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.

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Ruta chalepensis L. is a tall perennial plant with woody base. It is one of the two Ruta species present in Saudi Arabia$). It is commonly known by the natives by the name "Shazab" using it in the skin diseases. In the writings of Dawoud El-Antaki, in his "Tazkaret Oli EI-Albab3) gave a detailed account of the drug 'in treating the skin diseases. From the free coumarin fraction of R. chalepensis we isolated by repeated preparative layer chromatography, the parent furocoumarin; psoralene'), bergapten", xanthotoxin'), isopimpenillin') and the two furocoumzrins which obtained from this species at first time; isoi~iperntorin~) and alloimperatorinl). From this previous studies, there was evidence for further constituent in the natural mixlure.The alcoholic extract of thc plant was saponified at room temperature, exlracted with ether, the aqueous layer was acidified with cooling, extracted with chloroform to give the free coumarin whicfi chromatographed on silica gel chromatography column, etution with 10% petroleuq ether in benzene solvent system removed a crystalline compound m.p. 78-80 idcntified as chalepensin which give a pale violete fluorescence under U.V. light and deep orange color after spraying with iodine/KI spraying reagent, R f 0.76 (benzene-ethyl acetate ; 9:l). The isolated compound dissolves in methanolic sodium hydroxide with yelIow color and is recovered unchanged upon acidification this behaviour indicates that the coinpound may be related to the coumarin group.The UV spectrum showing maxima at A,,, 214, 241, 248, 286 and 325 nrn (log E: 4.41, 4.43, 4.4, 4.1 and 3.9 respectively) concordant wilh a furocouniarin constitution. The 1R spectrum showiiig bands at 3000, 2850, 1735, 1640 and 1590. The mass spectrum exhibited fragmentations which are in agreement with chalepensin including M + (m/e 254) (100%) and ions at m/c 240, 239, 225, 211, 199, 183, 171, 165, 1.55, 128, 115, 77 and 41 resulting from M + by expulsions of CHI, CO and CHI respectively.The nmr spectrum of the isolated material contained all the signals expected to arise from chalepensin; with signals at 6 7.7-7.6 (3H), 7.40 (IH), 6.83 (IH), 6.5-6 (lH), 5.3-4.9 (ZH), 1.52 (48).Chdepensin was found to have molIuscicida1 activity2) at a concentration of 2 ppm. when tested against Agstralorbis glubratus. Further elution of the column with benzene removed a fraction which gave the typical reaction of coumarin. The identity of the compound present was confirmed by m. p. (67-68"), m. a. P.i