chemical physics of discharges - Argonne National Laboratory

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A summary of the products formed in the R. F. powered dfscharge is presented
in figure L. It should Fe noted that these results am normalized to exclude
recovered tolmne, non-oondensatle cases and polymeric materials which in
combination accounted for 3 to 6 of the oripinal toluene vapor.
The formation of Substantial amounts of dimer biaryls; bibenzyl, diphenylmethane
and biphenyl is of prticulqr signifiance since, as noted above, the formation
of only trace amounts of dlmer bisryls vas observed in the microwsve
discharqe. Additional experiments -re condmted in which a helium carrier gas was
uaed, and no substantial difference in the nature and amounts of products formed was
noted. The chanp in product ratios, therefore, appears to reflect a change In
mechanism vhich results frm the use of an R. F. povered discharee.
The products formed jn this study, in particular the large amounts of biaqls,
indicate that free radicals am the major intennediatee leadlne to the formation of
products. The compounds fonned can be readily explained by the series of radical
combination and hydrown sbstraotion reactions involving benzyl, phenyl and methyl
radicals which are outlined in fieurn 5.
It should be noted that minor, but definite amounts of the three xJrlene isomers
-re fomed In the radiofrequancy discharee. The trace amounts of these isomers
which were formed from the dlssociation of toluene raper in a microwave powered discharge
hss previously been used aq an a ament against the presence of methyl
radlcals in the mlcrowave discharp. (3f As is shovn in figure 6, however, it has
been demonstrated that methyl radlcals mact vith toluene wp r with 3 hundred fold
preference for the side chafn rathe- than the rinc positions.~4) It is, therefore,
entirely co~slstent with the presence of methyl radicals In the R. F. disoharee that
msctfon should take place predominantly vith the toluene slde chain to fonn
ethylbenzene rather than with the rlng positiona to yield xylene isomers, although
the formation of small amounts of qlene should be anticipated.
Experiments in which speoifically labeled deuteriotoluene was passed through
the R.F. disoharee afforded additional experimental data which supported the
importance of radioal intermediates. The producte formed from the labeled toluene
were collected, separated by chromatoyraphio techniques, and the distribution of the
deuterium label determined by infrared and nuclear magnetic resonance epctroscopy
and mass spctmmetry.
The slde chain of the recovered ethylbenzene was found to be heavily deuterated.
The partisl pass spectrum of the recovered ethylbenzene compared to the mass
speotrm of un-deuterated raterfal is shevn in figure 7. The parent peak of the
recovemd material is five mass units higher than the corresponding peak in the
un-deuterated material, and deronstrates the incluslon of five deuterium atoms in
the molecule. The bene pak of the unlabeled ethylbenzene molecule resulte from a
P-15 cleawae correspondinp to the lam of a methyl group. In the spectrum of the
reoovemd saterial, the base peak results from P-18 cleavaae and clearly
demonotrataa the rethyl group of the side chain to be fully deuterated. This
molecule preswmbly forms from the combination of C7H5D2 radical vith a CD3 radical.
The distribution of the deuterium label in the benzyl frapent can be demonstrated
by considerlnc the WR epectrm of the recovered bibenzyl. In figure 8, the
100 VC. proton rapetic ma30nance spectrum of un-deutered bibenzyl and that of the
mcovered material ere compared. Ria important to note that the 5:2 ratio of
aromatic to sethylene protons observed in the non-deutereted material vould also be
observed with the prtjally deutemted nolecule if the deuterium label vere uniformly
distrituted. The ematly reduced methylene proton peak in the spectrum of the
remwred material, however, clearly demonetrates that the label remains localiced
fn the e!de chain of the hnayl fragment, and is not distributed throughout the
ring.
This result is entfrely consistent vlth the presence of fme radical interlpsdiatee
in the discharge slnce, as io show in fim 9, it has been demonstrated
that the un=ioniced benzyl radical does not undergo any type of rearrangement vhkb
t in the randomlzatlon of a label, such as the fornation of the tropyliw
z%a?eg By contrast, the benzyl cation doee maergo an immediate arranqement to
1ltm lon, which vould result in uniform distribution of the deuterium
labl. Since this is not observed, the benzyl catien can be ruled out as a
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