2234
NOTES
On the Competition between Unimolecular Dissociation and Ion-Molecule Reaction of
36
&-%Butene Molecular Ions 32
by Stephen J. Wisniewski, Roger P. Clow, and Jean H. FutrelllaIb 28
Department of Chemistry, University of Utah, Salt Lake City, Utah 8.411) (Received December 89, 1969)
24
A number of recent papers have been concerned with 0 0 ion-molecule reactions of unsaturated X 1 A frequent observation for these systems is an apparent -.--w 2 0 inhibition period before a substantial amount of reaction of the molecular ion is reflected through a decrease in the intensity of that ion; occasionally, there is even an increase in the intensity of the ion such that it goes through a maximum as a function of pressure of the 12 neutral species in the mass spectrometer ion ~ o u r c e . ~ ~ * ~ ~ Generally, it has been possible to account for these features by invoking an essentially charge exchange 08 reaction of certain fragment ions from these systems with the neutral molecule. I n some cases it has been demonstrated unequivocally through studies involving a tandem mass spectrometer that this is the correct interpretation2 One recent report of ion-molecule reactions in the isomeric butene@ demonstrated, however, that the initial increase of the parent ion, C4Hs+, 20 40 60 80 io0 SOURCE PRESSURE (MICRONS) in cis-2-butene is much greater than could be accounted for by charge exchange. Both cis- and trans-Qbutene Figure 1. Pressure dependence of major ions in t h e mass reflect a very large increase in intensity of the parent spectrum of cis-2-butene. High-pressure study in conventional mass spectrometer, d a t a of ref 2c. ion; in the cis isomer, it increases by about 17% of total ionization, This behavior is illustrated in Figure 1, which shows the dependence of ion intensity for the tions6 both suggest that long-lived vibronically excited major ions of cis-butene on pressure in the ion source states are anticipated as a general feature for unsatuas observed in a high-pressure time-of-flight mass rated systems. If this explanation is correct, however, spectrometer.2C it implies relatively specialized conditions for the comThese workers postulated that the initial increase in petition t o be observed in the case of the cis isomer and parent cis-2-butene occurred via collisional stabilization not for other butenes. Specifically, it calls for a fairly of excited parent ions, reflecting the competition beclose match between the rates of the unimolecular tween the primary decomposition reaction C4H8+”-+ fragments
(1)
and collisional stabilization of vibronically excited molecular ions C&+”
+ CdH, +C4H.9’’ + C4Hs
(2)
It is likely that this effect is particularly pronounced for the cis-2-butene molecular ion because the cross section for subsequent ion-molecule reactions consuming C4Hs+is somewhat lower than for molecular ions from the other butene isomers. Figure 1 illustrates the pressure dependence of the major ions, as observed in the high-pressure mass spectrometer. The observation for the butene isomers of intense metastable ions2c and theoretical consideraT h e JOUTTU~ of Physical Chemistry
(1) (a) Alfred P. Sloan Fellow, 1968-1970. (h) This investigation was supported in part hy a Public Health Service Research Career Development Award, 1 KO4 GM42390-01 GMK, from the National Institute of General Medical Sciences. (2) (a) J. H. Futrell and T. 0. Tiernan, J. Phys. Chem., 72, 158 (1968); (b) T. 0. Tiernan and J. H. Futrell, ibid., 72, 3080 (1968); (c) F. P. Ahramson and J. H. Futrell, ibid., 72, 1994 (1968); (d) B. M. Hughes, T. 0. Tiernan, and J. H. Futrell, ibid., 73, 829 (1969). (3) A. G. Harrison, Can. J . Chem., 41, 236 (1962); A. A. Herod and A. G. Harrison, J. Phys. Chem., 73, 3189 (1969); J. J . Myher and A. G. Harrison, Can. J. Chem., 46, 101 (1968); J. J. Myher and A. G. Harrison, J . Phys. Chem., 72, 1905 (1968). (4) (a) V. Aquilanti, A. Galli, A. GiardinrGuidoni, and G. G. Volpi, Xrana. Faraday Soc., 63, 926 (1967); (b) S. Wexler, A. Lifshits, and A. Quattrochi, Advances in Chemistry Series, No. 58, American Chemical society, Washington, D . C., 1966, p 193. (5) J. L. Beauchamp, L. R. Andrews, and J. D. Baldeschwieler, J. Amer. Chem. Soc., 89,4569 (1967). (6) M. Vestal, personal communication.
2235
NOTES dissociation (reaction 1) and the collision frequency of the stabilization reaction 2 under the conditions of the experiment illustrated in Figure 1. Consequently, one would not expect that the same type of behavior-observed in at least two high pressure mass spectrometer experiments-would also maintain, if one could change the time scale of the apparatus significantly.
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