and Kates119] based on the deuterium isotopic perturbation effect. An additional factor that supports this conclusion can be found by analyzing further the CH stretching region. In the hypothetical classical 2-norbornyl cation, the hyperconjugative interaction of the C ( 3 j H bonds with the formally empty p orbital at C(2) should exert a significant stabilizing effect. This would lead to a weakening of the C(3)-H bonds, which should be reflected in a sizeable decrease in the frequencies of the corresponding CH stretching modes. Such a shift was recently demonstrated for the I-methylcyclopentyl cation,[51 where strong hyperconjugative interaction of the C(2)-H bonds with the cationic center at C(l) caused a decrease in the frequency of the C(2)H stretch of about 220 cm-* to 2775 cm-', in perfect agreement with accurate calculations for the parent cyclopentyl cation. [20] In a delocalized, bridged structure ascribed to the 2-norbornyl cation, such an interaction should be small and the only bonds that could contribute to a hyperconjugative stabilization are C(3)-H and C(7 j H . The normal mode analysis of the computed spectrum shows that the modes corresponding to the two lowest CH stretching modes, computed at 2991 and 2987 cm-', indeed involve hydrogens at C(3) and C(7). The absence of a significant hyperconjugative decrease in the stretching frequencies is additional, albeit negative, evidence for the bridged structure of the cation. The same conclusion was reached some twenty years ago[''] on the basis of secondary kinetic isotope effect measurements in solvolysis of 2-norbornyl brosylates.[221