poster at the Symposium des Arbeitskreises (' : *) proportions. Furthermore, the kinetic energy KSAM der GDCh-Fachgruppe ,,Analytische Chemie" in Martinsried (Germany), March 1981.[2] Syntex P2, four-circle diffractometer with modified LT-I cooling unit, Mo,,,, 20,,,, =60", 762 reflections, of which 722 observed (Fo>3uF), program system EXTL, R , =0.026.[3] The crystal structure of [(HOC2H,),NH]+SH-(unpublished) shows the release, T, for the I3CH3-and "CH3-eliminations, would be expected to be closely similar. If, however, the internal energy of (2,* is distributed non-statistically ciation (i. e. the behavior is non-ergodic), then-despite the equivalence Of both methyl group-the two I3CH3 and to same geometry for the singly protonated molecule. One of the central questions in reaction dynamics today is whether highly excited molecules behave ergodically['], i. e. whether unimolecular dissociations are slower than intramolecular vibrational energy redistribution. It has been shown for neutral molecules[21 that energy randomization is completed within a few picoseconds and that molecules behave non-ergodically only under special conditions['"]. If similar non-ergodic behavior were found in ionic systems, then one of the basic assumptions of the quasi-equilibrium theory ( Q E P of mass spectra'31 would be violated.Evidence for the non-statistical behavior of the enol radical cation of acetone ( I ) , through measurements of kinetic energy release distributions T of the dissociation fragments, has recently appearedl4]. In that study and in previous ones[51, [2H]-labeled isotopomers of (1) were employed and there was the ever present possibility of kinetic isotope effects. In order to circumvent this problem, we have now prepared the [13C]-labeled isotopomers (la) and (lb). We report here on the unimolecular dissociation of this radical cation in the gas phase and present evidence indicating that this system is characterized by non-ergodic behavior.The study of ['HI-labeled ions (l), strengthened by thermochemical has shown that the CH3-cleavage from metastable ions (1) (lifetimes = lo-' s) does not take place by direct a-cleavage ((1)#(3) + CH3), but by dissociation to ion (5) following a rate determining [1,3]-Hshift ((1)-(4)+(2)) ( Fig. 1). A highly excited intermediate acetone radical cation (2)* is formed, whose lifetime has been estimated to be 5 x If (2)" were to behave ergodically, one would expect, on the basis of the chemical equivalence of the two methyl groups, that the two specifically ['3C]-labeled ions (la) and ( l b ) would eliminate the radicals I3CH; and "CH; in d4].