Assuming that in the solid phase experiments the disproportionation reaction

(8) [CH CD (or CD2CH)+D (or H)]→ CHCD+HD

occurs with a low probability, it may be concluded that molecular hydrogen elimination processes occur via the same excited state in the gas as in the solid phase independent of the energy of the photon. In gas phase photolysis experiments the yield of acetylene is always considerably higher than that of "molecular" hydrogen (C2H2/H2=2.8 at 10 eV). The occurrence of process 3 (process 7 in the case of CH2CD) accounts for this, as is clearly illustrated by the fact that in all gas phase CH2CD experiments the abundance of CHCD in the acetylene fraction is considerably higher than that of HD in the hydrogen fraction. In the solid phase photolysis at 11.6-11.8 eV and in the radiolysis the abundance of CHD in the acetylene fraction is somewhat higher than that of HD in the hydrogen fraction indicating that at these energies reaction 7 may occur. Disproportionation reactions involving CH2CD CD CH and any other radical are however a more likely source of the excess CHD in the solid phase.

The absence of HD in the photolysis and radiolysis of C2D4:C2H4 (1:1) mixtures demonstrates that if H(D) atoms are indeed eliminated in the primary process they do not combine with other H atoms to form hydrogen in these experiments. The most probable fate of these H atoms especially if they are formed with excess kinetic energy, is addition to ethylene to form ethyl radicals:

C2H+C2H1→CH 138.3

× 10-10cm3/molecule-s.

In the presence of a charge acceptor (CA) such as (CH3)2NH or NO, the C4H ions have been shown to react to form C4H8 products [16]

whose structures are assumed to correspond to the structures of the precursor C4H ions. In the gas phase at pressures of 100 torr or less, reaction sequence 10-11 leads to the formation of 2-butene and isobutene: the relative amounts of these isomeric C4H8 products depend on the energy in the CH ion [17]. As the pressure is raised, for example, the formation of iso-C4Hs (i.e., iso-C4H) is quenched. If reaction 10 can compete with neutralization of the C2H; ions in the solid phase, it is possible that the resulting C4H ions might lead to the formation of C4Hs products. through neutralization of the ion:

It has actually been suggested before [3c] that in the solid phase radiolysis reaction 10 followed by 12 might account for the formation of 1-butene.

The results given in table 1 show that 1-butene is the most important C4H8 product in all the solid phase experiments. More than 80 percent of the 1-butene formed in the photolysis and radiolysis of C2D-CH; (1 : 1) mixtures (table 4) consists of C4H8, CH1D, and CDs. That is, 1-butene seems to be formed mainly by the combination of two C2H4(C2D4) units.

As mentioned above, in the gas phase very little. 1-butene is formed in reaction sequence 10-11; isomerization of the CH ions formed in reaction 10 to the 1-CH structure is evidently an improbable. process under those conditions. The fact that the yield of 1-butene is relatively large in the 8.4 eV photolysis where ionization is presumably unimportant, and furthermore undergoes a relatively small increase with increasing energy, seems to indicate that the 1butene is at least in part formed through a nonionic mechanism. A plausible mechanism which would lead to the observed isotopic distribution in the C2H1-C2D1 : 1) experiment would be H atom elimination from ethylene and addition of the hot H-atom to a neighboring ethylene molecule (reaction 9), followed by a recombination in the cage of the two radicals

(reaction 13).

[CH2CH+CH2CH3] → CH2 =CHCH2CH3.

It is of interest that the increase in the yield of 1butene with photon energy parallels the increase in the yield of n-butane, which as we have shown above, is formed in a reaction sequence involving H atom addition to ethylene. The presence of deuterium labeled butenes other than C4Ds and CDH in the photolysis and radiolysis of C2H4-C2D, mixtures might be explained by the participation of diffusive recombination of vinyl and ethyl radicals in the overall 1-butene production. Increased diffusion would explain the observation that butenes such as C1H2D, C1D2H, CHзD, and C3H5Dз are formed at relatively higher yields in the liquid phase [3d] than in the solid phase radiolysis and that in the liquid phase they increase relative to the yields of C4H8, C4H4D4 and C4D8, with an increase in temperature [3d].

It is understood that proposed mechanism is a tentative one and that other mechanisms such as those proposed in previous studies cannot be ruled out.

Of the C4H products formed in the solid phase irradiations of ethylene (table 1) there is one product, cyclobutane, which seems to be formed via a C2H intermediate. This statement is based on the fact that the relative yield of cyclobutane is very small in the photolysis with 8.4 eV photons where presumably few ions are formed (cyclobutane was not even detected in the earlier study [1] at this energy), and increases by nearly an order of magnitude when the photon energy is raised to 10.0 eV and again increases when the energy is raised to 11.6-11.8 eV. This large increase in yield with energy can be contrasted with the yields of the butene products, which increase by less than a factor of two when the energy is augmented from 8.4 eV to 11.6-11.8 eV. Furthermore, the fact that approximately 90 percent of the cyclobutane formed in the photolysis and radiolysis of C2D4-C2H4 mixtures consists of C4H8, C4H4D1, and CDs (see Results) indicates that this product is formed mainly in a reaction of C2H4 (C2D4) entities as has been shown before [3d] in the liquid phase radiolysis of ethylene. Cyclobutane is also formed in the mercury photosensitized photolysis [18] of ethylene at a pressure of 700 torr, with a quantum yield of 3.8 × 10-6. In those experiments, the cyclobutane product was suggested to be formed as a result of a reaction between triplet state ethylene and ground state ethylene:

Cyclobutane has been reported [19] as a product (G~0.1) in the gas phase radiolysis of C2H4 (pressure: 100 torr) and ascribed to the participation of triplet ethylene. Reaction 17 cannot be operative in the 10 and 11.6 eV photolysis experiments, since the ejected electrons will have insufficient energy to bring about the optically forbidden transition to the lowest triplet state at 3.6 eV [20].

The 2-butenes are formed as minor products in all the solid phase irradiations given in table 1. Their yield shows little or no variation with increasing energy; hence, their formation is probably not associated with an ionic process. This is of interest, since, as indicated above, in the gas phase at pressures of 100 torr or less, it has been shown that reaction 10 of the ethylene ion with ethylene leads mainly to the formation of 2-C4H ions. Thus, at any rate, the absence of 2butene as an important product in any of the solid phase experiments demonstrates that C4H ions formed in reaction 10 do not undergo neutralization to form 2-C4H8, under these conditions.

Methylcyclopropane is also formed in small yields in all the solid phase photolysis and radiolysis experiments reported in table 1. In experiments carried out with C2H4:C2D4(1:1) mixtures, about 75-80 percent of this product consists of C4H8, C4H4D4, or C4D8. In an earlier study [1] of the solid phase photolysis of ethylene at 8.4 eV, the formation of methylcyclopropane was attributed to a reaction of an ethylidene with ethylene:

Although this mechanism which requires a rearrangement of a long-lived excited ethylene molecule would indeed account for the isotopic distribution of methylcyclopropane products formed in the C2H4:C2D4 mixture, a free radical mechanism occurring in the cage may also explain the experimental observation. Addition of CH2CH to C2H, occurs with a low activation energy (0.14 eV) for thermal CH2CH radicals and may involve a neighboring molecule, especially if, as in the gas phase, the CH2CH retains some internal energy after its formation. Such an addition process would lead to the formation of the 3-butenyl radical [3f] and conceivably also of the methylene cyclopropyl radical. These two radicals may capture a neighboring hydrogen atom to form dimeric 1-butene and methylcyclopropane respectively.

phase photolysis and radiolysis of ethylene. Indeed, the isotopic composition of the cyclopropane produced in the photolysis and radiolysis of a C2H4: C2D4 (1:1) mixture (see Results) indicates that more than 80 percent of the cyclopropane consists of c-C3H6, c-C3H4D2, c-C3D4H2, and c-C3D6. This distribution is consistent with the formation of cyclopropane through addition of CH2(CD2) to C2H4(C2D1). Insertion of a CH2 species into a C-H bond of ethylene would also lead to the formation of propylene

4.4. The Formation of Hexenes

Several C6H12 products are formed in the solid phase irradiations. The total yields of these products are listed in table 1.

The yields of the hexene products are relatively small in the 8.4 eV photolysis, but undergo a large increase when the energy is raised to 10.0 eV, and more than double again when the energy is further raised to 11.6-11.8 eV. These observations are very similar to those made above concerning the yields of the cyclobutane product. As in that case, we can infer that the large increase in yield when the energy is raised to a point where ionization is certainly of importance (10.0 eV) may be related to participation of ions in the formation of the products in question. It has been reported before [3c] that the 2-hexene product formed in the radiolysis of a CD, -C2H, (1 : 1) mixture at 77 K consisted of more than 75 percent C6H12, C6H1D8, CH&D, and C6D12. In the present study (see Results) it is seen that the 2-hexenes, as well as the 1-hexene product have a similar isotopic composition when an equimolar ethylene mixture is irradiated with 11.611.8 eV photons. That is, most of the hexene products are evidently made up of C2H1(C2D4) units.

In the earlier radiolysis study [3c] it was suggested that hexene is formed in a process initiated by reaction of the CH; ion with ethylene and terminated by electron recombination with a CH species. The present

[1] Tschuikow-Roux, E., McNesby, J. R., Jackson, W. M., and Faris, J. L., J. Phys. Chem., 71, 1531 (1967).

[2] For a review see Holroyd, R., in Fundamental Processes in Radiation Chemistry, Ausloos, P., Ed. (1968).

[3] (a) Chang, P. C., Yang, N. C., and Wagner, C. D., J. Am. Chem. Soc. 81, 2060 (1959); (b) Collinson, E., Dainton, F. S., and Walker, D. C., Trans. Faraday Soc. 57, 1732 (1961); (c) Wagner, C. D., J. Phys. Chem. 66, 1158 (1962); (d) Holroyd. R. A., and Fessenden, R. W., J. Phys. Chem. 67, 2743 (1963); (e) Wagner, C. D., Trans. Faraday Soc. 64, 163 (1968); (f) Fessenden, R. W., and Schuler, R. H., J. Chem. Phys. 39, 2147 (1963); (g) Brash, J. L., and Golub, M. A.. Can. J. Chem. 46, 593 (1968); (h) Klassen, N. V., J. Phys. Chem. 71,2409 (1967); (i) Wagner, C. D., J. Phys. Chem. 71. 3445 (1967).

[4] Rebbert, R. E., and Ausloos, P., J. Chem. Phys. 46, 4333 (1967).

[5] Scala, A. A., and Ausloos, P., J. Chem. Phys. 47,5129 (1967)) [6] Gorden, R., Jr., Rebbert, R. E., and Ausloos, P., Natl. Bur. Std. Technical Note 496 (1969).

[7] (a) Chupka, W. A., Berkowitz, J., and Refaey, K. M. A., J. Chem. Phys. 50, 1938 (1969); (b) Botter, R., Dibeler, V. H.. Walker, J. A., and Rosenstock, H. M., J. Chem. Phys. 45. 1298 (1966); (c) Brehm, B., Z. Naturforschg. 21a, 196 (1966 ; (d) Al-Joboury, M. I., and Turner, D. W., J. Chem. Soc.. 4434 (1964).

[8]

[9]

Vermeil, C., Matheson, M., Leach, S., and Muller, F., J. Chim. Phys. 34, 596 (1964).

Gorden, R., Jr., and Ausloos, P., J. Chem. Phys. 47, 1799 (1967).

[10] Tiernan, T. O., and Futrell, J. H., J. Phys. Chem. 72, 3080 (1968).

[11] (a) Stevenson, P., Rad. Res. 10, 610 (1959); (b) Ausloos, P.. and Lias, S. G., Actions Chimiques et Biologiques des Radiation, 11, 1 (1967).

[12] (a) Sauer, M. C., and Dorfman, L. M., J. Chem. Phys. 35, 497 (1961); (b) Okabe, H., and McNesby, J. R., J. Chem. Phys. 36,601 (1962); (c) Ausloos, P., and Gorden, R., Jr., J. Chem Phys. 36, 5 (1962).

[13] Back, R. A., and Griffiths, D. W. L., J. Chem. Phys. 46, 4839 (1967); and Borrell, P., Cashmore, P., Cervenka, A., and James, F. C., J. Chim. Phys. 229 (1970).

constants.

1. Introduction

Recent publications from this laboratory [1] have illustrated that a photoionization mass spectrometer is advantageous for studying the reactions of parent hydrocarbon ions. In some cases selective ionization of hydrocarbons in the presence of organic or inorganic additives may be achieved by proper utilization of the line emission (123.6, 116.4, or 106.7 nm) of rare gas resonance lamps. Furthermore, when ionization is induced by photoabsorption at energies only slightly above the ionization threshold, it is possible to investigate the reactions of parent ions with parent molecules in the absence of those complicating reactions associated with those fragment ions which would be produced at higher energies. The fact that parent ion-parent molecule reactions could be investigated at thermal kinetic energies in such an instrument at room temperature over a considerable pressure range was instrumental in the elucidation of the kinetics of formation and reaction of alkane dimer ions (CnH2n+2)[le, lf].

The present photoionization study differs from those mentioned above in that C4-C alkanes were irra

"This research was supported by the Atomic Energy Commission. **NBS Postdoctoral Research Associate 1968-70.

diated with photons of sufficient energy to induce unimolecular fragmentation of the parent ion. The reactivities of the resultant fragment ions towards the alkanes were derived from the variation of the composite mass spectrum of a particular system as a function of pressure. The ion-molecule chemistry occurring in some of these alkanes (n-C4H10, i-C4H10, and neo-CsH12) has been investigated previously by kinetic mass spectrometry using high energy electrons [2]. In those studies, however, the temperature of the ion source was approximately 500 K and the electron energy was well above that of the photons used in this investigation. Consequently, a considerable variety of fragment ions were produced and it was difficult in some instances to determine which of the many fragment ions were responsible for the formation of ionic products.

It was also considered of interest to compare rate data obtained from this study with that derived from analysis of the neutral products of ion-molecule reactions occurring in alkane systems [3]. Such a comparison is of particular importance since end product analysis has recently revealed the formation of alkyl and olefinic ions with more than one structure in the unimolecular decomposition of n-alkane and cycloalkane parent ions [4]. Since isomeric ions are known to exhibit different reactivities, one might expect to de