chemical physics of discharges - Argonne National Laboratory

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The Dlssocfatior of ToIuere VaFor in a Radiofrequency Discharge
Frank J. Dinan
Departrent of Cher.istry, Carisius Collem , Buffalo, Nev Tork
The emission spctra resulting from the excitation of toluene wpcr in electrode
discharges have teen investiqatec' by Schuler as a part of his extenelre work
dealine dth the behavior of Folecules in discharps.(l) He found that two distinctly
diffennt spectra worn emitted. One, the normal toluene emission rpectrum,
WE centered in the 2600-3000 A' naion snd was related to the forbidden benzenoid
abeorption band of toluene in the crude minor Image relationehip vhich characbriz%s
fluorescence emission. This spectrum, as it was obtained In the premnt study,
le shown In figure 1. In addition to this ultraviolet eniselon, Schuler also
observed a vfsible erission spectrum from tolusne vhich appeared In the 4300 to
5000 Ao reqlon, the details of vhich ven not published. This spctrum was
designated the "blue spectrum" and was aesipned to the benzyl radial vithout stronk,
supportine evidence.
In a subeequert study of the react1 n
of toluere vapor in a high voltage elec-
trode discharce, Kraaijveld and Watenran92f investigated the fornation of the
bibenzylmolecule fmr toluene and found that, under optimum conditions in a 2400 v
arc, LO$ of the toluene vapor could te converted to tiberzyl. The other products
formed under these conditione vere not lnveetigabd. This result, ae vel1 as those
obtained by Schuler, imply the existence of benzyl radicals as the major species
present in electrode ivduced tolwne discharges.
Yore recently, Stmitwieser and Ward conducted the first comprehensive study
vhich vas concerned vith the ranee of r ducts formed from the electrodelees
rnicrovave excitation of tolwne vapor.p3? In this study, flowina tolmne vapor in a
helium csrrier rrae stream was excjted by a 3 RMc lricrovave peneratar, The epsctrm
of the lfaht enitted vas not investigated, hovever the products were carefully
detemined. The composition of the mixture of producte obtained is sbovn in figure 2.
The mv minor amounts of diger biaryla which mm formed in this discharge
sueepsted that radical intenredfated ven not of dominant Importance. The lack of
fornation of the xylene isorera vas also interpreted as eupportlng the absence of
methyl radicals in the plasm.
Two other possibilities, the fiolecular cation and anion vere coneidemd as
likely intermediates in the rnicrovave discharge. Results obtained from labeling
experiments ruled out consideration of the cation, and it was tentatively concluded
that anion irtenrediates remained as the moat likely peesibility.
This result contrasted intereetingly vith the conclusione which had been
previously dravn regarc!inu the douinance of radjcals in toluene discharges and =st
some doubt on these conclueions. Hovever, work vhicb had been done in W. D. Cooke's
laboratory at Cornel1 University siivested that very significant differences might
be anticiptcd 'ietveen the products resulting from the excitation of organic vapors
in a microvave povend discharge ard the excitation of these vaprs vlth a radlo-
frequency source. It vas, therefore, decided to investigate both the spectre and the
prodrrets obtained vhen toltrepe vapor 1.88 excfted in a 28 Mc. mdiofrequency dis-
charge.
The apparatus used in this study in ehom in block diagram form in figure 3.
?loving tol-ve rapr vas pemd throuqh a discharge povered by an R.P. transmitter
opemtinp at 28 mgcycles and 100 watts output. The pressure of the vapor uas
maintajned constant at 0.10 to 0.15 mm. Materials fomed in the plam wm
collecbd in traps maintained at Qo and 80° and vere aubseqwntly investigated by
chromatoer%phic and smctroscopfc techniques. The liqht emitted from the discharge
VB- focumd into a scannine Fonochroaator and detected by 3 IP-28 photomultiplier
tube. The a~pllfierl output of the photomltiplier vas recorded electronically.
Dnder the conditions used in these experiments, tolucne vae converted
to products jn 12 to 1s yield vfth 75 to 8% of the toluene being recovered unchaneed.