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Proton magnetic resonance spectra of some norbornane derivatives having double bonds and (or) a benzene ring in the skeleton were studied. Differences in the chemical shifts between the bridge methylenes of norbornene and benzonorbornene and that of norbornane result from the diamagnetic anisotropy effect of a double bond or a benzene ring being in good agreement with the values calculated theoretically. However, in the norbornadiene derivatives which have two double bonds or one olefinic double bond with a benzene ring in a part of the skeleton, the signals of their bridge methylenes show extraordinarily larger downfield shifts by about 0.8 p.p.m. than those expected from the additive shielding due t o each T-electron system. This anomaly is not observed in bicyclo[2.2.2]octane ring series. A large transannular interaction between two a-electron systems in that system would be a major contribution to the above anomaly. , I N T R O D U C T I O SI11 the past few years a number of proton magnetic resonance (n.1n.r.) spectra of bridged ring systems have been investigated, reporting the relations between the spin coupling constants of ring protons and the stereochemistry (1-lo), various types.of long-range spill couplings (3, 5 , 6, 8-11), the establishment of the configuration of substituents by the evaluation of diamagnetic shielding effects of a double bond or a benzene ring (9, 12-15), and so forth (16)(17)(18)(19). However, the substantial features of n.m.r. spsctra of bicyclo[2.2.l]heptane derivatives have not received much attention. By the studies of various ~nonohydroxyl derivatives of bicyclo[2.2.l]heptane1 RIusher (8) has recently reported that (i) a striliitlgly large difference in the magnetic shielding between an exo and an endo proton was obserr~ed, showing larger deshielding on the exo proton, and this difference can not be explained only by the diamagnetic anisotropy of the C-C bonds in the ring; that (ii) the coupling constants between each pair of adjacent protons, JHx-Hx and JIIn-H,*, are not equal; and that (iii) fairly large long-range spin coupling constants were found. On the basis of these results, he suggested the presence of some n-characters in this ring system. I11 a previous paper (20), we also suggested the presence of this character on the basis of the increased s-character of the C-H bonds, which was reflected in the strikingly large values of carbon-hydrogen spin coupling constants J G~~-H obtained from the C13-satellites (21)

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