble alkylation products. Studies of the molecular weight distributions and spectroscopic properties of the alkylation products obtained with different metals and different electron transfer agents are underway to resolve this issue. 

Another electronic property associated with aromaticity is magnetic susceptibility, which is determined by measuring the force exerted on the sample by a magnetic field. Magnetic susceptibility is closely related to polarizability and is different in the plane and perpendicular to the plane of the ring. It can be determined by various spectroscopic measurements, as well as by using an NMR spectrometer. It is observed that aromatic compounds have enhanced magnetic susceptibility, called exaltation (A), relative to values predicted on the basis of the localized structural components. 

Spectroscopic Properties. The phosphorescence quantum yields of the initiators at 77 K in 2-propanol are compared in Table IV. These were obtained upon excitation at the absorption maximum of 290 nm for all the compounds. Both the absorption and phosphorescence spectra of the initiators were similar to those of benzophenone. The quantum yields on the otherhand were variable. The phosphorescence emission spectra of all the compounds had maxima at 414, 443 and 474 nm respectively. No fluorescence was observed which is typical of aromatic ketones indicating a high rate of intersystem crossing. 

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