Free Radicals by Mass Spectrometry. XLIII. Ionization Potentials and Ionic Heats of Formation for Vinyl, Allyl, and Benzyl Radicals'

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1 Free Radicals by Mass Spectrometry. XLIII. Ionization Potentials and Ionic Heats of Formation for Vinyl, Allyl, and Benzyl Radicals' F. P. LOSSING Division of Chemistry, National Research Colrncil of Canada, Otta~va, Canada Received August 27, 1970 Using an energy-resolved electron beam, ionization potentials for the following free radicals have been measured: vinyl 8.95 V, allyl 8.07 V, benzyl 7.27 V. For vinyl and allyl ions new measurements of thresholds for dissociative ionization give AHf(C2H3+) = 266 kcal/mol and AHf(C3H5+) = 226 kcal/mol, leading to neutral radical heats of formation AHf(C2H3) =, 59.6 kcal/mol and AHr(C3H5) = 40 kcal/mol. The data for benzyl radical and ion give AHr(benzyl cat~on) = 213 kcal/mol. Canadian Journal of Chemistry, 49, 357 (1971) Introduction The ionization potential values available for vinyl, allyl, and benzyl free radicals were measured some years ago, using conventional mass spectrometer ion sources. It is now well-known that, as a result of low resolution in energy, ionization thresholds determined in this way are generally higher than the adiabatic IP by V, the error becoming larger with unfavorable Franck-Condon factors in the threshold ionization region. Recent electron impact measurements, either by the Retarding Potential Difference (RPD) method or by the use of true energy-resolved electron beams, are in satisfactory agreement with the more precise results obtainable by photoionization (PI) or photoelectron spectroscopy. The results of a recent study (1) of the ionization potentials of C,-C, alkyl radicals, using an energy-resolved electron beam from a two-stage double-hemispherical energy selector, show that good agreement now holds among the IP values for these radicals as measured by the RPD method, photoionization, and monoenergetic electron impact, even in cases where the ionization efficiency curves in the threshold region are decidedly non-linear. The present work reports new IP values for vinyl, allyl, and benzyl radicals using a monoenergetic electron beam with an energy width at half-height of about 0.07 V. For vinyl and allyl radicals, appearance potentials for formation of the corresponding C2H3+ and C3H5+ ions in dissociative ionization of suitable com~ounds have also been obtained. Heats of formation of the ionic and neutral 'NRCC No species have been derived, and compared with available literature data. The situation is more complicated for benzyl ion, since C7H7+ ions formed by dissociative ionization processes have almost certainly the tropylium ion structure (233). Experimental The measurements were carried out using the electron energy selector, ionization chamber, ion extractor, and quadrupole mass filter previously described (4). Radicals were produced in sufficient concentrations by pyrolysis of suitable compounds (see below) at pressures of about Torr in a fused-silica capillary furnace (5). Performance characteristics of this apparatus for some simple molecules (6) and radicals (1, 7) have been reported previously. Results and Discussion Vinyl Radical and lon In earlier measurements on vinyl radical (8) the radical was produced by the pyrolysis of methylvinyl mercury. In the present work the radicals were obtained more conveniently and in somewhat better yield from the pyrolysis of divinylsulfone. Some C2H, and C,H, were also formed by secondary reactions. Contributions from these species to the mle 27 peak were shown to be negligible with electron energies in the range used for measuring the threshold ionization of C2H3 radical. A typical threshold ionization curve for C2H3 radical is shown in Fig. 1, along with that for krypton added to the gas stream for calibration of the energy scale. This curve appears to exhibit one or more short linear sections, possibly corresponding to vibrational levels of the C2H3+ ion.

2 358 CANADIAN JOURNAL OF CHEMISTRY. VOL. 49, 1971 t l1 VINYL RADICAL '"1 VINYL RADICAL FIG. 1. Threshold ionization curve for vinyl radical produced by pyrolysis of divinyl sulfone. The reality or otherwise of this structure may be judged from Fig. 2, which gives the first derivative of the data points of Fig. 1. No smoothing was employed. Extrapolation of the first "linear" segment of Fig. 1 gives 8.95 V for the ionization potential of C2H3 radical. An upward break at around 9.4 V appears to correspond to the earlier values of V obtained for IP(C2H3) using conventional ion sources (8, 9). Owing to lack of energy resolution in the earlier measurements, the lowest segment of the curve was not observed. It is difficult to say whether the present result represents the true adiabatic IP, or whether it should be considered merely an upper limit. From Fig. 1 it appears improbable that the curve extends below 8.95 V but this possibility cannot be entirely ruled out. There appears to be no other literature value, either experimental or theoretical, for this ionization potential aside from the earlier low resolution results cited above. It may be noted for comparison that this value for the ionization potential of vinyl is 0.9 V lower than that of CH, radical (9.84 V) and about 0.6 V higher than that of C2H5 radical (8.38 V) (1). Current theoretical treatments for ionization energies of radicals appear to be insufficiently reliable to enable an estimate for IP(C2H3) to be made FIG. 2. First differential curve for vinyl radical ionization. The arrow marks the ionization potential obtained from Fig. 1. Three recent measurements of the photoionization threshold for the dissociative process have been made, giving the appearance potential AP(C2H3+) as V (lo), k 0.03 V (ll), and V (12). It seems probable that the lowest value is to be preferred. From standard heat of formation data this value gives AH,(C2H3+) = 266 kcal/mol. The reason for the difficulty in measuring this threshold, even by the usually precise PI method, is the low probability for C2H3+ ion formation near the energy threshold, resulting from the competing process [3 1 CzH4 + hv -t C2H2+ + Hz + e for which the appearance potential is slightly lower (10, 12). Earlier electron impact values for AP(C2H3+) are in the range V using a conventional ion source (8, 13) and V using RPD electron impact (14). In view of this difficulty no attempt was made to remeasure this appearance potential in the present work. As an alternative, the appearance potentials for C2H3 + ion formation from vinyl chloride and vinyl bromide were examined. The curves found for C2H3+ ion formation from these compounds exhibited a feature not observed for other fragment ions examined so far with this apparatus. As can be seen in Fig. 3, the ionization efficiency curve for C2H3+ production showed a "tail" of low C2H3+ yield, having a nearly linear form, followed by an abrupt upward break corresponding to a sudden large increase in C2H3+ yield. Comparison with the shape of the krypton standard curve indicates that this break is at

3 LOSSING: FREE RADICALS BY MASS SPECTROMETRY. XLIII VINYL CHLORIDE FIG. 3. Appearance potential curve for formation of C2H3+ ion from vinyl chloride. The arrow marks the onset for the production of C2H3+ and neutral C1 atom. about 12.5 V. It seems probable that the "tail" can be associated with the ion pair process and that the upward break corresponds to the threshold for the process with this assumption, AP(C2H3+) = 12.5 V gives AHf(C2H3+) = 268 kcal/mol. A curve of nearly identical form was obtained for vinyl bromide, with an upward break at 11.9 V, corresponding to AHf(C2H3+) = 266 kcal/mol. These results are in good agreement with the photoionization value (12) of AHf(C,H3+) = kcal/mol. No definite value could be obtained for the onset of the "tail" portion of the C2H3+ curves, owing to the low probability for this process and the uncertainty as to the true shape of its ion yield curve. From AH,(C2H3+) = 266 kcal/mol and IP(C2H3) = 8.95 V one gets AHf(C2H3) = 59.6 kcal/mol, corresponding to a bond dissociation energy D(C2H3-H) = 99.2 kcal/mol. Aside from the earlier electron impact work, which is based on erroneously high values for AP(C2H3 +) and IP(C2H3), experimental data for this bond are rather sparse. A pyrolytic study of divinyl mercury using the toluene carrier technique gave D(C2H3-H) kcal/mol (15, 16) with a rather large uncertainty. From unpublished results on the reaction of HI with vinyl iodide, Golden and Benson (17) deduced D(C2H3-H) f 2 kcal/mol, correspond- ing to AHf(C2H3) > 68.4 kcal/mol. Assuming the value of AHf(C2H3+) = 266 kcal/mol to be correct, this heat of formation for C2H3 would require IP(C2H3) to be < 8.57 V, about 0.4 V lower than the presently reported value. This dis- crepancy is large enough to raise the question as to whether the C2H3+ ion formed at the dissociation threshold for ethylene and vinyl halides has some other minimum energy configuration differing substantially from the "vinyl" structure. Ally1 Radical and lon A good yield of allyl radical was obtained by the pyrolysis of allylmethyl nitrite, prepared from a sample of 3-butene A threshold ionization curve for allyl radical is given in Fig. 4. The curve exhibits one or perhaps two breaks above the threshold, which is at 8.07 V with a probable uncertainty of about f 0.03 V. A first differential curve for these data points is given in Fig. 5, which shows clearly a second "step" at 8.22 V and just possibly a third at about 8.45 V. Earlier FIG. 4. Threshold ionization curve for allyl radical, produced by pyrolysis of allylmethyl nitrite.

4 360 CANADIAN JOURNAL OF CHEMISTRY. VOL ALLYL RADICAL I O I I FIG. 5. First differential curve for allyl radical ionization, showing two and possibly three "steps" I ELECTRON ENERGY iev) FIG. 6. Threshold curve for formation of C3H5+ ion from propylene. Curves for C3H5+ formation from 1- and 2-butene were of the same form. measurements of 8.16 V (18) and V (9) for IP(ally1) using conventional ion sources presumably correspond approximately to an unresolved maximum Franck-Condon factor at an energy of about 8.2 V. With some reservations, one can assign the onset at 8.07 V to the adiabatic IP. The presence of another "segment" below this value looks improbable but cannot be entirely ruled out. The appearance potentials for C3H5+ fragment ion from propylene, 1- and 2-butene were also measured. The efficiency curves were all of the apparently structureless type shown in Fig. 6. The estimated thresholds are given in Table 1, along with some earlier values measured by conventional methods (19) and by a second differential ionization method (20). No photon impact or RPD electron impact values for C3H,+ appear to have been published. From standard heats of formation these appearance potentials lead to AHf(C3H5+) = 226 kcal/mol with a probable uncertainty of 1 or 2 kcal/mol. Subtracting IP(ally1) = 8.07 V gives AHf(allyl) = kcall mol, in reasonable agreement with recent measurements by kinetic methods of 41 + l kcal/mol (17, 21). Appearance potential data for C3H5+, as collected in a recent compilation (22), give a wide range of values of AHf(C3H5+), from 216 to more than 260 kcal/mol. This indicates that although m/e 41 is a relatively abundant ion in the mass spectra of many hydrocarbons and allyl derivatives, its formation is not a particularly simple process (23). Benzyl Radical and Ion A reasonably good yield of benzyl radical was obtained by the pyrolysis of benzylmethyl nitrite at moderate furnace temperatures (24) The threshold ionization curve for benzyl radical (Fig. 7) was strongly curved, and showed no evident structure over the first 2.5 V portion. In the absence of any linear or quasi-linear section at the onset, an extrapolation to a well-defined threshold cannot be made. A comparison of the curved foot in the inset of Fig. 7 with the curvature of the foot of the krypton standard curve, resulting from the 0.07 V energy half-width, indicates that the onset of the benzyl curve cannot be lower TABLE 1. Appearance potentials for C3H5 + ion formation Compound AP (V) AH,(C3H5+) (kcal/mol) Previous AP (V) Propylene " 1-Butene a,11.4b 2-Butene "

5 LOSSING: FREE RADICALS BY MASS SPECTROMETRY. XLIII 361 CsHsCHz* -+ C7H7+ (tropylium) + e Formation of the excited state C6H5CH2* by photon impact could be spin-forbidden, making this mechanism inoperative or improbable for photoionization. It is of interest in this respect, particularly in view of the well-known isomerization (2, 3) _ ' j2.$4 ' $6 ' ik ' 20 ' 8'2 ' a' ELECTRON ENERGY I ev) FIG. 7. Threshold ionization curve for benzyl radical, produced by pyrolysis of benzylmethyl nitrite. Points A and B are estimated lower and upper limits for the onset of ionization. than point A (7.25 V) nor higher than point B (7.30 V). The best way to express this result is as the limit IP(benzy1) < 7.27 t V. This is appreciably lower than the value of 7.76 V obtained earlier by electron impact in a conventional ion source; evidently the curvature of the foot of the benzyl curve was seriously underestimated in the earlier measurement. There is, however, an unresolved discrepancy between the present value, and a value of 7.63 t obtained by photoionization (25,26). There seems no reason to suspect in either experiment that isomerization of benzyl radicals to tolyl or cycloheptatrienyl radicals, which are less stable than benzyl, has occurred. In any case, the IP of cycloheptatrienyl, V (26,27) is much lower than either of the experimental values for benzyl. The ionization potential of tolyl radical, CH,C,H,, is probably not too different from that of toluene2 and consequently too high to be a source of confusion in these experiments. The possibility of a real difference between the electron and photon impact experiments does exist, although it is rather unlikely. Ionization of benzyl radical by electron impact could conceivably occur through formation of a discrete excited neutral state, which then rearranges and ejects an electron to form tropylium ion. 'Compare, for instance IP(pheny1) and IP(benzene) (ref. 22). to calculate heats of formation for these two ionic species from the current data. Taking IP(cyc10- heptatrienyl) = 6.24 V (26,27), D(C7H7-C7H7) in ditropenyl = V (26), and AH,(ditropenyl) = 94.6 kcal/mol (28), one ob- IJIII/.I.11/1 tains AH,(cyclo-C7H7+) = 212 kcal/mol. A slightly lower value for AHf(C7H7+), 209 kcall mol, has been suggested from a measurement of the enthalpy of dissociation of bitropenyl (29). Taking AH,(benzyl) = 45 kcal/mol (16) and IP(benzy1) = 7.27 V, one obtains AHf(C,H5- CH2+) = 213 kcal/mol. On this basis, reaction 6 is thermoneutral or only slightly exothermic. This result is surprising in view of the universality of this reaction for benzyl ion and its analogs (3), and tends to support the possibility that the present electron impact IP for benzyl is too low because of some rearrangement as suggested above. 1. F. P. Lossm~ and G. P. SEMELUK. Can. J. Chem. 48, 955 (1970). 2. P. N. RYLANDER, S. MEYERSON, and H. M. GRUBB. J. Amer. Chem. Soc. 79, 842 (1957). 3. H. M. GRUBB and S. MEYERSON. In Mass Spectrometry of Organic Ions. Edited by F. W. McLafferty. Academic Press, New York Chap K. MAEDA. G. P. SEMELUK. and F. P. LOSSING. Intern. J. hiass Spectry. Ion phys. 1, 395 (1968). 5. J. B. FARMER and F. P. LOSSING. Can. J. Chem. 33, 861 (1955). 6. F. P. LOSSING and G. P. SEMELUK. Intern. J. Mass Spectry. Ion Phys. 2, 408 (1969). 7. T. MCALLISTER and F. P. LOSSING. J. Phys. Chem. 73, 2996 (1969). 8. A. G. HARRISON and F. P. LOSSING. J. Amer. Chem. Soc. 82, 519 (1960). 9. D. BECK and 0. OSBERGHAUS. Z. Phys. 160, 406 (1960); D. BECK. Discuss. Faraday Soc. 36, 56 (1963). 10. R. BOTTER, V. H. DIBELER, J. A. WALKER, and H. M. ROSENSTOCK. J. Chem. Phys. 45, 1298 (1966). 11. B; BREHM. Z. Naturforsch. 21a, 196 (1966). 12. W. A. CHUPKA, J. BERKOWITZ, and K. M. A. REFAEY. J. Chem. Phys. 50, 1938 (1969). 13. C. LIFSHITZ and F. A. LONG. J. Phys. Chem. 67, 2463 (1963). 14. J. E. COLLIN. Bull. Soc. Chim. Bel. 71, 15 (1962).

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