PERSISTENT ION-MOLECULAR COLLISION COMPLEXES OF ALKYL HALIDES
June, 1959
877
PERSISTENT ION-MOLECULE COLLISION COMPLEXES OF ALKYL HALIDES1 BY ROSWELL F. POTTIE A N D WILLLAM H. HAM ILL^ Received Februaru 11, 1969
The first examples of persistent collision complexes between an ion and a molecule have been observed by mass specfor ethyl iodide, propyl iodide and ethyl bromide. Methyl trometry. The processes are of the type R X + RX iodide and methyl bromide gave negative results butas%$ was observed with a mixture of methyl and ethyl iodides. Reaction cross-section falls to zero above a critical ion velocity.
+
+
electrons. The lower value corresponds to the position of the maximum in Fig. 1. A number of ion-molecule reactions are known to exhibit such an inverse dependence of ion abundance upon repeller field strength,8 although an inverse square root dependence is considered normal. The resolving power a t m/e = 312 is unfavorable CzHJ + CzHsI +CaHloIz (1) and special care was necessary to identify the ion. This reaction, if verified, would be of particular in- This was accomplished, in part, by using the metaterest as the first example (to our knowledge) of a stable suppressor electrode to improve the resolupersistent collision complex, or “sticky collisi~n.”~ tion. The mass spectrum in this region was Although previously unobserved in mass spec- calibrated a t mass-to-charge ratios of 298, 310 and trometry, such collision complexes are not unantici- 338 using propylene diiodide, tetramethylene dipated. In conventional gaseous systems the di- iodide and hexamethylene diiodide, respectively. meric ion should occur more frequently because it In this manner it was definitely established that an is susceptible to collisional stabilization by removal ion-molecule reaction product of m/e = 312 had of excess energy. Similarly, it can be expected to been formed from ethyl iodide. Furthermore, it grow by accretion of additional molecules and this follows that reaction (1) is occurring. concept is the basis of Lind’s “cluster t h e ~ r y . ” ~ As a final, confirmatory test we compared the According to a recent simple treatment5 of ion- ionization efficiency curve for the parent moleculemolecule “sticky collisions” the lifetime T of the ion C2H61+with that of the daughter. The vanishcomplex is given by ing current method gave an appearance potential a t m/e = 312 which agreed, within experimental 7 v - ‘ ( l - Eb/E)’-a (2) error, with that a t m/e 156. Rather unexpectedly, where E is the total internal energy, E b the energy the curves show a striking difference. The ionirequired to dissociate the complex and a the number zation efficiency curve for the primary ion (see of effective degrees of vibrational freedom. To be Fig. 1) is normal. I n contrast, the curve for detected in a mass spectrometer the complex must C4Hlo12+rises sharply from the onset of ionization sec. have a lifetime approximating to a well-defined maximum about 1.5 v. higher. Then, following a small subsequent decrease, the Results curve behaves normally over an interval of several The general experimental procedure for this work follows Stevenson and Schissler.6 Details volts and reaches a plateau only 8 v. above its appearance potential, decreasing slightly a t 70 v. have been described el~ewhere.~ I n order to confirm the reaction 1, as indicated The similarity and difference of the two curves in by preliminary experiments, it was next established Fig. 1 have been emphasized by choosing an approthat peak height tentatively assigned to C4H1012+ priate scale factor to equalize ion abundances a t varied as the square of the inlet pressure of ethyl higher ionizing voltages. A similar reaction has been found yielding iodide. Further, its ion intensity varied inversely C4H10Br2+ from ethyl bromide, C6H1412+ from propyl with the repeller field strength a t values in excess of 10 v. cm.-l both for 70 v. and for 10.5 1’. iodide and C3H812+ from a mixture of methyl and ethyl iodides. No reaction was found for methyl (1) Contribution from the Radiation Project operated by the iodide, methyl bromide or propyl chloride. The University of Notre Dame and supported in part under Atomic latter is an unfavorable case for test because of a Energy Commission Contract AT-(] 1-11-38, low abundance of C3H7C1+. The calculated cross (2) T o whom correspondence and requests for reprints should be sent. sections u for reactions in one-component systems (3) Dr. N. A. I. M. Boelrijk, working in these laboratories, has at 70 v. ionizing voltage and 4 v. cm.-l repeller examined many systems for evidence of this phenomenon, without field strength appear in Table I. success. J. L. Franklin, F. H. Field and F W. Lampe, in a preprint The ion abundance curves for C2H6Br+ and of a paper presented a t the Joint Conference on Mass Spectrometry, London, September, 1968, have remarked upon the absence of peaks C4HloBrz+ strongly resemble the corresponding attributable to such species in mass spectra containing evidence of there is slight evicurves in Fig. 1. For C6H1412+ other ion-molecular reactions. dence of structure in the ion abundance curve and (4) S. C. Lind, “The Chemical Effects of Alpha Particles and Electhe abundance ratio of primary to secondary ion is trons,” 2nd ed., Chemical Catalog Co., New York, N. Y., 1929. (5) M. Buiton and J. L. Magee, THIS JOURNAL, 66, 842 (1952). substantially constant over a 10 v. range above (6) D. P. Stevenson and D. 0. Schissler, J . Chem. Phys., 29, 282 (1 958). (8) G. Gioumousis and D. P. Stevenson, J . Chem. Phys., 29, 294 In the course of a study of ion-molecule reactions of ethyl iodide in the ionization chamber of a mass spectrometer we found evidence for an ionic species at, or close to, m/e = 312. This mass corresponds to C4Hlo12+which suggests its formation in the bimolecular process +
+
(7) R. F. Pottie, R. Barker and W. H. Hamill, R a d . Res., in press.
(1958).
ROSWELL F. POTTIE AND WILLIAM H. HAMILL
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Vol. 63
At 20 v. ionizing voltage, corresponding t o the maximum in the ionization efficiency curve, the observed ion intensity dependence for m/e = 80 was characteristic of primary ions, as expected. In contrast, for ethyl iodide a t 70 v. ionizing voltage, where the contribution from the low energy component should be negligible, the peak height for the ion a t m/e = 312 decreased in a manner characteristic of secondary ions with increasing repeller field strength. When the ionizing voltage was then decreased to a value corresponding to the maximum in Fig. 1, the peak height again decreased in a similar manner with increasing repeller field strength. We conclude that the energy-rich precursor for the ion a t m/e = 312 is an ion, and not an excited neutral molecule, over the entire range of ionizing voltage. Ions at double the parent mass were not found for methyl bromide or methyl iodide a t 70 v. ionizing voltage but there is still the possibility of a resonance process a t some lower electron energy. The appropriate mass regions were therefore scanned repeatedly, over small voltage intervals, from 2 v. below to 10 v. above the appearance potentials of the parent molecule ions. No collision complex ion was detected in either instance. Discussion 12 14 16 18 20 Stevenson and Schissler6 have pointed out that VI. parent ion-daughter ion relationship is better Fig. 1.-Peak height P us. ionizing voltage VI for C4H&+ established by constancy of their abundance ( 0 ) from C2HhI+(0)a t 4 v. cm.-I ion repeller field strength. ratios a t low ionizing voltages than by simple agreement of their appearance potentials. The TABLE I ions C4Hl&+ and C4HloBrz+are exceptional in this CROSS-SECTIONS FOR PERSISTENT COLLISIONCOMPLEXES respect over the lower range of voltage while c x 101s cm.*at 4 V. agreeing excellently over the medium and higher om.-' range. The curves in Fig. 1 strongly suggest two C2HJ+ C2HJ -+ C4HloI2+ 1.5 processes and, if so, the facts require that C2H61+ C2H6Br++ C2HbBr 4CaHloBrz+ 3.2 is involved in each. To account for the results it C ~ H T I + CYH~T 4C O H I ~ I ~ ' 12 is postulated that there are two isomeric ionic their common appearance potential. The ion species of the empirical formula CzH61+and of abundance curve for C3H81z+resembles Fig. 1 in unequal reaction cross-sections. If one species having a maximum 1.5 v. above the appearance behaves normally in giving a constant parent ionpotential. I n addition, there is a second maximum daughter ion ratio, the contribution of the ababout 3.5 v. above the appearance potential, fol- normal variety can be isolated by subtracting the lowed by the plateau some 5 v. higher which parent curve from the daughter curve. This difference curve is plotted in the panel of Fig. 1 and persists t o 70 v. For the three reactions in Table I the difference suggests a resonance process. Before attempting to explain this effect i t is between the ionization potential of the primary ion and the appearance potential of the secondary ion first necessary t o consider the possibility that it is does not exceed 0.2 v. For C3H&+ the primary merely an artifact. Appearance potentials were ion cannot be distinguished because the ionization measured at repeller field strengths of 4 v. cm.-l in potentials of methyl iodide and of ethyl iodide dif- order to maximize secondary ion current, but a t the risk of instability. The ionization efficiency fer by less than 0.2 v. Although no difference could be distinguished curves for primary ions were all normal, as for the between appearance potentials of primary and one illustrated, while those for collision complex secondary ions, the maxima in the ion abundance ions exhibit the features mentioned. Also, ionicurves suggested contributions to the observed zation efficiency curves for many secondary ions reactions from resonance excitations of parent which are not collision complexes have been determined with the same instrument and a t the molecules. Reactions of the type same repeller field strength but with no evidence A* + A + A*+ + eof the anomalies reported here, not excluding such are well known for atoms and have been observed ions resulting from alkyl halides. by mass s p e c t r ~ m e t r y . ~For comparison with There is no necessary inconsistency between the alkyl halides we measured the Arz+ intensity the structure reported for the secondary ion abunfrom argon as a function of repeller field strength. dance curves and the apparent lack of structure in the corresponding primary ion curves. The (9) J. A. Hornbeck and J. P. Molnar, Phya. Rev., 84, 621 (1951).
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June, 1959
RADIOLYSIS OF LIQUID12-PENTANE
electron energy resolution of the instrument is so low that negative evidence is simply inconclusive. One is inclined t o attempt t o correlate the maxima in the secondary ion abundance curves with the double series of electronic states observed for the alkyl halides. lo These series lead to ionization potentials separated by 0.64 and 0.59 v. for methyl and ethyl iodides and by 0.33 and 0.33 v. for methyl and ethyl bromides. A serious deficiency of this propose1 is that it cannot account for the break in the curve for C3H&+ a t an electron energy ca. 3.5 v. above the ionization potential. It would also be necessary to assume that formation of one of these states was sharply resonant, although the probability of ion formation by electron impact is normally linear in the excess energy. Whatever may be the detailed explanation of these effects, it is rather likely that the sensitivity of the collision complex to small energy differences will amplify the effect, particularly so if only a small fraction of complexes survive long enough to be collected. Referring to equation 2 let E = Eb f Ef-I- E, -I- 3kT Ef is the kinetic energy of the primary ion arising from the repeller field. Its maximum value was 0.5 e.v. and since u varies as Ef-llZ we take Ef = 0.1 e.v. as representative. E, is the vibrational energy of the primary ion resulting from vertical ionization. The ionization potential of ethyl iodide by electron impact" is 9.47 e.v. and the spectroscopic value'" is 9.34 e.v., giving E , = 0.12 e.v. At the temperature of the ionization chamber (250°), 3kT is 0.14 e.v. The value of E b is very uncertain but 1 e.v. is a plausible value. Letting (IO) W. C . Price, J . Chem. Phys., 4, 539, 547 (1936). (11) J. D. Morrison and A. J. C. Nicholson, ibid., 20, 1021 (1952).
879
7 = sec. and Y = 10laset.-' leads to a required minimum 13 degrees of vibrational freedom. Considering the high internal energy, heavy atoms and relatively weak C-C bonds, this is an acceptable value. For these values of the parameters, decreasing the internal energy of the collision complex by only 0.03 e.v. would more than double the value of r. If the very small cross-section for C4H1&+ can be interpreted as inefficient collection due to extensive decomposition of the collision complex, then doubling the value of 7 will have a large effect upon u. Considering the sensitivity of the value of T to the internal energy of the complex we may consider that r changes discontinuously to zero a t some critical value, E,, of the translational energy of the primary ion. There is a corresponding critical limit, Z,, of the length of the ion track (measured from the electron beam toward the exit slit), along which a viable complex can form. This distance is shorter, the greater the repeller field strength, F. The yield of complexes is proportional to
The cross-section for collision, u(E), is presumed to depend upon E-'/z but it is clear that regardless of the functional dependence, the yield of secondary ions varies inversely with field strength, provided that E, is less than the maximum energy attainable by the primary ion.12 Acknowledgment.-It is a pleasure t o acknowledge helpful conversations with Professor John L. Magee. (12) N. A. I. further detail.
M. Boelrijk
and W. H. Hamill, t o be published in
RADIOLYSIS OF LIQUID n-PENTANEl BY ADOLFE. DE VRIES~ AND AUGUSTINE 0. ALLEN Contribution from Department of Chemistry, Brookhaven National Laboratory, Upton, N . Y . Received February 11, 1969
Product distribution in n-pentane radiolysis is the same for beams of 14 MeV. H e + +ions as for 2 MeV. electrons, in marked contrast to the behavior of aqueous solutions. The various branched- and straight-chain products heavier than pentane contain little or no unsaturation and their relative yields are consistent with their being formed by random combination of free radicals produced by breakup of pentane without rearrangement. Hydrocarbon products lighter than pentane must be formed by some other mechanism.
Introduction A good deal of work has recently been reported on radiolysis products of liquid paraffinic hydrocarbons, using radiations of low ionization density, such as fast electron^.^-^ The present work was undertaken to determine the effects of changing radiation quality on the product distribution in the radiolysis of n-pentane. (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) Lab. v. Massaspectrografie, Amsterdam, Netherlands. (3) H. A. Dewhurst. THISJ O U R N A L61, , 1466 (1957). (4) H. A. Dewhurst, ibid., 62, 15 (1958). ( 5 ) J. J. Keenan, R. M. Lincoln, R. L. Rogers and H. Burwasser, J . A m . Cliem. Soc., 1 9 , 5125 (1957).
Experimental Matheson's best pentane was washed with H2S04 and KOH, fractionated through a 20-plate column and assed through 1.5 m. of silica gel. One-gram samples o?thoroughly deaerated pentane were sealed under vacuum into small radiation cells fitted with thin glass windows. Electron-beam irradiations were performed at 2 MeV. and 0.5 pamp. with a Van de Graaff generator t o total doses of 1.5 X loz1or 0 X loz1e.v. Cyclotron irradiations were performed with He++ ions (or-rays) having their energy reduced by absorbers t o 14 MeV. (their original energy is 40 MeV.); the current was about 0.06 pamp. and the total dose was about 3 X 1 0 2 1 e.v., but was determined only approximately because of difficulties in reading the current, due to the low electrical conductivity of pentane. After irradiation the sample tubes were opened on a vacuum line; Hz and CHI were pumped from the sample at
