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of these two halotoluene ions. For m-iodotoluene ions, the dissociation channel of lowest activation energy is direct cleavage of iodine atom to yield w-tropyl product ions. For m-bromotoluene ions, at least one rearrangement channel has lower activation energy than direct cleavage, and a mixture of benzyl and tropylium product ions results from traversing this rearrangement pathway.As a secondary conclusion, we put high confidence in the thermochemistry displayed in Table I and in Schemes I and II (except that the benzyl ion's heat of formation still seems rather uncertain). In particular, the value given for the m-tropyl ion is worthy of respect: All three tropyl ion values are derived by plausible analogy with the accurately known halobenzene ion dissociation thermochemistry and should be reasonable estimates, but the w-tropyl ion in particular has its heat of formation accurately confirmed by the photodissociation threshold value reported here for m-iodotoluene ion.Hanratty, Paulsen, and Beauchamp19 have pointed out in a particularly graphic way the different roles played by the disso-

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