J . Phys. Chem. 1984, 88, 5320-5324
5320
Some Ion-Molecule Reactions Pertinent to Combustion Nora M. Semo and Walter S. Koski* Department of Chemistry, The Johns Hopkins Uniuersity, Baltimore, Maryland 21 218 (Received: February 15, 1984)
Some ion-molecule reactions pertinent to combustion processes have been studied by using tandem mass spectrometrictechniques under binary collision conditions. The proton transfer from HCO' to H 2 0 and C6H6 and from D30t to C6H6and naphthalene has been examined as a function of projectile ion kinetic energy. These reactions are all exothermic and proceed with high cross sections at low projectile ion kinetic energy. The hydride ion abstraction in the reaction H30+ + C6H6 to produce C6H5+was examined. The results suggest that the C6H5' product is linear rather than cyclic. The reactions of C2H2+and CzH2and of C2H3' and C2H2were also studied. The collision-induced dissociation mass spectra of the cluster ions (C2H2.C2H2)+, (C3H7*C2H4)',and (C2H3.C2H2)+were studied, and the binding energies were measured.
Introduction In recent years there has been an increasing interest in the role of ions in combustion processes, and various ion-molecule reactions have been postulated in an attempt to explain the ionic aspects of combustion phenomena. For example, the following series of ion-molecule reactions has been suggested' as being pertinent to soot formation.
1-
H2
1-
HZ
etc.
There is a dearth of information on many postulated ionmolecule reactions. Their reaction cross sections have not been measured, and in some cases it is not known whether a proposed reaction is exo- or endothermic because of the lack of thermodynamic information. We have initiated a program in our laboratory to study some of the ion-molecule reactions that appear to be potentially important in combustion, and in this report we give some of our initial results. Examination of the above sequence of reactions shows that it consists of proton transfer, condensation, and hydrogen elimination reactions. Indeed, it has been pointed out that, in C2H2/02flames, the ions observed have the general formula C,,H,', with m being an odd n ~ m b e r . ~The , ~ presence of ions with an odd number of hydrogens may imply the presence of proton transfer or hydride ion abstraction reactions, although this point may be disputed as far as heavy ions in sooting flames are con~erned.~In this study we have measured the cross sections of four proton transfer reactions involving HCO' and D30+, a hydride ion abstraction reaction, and two condensation reactions using tandem mass spectrometric techniques. Results obtained for the latter reactions appear to be at variance with observation of ions in flames. An examination of the apparent discrepancies suggests that the role of ionic clusters in combustion processes should be considered.
Experimental Section The instrumentation used in this study is of two types: (1) a spectrometer capable of measuring the angular distribution and (2) an in-line tandem spectrometer. Both of these instruments have been previously d e ~ c r i b e d ,so ~ ,only ~ a brief description of (1) Vinckier, C.; Gardner, M. P.; Bayes, K. D. "Sixteenth Symposium (International) on Combustion"; The Combustion Institute: Pittsburgh, PA, 1911; p 881. (2) Michaud, P.; Delfau, J. L.; Barassin, A. "Eighteenth Symposium (International) on Combustion"; The Combustion Institute: Pittsburgh, PA, 1981; p 443. (3) Olson, D. B.; Calcotte, H. F. "Eighteenth Symposium (International) on Combustion"; The Combustion Institute: Pittsburgh, PA, 1981; p 453. (4) Delfau, J. L.; Michaud, P.; Barassin, A. Combust. Sci Technol. 1979, 20, 165.
0022-3654/84/2088-5320$01.50/0
the principal features is given here. The first instrument consists of two quadrupole mass filters and hemispherical electrostatic analyzers' in tandem. Cylindrical lenses were used throughout the instrument, and the optical properties of these lenses are based on the calculations of Kuyatt and others.* All voltages for the lens elements are referenced to the exit aperture of the ion source chamber. Ions are formed by electron bombardment of suitable gas mixtures in the chamber. The ions issuing from the source are focused into the primary quadrupole mass filter. The ion beam is then decelerated into the first electrostatic analyzer. From the analyzer, the ions are accelerated and focused into the reaction cell. Although the instrument is capable of measuring the angular distribution of the products over a wide angle, in the experiments reported here, measurements were made only at 0' to the beam direction. The secondary lens system is similar to the primary ion beam section. The ions leaving the reaction cell are energy analyzed by another 180' electrostatic analyzer and are mass analyzed by the secondary quadrupole mass filter. The lens elements of the detection system are connected to the reference voltage of the ion source through a variable offset voltage and a ramp voltage. By proper settings of these voltages with respect to the reference voltage, an energy scan of the ions exiting the reaction cell is made. The ions pass through the secondary quadrupole mass filter and strike a continuous channel electron multiplier. Pulse-counting techniques are used. The second instrument used in this work is an in-line tandem mass spectrometer. This instrument is used for cross-section and threshold measurements. Ions are formed by electron bombardment of suitable gas mixtures. The ion beam is mass analyzed by a quadrupole mass filter and is energy analyzed by an electrostatic analyzer before passing through a reaction chamber. The products are then mass analyzed by a 60' Nier type mass spectrometer and detected with an electron multiplier.
Results and Discussion One of the assumptions that frequently has been made in estimating reaction rate coefficients is that the reaction HCO+(H20,CO)H3Ot is the dominant ion-molecule path in flames, and apparently, under flame conditions, equilibrium exists between HCO+ and H30+.' It appeared appropriate, therefore, to examine this reaction. Figure 1 gives a typical result for the variation of the cross section for the production of H 3 0 + as a (5) Wendell, K.; Jones, C. A,; Kaufman, J. J.; Koski, W. S. J . Chem. Phys. 1975, 63, 750.
(6) Watkins, H. P.; Sondergaard, N. A,; Koski, W. S. Radiochim. Acta 1981, 29, 87.
(7) Simpson, J. A. Rev. Sci. Instrum. 1964, 35, 1698. (8) Natali, S.;DiChio, D.; Kuyatt, C. E. J . Res. Nutl. Bur. Stand. ( U S . ) 1972, 76, 21.
(9) Calcotte, H. F. "Ion-Molecule Reactions"; Franklin, J. L., Ed.; Plenum Press: New York. 1972.
0 1984 American Chemical Society
Ion-Molecule Reactions Pertinent to Combustion
6.0
-
4.0
-
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5321
IO
-
-
8-
N
‘5 ’0
6-
4-
2-
I
01
I 2
I
4
I
6 E (eV) lab
I
I
8
IO
Figure 1. Proton transfer reactions from HCO’ to benzene and H20. HCO+(C6H6,CO)C6H7(A) and HCOt(H20,CO)H30t (0). “Error”
I
2
3 E k V ) lab
4
5
6
7
Figure 2. Proton transfer reactions from D30t to benzene and naphthalene: D30+(C6H6,H,0)C6H6D+ (A)and D30’(CloH,,D20)CloH,D’
(0).
bars indicate the spread in four measurements on each point in all figures.
function of projectile ion kinetic energy. The HCO’ was produced by electron bombardment of acetaldehyde. Under these conditions only the formyl positive ion is produced.1° The cross-section curve is one typical of an exothermic ion-molecule reaction in which the cross section increases with decreasing projectile ion kinetic energy. It will be noted that, at the energies generally associated with flames (-0.2 eV), the cross section becomes large. Proton transfer to aromatic systems is also of interest in combustion, and we have chosen to measure the proton transfer from the positive formyl ion to benzene. The reaction is exothermic by 1.45 eV. The results are given in Figure 1. The behavior is that of an exothermic reaction, and at low energies it has a high proton transfer cross section. Both of these transfer processes can be expected to proceed well under flame conditions. In acetylene/oxygen flames, H30+is the most abundant ion present when the equivalence ratio is about unity; hence, proton transfer from H30+is of interest. Two examples are given in Figure 2 where the cross-section curves are shown for deuteron transfer from D 3 0 +to benzene and naphthalene. D30+is used to get a better separation in product mass peaks. Both of these reactions are exothermic, and in the case of benzene, the exothermicity is 0.56 eV. The curves are almost superimposable, and the reactions proceed with high cross sections at low energies. The reaction H30+(C6H6,H20,H2)C6H5+ which corresponds to hydride ion abstraction from benzene is expected to be endothermic and probably does not take place in flames. However, the product ion so produced may be of interest in combustion; hence, the results are presented here. The cross-section curve for hydride ion abstraction is given in Figure 3. The interesting point about this curve is the apparent threshold value. If the product ion was the cyclic C6H5+,one would expect a threshold of 3.4 eV. If one carries out the customary extrapolation” of the linear part of the cross-section curve, one obtains a threshold of 6.8 eV. This (10) Semo, N. M.; Koski, W. S . J . Phys. Chem. 1983, 87, 2302. ( 1 1 ) Tiernan, T. 0.;Hughes, B. M.; Lifshitz, C. J. J . Chem. Phys. 1971, 56, 5692.
E(@/)lab
-
Cross section for the production of C6HSt from H30t(C6H6,H20,H2)C6H,+as a function of projectile ion kinetic energy.
Figure 3.
suggests that the C6H5+product is linear rather than cyclic since the heats of formation of all linear C6H5+ions are higher than that of the cyclic C6H5+.12 Increasing the heats of formation would increase the observed reaction threshold. Examination of the literature shows that Momigny et al.I3 report a heat of formation for C6H5+of 348 kcal/mol obtained from electron impact using the target compound -CrC-C=C-C=C. The value of the heat of formation of 348 kcal/mol is compatible with our threshold measurement. It is also interesting to note that Calfor the cotte14 suggested the structure -C=C-C+=C-C=C C&5+ ion found in flames. In view of the uncertainties associated with heats of formation the above good agreement may be for(12) Rosenstock, H. M.; Draxl, K.; Seiner, B. W.; Herron, J. T. J . Phys. Chem. ReJ Data, Suppl. 1977, 6, 1 . (1 3) Momigny, J.; Brakier, L.; DOr, L. Bull. CI.Sci. Acud. R . Eelg. 1962, 48, 1002. (14) Calcotte, H. F. Combust. Flume 1981, 42, 215.
5322 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
Semo and Koski
)I t ._
VI c
2
-c
Figure 5. Energy spectrum showing the peak due to the projectile ion
C2Hz+ (a) and the product ion C4H3+ (b) for the reaction C2Hz+(CzHz,H)C4H3+. The arrows from left to right indicate the expected product peak positions for persistent complex formation and for reaction proceeding by a direction mechanism, respectively.
6 8 E (EV) lab
10
+
Figure 4. Cross sections of the reactions of C2H2+ C2H2to give C4H,+ (D), C4H3+(0),and C4H2+(A) as a function of projectile ion kinetic
energy.
C4H3+of 0.45 when they bombarded C2H2with electrons of 115-eV energy. Butrill18 found that the ratio was a function of the u2(C-C stretch) vibration of C2H2+. At u2 = 0 he obtained a ratio of 0.48 and at u2 = 1 a ratio of 0.35. The ratio of our cross sections in the 2-8-eV region gives an average of 0.35. This result is consistent with published results, and the ratio we obtained suggests that our projectile C2H2+has some vibrational excitation as is expected from the mode of preparation and from Butrill's results. In view of the fact that the condensation product, C4H4+,is formed in the reaction C2H2+ C2H2at low energies, it is of interest to examine the energy spectrum of the product ion, C4H3+, to see if it is formed by the dissociation of C4H4+or whether it is produced by a direct mechanism. The work of Ono and NgI5 on the photodissociation of neutral C2H2.C2H2clusters shows that C4H3+and C4H2' can be formed by both sequential and parallel processes, and Lifshitz et aLl9 report observing two metastable decompositions of C4H4+,indicating parallel reactions.
+
tuitous. However, the fact that there is a large difference between the thresholds for the formation of cyclic and linear C6H5+does suggest that t h e C6H5+ product in t h e reaction H30+(C6H6,H20,H2)C6H5+ is linear. In conclusion, therefore, as far as the proton transfer reactions are concerned, there are no surprises. Those reactions that were expected to be exothermic behaved in an exothermic manner, and their cross sections decreased with increasing projectile ion kinetic energies. At energies corresponding to flame temperatures, their cross sections are high. Two condensation reactions have been given some attention in this preliminary investigation. The first is the reaction of C2H2+ with C2H2,and the second is the C2H3' + CzH2reaction. The C2H2++ C2H2reaction has been studied by many investigators over the years. See Ono and Ng and references therein.15 For our measurements, C2H2+was prepared by electron bombardment of a C2H2/Xe mixture with the Xe being in large excess. Since the ionization potentials of Xe to the Xe+(2P3/2)and Xe+(2Plj2) states are 12.130 and 13.43 eV, respectively, and of C2H2 to C2H2+(211u)and C2H2+(21;) are 11.4 and 16.3 eV, respectively, one would expect only the % state I1 of C2H2+to be formed by charge exchange between Xe+ and C2H2. There will be, however, sufficient excess energy to produce the C2H2+ion with some vibrational excitation in the state. The cross-section results for the C2H2+ C2H2reaction are presented in Figure 4. The product ions C4H3+and C4H2' are readily observable, and even the C4H4+ion is present if the projectile ion energy is kept low enough. It is of interest to examine the ratio of the yields of C4H2' to C4H3+. Munson16 and Futrell et al.17found a ratio of C4Hz+to
+
(15) Ono, Y.; Ng, C . Y. J . Chem. Phys. 1982, 77, 2947. (16) Munson, M. S. B. J . Phys. Chem. 1965, 69,572.
-
C4H4+
+
+ C4H2" + H2 C4H3+ H
Apparently, both sequential and parallel processes can go on, depending on conditions. In an attempt to shed some light on this matter, we measured the energy spectrum of C4H3+ formed by the reaction C2H2+(C2H2,H)C4H3+ at a projectile energy of 2 eV in the laboratory system. If the C4H3+ion arises from a persistant complex (in this case C4H4+)that lives for a time longer than s, the rotational period of the system, one would expect the velocity of the C4H3+ion to be the same as the velocity of the center of mass. Indeed, this is the case. The energy spectrum is given in Figure 5. The arrow on the left indicates the expected peak position if the product ion was formed by a complex mechanism. The arrow on the right shows the expected peak position if the reaction was proceeding by a direct mechanism. The energy difference is small but measurable. However, the width of the product peak, which is probably due to the internal energy possessed by the product ion, is such that we are not able to say unambiguously that the product is formed by one mechanism or the other. The (17) Futrell, J. H.; Tiernan, T. 0. .I Phys. . Chem. 1968, 72, 158. (18) Butrill, Jr., S . E. J. Chem. Phys. 1975, 62, 1834. (19) Lifshitz, C.; Gibson, D.; Levsen, K.; Dotan, I. In?.J . Mass Spectrom. Ion. Phys. 1981, 40, 157.
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5323
Ion-Molecule Reactions Pertinent to Combustion
0.e
-g0.6 9 N -
os Y)
0.4
0.2
0. 4
8
I2
16
20
24
28
E (ev) lab 0.4
-g0.3
?3 9
0.2
0.1
0.
Figure 6. Collision-induceddissociation cross sections for dissociation of C4H4+and C4H,,+in helium to produce C2H2+(A) and C3H7+(0),
was used as a target gas. The CID mass spectrum of C4H,,+ in He is dominated completely by a single ion, C3H7+,in the energy range convered by this study. The upper portion of Figure 6 gives the apparent cross section for the production of C3H,+ as a function of projectile ion kinetic energy. The measured threshold value gives a binding energy between C3H7+and CHI of 5.8 kcal/mol. Hiraoka and Kebarle20 have also studied this ion using thermochemical methods, and they arrived at a value of 3.4 kcal/mol. Considering the small value of the dissociation energy, these values are probably well within our experimental error. The point is that C4H11+is a cluster ion in which C3H7+is bound to CHI by a few kcal/mol of energy. The second ion on which we made some CID measurements is C4H4+,the results for which are given in the lower portion of Figure 6. Again, helium was used as a target gas and C2H2+was the only ion observed in the energy range covered. The threshold value when converted to the center of mass system is 1.1 eV, and since our estimate of the error is about 0.15 eV, this is in good agreement with the more accurate value of 0.98 eV reported for this cluster by Ono and Ng.15 A question that naturally arises at this point is what fraction of the mass 52 ion is due to cluster ion, C2H2+C2H2, and what fraction is due to covalently bonded C4H4+ions which do not contribute to the CID spectrum in our kinetic energy range? This question is difficult to answer unambiguously. However, there are some remarks that can be addressed to the point. The ion C4Hll+is a nonclassical ion, so it does not have a contribution from a completely covalently bonded structure. Yet, its CID behavior is very similar to that of C4H4+. The second point is that if the cluster ion was present in only trace amounts in the mass 52 beam, one would expect the slope of the curve in Figure 6 to be small. In addition, a more quantitative comment can be made. Rebick and Levine*l have pointed out that their statistical theory of collision-induced dissociation suggests a threshold law
respectively. results, however, are not inconsistent with the finding of others that both mechanisms are at work. A similar conclusion is drawn from the energy spectrum of C4H2'. An analogous series of measurements were made on the C2H3+ C2H2reaction. C2H3+was prepared by electron bombardment of a C2H2/Xemixture. The products C4H4+, C4H3+,and C4H2+ were readily detected. N o C4HS+were observed, indicating that if C4H5+is formed, its lifetime is less than the transit time in our instrument, i.e., about 10 p s . One is then confronted with an apparent discrepancy. In flames and in the Vinckier et al.' experiments, the ion C4H5+is readily observed, but no evidence is found for it in the tandem mass spectrometer measurements. One is, therefore, forced to conclude that the C4HS+ion observed in flames does not arise from binary collisions between C2H3+and C2H2. There are various possible explanations for this apparent discrepancy. Firstly, the C4H5+may be formed by alternative reactants; for example, C2H5+ C2H2can yield C4H5+.This process is a feasible one in flames; however, it does not look like a plausible explanation for the Vinckier et al.' results. Secondly, the pressure conditions in flames are generally such that many three-body collisions can take place, resulting in collisional stabilization of C4H5+.Thirdly, we are considering as an explanation the role that clusters may play. In view of the long range of ion-induced dipole forces, the low relative kinetic energy of reactants, the relatively high pressure, and the high polarizability of the unsaturated neutral reactants in flames, one might expect that conditions would be favorable for ion cluster formation. We have made several observations that have some bearing on this point. In spite of the fact that we did not see C4Hs+as a product from the binary collision of C2H3+ and C2H2,we readily see this ion exit from an ion source where acetylene at a pressure of 0.5 torr was bombarded with 100-eV electrons. There were also other ions which appeared pertinent to our study, so we examined the ions C4Hll+,C4H4+,and C4H5+ by studying the collision-induced dissociation (CID) mass spectra as a function of the kinetic energies of the projectile ions. Helium
+
+
where u is the cross section and E , E,,,, and Eo are the projectile kinetic energy, internal energy, and threshold energy, respectively. This threshold law has been used to advantage by a number of investigators.22 Rebick and Levine2' report that for direct CID the exponent n was invariably in the 1.9-2.2 range. Parks et al.23 have reported similar results and give a theoretical value for n of 1.8-1.9. Analysis of the results obtained for CID of C4Hll+ and C4H4+gives values for n of 1.85 and 1.80, respectively. If the cluster ion was present in small amounts in the mass 52 peak, its rise with energy would be too small to give the power dependence expected by theory. It appears that, in spite of the greater stability of the covalently bonded C4H4+ions, the binding energy of the cluster and the barrier to its rearrangement to the covalent structure are sufficiently high so that, under the mild kinetic energy conditions in the ion source, a significant fraction of the mass 52 ions is in the cluster form. To emphasize the possibility that the ions of mass 52 are not completely in the cluster form, we have designated the cross section as apparent in Figure 6. The CID mass spectrum of C4HS+was also examined, and the product ion (C2H3') was measured over the energy range from 10 to 70 eV in the laboratory system. The threshold value was found to vary with ion source pressure. The values ranged from 10 to 1.6 kcal/mol. The lower ion source pressure gave the lower threshold value. It appears that the binding energy is influenced by the internal excitation of the C2H3' ion. In a recent studylo where we examined the proton transfr reaction between HCO+ and C2H2to produce C2H3+, the measured energy spectrum of the product ion indicated that a significant fraction of the available (20) Hiraoka, K.; Kebarle, P. Adu. Mass Spectrom. 1978, 7 8 , 1408. (21) Rebick, C.; Levine, R. D. J . Chem. Phys. 1973, 58, 3942. ( 2 2 ) Ervin. K.: Loh. S. K.: Aristov. N.: Armentrout. P. B. J . Phvs. Chem. 1983, 87, 3953. Armentrout, P. B.; Beauchamp, J. L. J . Am. &ern. SOC. 1981. 103. 784. (23) Parsk, E. K.; Sheen, S. H.; Dimoplon, G.; Wexler, S. In "State to State Chemistry"; Brooks, P. R., Hayes, E. F., Eds.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No. 56, p 94.
5324
J. Phys. Chem. 1984, 88, 5324-5321
reaction energy was converted to internal energy of the ion. This would tend to shorten the lifetime of any persistent complex that C2H2reaction and may be a might be formed in the CZH3' contributing factor our failure to see a long-lived complex in this reaction under binary collision conditions.
+
Acknowledgment. This work was carried out under the auspices the U.S. Department of Energy. Registry No. HCOt, 17030-74-9; C6H6, 71-43-2; naphthalene, 9120-3.
Thermal Decomposition of Ions. 1. Pyrolysis of Protonated Ethers. Activation Energies and A Factors L. Wayne Seck* and Michael Meot-Ner (Mautner) Chemical Kinetics Division, Center f o r Chemical Physics, National Bureau of Standards, Washington, D.C. 20234 (Received: February 28, 1984)
Unimolecular rate constants (kd) have been measured at the high-pressure limit for the thermal decomposition of protonated dimethoxyethane (glyme) and bis(2-methoxyethyl) ether (diglyme). In both cases the decompositioninvolves loss of CH30H and formation of an oxycarbonium ion. The variation of kd with temperature gives log A = 11.1 and E, = 21.1 kcal-mol-' for the pyrolysis of (glyme)H+ and log A = 13.7 and E, = 30.7 for (diglyme)Ht. To the best of our knowledge these are the first Arrhenius parameters ever determined for the decomposition of cations in the vapor phase under equilibrium conditions at the high-pressure limit. The data and thermochemistry are consistent with a transition state in which electron shifts result in the disruption of an internal hydrogen bond.
Introduction The thermally activated unimolecular decomposition and rearrangement of neutral molecules is one of the most thoroughly documented areas of chemical kinetics, both experimentally and theoretically. Much attention has also been given to the fragmentation of energetic ions, such as those formed by electron impact.' However, with the exception of two studies involving carbonium ions in the lower pressure "falloff" region of unimolecular kinetics,* no quantitative information concerning the thermally induced decomposition of gaseous ions is available. Pulsed high-pressure mass spectrometry provides a unique tool for investigating the temporal behavior of reactive ionic systems. Although the main application has been in the area of equilibrium measurement^,^ the combined capabilities of high pressure, wide temperature range, and time resolution also offer a powerful method for quantifying the unimolecular decomposition kinetics of thermalized ions. In this study we have examined two protonated methoxy ethers. Protonated ethers were chosen as models because the pyrolysis results solely in the elimination of C H 3 0 H 4at moderate temperatures (C650 K), which simplifies the kinetic analysis. Furthermore, the analogous vapor-phase acid-catalyzed chain decompositions of acetals and ethers have been extensively studied5 under static conditions, and polar transition states have been invoked to explain the magnitude of the frequency ( A ) factors and activation energies (E,) for CH,OH elimination, which again is the only mode of decomposition. Our goal, then, was to attempt to evaluate A and E , for these systems at the high-pressure limit using pulsed mass spectrometry, interpret the values in terms of the overall thermochemistry, and generate a consistent description
of the transition state (mechanism) for the decomposition.
1979, 50, 51.
Experimental Section All measurements were carried out with the NBS pulsed high-pressure mass spectrometer.6 The ion source (reaction chamber) is fabricated from a single block of stainless steel 12 cm in length and 5 cm in diameter. Uniform temperature regulation is provided by three 100-W cartridge heaters implanted in the walls of the chamber along its long axis. Temperatures are monitored at various interior points with thermocouples, and to ensure efficient heating, the gaseous sample passes through a 3-mm-diameter bored-out section of the chamber walls for a distance of 8 cm before introduction into the reaction volume. Mixtures of the compounds of interest were prepared in an external vacuum manifold prior to introduction into the reaction chamber via micrometering valves. Cyclohexane vapor was used as the bath gas, and dilution factors varied from lo2 to lo5, depending upon the particular measurement. Total source pressures normally fell within the range 0.1-1.5 torr at temperatures between 400 and 640 K. The experimental-kinetic sequence may be summarized as follows: (i) the desired total pressure of the ether-cyclohexane mixture was first established at the chosen temperature; (ii) the electron beam circuitry was adjusted to provide an ionizing pulse of 0.6-1-keV electrons having a typical duration of 0.1-0.5 ms repeated every 10-20 ms; (iii) since the bulk component was cyclohexane,the energy initially deposited during the electron pulse lead to the instantaneous formation of species such as C6HI2+, C6Hll+, C6HIO+,C5H9+,C4Hs+,etc.; (iv) these moieties, in turn, reacted with the ether via proton transfer and/or charge exchange to generate protonated parent molecules and/or fragment ions characteristic of the ether (mainly ROCH3.H+). Since fragment cations derived from the ether are unreactive toward cyclohexane, essentially all of these reacted further with the ether via proton
(2) (a) Moet-Ner, M.; Field, F. H. J . Phys. Chem. 1976, 80, 2865. (b) French, M.; Kebarle, P. Can. J . Chem. 1975, 53, 2268. (3) Kebarle, P. Annu. Reo. Phys. Chem. 1977, 28,445. (4) Mautner M. (Meot-Ner), J. Am. Chem. SOC.1983, 105, 4906. (5). Failes, R. L.; Shapiro, J. S.; Stimson, V. R. in "The Chemistry of Functional Groups"; Patai, S. Ed.; Wiley: New York, 1980; Supplement E, Part 1.
( 6 ) Meot-Ner (Mautner), M.; Sieck, L. W. J Am. Chem. SOC.1983, 205, 2956. (7) 1 kcal-mol-' = 4.184 kJ.mol-I. Unless otherwise stated, all AHfovalues have an absolute uncertainty of f l . 0 kcal.mol-I and all AS' values have an absolute uncertainty of f l . 5 cal-deg-l.rno1-l.
(1) Brenton, A. G.; Morgan R. P.; Beynon, J. H. Annu. Reo. Phys. Chem.
This article not subject to US.Copyright. Published 1984 by the American Chemical Society
