3600
For other trajectories, the I3CH3+ strips a C from the CzH4, and only dissociations to the left are allowed. This model envisions a potential surface of two reaction paths with very similar scrambling and energy partioning features. It is also similar to model IV in using reaction 2 data for the theory rather than using the theory to fit the reaction 2 data. The closeness of the original model I fit to the reaction 2 data, however, means the revised model outlined here has no significant effect on the predicted cross sections for reactions 3 and 5. Of the two model I mechanisms, the revised version just presented rests upon the more reasonable assumptions. The existence of two mechanisms with similar net results seems more likely than the reversal of C atom momenta with each proton jump. Yet, without the experimental kinematic information, the original static version of model I would have been acceptable and simpler. This reaction thus illustrates the importance of examining the experimental reaction dynamics when considering simplified models for a reaction, and warns of the danger of viewing a reactive event in terms of static intermediates.
V. Conclusions 1. The reaction takes place via a direct mechanism, although considerable C and H scrambling occurs. All ionic products are predominantly formed forward of the center of mass. 2. In our energy range, the isotopic scrambling can be accounted for by a simple model involving linear intermediates. 3 . Despite any scrambling processes, or the likelihood of two separate mechanisms, the kinematics of all products are similar. This behavior is more clearly illustrated by reaction 1 than the previously studied methyl cation-methane reaction,” but may prove typical for many reactions in which scrambling occurs. Experiments on other carbonium ion reactions are in progress. In addition, theoretical trajectory
studies of this or similar reactions,20 while difficult, might clarify the puzzling behavior of this system.
Acknowledgment. Research support from the National Aeronautics and Space ’ Administration is gratefully acknowledged. References and Notes (1) M. Saunders, P. VoQel, E. L. Hagen, and J. Rosenfeld, Acc. Chem. Res., 6, 53 (1973). (2) J. T. Eursey, M. M. Bursey, and D. G. I. Kingston, Chem. Rev., 73, 191 f19731 -. (3) 2.Herman, A. Lee, and R. Wolfgang, J. Chem. Phys., 51, 452 (1969);2. Herman, P. Hierl, A. Lee, and R. Wolfgang, ibid., 51, 454 (1969); A. Ding, A. Henglein, and K. Lacmann, 2. Naturforsch., Teil A, 23, 2084 (1968). (4) W. T. Huntress, Jr., J. Chem. Phys., 56, 5111 (1972);T. 0. Tiernan and J. H. Futrell, J. Phys. Chem. 72, 3080 (1968). (5) J. Weiner, A. Lee, and R. Wolfgang, Chem. Phys. Lett., 13, 613 (1972). (6) J. Weiner, G. P. K. Smith, M. Saunders. and R. J. Cross, Jr., J. Am. Chem. Soc., 95, 4115 (1973). (7) P. C. Hariharan, L. Radom. J. A. Pople, and P. v. R. Schleyer, J. Am. Chem. SOC., 96, 599 (1974): L. Radom, J. A. Pople, V. Buss. and P. v. R. Schleyer, ibid., 94, 31 1 (1972). (8) Z. Herman, J. D. Kerstetter. T. L. Rose, and R. Wolfgang, Rev. Sci. Instrum., 40, 538 (1969). (9) S.G. Lias, R. E. Rebbert, and P. Ausloos, J. Am Soc., 92, 22 (1970). (10) D. J. McAdoo, F. W. McLaffnrty, and P. F. Eente Ill, J. Am. Chem. SOC., 94, 6 (1972). (11) J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron. K. Draxl. and F. H. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26 (1969). (12) F. P. Lossing and G. P. Semeluk, Can. J. Chem., 48, 955 (1970). 634 (1972). (13) S.-L. Chong and J. L. Franklin, J. Am. Chem. SOC., \/ys.Energ., 7, (14) N. V. Kir’yakov, M. I. Markin, and V. L. Tal’roze, 94 (1973): 178 (1973). (15) R. Wolfgang and R. J. Cross Jr., J. Phys. Chem., 73, 745 ,7969). (16) W. B. Miller, S. A. Safron, and D. R. Herschbach. Discuss. Faraday Soc., 44, 108 (1967). 17) R. C. Lord and PI Venkateswarlu, J. Opt. Soc. Am., 43, 1079 (1953). 9
,
- I .
18) The rotational period can be calculated from the angular momentum, estimated by L = fibg where is the reduced mass of the colliding reactants, g is the relative collisional velocity, and b Is the maximum impact parameter leading to reaction, estimated from the cross section by b = (u/z)’”. Then 7 = 2al/L where IS= %lrb2. 19) Consider the reaction of ref 5, CH3+ CH4 C2X5+ XZ, in which the majority of all isotopic ion products are forward scattered. This occurs regardless of the end of the C2H7+ complex from which the X2 dissociates. (20) R. Hoffmann, D. M. Hayess, and P. S. Skell, J. Phys. Chem., 76, 664 (1972), have done some calculations of the isoelectronic CHP CZH4 surface.
+
-
+
+
Gaseous Ionic Acetylation of Cresols Dale A. Chatfield’ and Maurice M. Bursey** Contribution from the Venable and Kenan Chemical Laboratories, The University of North Carolina, Chapel Hill, North Carolina 27514. Received October 4 , 1974
Abstract: Rates of the ion-molecule reaction of 0 - , m-, and p-cresol with CH3COCO(COCH3)CH3+, CH3CO(COCHj)CH3+, and CH3CO+ were measured by ion cyclotron resonance and compared with calculated results. The relative insensitivity of the rate to the choice of neutral isomer was duplicated, but only modest agreement was found on examining sensitivity to the choice of ionic acetylating species.
reaction rates to very complex organic reactions, we have There have been several approaches to the study of reacnow studied the rates of acetylation of the three isomeric tions resembling electrophilic aromatic substitutions in the cresols using two different precursors of the acetyl ion, acegas phase. One approach has been through the ion cyclotron tone and biacetyl. Conventional ion cyclotron resonance resonance t e ~ h n i q u e . ~The - ~ other has been through high(ICR) techniques were used. energy ion chemistry.6-8 In addition, the acylation of oxygen-containing compounds has been studied by ion cyclotron resonance with the use of a variety of p r e c ~ r s o r s . ~ , ~ ~ Experimental Section In connection with these studies, and in a n attempt to exA Varian V-5900 ICR spectrometer was used for all experiplore the applicability of accepted theories of ion-molecule ments. It was equipped with a standard (1.27 X 2.54 X 14 cm) Journal of’the American Chemical Society
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June 25, 1975
3601 three-section cell, grid modulation, split drift voltages, and a 20 l/s Noble Ion pump. Pulsed double resonance experiments were conducted while observing the usual precautions of low emission current (
