The electronic absorption spectra of coumarin and its derivatives umbelliferone (6-hydroxycoumarin), esculetin (6,7-dihydroxycoumarin), and scopoletin (6-methoxy-7-hydroxycoumarin) were investigated. The n → π* transition was not observed in the spectrum of coumarin but was observed in the spectra of some of its derivatives. Molecular orbital calculations, using the INDO procedures, were carried out on coumarin and some of its derivatives. The activities of the different sites of the molecules are discussed in terms of the coefficients of the atomic orbitals constituting the HOMO and the LUMO. The correspondence between the calculated and experimental transition energy and band intensity is satisfactory.

The charge transfer complexes of 2,2′-bithienyl, 5,5′-dimethyl-2,2′-bithienyl, and some thiophene derivatives as donors with tetracyanoethylene as an acceptor have been studied spectrophotometrically. In addition, complexes of the first donor with chloranil and iodine have been studied. The formation constants and extinction coefficients of the molecular complexes formed have been determined by graphical as well as iterative methods. From measurements at different temperatures, the thermodynamic functions of the complex formation have been calculated. The ionization potentials of the donors have been obtained from the energies of the charge transfer transitions.

ABSTRACT:The thermal decomposition of formyl, acetyl, and benzoyl azides to the corresponding isocyanate and nitrogen has been treated theoretically using ab initio molecular orbital calculations at the Møller-Plesset type 2 (MP2)(full)/6-31G* level. The reaction is stimulated by elongation of NON bond and is followed until the formation of the isocyanate and expulsion of nitrogen. The decomposition of formyl azide proved to be a concerted one-step reaction without the formation of a nitrene intermediate. In contrast, the conversion of both acetyl and benzoyl azides to the corresponding isocyanate and nitrogen is a two-step reaction, and a nitrene intermediate is formed. One transition state is located and identified during the course of the conversion of formyl azide, but two transition states are located and identified during the course of the conversion of acetyl and benzoyl azides. The thermodynamic functions, ⌬E r and ⌬H r , of the studied reactions are calculated. The results predict that the ease of conversion of the acyl azide to the isocyanate and nitrogen goes in the order: formyl azide Ͼ acetyl azide Ͼ benzoyl azide.

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The electronic absorption spectra of 2,2′-bithienyl and a number of its derivatives have been investigated. The predominant conformer as well as the polarity of the molecule were predicted from the spectrum. Molecular orbital calculations were performed on different conformers of 5-formyl-2,2′-bithienyl and the s-trans geometry proved to be the predominant conformer of the molecule. A good agreement is observed between calculated and observed transition energy and intensity.