A ring-current model has been used to calculate the magnetic anisotropy of a cyclopropane ring. This method has proved to be convenient and rapid for estimating the shielding contribution of a cyclopropane ring to the total chemical shifts of neighboring protons with only the aid of molecular models. Shift contributions calculated on the basis of a ring-current model are applied to over 40 different compounds of known geometry.Cyclopropane rings possess a wide variety of unique chemical and physical properties as a result of bond angle deformation. It has been reported that cyclopropane has a molar susceptibility of -39.9 X 10~6,3 while the value predicted from Pascal's numbers is ca. -32 X 10'6. The enhanced suscepti-(1) This work was supported in part by the National Science Foundation and the Petroleum Research Fund, administered by the American Chemical Society.(2) (a) This investigation was supported in part by National Insti-

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Ultraviolet and infrared spectroscopy have been used to study the photochemistry of 1,3-cyclohexadiene (I), cis-l,3,5-hexatriene (II), and transA,3,5-hexatriene (III) suspended in argon matrices at 20°K. It was found that I photolyzed rapidly and irreversibly to II, which photolyzed slowly and irreversibly to III and several thermally unstable photo products. Various possibilities are considered for the latter and 1,2,4-hexatriene (IV) and exo-2vinylbicyclo[1.1.0]butane (XI) are suggested as likely candidates. Surprisingly, III was found to be photostable under these conditions. The photochemistry of 1,3-cyclohexadiene (I) and the valency isomeric cis-1,3,5-hexatriene (II) and Zranx-1,3,5-hexatriene (III) has been the subject of considerable interest.3 Vapor-phase photolysis of I or mixtures of II and III were reported to yield hydrogen, a mixture of C2 hydrocarbons, 1,2,4-hexatriene (IV), benzene, and polymer.4 In addition I, II, and

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