J . Chem. Soc., Faraday Trans. I , 1982,78, 1667-1675 Ion-Molecule Reactions in Gaseous Hydrogen + Pentane Mixtures BY M ICHAEL NEUM ANN-S PA LLART Institut de Chimie Physique, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland Received 17th August, 1981 The reactions of H: and D: with n-C,D,, and n-C5HI2, respectively, have been studied. The protonating species have been generated from hydrogen by 6oCo y-rays at 10Torr or by electron impact in an ion-cyclotron-resonance apparatus at 1 OP4 Torr. Using both techniques the relative probability of fragmentation reactions of the pentonium ion was measured and the reaction mechanism is discussed. The transferred proton is shown to be retained to a different extent in the fragment alkyl ions. The pentyl ion isomerizes to the tertiary structure and yields mainly 2-methylbut-2-ene, 2-methylbut- 1 -ene, and isobutene after being neutralized.Ion-molecule reactions of hydrocarbons in the gas phase have been the subject of many investigations carried out in the field of mass spectrometry as well as radiation chemistry over the last few decades.l Chemical ionization has been shown to be a useful tool in deriving mechanistic and kinetic knowledge about the often rather complicated reaction schemes found in hydrocarbon systems. More recently ion-cyclotron-resonance (i.c.r.) studies have helped to define the role of the various ionic precursors and fregmentation channels.,? Information about ionic structures, reactions at elevated pressure and neutralization reactions has mostly been derived from steady-state y- and pulse-radiolysis investigations.Chemical-ionization reactant ions such as CH:, C,H,+ and HZ generated from methane or hydrogen, respectively, are chosen since they easily transfer protons to or abstract hydride ions from hydrocarbons. In the direct radiolysis of pentane, C5HT2 is formed. Most of the parent ions fragment, the fragment ions undergoing ion-molecule reactions. Some parent ions become unreactive and are neutralized by electron^.^ In chemical ionization C5Ht3 is formed by protonation and all C5H:3 ions undergo fragmentation, as will be shown below. The chemical ionization of perdeuterated n-pentane with HZ has already been investigated at atmospheric pres~ure.~ This investigation showed that a proton is quantitatively transferred to pentane.Fragmentation reactions of the C5D12H+ ion thus formed were discussed on the basis of the radiolytic end-products, assuming that the transferred proton is entirely incorporated into the neutral fragment. Yet it has been found in the chemical ionization of n-hexane with methane that there is some retention of labelling in the butyl and propyl ions. Thus, we became interested in seeing if the same effect could be found in n-pentane when the hydronium ion is taken as the reactant. In the present study we therefore reinvestigated the hydrogen + pentane system. Radiolysis experiments of H, + C5D1, mixtures were carried out at low pressure (10 Torr H,) and pentane concentrations of 0.5 mol%. In parallel, i.c.r. experiments were performed with D, + C5H12 mixtures.Another object of this study was the fate of the pentyl ion. This ion is supposed I6671668 ION-MOLECULE REACTIONS I N H,+C,H,, to be formed in high yield via the hydride abstraction from pentane of all fragment alkyl ions. Pentyl ions are obviously unreactive in the system. Thus neutralization is the most probable reaction path; indeed typical products of this were found in our radiolysis experiments. In some experiments oxygen was added as a scavenger to exclude contributions from radical reactions. EXPERIMENTAL MATERIALS The purity of the hydrogen used was 99.995% and that of the deuterium (L'Air Liquide) 99.4%. Perdeuterated pentane was prepared catalytically.6 It contained 10% C,D,,H and, after distillation, 0.1 % isopentane as main impurities.It was dried over molecular sieve (4A) and air was removed by repeated freeze-pumpthaw cycles. For y-irradiations spherical Pyrex vessels (500 and 4000 cm3) connected via metal-glass joints to metal valves equipped with indium seals were used. Each vessel was checked for leaks with a helium leak detector. After pimping out to IO-'Torr they were filled with pentane and hydrogen at least 10 h before iiradiation to allow mixing of the gases to take place. IRRADIATION A N D ANALYSIS Radiolysis was carried out in a Sulzer irradiation chamber (50 kCi yGOCo). The dose rate was determined with an ethylene dosimeter taking G(H,) = 1.36.' The dose rate in hydrogen was calculated by correcting for the stopping-power ratio of hydrogen and ethylene for electrons from Pyrex.8 Values of 1.35 x lozo and 9.83 x IOl9 eV g-' h-' for the 500 and 4000 cm3 vessels, respectively, were obtained.The absorbed dose was varied between 2 x 1919 and 9 x loz0 eV g-l. After irradiation the vessels were immersed in liquid nitrogen for 2 h and hydrogen was then pumped off. Subsequently the radiolytic products were allowed to condense into a short injection loop (20 cm) at liquid-nitrogen temperature for I h. Instead of condensation aliquots were taken by expanding the products into a relatively long injection loop (4m) for the determination of methane, ethane and ethylene, since these substances could not be condensed completely at 76 K or had already been partially pumped off with the hydrogen. The sample loop was then attached to a gas-chromatography-mass-spectrometry system or to a dual column gas-chromatography unit.The latter has already been de~cribed.~ However, the sample introduction system was modified by connecting it to a high-vacuum line. Chromatogramms were run for 80 min at 300 K and then temperature programmed at 4 K min-' up to 336 K. Peak areas were integrated with an Autolab computing integrator (Spectra Physics). The gas-chromatography-mass-spectrometry system consisted of a Perkin-Elmer F- 1 1 chromatograph and a Durapak-Squalane combined single column (resolution 4000 theoretical plates for both injection loops) as described above. The exit of the column was connected to a flame ionization detector (FID) and via a silicone-membrane separator (1 in. diameter, 2/1000 in.thick) held at 353 K (Beta Peripherals) to a QMG 31 1 quadrupole mass filter (Bakers). Signals from the electron multiplier were amplified with a Keithley electrometer and averaged with a Nicolet signal averager. Signals from the FID were treated by a Hewlett-Packard laboratory data system. Substances were identified by their retention times and mass spectra. Absolute amounts were calculated from calibration runs. Principally, hydrocarbons from C, to C,, could be detected by the double-column system. The i.c.r. measurement has alreay been de~cribed.~ RESULTS The amounts of different products formed in steady-state y-radiolysis were very low in most of the experiments (partial pressure 10+ Torr for some products and total quantities down to 10-lo mol); thus carrying out calibration and analysis was difficult.However, in spite of this relatively good reproducibility was obtained ( f 5 %).M. NEU M A NN-S P A L LA RT 1669 TABLE 1 .-G-VALUES OF y-RADIOLYSIS PRODUCTS OF H, + C,D,, MIXTURES AT 10 Torr (i) 7.6 x lo1, eV g-l, 0.5 mol% C,D,,; (ii) 7.6 x lo1, eV g-l, 0.5 mol% C5D12, 0.1 mol% 0,; (iii) 7.6 x lo1@ eV g-l, 0.06 mol% C5D12; (iv)1.2 x loz1 eV g-l, 0.06 mol% C5D12; (v) 7.5 x lozo eV g-l, 0.06 mol% C5D12, 0.1 mol% 0,. methane ethane ethene propane propene isobutane butane isobu tene isopentane 2-methylbut-1 -ene 2-me thy1 but-2-ene 2.0 0.8 0.5 0.9 0.18 0.17 1 .o 0.4 (0.1)" 0.31 (0.7)a 0.72 (1 .O)a - 1.8 0.7 0.6 0.9 0.18 0.06 1.1 0.1 0.4 1 .o - 0.5 0.15 0.4 0.06 0.07 0.9 0.14 (0.4)a 0.3 (0.1)" 0.31 (0.7)a 0.71 (1.0)" ~~ - 1.7 0.6 0.3 0.04 0.07 0.5 0.3 0.02 0.03 0.16 0.01 0.9 0.7 0.02 0.7 0.01 0.04 0.03 0.11 0.17 - a 1.8 x lo1, eV g-l.TABLE 2.-ISOTOPIC COMPOSITION OF 7-RADIOLYSIS PRODUCTS (%) OF H, + C,Dl, MIXTURES AT 10 Torr (VALUES NOT CORRECTED FOR THE ISOTOPIC IMPURITY OF THE STARTING MATERIAL) (1) 5.5 x 1019 eV g-l, 0.5 mol% C,D,,; (ii) 8.8 x 1020 eV g-l, 0.5 mol% C6D12; (iii) 8.8 x 1020 eV g-l, 0.5 mol% C5D12, 0.1 mol%O,; (iv) 8.8 x 1020 eV g-l, 0.06 mol% C5D1,; (v) 8.8 x 1020 eV g-l, 0.06 mol% C5D1,, 0.1 mol% 0,. product 0) (ii) (iii) (iv) ( 4 CD3 H CD4 c2 D, H c2 D, c3 D, H C4 D, H '2 D4 C2D5H '3 D8 '4 D I O i-C, D, H3 i-C, D,, H, i-C, D,, i-C, D,, i-C, D, H,-en- 1 i-C, D, H-en- 1 i-C, Dl0-en- 1 i-C, D, H,-en-2 i-C, D, H-en-2 i-C, D,,-en-2 55 45 22 78 12 88 3 23 74 0 21 79 28 72 61 39 26 74 13 87 12 32 30 26 6 30 64 6 27 67 46 54 16 84 9 91 3 34 63 0 27 73 80 20 28 72 71 29 31 69 13 87 17 40 28 13 9 37 54 5 29 66 67 33 22 78 9 91 4 34 62 0 27 731670 ION-MOLECULE REACTIONS I N H,+C,H,, The G values under various conditions are listed in table 1.Assuming a G( - pentane) value of 5 the conversion of pentane was 2.6% at a dose of 8.8 x 1019 eV g-l. Traces of compounds in the C, to C,, region were found and were ascribed to the direct radiolysis of pentane. The observed decrease in yield of some lighter products at lower pentane concentrations can only partially be explained by contributions from the direct radiolysis of pentanelO at 0.5 mol% pentane. The isotopic compositions of the radiolysis products were calculated as follows.Mass spectra obtained from calibration runs for each non-deuterated compound were used to calculate the mass spectra of the partially deuterated compounds assuming no isotope effect on fragmentation. These spectra were used to estimate the isotopic compositions given in table 2. The spectra of some of the products listed in table 1 could not be measured because of the intensities of the M+, ( M - I)+, etc. ions were too low. The G values and isotopic distributions presented are mean values of several determinations. In the i.c.r. experiments mixtures of D, + n-C,H,, were used [p(D,) = 4 x lo-, Torr, p(C5H12) = 1 x lo-, Torr]. Relative fragmentation probabilities of C,H,,D+ formed by deuteron transfer from D: were monitored by modulated ejection of D3+.Their values are given in tables 3 and 4 together with the values for the corresponding products (see Discussion) from y-radiolysis. (G values are converted to M/N+ using M/N+ = G W/ 100. W, the energy required fbr the formation of one ion pair is 36.3 eV for electrons in hydrogenll and CM/N+ = 1 for ionic products.) TABLE 3.-LABEL RETENTION IN FRAGMENT ALKYL IONS (I.C.R. OF D, + c5 Hl, MIXTURE) AND CORRESPONDING STABLE END-PRODUCTS (7-RADIOLYSIS OF H, + C5Dl, MIXTURES AT 10 Torr, VALUES CORRECTED FOR ISOTOPIC IMPURITIES OF THE STARTING MATERIAL) ion relative y-radiolysis relative (i.c.r.) intensity product abundance c3 H;: c4 H,+ C3 H, D+ C, H, D+ 82 18 96 4 TABLE 4.-FRAGMENTATION OF C,X:3 FORMED FROM D, + C5H12 MIXTURES (I.C.R. EXPERIMENT) AND H, + C5 D,, MIXGURES (~-RADIOLYSIS EXPERIMENT) heat of ion product reaction/ (i.c.r.) Ii/ZIi (y-radiolysis) Mi/N+ kcal mol-I '5 'Tl c4x; + c3x; 0.20 x2 (0.22)" 0.32 '4 x10 + '3 x6 0.44 0.48 c,x, 0 .3 4 0.21 c4x,o 0.37 0.1 1 C3X6 0.7 1.9 8.9 13.7 a Value calculated using M(C,Xtl)/N+ = 1 - [M(C,X,+)/N+ + M(C,X,f)/N+ + M(C,X;)/N+].M. N EU M AN N-S P ALL ART 1671 DISCUSSION When hydrogen is irradiated with y-rays or electrons, it is ionized and transfers a proton to another hydrogen molecule: H l + H, + H3+ + H AH = - 38 kcal mol-l. (1) The reactant ion thus generated will transfer a proton to the added hydrocarbon in an exothermic process: (2) The heat of reaction (2) is given by the difference in proton affinities (A,) of hydrogen and pentane (see Appendix for all the heats of reactions) and may differ by several kcal mol-1 according to the assumed structure of the C5H;t, ion.This ion therefore carries a large amount of internal energy. According to ref. (12) the transferred proton is localized in a three-centre bond when a hydrocarbon is protonated: H i + C, D,, + H, + C , D,, H+ AH = - 56 kcal mol-l. Subsequent scission of such a bond leads to the formation of an alkyl ion and a neutral molecule, as is also indicated by the experimental results: Reaction enthalpies are estimated from the heats of formation of the non-deuterated compounds (see the Appendix). The formation of P-C,X;~ (X = H,D) is clearly not favoured because of its high endothermicity (AH = 21 kcal mol-l) as it was demonstrated in an i.c.r. experiment on the chemical ionization of [ 1 ,5-D6]pentane with CH, where the pentyl ion formed via CHZ as precursor contained 99% C5H5D,+.13 Fragment alkyl ions were detected in the i.c.r.experiments with mixtures of D, and C5H12 (see tables 3 and 4). The postulated reaction5* l4 C,Xt3 + C,X$ + C,X, AH = 20 kcal mol-l (9) does not play an important role in the reaction scheme : only very weak signals ( < 1 % total ionization current) of m / z 29 and 30 were observed. Alkyl ions from reactions (3)-(8) are known to isomerize rapidly to the secondary structure1 and to abstract H- from pentane (rate constant z cm3 molecule-l s-l):15 (10) (1 1) (12) (13) AH = - 1.8 kcal mol-l AH = - 5.9 kcal mol-l. I s - C ~ D i + C5 D1, + C4 D1, + s - C ~ D;i S-C4 D, H+ + C5D12 + C, D, H + s-C, Dt1 s-C~DT + C5D12 + C3 D i + s - C ~ Dt1 S-CaDBH++ C5 D12 -+ C, D, H + s-C, Dl11672 ION-MOLECULE REACTIONS I N H,+C5Hl, The G value of butane and propane and the corresponding products (methane and ethane, respectively) of a given isotopic composition indicate the probabilities of fragmentation channels (3)-(8).According to the proposed reactions [reactions (3)-(8) and (10)-(13)] all ions are ultimately converted to pentyl ions (the so-called funnel principle). Their reactions will be discussed below. Butane originating from ion-molecule reactions possesses an unbranched structure, as is shown from the stable end-product analysis of y-radiolysis experiments. The formation of isobutane was surpressed when oxygen was added as a radical scavenger and is thus interpreted as a radical process.Accordingly, butyl ions maintain their initial unbranched structure under radiolysis conditions within the nanosecond time-scale. This was recently confirmed by Shold and Ausloos.16 It is shown (table 3) that the incoming proton is almost entirely incorporated in the neutral fragment formed by methane loss, in agreement with experiments on the methane + n-hexane system. The formation of C,Hz probably occurs by further fragmentation of butyl ions which have isomerized to the tertiary structure: (14) It was shown that all carbon atoms are equivalent in this process.17 Consequently the tertiary structure is ascribed to butyl ions fragmenting. Hydride abstraction is also thought to be the main reaction for C,X; ions formed in reaction (14): c, x,+ -+ c, x; + c x , .C,X,++C,X,, -+ C,X,+C,X[~. For the propyl ion high retention was found: C,H,D+/C,X; = 0.20 (i.c.r.) and C,D, H/C,X, = 0.18 (y-radiolysis). These values are lower limits since some C, H;f (C, D8) arises from reaction (1 6): S-C,X;~ -+ n-C,X:+C,X, AH = 44.5 kcal mol-l. (16) The same process needs only 28.5 kcal mol-l when s-C, X; is formed in a rearrangement reaction, as was found for hexyl radicals (formed by H- abstraction from hexane by C,X;) losing ethylene., Such a process might well be favoured because of the higher exothermicity of the initial proton-transfer reaction when H: or D3+ is taken as a reagent. In pentyl ions low label retention was found in the i.c.r. experiment: C, Hl,D+/C,X~l = 0.04. Since pentyl ions arise from different secondary reactions, no conclusions about reactions (3) and (4) could be drawn from y-radiolysis experiments, as it was not possible to determine the radiolytic hydrogen yield in the large amount of the reacting gas, hydrogen.Generally, label retentions in y-radiolysis are also reflected (as a mirror-image of those of fragment alkyl ions) by the isotopic composition of the neutral products of fragmentation reactions (3)-(8). Also, the G values of the corresponding products should be the same. For ethane (G = 0.8) and propane (G = 0.9) [table 1, column (i)] formed in reactions (7) and (8) and (12) and (13), respectively, this is in fact the case. Combining these G values with the isotopic abundances (table 2) reasonable agreement was observed for the corresponding products [G(C, H, D) = 0.43, G(C,D,) = 0.73 and G(C,D,) = 0.35, G(C,D,H) = 0.21)], taking into consideration that at least a part of C,D, originates from the fragmentation reaction (16).Comparing the products from reactions (5) and (6) and from subsequent reactions, reaching (lo), (1 l), (14) and (1 5), in the same way, G(butane) + G(propene) = G(buty1 ion) [reactions (5) and (6)] and G(methane) (after substraction of G(CD,) [formed in reaction (1 4)], which is equal to G(C, D6) = 0.1 8} should be equal. Yet the comparison [G(butane) + G(propene) = 1.18, G(methane) - G(propene) = 1.71 suggests an addi- tional path to methane formation, as will be discussed below.M. NEU M ANN-S PA L LAR T 1673 Summing up the i.c.r. intensities of C,X; and C,X; and comparing them with the intensities of C,X: and C,Xrl (table 4) the distribution does not correspond to the sequence of reaction enthalpies as it does in the CH, + C,H,, system (except for the hexyl ion)., One reason is the further fragmentation of the pentyl ion [reaction (16)], another might be the difference in the heats of formation of pentonium ions of different structures. Relatively large differerlces are to be expected between C-H and C-C protonated forms, which possibly may provide an explanation for the unexpectedly low reaction probability leading to the parent alkyl ions, as was found for both the CH,+C,H,, and CH,+C,H,, systems.It also follows from table 4 that secondary fragmentations play a much greater role in the low-pressure i.c.r. experiments, e.g.comparing the ratio of propane to butane and propyl ion to butyl ion. It is therefore difficult to estimate the relative amounts of initially formed fragment ions by y-radiolysis. Yet it is shown that at 10 Torr the ratio of propane to butane follows the order of reaction enthalpies for reactions (5)-(S), in contrast to the i.c.r. case. The very low retention in the butyl ion reflects the same trend, namely that methane loss, which demands less energy, is a faster process than ethane loss leading to propyl ions. The reactions forming pentyl ions [reactions (3) and (4)] have to be treated separately (see above). The estimated radiolytic yield (M(penty1 ion)/N+ = 1 - [M(propane)/ N+ + M(butane)/N+ + M(propene)/N+] = 0.22) is close to the value found in i.c.r. experiments (tables 3 and 4).A further comparison of the distribution of pentonium-ion fragments in i.c.r. and y-radiolysis shows that the most endothermic reaction, the formation of propyl ion, is relatively faster in the i.c.r. case. This can be explained by the fact that the protonating ions, Hi, are internally excited under i.c.r. conditions at lo-, Torr, as was demonstrated by Bowers et aZ.lS A relatively lower extent of proply-ion formation accompanied by a higher degree of label retention (33%) was measured by Ausloos in the y-radiolysis of a hydrogen+pentane mixture at 760 Torr,, if we interpret the data given in this reference in the manner given above; i.e. reaction (9) does not take place. The effect can be explained by de-excitation of H i and/or C,X;t, at high pressure.From reactions (3)-(S), (10)-(13) and (15) it follows that all ions initially formed should ultimately yield C,Drl with a theoretical M/N+ value of 1 (corresponding to G = 2.76). C,D;, plays the typical role of an unreactive ion in the system. It has been shownlo? l8 to undergo rearrangement to the tertiary structure. Neutralization must be considered as a predominant reaction for t-C,Dfl ions t-C, Drl + e- (t-C, D,,)* (17) forming highly excited pentyl radicals. In the y-radiolysis of n-pentanelO and the pulse radiolysis of neopentane,19 isopentenes and isobutene were found, inferring an ionic precursor of branched structure, namely the tertiary pentyl ion. Probable reactions of excited pentyl radicals are fragmentations : (t-C, Dll)* --* (CD,), C=CD CD, + D (1 8) (19) -+ CD=C(CD,)CD, CD, + D + (CD,), C=CD, + CD,.The products of reactions (1 8)-(20) were indeed found in our system, and no linear pentenes were detected. Thus it is proved that all pentyl ions have time to isomerize to the tertiary structure before neutralisation under the conditions given here. CD, radicals formed in reaction (20) are thought to combine with H atoms, present at high concentration in the system, thus explaining the relatively high G value of CD,H discussed above.1674 ION-MOLECULE REACTIONS I N H,+C,H,, Table 1 shows that the G values of all unsaturated products decrease strongly with increasing dose accompanied by an increase of G values of some radical products like isopentane, which can be suppressed almost completely with oxygen. On the other hand, oxygen, when added, increases the yields of unsaturated products by the protection of double bonds from radical attack.The isotopic distribution of 2- methylbut-1 -ene and 2-methylbut-2-ene shifts towards higher concentrations of higher deuterated compounds at lower conversions (table 2). The same effect is seen at high conversion when oxygen is added as a free-radical scavenger. This may be interpreted as the consequence of hydrogen-radical attack at the double bond forming isopentyl radicals, the disproportionation of which would again yield branched pentenes and isopentane. Their recombination product with hydrogen atoms is also isopentane. Indeed an increase of G(isopentane) with dose was observed. The same trends and patterns of isotopic distribution of isobutane (data not given) indicate that similar processes take place when isobutene is attacked by hydrogen atoms.The sum of G values of 2-methylbut-1 -ene, 2-methylbut-2-ene, isobutene and isobutane ( = 2.2) at the lowest dose used (table 1) accounts for 80% of the pentyl ions expected theoretically when all ions ultimately react to give these ions. The author thanks Drs R. Houriet and J. Dawson for help with the i.c.r. experiments, Ammanz Ruf for operating the gas-chromatography-mass-spectrometry system, Dr S. Lukac for many discussions and Prof. T. Gaumann for encouraging this work, which was supported by the Swiss National Foundation. APPENDIX The following values were used for the calculations of heats of reaction, AH: A,(H,) = 101 kcal mo1-l A,(C5 HI,) = 157 kcal mol-l AH,(s-C, HTl) = 176 kcal mol-1 AH,(p-C, Htl) = 195 kcal mol-I (estimated by extrapolation from known proton affinities of the lower hydrocarbons).’ and were calculated from AH,(s-C,H,,) = 8 kcal mol-l, AHf(p-C,Hll) = 11 kcal mol-l, ionization potential (s-C,H1,) = 168 kcal mol-l, ionization potential (p-C, Hll) = 184 kcal mol-1 [all estimated by extrapolation from data of ref.(20)]. AH,(s-C,H;) = 183 kcal mo1-l AH,(s-C,H;) = 192 kcal mol-I AHf(Cz H$) = 119 kcal mol-’ were taken from ref. (20). S. G. Lias and P. Ausloos, Ion-Molecule Reactions-Their Role in Radiution Chemistry (American Chemical Society, Washington, D.C., 1975). R. Houriet, G. Parisod and T. Gaumann, J. Am. Chem. SOC., 1977,99, 3599. R. Houriet and T. Gaumann, Int. J. Mass Spectrom. Ion Phys., 1978, 28, 93. M. Neumann-Spallart and S. LukaC, Radiat. Phys. Chem., 1980, 15, 723. P. Ausloos and S..G. Lias, Discuss, Faraday Soc., 1965, 39, 36. T. Gaumann, H. Oz and 0. Piringer, Helu. Chim. Acta, 1978, 61, 258. I. Janovsky, J. Radiat. Phys. Chem., 1976, 8, 396. D. W. Huyton and T. W. Woodward, Radiat. Res. Rev., 1970, 2, 205.M. N E U M A NN-S PALL A R T 1675 S . LukaE, Chromatographia, 1979, 12, 17. lo S . LukaE, Radiat. Phys. Chem. 1980, 15, 713. l1 A. Henglein, W. Schnabel and J. Wendenburg, Einfiihrung in die Strahlenchemie (Verlag Chemie, Weinheim, 1969). K. Hiraoka and P. Kebarle, J. Am. Chem. Soc., 1976, 98, 61 19. Spectroscopy, ed. H. Hartmann and K. P. Wanczek (Springer, Berlin, 1978). 13 P. Houriet and T. Gaumann, Lecture Notes in Chemistry, vol. 7, Ion Cyclotron Resonance l 4 P. Ausloos, S. G. Lias and R. Gordon Jr, J . Phys. Chem. 1963, 39, 3341. l 5 L. W. Sieck and S. G. Lias, J. Phys. Chem. Ref. Data, 1976, 5, 1123. I6 D. M. Shold and P. Ausloos, J. Am. Chem. SOC., 1978, 100, 7915. 1’ Hei-Wun Leung, Chun Wai Tsang and A. G. Harrison, Org. Mass Spectrom., 1976, 11, 664. T. Su and M. T. Bowers, J. Am. Chem. SOC., 1973,95, 761 1. l 9 R. E. Rebbert and P. Ausloos, J. Res. Natl. Bur. Stand. Sect. A, 1972, 76, 329. 2o F. P. Lossing and G. P. Semeluk, Can. J. Chem., 1970, 48, 995. (PAPER 1 / 1325)