difference between the two transition states is substantially lower than that between I and the first transition state for ionization. On the other hand, a single-maximum potential energy diagram is sufficient for nonpolar solvents. ConclusionThe complementary evidences gained from this spectral, kinetic, and equilibrium study lead to the conclusions that (a) the colored transients from indolinospiropyrans are zwitterionic; (b) these colored transients exist in solution as an equilibrium distribution distribution of steroisomers; (c) the dominant isomer in nonpolar solvents is a planar irois-monomeric species, while that in polar and hydroxylic media is a dimeric and/or associated species. It is shown that the effect of solvent on the visible absorption spectra of the transients from indolinospiropyrans parallels that for the ionic cyanine dyes.20 This with the kinetic and equilibrium data presented indicate that contributions from quinoid species are probably minimal rather than 5671 dominant, as was previously thought17 from spectral comparisons with the nonionic merocyanines.An intriguing, but as yet unexplained feature of this work is the extremes of spectral and kinetic behavior exhibited by the indolinospiropyrans and the simple pyran and benzopyran.46 In the latter cases the transient photoproduct appears to be a molecular species. In contrast, the action of the indolino nucleus in conferring definite zwitterionic character to the transient photoproduct from spiropyrans is dramatic. This apparent ability of the indolino substituent to stabilize ionic structures is of continuing research interest in this laboratory.

Abstract-The decomposition reactions of [CaH50]+ ions produced by dissociative electron-impact ionization of 2-propanol have been studied, using 13C and deuterium labeling coupled with metastable intensity studies. In addition, the fragmentation reactions following protonation of appropriately labeled acetaldehydes and ethylene oxides with [H,]+ or [D3]+ have been investigated. In both studies particular attention has been paid to the reactions leading to [CHOlf, [C,H31+ and [H&l+. In both the electron-impact-induced reactions and the chemical ionization systems the fragmentation of [C2H50]+ to both [H,O]+ and [C,H,]+ proceeds by a single mechanism. For each case the reaction involves a mechanism in which the hydrogen originally bonded to oxygen is retained in the oxygen containing fragment while the four hydrogens originally bonded to carbon become indistinguishable.The fragmentation of [C2H50]+ to produce [CHO]+ proceeds by a number of mechanisms. The lowest energy route involves complete retention of the cc carbon and hydrogen while a higher energy route proceeds by a mechanism in which the carbons and the attached hydrogens become indistinguishable. A third distinct mechanism, observed in the electron-impact spectra only, proceeds with retention of the hydroxylic hydrogen in the product ion. A more general study of the electron-impact mass spectrum of 2-propanol has been made and the fragmentation reactions proceeding from the molecular ion have been identified. THE [C2H50]+ ion is prominent in the mass spectra of many classes of oxygen containing molecules including alcohols, ethers and esters, and a knowledge of the possible structures and mechanisms of fragmentation of [C2H,0]f is of considerable importance in attaining a greater fundamental understanding of the electron-impactinduced fragmentation of oxygen containing molecules.Several structures are clearly distinguishable on the basis of their enthalpies of formation and these are summarized in Table 1. Although several additional structures can be written there are no reliable thermochemical data to characterize them. Appearance potential r n e a s~r e m e n t s~~~~~ indicate that ions of structure a [CH,CH=OH] are formed from 2-alkanols (RCH(CH,)OH) at the appearance potential threshold, while isotopic labeling experiments in 2-propanoP and 2-butano15 indicate that the alkyl group R is lost without prior rearrangement of the carbons or hydrogens. Ions of structure c [CH,O+=CH,] are formed from alkyl methyl ethers, as indicated both by appearance potential measurements2*a and isotopic labeling experiments.2 In contrast, the thermochemical datum for structure b is derived from a study' of

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The exchange reaction between CH3+ and D2 leading to CH2D+ and CHD2+ is shown to proceed through a collision complex in which the hydrogens and deuteriums become equivalent. The rate coefficient for reaction of CH3+ with D2 is 6.1 ± 0.1 × 10−10 cm3 molecule−1 s−1.

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