- Email: [email protected]
On the mechanisms of C6H6 ionization and fragmentation
OF C,&
ONTHEME-
Institute
for Ma-
Reeamh_
National
IONiZATION AND FBAGBIEHTATION
Ruremr
Of Stondprdr.
Wdtgon,
D.C. 20234
(U.S.A.)
National Reoemdr (piireceived
Council., Ottawa KIA
OR6 (Chada)
21February1977;infinalfo~n19
April 1977)
ABsrRAcJ? Photoionizationand electron monocbromatorstudi~havcken~outonbenzone, 1.5-bexsliyne. 2.4heradipne pyxidine andsomeCeF& isometatodev&pnew infonnationconcenIing thedetailso~benzeneionfragmentation Reaemtresul~tagetber withthoseofotherworLerqindiatatbat&numbetof~~ion~~tnarOisa
comparPbletothatofthe~~dtedotateof~e mediatestatesinmkeletal
benzemeion.Theseco&dactasintur-
ofthebenzens ion andaccoantfortbekbsarpsd a3all~etic~~~onvinylaee~l~andbuhtriaesin~~thrt~~ ionstru* taresare notformedatthefrasmen~ti~~oldinknzsna~~bnl,S-heEadiyne ~dppridineiPdicatetbef~~nof&~~C~ ionstructuru,more stable t&n the tmoiinearThe evidence for lnck of competition between hydrogun 10s~and &cletal fragment8tioninthebenzene ioni8mvkwed.Photcbionizationof_banxansreveal8 autoionizationcont6~tionstoparent ion and fragment ionproduction;t~tuof the autoionizing levels are gkmn The phOtai0niution b&SViOr Of l,S-heXadiyx208ugge tbattbisalso ~~~~taopairsofnoncompetiPgreectio~,~~~for benzenk INTRODUCI’ION
fragmentation
_ =;-
3%; _-
-
.-
, _
-
_ -_ _
I
.
benzeneionfollowedbgrunimo~decomposition. byintemalconve&onofthe (iii)Fondation ofc~andc+LI;occuued bfmzeneiontotheSrstexcitedstateofthebeI%XSle ekctronically~ci~ ion or some isoat the sanieenergy.again foJlowedbyunimdecular decomposition, (~~)~eshxttueoftiefkqrnentsfoxznedat tfvesholdwere C&&phenyZ Csnt,phenylene,~3,cyclopropenyl,and CJ?lf,wbichwasnotavinyl~~~estrucftrrebut~Iysrinnisomerwhich~a~can~yloiRer heatofformation,ca285~~ol",~tbe~y~l~eion(v)The shape of.thG went ion yieId curves in the threshold region des&b&dbyquasiequilibriumtheory(QET)appl.iwi couldbequanwatively occuningontheground totsvoindepesnientpakofcompe%ngreactions lsoenergetrcisomersurfWerfspecstatesur&ceandtheSrstex&edstateorassumptionthatthe tively,TheQETcalc~~Iktio~~-requkedtheadditional stepfunc~onphotob~on~~vaZidovertheenergyrangeofinterest (ca2eV)andtbat~~~onwas~port3urt~tba+,region. (vi)Theabovefrsgmentationmodelimpliedver3i~~tgineticshiffs in ikagmention ‘%brershoId"vahxesandled to predicted residence time bZXTV&~ifiXlIieffixtsonion&gnxentf&ionfhregholdsw%ichshouIdbeo brationprobkmsco&dbeoqercome, (vii)The model accounted&te~riIyfortheexperimentalIy formingC&asdeterminedhy determined rateenergy~efortheprocess AndlauerandOttinger-[BJandwasin~t~entwiththeir ~~usrewrltforc6H;formkt;on,whichappeared~entallytoshow amucbweakerenergydependence. (viii)Thebranching~o6scalculated foreachoftbetwopa&soffkagmentatfonproceses wereino~yqgalit&ve agreementwitbtheintenaityratios oftheeorrespondingpairs of_metaskhle b8IlSitiOnS_ Sincethattime anumberofadditional. experimentalst~~dieshavebeenundertaken. 'lIxseinch1dedphotoionizationof1,5-hexwSyneand 2,4&e yneasweIlasamorecareful studyofbenzeneinthe+ramnentationregion_ Electronmonocbxomatorstudieswerecarried outonionizationand/orpertinentfragmentationp~in benzene, l,fi-hexediyne,vinylxetylene, butadiens pyxidine-and s%xo benze33e_Ti1eresultsoftheseexp&ments, togetb~withnewinformationpa~~byotherwo~~itpOssible torwxamketheresuRs8ndax&u&onsqftbeeadier6tudyinamoxecx5* cal~on_ThefoRo~tD~~bediscussed:
.-
-
: _
-
__ _
*
. -
Ref.
9,98t 0,06 TM work 8,OO t 0,06 Thb work
PI
pot&& of benxenehmen, In eV FM
‘MS work
Eh!
9,96t 9,06
6”
9,O’ II,86
6 6
*
6
9,60
’
es
ii, 9J6 9J6 lo,86 ’ 9,ao
Bib
r a
9 Othervrlubrhive beenreportedin the literature[84], but tbe present#tt pemlb a correciionfor ~0~8eVg&ematioerrorby’com&rlqn “th moreaccuratereaulbfor 1,6-and 2,4-Hexadlyne,
a
2; 3
Rei
838
a,91f 0‘02 -9,411
-10,o a
PE
l‘A&mbrtlc ionltatlonpotentialestimatedby UIfromphotoelectronspectrumdIegram,
$otkw I’
Fht ickitlon
‘I[IABLE 1’
ablecorrectionforspstematicenorsinanolderel~nimpact~dy[6] on some isomers pezmit us nowtodeterminetb~location ofanumber of isomerionstateswi~respedto benzene ion statesona commonenergy scale.Theindividualmeawxrementsare summarizedinTable1. Wititheexception of ahe3.tof foxmationof1,5&exzwSyneintheliquid phase [7] there 8re noavaitableexpe+1entalv8hxesfortheisomers.They can,ho~~~,tKestirnrded~sufEiclentaccuracy(*2kcal/mol'~ orless) bymeansofthe %imai&ionscheine of Benson [S,S].TheheatofformatIon forthe1,5hexsdiyneIs~nv~toagasp~~uebyuginga?kcal/ mol-' eb'. ‘_fortfieheatofvaparization,Theestimatesare~~inTable 2, alongwit out choice of i-on pofzntiab drawn from the variousmeasuredvaluesgivenin flrble1,andfinaUy~eionicheatoffonnationvaIues.
Also,forcomparisonpurposes aseriesof 4zperimental[IOJ andf?stimated vah~esforsomeC~andC~hydroca&o nsisghwntoiUustratethereliahili~of the .. ISonschequzforanandogousset ofmolecdes(excep~ofcourse, Dewarbenzene). Weconchxdethatonecanplizemanyoft&se isomerion st&esonacommonenergyscaIewithanaccuraq of-O.ZeV. TABLEt Heatsoff ormation ofaomeC4,C6
~~.~entd(kartmol-~)
rsomer
~~ Benzene
1,~D l&He
andCshydmauho
--_
Ref.
w 19.81= 0.13 19.812 0.13
ran, E, 95-2 9822 99r2
I,tM&.x.ad.i~6-~e 1,2,4,S_Hesatetnrene 98 = 2 l%warBenzent 92s5 mlxvene 51* 5 3,4Dime!thyla=cgcbbutam ??a5 18Baatadiene 38.97 1.3Bxtadime 26.63 l$Pentad%ne 34.02 1,3PeIltzuliem 21.95 IpPeaQuiiene 24.76 Penti3+mz-I-yne 61.68 3Ll3 37.46
10 10
Ionization Potential (ev) 9.25
7,ll 12
1r.49 8.95' 9.451 9-96 8.91 %0* s:53 9.0 8.36 8.80
37.78= 26.11 f 33.62 * 19.13s 26.25 f
0.14 0.15 0.16 0.24 0.2
Fz4*0.28 37.4st 0.37
~EtkstmmimpacLvllaeminnsO.3eV.
bmisi68noldvdneofvely-
ni8omersandion.s
-
-:
=Vabaefmmref.10with?kcdnd--~edmatedh~d
-
10 10 Z8
-
:: 10 10
-
A
A&Ion (kcalnlor~) 233.1 235 301*5 316r5 929*2 295*5 316-5 299*S 295i5 300*5 244*5 280*5 -
Now, the adiabatic ionization potential leading to the formation of-the benzene ion in the first excited E* state is 11.49 eV [ 131, corresponding to a benzene ion with A& = 285 kcal/mol- l. Prom a comparison with Table 2, it is clear that there are a number of isomer ion stales which are close enough inenergyto~excitedstatetobethe~~~isomer9&tevLa~~ the skeIet& f&xgmentation proceeds. However the present considerations at least put some sufficient& accxux& thennocbcmistry bebind tie eadier conjectums [1,6]. It is also intere&ng that Dewar benzene, fMvene and 3.4&methylene cyclobutene ions appear to be thermochemically accessible in the isomerization process, since it is known that complete randomization of ti topically labeled carbon atoms occurs in the skeletal frsgmentation process at the fragmentation threshold [14,15]. In addition, Allan and Ma&r have csrrifxi out experiments in which they were able to observe fluorescence of excited radical cations of various fluorint+substituted henzenes [16] although they ohsenred no flu0 rescence IYom the benzene ion itself. This suggests t&at the bigger electronic states undergo an isomerization which is rapid compared to radiative lifetimes. TI4ERbsocHEMIsTRYANDsTRu-
OFCsHjANDC&FRAGMEWk3
.
In the earlier study it wss shown that the o bservable &qynentation onset for C& was too low in energy to permit consideration of the linear propargyl ion 8s the product at this threshold. Consequently the frsgtnent ion had necess&ly to be the cyclopropenyl ion (requiring isomer&&on at ttueshold). The close parallel in hehaviour of the C& and CjI: ion yield cuw~es suggested that the processe s had identical tbermochemicsl thresholds, which Ied to poduhtion of a Ca ion zqecies of heat of formation c& 285 kcal/ mol-‘, significantly lower than the value for the vinyl acetylene ion based on non-monoenergetic electron impact meaauements. We have csrried out a number of new ionization and fmgmentstion threshold measurements which are summarized in Table 3. The results on vinyl scetylene 821dbutatziene provide reliable values for the heats of formation of the two linear isomers. They are siguifrcsntly higher than the vslue postulated for the CS &agment fram benzene. Both the photoionization and electron monochromator measurements on C&G from 1,5heradiyne revealed very poorly defined Yhreab~kis’~. The photoionization yield me isshoumin Fig. l.There is a grsdual rise followed by a noticeable change in dope. We surmise that the gm$tswl,uxwl~!~?J. to collisional proceaees and express our-doubts by assk@q certainty of SOA. The com@etely different electron rnonochrom~~ erperimeatgmesavergnim;tnrf~old~~value,see~Z.Itisnoted~~~ %ur!shokP values lie only.& 0.4 eV above the molecular ionization w hoMgothatIdnetic~effectscantre~~smd~eresnlts!eadtoaC~ species of forma+n of-y 286 kcsllmol”. We feel that the obsezrrablefmg. w ion opBetDlie &fimt& s&d GgnBcanw Mow the t&mrmch~
.__...._
-._-----
- --
I
._
- Cd,+ + CaHa -, Cd,+ t CIHa -, C4Hs+t C$I1 C&H/$‘4(pyridlne) * C,H,+ + HCN ,C& (bonzeu) - C&H,+ + CaHg C&(kwns) - O,Hd++ C2Ha 4f&Ii6(bmae) m,‘C,H,++ CaHa
,licIWH~H~CH :H#C=MG-CH~
_
__
-
._
-
-
xi * Oaos lo:42 t 0,08 10.47* OS 11.27t 0,2 ’ 12,84f 0,l 14,17f 0,08 19.04f 0,l 14,lOf 0.8
9,bB* 0,02 OJ8 f 0,OB OAM* 0,08 (vertical)
rppeuance potenthI(8V)
Ionlza#onor
-
Eh! PI EM PI EM EI
;u91
:[a31
PI [ 371
AMhod
ofOJi~+bornera andobwvrble fragmsnton&
,HC@=CH&H~CH
He& offormtlon
_.
._
OA2 0,47 2,37 a,07 4,02 4,09 4,04
;‘;;” 0
-___
f2 *a
-
.-
_.
-
--
09 * 2 :eo *2 i2 84,66* 0.12 19,8x* 0.18 19,81f 0.18 19,81* 0.18
::
72,00 72,00
&ffapI reactant (kcnlmol’l)
.
.
9 t 10 10 10 10
9
17 17
bf,
.-
__
L_.
-
14 k2 2 a fat2 I6 ‘t a) 1: 16 t a) (2 Id , t 6) (2 I7 f a) (2 12 t 2) (2 I7 f 2) (2 13 t 7)
;;108,7 t 0,6 208,7* 0,6
t
--,
AtiapI ’ ion (kcalmol-1)
I.5-HEXADlYNE
t
._
1300
1260
Q20
I220
I200
II80
1160
WAVELENGTH.
1220
I
f
1200
_r:-_
lleo
1160
A
ofC6~parent.C~~
Fig_ 1_WotoiontJansholdcnme~
_=-
C&l
andC&J$ionsofl.S-hcradiyne.
I
I
8
II9
I
I
I
loo
I
T
90 80
0 D
70
0
0
0 D
50
0
D
40
.
BEWENE
-
,’
UnEY 0
20
IO I
_2
_4
.6
I22
.4
_6-
.8
threshold for foxmation of linear isomers. The formation of C*rr; was also omed in 2&h-e where it is, not surprising&, 8 weak p-. The observable fragmentation onset is given in the table with au uncertGnty &g$eD.D.ZOA_ - has mass 52 as a rnwr ion fragment. Accumassspecpum ratemsssmea~~lremeat~thatitisC~accornpaaiedbylassofHCN [18]_ The observabk! fragmentation olzBet of mass S2 d _ - edbyeIec_n monochromator techniques leads to a heat of fannation of 287 f 2 kcall mol-‘_ Interestingly enough this onset lie6 3 eV above the molecular ion%+ tion threshoId_ This again raises kinetic shift questions, In any event one can ent gives an upper bound to a CS heat of forconclude that Se rnmation of 237 f 2 kcal~ol”The obsen~&le went&ion onsets demed in benzene will be discussed laterThe above results suppoe the earlier suggestion that there is a stable gaS c8 8 kcal/mol-’ lower ph= 6% species which has a heat of formation than either vinyIace~lene or butadiene ion. Evidence is beginning to accumulate that the neutral species cyclobutadiene and tetrahedrane have at Ieast a transient exzstence 3s reaction intermediates [19-221, The analogy is -pting-
The availahle and new m easumments bearing on CT& enwetics are summanzed in Table 4, It is now clear that the phenyl ion has a heat of formationintherauge266--270 kcal/mol"tpldwtththisvaluetheratevs.energy dependence of halogen loss of halobenzene ions has been explained satXactori.& in terms of wum theory [ 25 ] _ The 0bserpabIe &agmentation onse* for the two lineartimers are considerably higher than this_ The data suggest the formation of a Ca ion having
Phcllyimdial
8.1 t 0.1 leemd@eM 10.1et 0.08 10s •t0.x 10.66~OA9 18a78rO.O8 1%74t 0.1
‘= -‘;-.
of formation of -231 kcal/mo1-%, implying a structure othe+um phenyl. Now the *‘-on schemes for neutral heats of formation indic& that the conceivable lineat isomers all have heats of formation ca 20 kcalj‘ mol”- Further, carban-hydrogen bond energies lie in the range 90-100 kcal/m01-~_ These values and the heat of formation of this C&X$ species imunreasonably low ~aradicalionizatisnpotentialqflessthan7eV,whichis for a linear rzxlia5.LThus the possibility must be considered that this C&G species is some kind of ring isomer, The fi8gment&on onsets~are close enough to the molecular ionization potential so that kinetic shift effects can bciguomd. The C.& went&ion onset palue in benzene is sufficient& close on the common energy scale to the values obtained from the linear isomers to raise the following question: Dots benzene form a linear Cs isomer of heat of formation -2S1-2S2 kcal/mol-’ or do thelinearC& isomersiaomerize to form phenyl ions in a pwithlargerkineticabift,orarethereinfact twodi&inctspecies? Similar questions may be asked about Cs formation in benzene since the obscreed fragmentation onset leads in a simpl~miuded interpretation to a CZ f&gment with a heat of formation quite close to tbelinesr Ca * mers, see Table 3. An attempt to answer these questions will be made in the course of the following Won on the evidence for independent pairs of competing 8
heat
reaCtiOnS.
THEEVXDWCEBORINDRPRNDENTPAIRSOFCOHP.EX7N
G REACIlONS
Our earlier analysis of the shapes of fragment photoion yields curves [l] led to the conclusion that there were two independent pairs of competing reactions.This in &elf makes it mely doubtful that one could interpret the observed went ion onsets in benzene as m oamuingdirectly from the excited state initia& populated at or very near thermochemical threshold without-kinetic shift, There is further evidence, Chupka and co-workers {26] have analyzed the first de6vative of the photoionization yields curve for the metastable transition of the bensend ion losing a deuterium alum, This analysis gave tbe.cnergy dependence of tke mte constant in the range lti-lOs s”_ The magnitaxdc and energy constants calculated on the dependancewasingoodaccordwithrate in normal benzene aaarmption that the anslogous fragmentation pion ground state, f-g phcnyl ions- These ~fIomthebenzem3 resultscon~theresul&ofAndlauarandOtt_ [2] whofoundanunfor the rate constant for H&tom loss, which _. recently tsimied out photoion-photoelccWn&KWith&thepNWSS tron GGncid& experiments on benz4GrThey 3r$+9ngloss of H~ns‘;iot.in-competi@cm+th~e~~~~-
bon--r#rbon
bond
nlptaare.
T&se
results
are
consistent
with
ourconcIusion
hydrogenloss about~of~ti~onbe~~tbet(ROsetsofp~ audcarbon~nho&rupture. AsfortheshuctureofC6H:formedfromthetwolinearisomers,noneof theabovediscusgionrulegoutthepossibilitythattheio~at~oldmay be phenyl ions formed via prior isomerization of the linear isomers to ex~tedgroundstatebe~eions.Ino~ertose#ae~questionitwouId beusefultosiudy camfUythepeak&apesofthecorrespo ndingmetastable &msitionsand/or ~etheehePIical~~~oftheC6H;ionsformed fromtbevariousprecurso rmoIecuIes_ Asforthe ~nbo~rnpftuereactions,inaddition~thefindingofaC&Z$species offherequird heatofformationwenotethefollowing~ ELand,inapho~ion_photoelectroncoincidencestudy[28] haafoundthat thebranebingratioforformationofCJI;audCJI;frombeuzenehadthe same value at 15_0,15_7and16_&V energy iuputaud thevaluewasvery in our earlierphotoionization study. In plater similartothat po&&ted haveidenstudy [27]hefoundtbattherateconstantsforthetwoproce~ses ti~energgdepend~~,Theseresultsshowthatthetwop~areverp closeIpcoupled,incom~titionwithoneanother.Toge~erwiththe~own thermocb~ form formationtbeymakeunavoidabletheconclusion involve kinetic sbXt;theycaunotbedes&bedas~fragthatbothprocewes mentationoecurring fromtfieexcitedstateiaitiaIlypolulatedintheionizationprocess.Wenote alsothatthisccmcluaionisako forced bytheC13isotipicIabehng&udiesonmetastable&msitiousinvolviugC~bondrupt+re wbichsbowtbatatthresboldtbe ~a.repmcededbycompIetecarbon randomization[14,l5]. istosomeextent The independentbehaviorofthetwopairsofpnxesses manifestedinthedifferrentenergpdependenceofthephotoionyield~~, asnotedearher.Studiesofphotoelectronangular~utionshaveshown that in the region where fragmentationiafirstobrerv~therearetwo IrrandB2,[29]_Cnecauspecu.latethatben~~YoverlapP~bands,~ zeneionsinitiallyformedinonestateallundergointemalconveraionleading uItimatelyto groundstate benzeneionandtho~formedintheotherstate undergoskeIetalisomeriz&ion_Thiswoulda . 3y bring aboutasituation inwhichtbereisnoccpmpetition,Itistobenotedtbatin benzenethetwo paimofproeeS%x don&have identicalonsetenergies,whereaswithineach pairtheyarevktuauyiudistiuguishabk Cnelagtpoiutconcems the relation oftheonsetenergyoffragmentation to tbelowenergy~oftheph~~~nband~tto~ electronstudies~eavarietyofvaiues,Amangthemostdetailedarethose of Asbringetal_[13]~dCadsonandAndetEion[29]_N~~ff~tabulated values,but&~~idrulerinterpoIatlonoftbeirpu~~~glPes) 13.70 and 13_64* 0.02 eV for the minimumenergy_Alsoarecentmekur+ m~tby~givefithevahxe13.70*0.02 eV[491.Thiaiaverycloseto the&gmentationon8etemzgieso~ here-It~OOneIl~gKO~,
Inspiteofthela%egineticshiftwhich~s~~byouranaly9is, itmsjr'mot veriEable_jn averpIongresidence~eapparatusfortPlesimpfere~n that&may nothepossihle toproducebenzeneionswitbsuffidentlylow excitationenergydueto Franc~ndonrestrictions.Anumberofconventi~~~nirll~tgtudieshavebeencarriedoutwhich~pnkinetic shifteffectsbystudyingtheeffectofionresidencetimeontheappearance potentialofC6N;in benzene[3~32].Themo~thoro~andcarefulstudp is that ofGordonandBeid[32]whofoundthattbeappearancepotentkl decreased from 14_2* O.leV to l2.'7-+O_leV as the ion residence time increasedfromca5~to1200~1.~in agreementwiththeearGr,lesscomplete study of L&hit?-. et al. [31]. The long residence time appearance potentialwasseveraltenthsofaneVbelowthetherm~ernicalthreshold calculaIx?dbyBosenstoc ketal. [l]. However, itisdisturbingthatitshows nosigpofIevelingoffatlongresidencetimes.Theverg~cantaspectof thisstudyis~atonthebasisofthephotoelectronspectrum th~ZSeI.lO optical&-allowed and Fkancmndon accessiblebenzene parent ion states between about 12.8 and 13.7eV [13]_ This raisesinterestingquestions aboutproductionofexcited benzeneionsinthisenergyrange;theycouPdbe formed by autoionization exclusively or by forbidden transitionsin the quartetmanifoldoftbeion.
be
AtiFI’OIONIZATION
AND FXUiGMl3NTATION
IN BEWZEXE
AND 1.5~HBXADIYNB
In~.3and4areshownthephotoionpieldcurvesforparentand~ generation~~ionsinbenzeneandl,~~e.Thebenzenedata were talus at OS-i% interpaZsandthosefor1.5-h~yneat6~ intervals_ approximatelyforscattered ligbt.ThiswasnecesThedatawerecorrected saryinthe1,5-hexadiyne ~ofthewidevaxiationsinthein~of andinthe ~tatctifferentpointsin~ehydrogenmany-linespectrum, benzene,which wasstudied~~theHopfi~dcontinuum.Tbeco~o~ weremadeto~~8swellaspossibletbeen~at~~ion~entationcouldGrstbeobserped_ ThesmaUwavel~SMervakwerecbosen topermito~~ofautoio~onstru~inIbenzene,ifp~~ thereisasuudlbutdefinite In thebenzeneparentphotoionyieldcurve. increase in yieId sta&ing at 90O+WSA (13.7~13.7OeV) which correspondsratWrcloselyfotheonset oftheE =,,bsndinthephotoelectronspectzum [13,29].Abovethisene%ythemostnoteworthyfeatureistheappearante of dktinct autoioCzing stxuckve at 850A and shorter wasmb@b. These comapond tataBydbesg sexies 433nw ta the ionizationlimit at 16_84eV, firstoM by Kl-Sayed &aL[33].'I%evibrationalktzuctarre desctibedq~~~.bytheseautbonr~aitfitbestructareof~e ~o~n~oftlh;elimitingioniccstate[34].~~y,15“‘ etaL[13]give~eIo~energy~atabout16.99~V~~Tbmeret agreeaiefltwiththespec&xxopic aL[34]6howthefir&p&kat1&84eV;in ~~Tbs~pt850-Aw~~~-an~qpanturnnumbern=
I
1
2
cd%+ x4
3’ ;__--__ f
1
/
f o---&c
Fig_3_PhotoknyieJdaxvu 750A and ~b)750-600 A Wms~tol6JMeV(736A),rciut~
ofblmzene _ parent and fbgment ion8 in the region (I) 950‘x%e vertxd -idiatupcwitionsofdous Rydberg
E
4PE
1
3
3zJwx
PE
EANDS
_-_-
__ _ ‘6% - -__
_---_ _-__ -_ -_--_ -_-_ -_ -._ --_ ---_ ---.-..-____
Ca”;
--__
C&T
__--
1SHEXADlYNE
_.__=.=_--_-__I-._I -
-_ -.-
..U)_;-
_
C&l,
rI_’
_”
=---------.
:a.-.._____ ___.--__ --__
-
--._
____.----=
._
--:-K.__ __‘-
-.__
--a ---_______ ----..__ -.
-=__
_:::A___
_-
.::::..
_.--
.
___-‘ _--,: ___-.W”; --.--y--.-..#-._‘___ -__-‘ ___--__A--+____-_-t ---“..-------C______-_____----=____---.= _-_____.---.----.. c4*2 _r:_.___--S....,Ul:zw~~ 1 , , 1200
mo
Ymo
9&l
---__
.I--.. ...;-
:-
c6pS _ _z-vs...” -__-
.-___--
__.---.-‘i.r-_~~___
_-_-
8(?0
-5-m
*‘k-l*..
700
WAVlXENGTH.A
aboveca17.3eV.Obvioualyautoioni2&ionisvexyimportantinpopulating fragmentingionstatesinthisinitiaIenergyrange. Thephotoionyieldcurvesofvariousionsinl,!%hexadiyneshow.anumber ofnotRworthyfeakues.Theparention~eisstron~ypeaked,vergLikely due to contributions from autoionizationofthefourelectrouictrausitions WekctxonbandbetweeulO aud11.5eV 131. thatcomprisethebroadpho TheionpieldsofC~~C~riSeinacloselgpropartionalfashionfrom their o-able at1225A (lO.leV)to neady1150A (10.8eV)following whichtk?yEmain essentialIyflatdownto900A(13.8eV).Thentheygradinyield. uallydecrease asthelikelydaugbterionsC+H;andC~~increase TheeneFgyatwhichtbeCsH:andC~ion~~toincrease(-lO.S eV)maybecompared~~thephotoeledtonspec~,whichintheonset region consistsofagroupoffouroverlappi.ugbands,atartingatjustunder lO.OeV and endiugat about11.3-ll,SeV [35]_ItappearsthatnotaU of theion electronic~~intbe~o~regionco~~~to~entation intheseChWlldl.~,tULtOi~on~Rydbergtezms,conv~to aezA,&a& witb an&iabatkionizationpotentialof-13.0 edim8tqi from Fis,6ofrei,3,alsoappearstobeineffectme.Perhapsfortuitously,~eo~ ~~onsetof~~t~~~toC+H;sndC4H;sfarEsoutvery nespandscl~yinenepggtothi8adiabatic 95OAwhichco ?!Taround inthisregionwilIhaveto lorUzaMnpotentiaLTlx¶?~eutation~ bestudkdmorecIoaeIy~
34+ -; GrossandAemi[36]have~~outanunurrual~o8sofhydrogeaatoms _ e,TheynotedthattheC~/C~ratiointfiemrres iu ionized l,v ~tivetotheresidencetimeiuthemassspectzospectrum==~ew meterionsource,incontragtto~eC~/C~~~in benzene, 2&hexadiyne and f,3-hex&ien*yne which were notvexymuchaffect&bythe resldencetimerrhftnne~m~l06sto-l0~3s.Thereasonforthisis~~ thatinthektterspeciesthereare moreaccesdde ionstateswhicharethermochemi~stableM,~thesameabsolutechan~inextent offragmentationintheenergyrange subjectto~entationinthistimescaleproduces IittIereMivechangeintheparent/daughterratio. Thebehaviourof~eCJI;endC~ionyield~~isquitedifferent;the (-16.5eV)and the latter to -365A (w-14-3 former&es out to -75OA eV),~~,gUtOiO~oaoftheRydbergstatesisvergeffectivehere_Thelack ofob~autoio~structureinthese~entionyield CZlXVEkiSprOb ably duetothenumberrmdbreadttroftheuppersCatesinvolved~inbenzene, tie linear fragment ion increase overalargeenergyrangeposes _ _ tere&ngquestionsof oscilMorskengthdi&ibutionaudtheenergydepenFace ofthe competitionbetween autoionizationandpredkocktionofthe Rydbergstates,Anotherpossibilitp,ofcourse,isthatthelinearfragment ion yieId curves are due to significant departures ftom step function behavior in the partial photoionization crogi-sections. However, theinteusityrektionshipsoftheindividualbandsin benzene [131 make this an unlikeIy possibiliQ. One hopes this is the case for l,!S-hexAiyne as well. The ion :yieId curves of 2&haadiyne do not present any unusual or spe&ic features thatmerit dwxssion or interesting interpretation. AU fragment ion8 show very gradual 0mtS, Theabove dkxssionsuggeststhattheprimarg ~eutation~ in1,5h~emayaZQorepresentasituationinvol~gtwoindependent pairspfcom~reactions,asinbenzene,Studiesemployiagallthediverse techu4ueshithertoappliedtobenzenewiUbevery&zitfuL
ACECNOWe woutd Eke to thank Dr_ ES_ Domalski forhelpfuldiscu&onson organicthermochemistryandtA~ermochemicalestim&~ ‘0nschemeqandDr.J. P.Maier forheIpNinforxnation.
REFERENCES
_ -.i--7$ 341 ,
_>“ . k
5 E Heilhro~er, Et. Gleita, R. Hops V. Hamung and k deMeijcm, Helv-Chim. &ta, 54(1971)783. 6 J. ==n&my. L BroLier and L D’Or. Bull. CL ScL. Acad R Be&&, 48 (1962) 1002. 7 H-A Skinner and k Snehon, Tram- Farslay Sot, 55 (1959) 404.
8 S-W- Benson, F-R.- Cmihhnk D-M- Golden. G-R- Hangen, H-Et_ iTNeal, A-S. Rodgas, R Shaw and R Whk. i?hem- Rev.. 69 (1969) 279, 9 S.W, F3enson, Thexm~wnical Kinetics, John Wilcp and Sona, New York, 1968.
10 J-D- Cox and G- Pilcher,Thexmoch aniskyofOrgauicandGrganomeblLicCompounds,AcademicPres,NewYor&l970, 11 EMbusted kcal/mol-xheatofvaporizationaddedtoA&(liq)ofret6_ 12 W-LO uguinine,C. R. Acad_Sci.,106 (1888)1472. The heatoffonnation of 2,4Hexadiynerepotihereis ofvug doabtfulaccuacy. 13 ~~O_Edqvirrt.E-LindholmandL.E.SeliqCheraPhpaLe~.5(1970)192_ 14 R-J-DickinsonandD_ Wti, J_Chem_Soc B.(1971)249_ 15 JH- Bepnon.R-M- Capric&W-O-Perry and W-E-Baitinger,J-kn.Chem.Soc_,94 (1972)6626_ 16 l!d_Allan and J_RMaier, Chem.Phy&Lett-,34(1975)442yaamicsofGrgatlic 17 D_R.StulI,EF-W~mandG.C.S~e,TheChenricalTherm~ Compounds,JohnWileyandSons,NewYork,196918 J-H- BeyPon,R-k~~deraandA-E_W~~e~Spectrao~OrganicMole~e~,Elsevier~New York, 1968.p.299. 19 L_Watts,J_P_FitzpatrickandRPettit,J.Am-Chem-Soc.87(1965)3255. 20 P_B_ShwlinandA_P-Wolf,J_Am.Chem-Soc_.92(1970)406. 21 L_B.RodewaldandH_-K.Lee,J-Am.Chem-&c_,95(1973)623. 22 W-J- LeNoble, Hi&lights of Grgaaic Chemistry. h¶arcel Dekker. Inc. New York, 1974. 23 Yu.Sergeev,M_E-Ak0pyanandG.L Viles~v.Opt.Spektronk,32(1972) 23O,combined with D(C~HJ--H)from D.M.Golden and S.W.Benson,Chem-Rep,69(1969) 125_ 24 d.L_Beauchamp,Advan_MassSpec&xn, 6(1974)717_ 25 T-Baez,BP_Tsai.D-SmitbandP_T.Marray.J-Chcze Pbya.64 (1976)2460. 26 W-A- Chupka, in C.SaadorZy.P~-Apslooaapd~.Robin(~).Ch~~Spec troscopy
and Photoch~~
in the Vacuum Ultraviolet, D. Reidel, Boston, 1974,
27 28 29 30 31
JaD-BlandandH_Schulte.J-C4emPhya,62(1975)3835. J_HD-Bland.Int_J_h¶assSpectrom-IonPhy~.13(1974)457. ~ACarlsanandCP,And~n.Ch~PhyaLett.10(1971)561. Scpe&ronx_,6(1972)827_ M.L.Grcnw,Org_lldaes C- Li&hitz.A-I@_ Peerq I& Weisand&¶_J_We~Advan_hfssSpectrom_.6(1974) 871_ 32 S-M_Gozdo~andN.W-Reid,Int_J-h%ssSpectro~IonPhy~,18(197S)379_ 'Ibm&a,J-Cbem.Phys.,34(1961)3S4_ 33 B&F~ElSayed.M.KashaandY34 D.W. Tumeq C- Bakeq AD_ Baker ad C-FL Bnandk. hfolemlar Photoelectron spec35 36 37 38 39 * 40
troscopy, Wiley-Iutersekncc, New York. 1970. Ret3,Fig_6ML Gmsn and ZLJs Aemi, J. Am. Chema Sm.. 95 (1973) 7875. V-H- Dibekr. ~pnblidmi workM-D- Van Hoorn, J- Electron Spectmm~ RslaL Wenom-. 6 (1975) 65. F- m E_ Heilhmnast, E- Klo&er-Ja usen. A Schmefra, A-S- Mum&a, and L_ nsdom, calan. Phys_. 4 (1974) 107, K_ Shgbahn. U&&sty of Upprrls Fubliatiom UUIP-940, June 1976.
IA
Pople
ONTHEME-
Institute
for Ma-
Reeamh_
National
IONiZATION AND FBAGBIEHTATION
Ruremr
Of Stondprdr.
Wdtgon,
D.C. 20234
(U.S.A.)
National Reoemdr (piireceived
Council., Ottawa KIA
OR6 (Chada)
21February1977;infinalfo~n19
April 1977)
ABsrRAcJ? Photoionizationand electron monocbromatorstudi~havcken~outonbenzone, 1.5-bexsliyne. 2.4heradipne pyxidine andsomeCeF& isometatodev&pnew infonnationconcenIing thedetailso~benzeneionfragmentation Reaemtresul~tagetber withthoseofotherworLerqindiatatbat&numbetof~~ion~~tnarOisa
comparPbletothatofthe~~dtedotateof~e mediatestatesinmkeletal
benzemeion.Theseco&dactasintur-
ofthebenzens ion andaccoantfortbekbsarpsd a3all~etic~~~onvinylaee~l~andbuhtriaesin~~thrt~~ ionstru* taresare notformedatthefrasmen~ti~~oldinknzsna~~bnl,S-heEadiyne ~dppridineiPdicatetbef~~nof&~~C~ ionstructuru,more stable t&n the tmoiinearThe evidence for lnck of competition between hydrogun 10s~and &cletal fragment8tioninthebenzene ioni8mvkwed.Photcbionizationof_banxansreveal8 autoionizationcont6~tionstoparent ion and fragment ionproduction;t~tuof the autoionizing levels are gkmn The phOtai0niution b&SViOr Of l,S-heXadiyx208ugge tbattbisalso ~~~~taopairsofnoncompetiPgreectio~,~~~for benzenk INTRODUCI’ION
fragmentation
_ =;-
3%; _-
-
.-
, _
-
_ -_ _
I
.
benzeneionfollowedbgrunimo~decomposition. byintemalconve&onofthe (iii)Fondation ofc~andc+LI;occuued bfmzeneiontotheSrstexcitedstateofthebeI%XSle ekctronically~ci~ ion or some isoat the sanieenergy.again foJlowedbyunimdecular decomposition, (~~)~eshxttueoftiefkqrnentsfoxznedat tfvesholdwere C&&phenyZ Csnt,phenylene,~3,cyclopropenyl,and CJ?lf,wbichwasnotavinyl~~~estrucftrrebut~Iysrinnisomerwhich~a~can~yloiRer heatofformation,ca285~~ol",~tbe~y~l~eion(v)The shape of.thG went ion yieId curves in the threshold region des&b&dbyquasiequilibriumtheory(QET)appl.iwi couldbequanwatively occuningontheground totsvoindepesnientpakofcompe%ngreactions lsoenergetrcisomersurfWerfspecstatesur&ceandtheSrstex&edstateorassumptionthatthe tively,TheQETcalc~~Iktio~~-requkedtheadditional stepfunc~onphotob~on~~vaZidovertheenergyrangeofinterest (ca2eV)andtbat~~~onwas~port3urt~tba+,region. (vi)Theabovefrsgmentationmodelimpliedver3i~~tgineticshiffs in ikagmention ‘%brershoId"vahxesandled to predicted residence time bZXTV&~ifiXlIieffixtsonion&gnxentf&ionfhregholdsw%ichshouIdbeo brationprobkmsco&dbeoqercome, (vii)The model accounted&te~riIyfortheexperimentalIy formingC&asdeterminedhy determined rateenergy~efortheprocess AndlauerandOttinger-[BJandwasin~t~entwiththeir ~~usrewrltforc6H;formkt;on,whichappeared~entallytoshow amucbweakerenergydependence. (viii)Thebranching~o6scalculated foreachoftbetwopa&soffkagmentatfonproceses wereino~yqgalit&ve agreementwitbtheintenaityratios oftheeorrespondingpairs of_metaskhle b8IlSitiOnS_ Sincethattime anumberofadditional. experimentalst~~dieshavebeenundertaken. 'lIxseinch1dedphotoionizationof1,5-hexwSyneand 2,4&e yneasweIlasamorecareful studyofbenzeneinthe+ramnentationregion_ Electronmonocbxomatorstudieswerecarried outonionizationand/orpertinentfragmentationp~in benzene, l,fi-hexediyne,vinylxetylene, butadiens pyxidine-and s%xo benze33e_Ti1eresultsoftheseexp&ments, togetb~withnewinformationpa~~byotherwo~~itpOssible torwxamketheresuRs8ndax&u&onsqftbeeadier6tudyinamoxecx5* cal~on_ThefoRo~tD~~bediscussed:
.-
-
: _
-
__ _
*
. -
Ref.
9,98t 0,06 TM work 8,OO t 0,06 Thb work
PI
pot&& of benxenehmen, In eV FM
‘MS work
Eh!
9,96t 9,06
6”
9,O’ II,86
6 6
*
6
9,60
’
es
ii, 9J6 9J6 lo,86 ’ 9,ao
Bib
r a
9 Othervrlubrhive beenreportedin the literature[84], but tbe present#tt pemlb a correciionfor ~0~8eVg&ematioerrorby’com&rlqn “th moreaccuratereaulbfor 1,6-and 2,4-Hexadlyne,
a
2; 3
Rei
838
a,91f 0‘02 -9,411
-10,o a
PE
l‘A&mbrtlc ionltatlonpotentialestimatedby UIfromphotoelectronspectrumdIegram,
$otkw I’
Fht ickitlon
‘I[IABLE 1’
ablecorrectionforspstematicenorsinanolderel~nimpact~dy[6] on some isomers pezmit us nowtodeterminetb~location ofanumber of isomerionstateswi~respedto benzene ion statesona commonenergy scale.Theindividualmeawxrementsare summarizedinTable1. Wititheexception of ahe3.tof foxmationof1,5&exzwSyneintheliquid phase [7] there 8re noavaitableexpe+1entalv8hxesfortheisomers.They can,ho~~~,tKestirnrded~sufEiclentaccuracy(*2kcal/mol'~ orless) bymeansofthe %imai&ionscheine of Benson [S,S].TheheatofformatIon forthe1,5hexsdiyneIs~nv~toagasp~~uebyuginga?kcal/ mol-' eb'. ‘_fortfieheatofvaparization,Theestimatesare~~inTable 2, alongwit out choice of i-on pofzntiab drawn from the variousmeasuredvaluesgivenin flrble1,andfinaUy~eionicheatoffonnationvaIues.
Also,forcomparisonpurposes aseriesof 4zperimental[IOJ andf?stimated vah~esforsomeC~andC~hydroca&o nsisghwntoiUustratethereliahili~of the .. ISonschequzforanandogousset ofmolecdes(excep~ofcourse, Dewarbenzene). Weconchxdethatonecanplizemanyoft&se isomerion st&esonacommonenergyscaIewithanaccuraq of-O.ZeV. TABLEt Heatsoff ormation ofaomeC4,C6
~~.~entd(kartmol-~)
rsomer
~~ Benzene
1,~D l&He
andCshydmauho
--_
Ref.
w 19.81= 0.13 19.812 0.13
ran, E, 95-2 9822 99r2
I,tM&.x.ad.i~6-~e 1,2,4,S_Hesatetnrene 98 = 2 l%warBenzent 92s5 mlxvene 51* 5 3,4Dime!thyla=cgcbbutam ??a5 18Baatadiene 38.97 1.3Bxtadime 26.63 l$Pentad%ne 34.02 1,3PeIltzuliem 21.95 IpPeaQuiiene 24.76 Penti3+mz-I-yne 61.68 3Ll3 37.46
10 10
Ionization Potential (ev) 9.25
7,ll 12
1r.49 8.95' 9.451 9-96 8.91 %0* s:53 9.0 8.36 8.80
37.78= 26.11 f 33.62 * 19.13s 26.25 f
0.14 0.15 0.16 0.24 0.2
Fz4*0.28 37.4st 0.37
~EtkstmmimpacLvllaeminnsO.3eV.
bmisi68noldvdneofvely-
ni8omersandion.s
-
-:
=Vabaefmmref.10with?kcdnd--~edmatedh~d
-
10 10 Z8
-
:: 10 10
-
A
A&Ion (kcalnlor~) 233.1 235 301*5 316r5 929*2 295*5 316-5 299*S 295i5 300*5 244*5 280*5 -
Now, the adiabatic ionization potential leading to the formation of-the benzene ion in the first excited E* state is 11.49 eV [ 131, corresponding to a benzene ion with A& = 285 kcal/mol- l. Prom a comparison with Table 2, it is clear that there are a number of isomer ion stales which are close enough inenergyto~excitedstatetobethe~~~isomer9&tevLa~~ the skeIet& f&xgmentation proceeds. However the present considerations at least put some sufficient& accxux& thennocbcmistry bebind tie eadier conjectums [1,6]. It is also intere&ng that Dewar benzene, fMvene and 3.4&methylene cyclobutene ions appear to be thermochemically accessible in the isomerization process, since it is known that complete randomization of ti topically labeled carbon atoms occurs in the skeletal frsgmentation process at the fragmentation threshold [14,15]. In addition, Allan and Ma&r have csrrifxi out experiments in which they were able to observe fluorescence of excited radical cations of various fluorint+substituted henzenes [16] although they ohsenred no flu0 rescence IYom the benzene ion itself. This suggests t&at the bigger electronic states undergo an isomerization which is rapid compared to radiative lifetimes. TI4ERbsocHEMIsTRYANDsTRu-
OFCsHjANDC&FRAGMEWk3
.
In the earlier study it wss shown that the o bservable &qynentation onset for C& was too low in energy to permit consideration of the linear propargyl ion 8s the product at this threshold. Consequently the frsgtnent ion had necess&ly to be the cyclopropenyl ion (requiring isomer&&on at ttueshold). The close parallel in hehaviour of the C& and CjI: ion yield cuw~es suggested that the processe s had identical tbermochemicsl thresholds, which Ied to poduhtion of a Ca ion zqecies of heat of formation c& 285 kcal/ mol-‘, significantly lower than the value for the vinyl acetylene ion based on non-monoenergetic electron impact meaauements. We have csrried out a number of new ionization and fmgmentstion threshold measurements which are summarized in Table 3. The results on vinyl scetylene 821dbutatziene provide reliable values for the heats of formation of the two linear isomers. They are siguifrcsntly higher than the vslue postulated for the CS &agment fram benzene. Both the photoionization and electron monochromator measurements on C&G from 1,5heradiyne revealed very poorly defined Yhreab~kis’~. The photoionization yield me isshoumin Fig. l.There is a grsdual rise followed by a noticeable change in dope. We surmise that the gm$tswl,uxwl~!~?J. to collisional proceaees and express our-doubts by assk@q certainty of SOA. The com@etely different electron rnonochrom~~ erperimeatgmesavergnim;tnrf~old~~value,see~Z.Itisnoted~~~ %ur!shokP values lie only.& 0.4 eV above the molecular ionization w hoMgothatIdnetic~effectscantre~~smd~eresnlts!eadtoaC~ species of forma+n of-y 286 kcsllmol”. We feel that the obsezrrablefmg. w ion opBetDlie &fimt& s&d GgnBcanw Mow the t&mrmch~
.__...._
-._-----
- --
I
._
- Cd,+ + CaHa -, Cd,+ t CIHa -, C4Hs+t C$I1 C&H/$‘4(pyridlne) * C,H,+ + HCN ,C& (bonzeu) - C&H,+ + CaHg C&(kwns) - O,Hd++ C2Ha 4f&Ii6(bmae) m,‘C,H,++ CaHa
,licIWH~H~CH :H#C=MG-CH~
_
__
-
._
-
-
xi * Oaos lo:42 t 0,08 10.47* OS 11.27t 0,2 ’ 12,84f 0,l 14,17f 0,08 19.04f 0,l 14,lOf 0.8
9,bB* 0,02 OJ8 f 0,OB OAM* 0,08 (vertical)
rppeuance potenthI(8V)
Ionlza#onor
-
Eh! PI EM PI EM EI
;u91
:[a31
PI [ 371
AMhod
ofOJi~+bornera andobwvrble fragmsnton&
,HC@=CH&H~CH
He& offormtlon
_.
._
OA2 0,47 2,37 a,07 4,02 4,09 4,04
;‘;;” 0
-___
f2 *a
-
.-
_.
-
--
09 * 2 :eo *2 i2 84,66* 0.12 19,8x* 0.18 19,81f 0.18 19,81* 0.18
::
72,00 72,00
&ffapI reactant (kcnlmol’l)
.
.
9 t 10 10 10 10
9
17 17
bf,
.-
__
L_.
-
14 k2 2 a fat2 I6 ‘t a) 1: 16 t a) (2 Id , t 6) (2 I7 f a) (2 12 t 2) (2 I7 f 2) (2 13 t 7)
;;108,7 t 0,6 208,7* 0,6
t
--,
AtiapI ’ ion (kcalmol-1)
I.5-HEXADlYNE
t
._
1300
1260
Q20
I220
I200
II80
1160
WAVELENGTH.
1220
I
f
1200
_r:-_
lleo
1160
A
ofC6~parent.C~~
Fig_ 1_WotoiontJansholdcnme~
_=-
C&l
andC&J$ionsofl.S-hcradiyne.
I
I
8
II9
I
I
I
loo
I
T
90 80
0 D
70
0
0
0 D
50
0
D
40
.
BEWENE
-
,’
UnEY 0
20
IO I
_2
_4
.6
I22
.4
_6-
.8
threshold for foxmation of linear isomers. The formation of C*rr; was also omed in 2&h-e where it is, not surprising&, 8 weak p-. The observable fragmentation onset is given in the table with au uncertGnty &g$eD.D.ZOA_ - has mass 52 as a rnwr ion fragment. Accumassspecpum ratemsssmea~~lremeat~thatitisC~accornpaaiedbylassofHCN [18]_ The observabk! fragmentation olzBet of mass S2 d _ - edbyeIec_n monochromator techniques leads to a heat of fannation of 287 f 2 kcall mol-‘_ Interestingly enough this onset lie6 3 eV above the molecular ion%+ tion threshoId_ This again raises kinetic shift questions, In any event one can ent gives an upper bound to a CS heat of forconclude that Se rnmation of 237 f 2 kcal~ol”The obsen~&le went&ion onsets demed in benzene will be discussed laterThe above results suppoe the earlier suggestion that there is a stable gaS c8 8 kcal/mol-’ lower ph= 6% species which has a heat of formation than either vinyIace~lene or butadiene ion. Evidence is beginning to accumulate that the neutral species cyclobutadiene and tetrahedrane have at Ieast a transient exzstence 3s reaction intermediates [19-221, The analogy is -pting-
The availahle and new m easumments bearing on CT& enwetics are summanzed in Table 4, It is now clear that the phenyl ion has a heat of formationintherauge266--270 kcal/mol"tpldwtththisvaluetheratevs.energy dependence of halogen loss of halobenzene ions has been explained satXactori.& in terms of wum theory [ 25 ] _ The 0bserpabIe &agmentation onse* for the two lineartimers are considerably higher than this_ The data suggest the formation of a Ca ion having
Phcllyimdial
8.1 t 0.1 leemd@eM 10.1et 0.08 10s •t0.x 10.66~OA9 18a78rO.O8 1%74t 0.1
‘= -‘;-.
of formation of -231 kcal/mo1-%, implying a structure othe+um phenyl. Now the *‘-on schemes for neutral heats of formation indic& that the conceivable lineat isomers all have heats of formation ca 20 kcalj‘ mol”- Further, carban-hydrogen bond energies lie in the range 90-100 kcal/m01-~_ These values and the heat of formation of this C&X$ species imunreasonably low ~aradicalionizatisnpotentialqflessthan7eV,whichis for a linear rzxlia5.LThus the possibility must be considered that this C&G species is some kind of ring isomer, The fi8gment&on onsets~are close enough to the molecular ionization potential so that kinetic shift effects can bciguomd. The C.& went&ion onset palue in benzene is sufficient& close on the common energy scale to the values obtained from the linear isomers to raise the following question: Dots benzene form a linear Cs isomer of heat of formation -2S1-2S2 kcal/mol-’ or do thelinearC& isomersiaomerize to form phenyl ions in a pwithlargerkineticabift,orarethereinfact twodi&inctspecies? Similar questions may be asked about Cs formation in benzene since the obscreed fragmentation onset leads in a simpl~miuded interpretation to a CZ f&gment with a heat of formation quite close to tbelinesr Ca * mers, see Table 3. An attempt to answer these questions will be made in the course of the following Won on the evidence for independent pairs of competing 8
heat
reaCtiOnS.
THEEVXDWCEBORINDRPRNDENTPAIRSOFCOHP.EX7N
G REACIlONS
Our earlier analysis of the shapes of fragment photoion yields curves [l] led to the conclusion that there were two independent pairs of competing reactions.This in &elf makes it mely doubtful that one could interpret the observed went ion onsets in benzene as m oamuingdirectly from the excited state initia& populated at or very near thermochemical threshold without-kinetic shift, There is further evidence, Chupka and co-workers {26] have analyzed the first de6vative of the photoionization yields curve for the metastable transition of the bensend ion losing a deuterium alum, This analysis gave tbe.cnergy dependence of tke mte constant in the range lti-lOs s”_ The magnitaxdc and energy constants calculated on the dependancewasingoodaccordwithrate in normal benzene aaarmption that the anslogous fragmentation pion ground state, f-g phcnyl ions- These ~fIomthebenzem3 resultscon~theresul&ofAndlauarandOtt_ [2] whofoundanunfor the rate constant for H&tom loss, which _. recently tsimied out photoion-photoelccWn&KWith&thepNWSS tron GGncid& experiments on benz4GrThey 3r$+9ngloss of H~ns‘;iot.in-competi@cm+th~e~~~~-
bon--r#rbon
bond
nlptaare.
T&se
results
are
consistent
with
ourconcIusion
hydrogenloss about~of~ti~onbe~~tbet(ROsetsofp~ audcarbon~nho&rupture. AsfortheshuctureofC6H:formedfromthetwolinearisomers,noneof theabovediscusgionrulegoutthepossibilitythattheio~at~oldmay be phenyl ions formed via prior isomerization of the linear isomers to ex~tedgroundstatebe~eions.Ino~ertose#ae~questionitwouId beusefultosiudy camfUythepeak&apesofthecorrespo ndingmetastable &msitionsand/or ~etheehePIical~~~oftheC6H;ionsformed fromtbevariousprecurso rmoIecuIes_ Asforthe ~nbo~rnpftuereactions,inaddition~thefindingofaC&Z$species offherequird heatofformationwenotethefollowing~ ELand,inapho~ion_photoelectroncoincidencestudy[28] haafoundthat thebranebingratioforformationofCJI;audCJI;frombeuzenehadthe same value at 15_0,15_7and16_&V energy iuputaud thevaluewasvery in our earlierphotoionization study. In plater similartothat po&&ted haveidenstudy [27]hefoundtbattherateconstantsforthetwoproce~ses ti~energgdepend~~,Theseresultsshowthatthetwop~areverp closeIpcoupled,incom~titionwithoneanother.Toge~erwiththe~own thermocb~ form formationtbeymakeunavoidabletheconclusion involve kinetic sbXt;theycaunotbedes&bedas~fragthatbothprocewes mentationoecurring fromtfieexcitedstateiaitiaIlypolulatedintheionizationprocess.Wenote alsothatthisccmcluaionisako forced bytheC13isotipicIabehng&udiesonmetastable&msitiousinvolviugC~bondrupt+re wbichsbowtbatatthresboldtbe ~a.repmcededbycompIetecarbon randomization[14,l5]. istosomeextent The independentbehaviorofthetwopairsofpnxesses manifestedinthedifferrentenergpdependenceofthephotoionyield~~, asnotedearher.Studiesofphotoelectronangular~utionshaveshown that in the region where fragmentationiafirstobrerv~therearetwo IrrandB2,[29]_Cnecauspecu.latethatben~~YoverlapP~bands,~ zeneionsinitiallyformedinonestateallundergointemalconveraionleading uItimatelyto groundstate benzeneionandtho~formedintheotherstate undergoskeIetalisomeriz&ion_Thiswoulda . 3y bring aboutasituation inwhichtbereisnoccpmpetition,Itistobenotedtbatin benzenethetwo paimofproeeS%x don&have identicalonsetenergies,whereaswithineach pairtheyarevktuauyiudistiuguishabk Cnelagtpoiutconcems the relation oftheonsetenergyoffragmentation to tbelowenergy~oftheph~~~nband~tto~ electronstudies~eavarietyofvaiues,Amangthemostdetailedarethose of Asbringetal_[13]~dCadsonandAndetEion[29]_N~~ff~tabulated values,but&~~idrulerinterpoIatlonoftbeirpu~~~glPes) 13.70 and 13_64* 0.02 eV for the minimumenergy_Alsoarecentmekur+ m~tby~givefithevahxe13.70*0.02 eV[491.Thiaiaverycloseto the&gmentationon8etemzgieso~ here-It~OOneIl~gKO~,
Inspiteofthela%egineticshiftwhich~s~~byouranaly9is, itmsjr'mot veriEable_jn averpIongresidence~eapparatusfortPlesimpfere~n that&may nothepossihle toproducebenzeneionswitbsuffidentlylow excitationenergydueto Franc~ndonrestrictions.Anumberofconventi~~~nirll~tgtudieshavebeencarriedoutwhich~pnkinetic shifteffectsbystudyingtheeffectofionresidencetimeontheappearance potentialofC6N;in benzene[3~32].Themo~thoro~andcarefulstudp is that ofGordonandBeid[32]whofoundthattbeappearancepotentkl decreased from 14_2* O.leV to l2.'7-+O_leV as the ion residence time increasedfromca5~to1200~1.~in agreementwiththeearGr,lesscomplete study of L&hit?-. et al. [31]. The long residence time appearance potentialwasseveraltenthsofaneVbelowthetherm~ernicalthreshold calculaIx?dbyBosenstoc ketal. [l]. However, itisdisturbingthatitshows nosigpofIevelingoffatlongresidencetimes.Theverg~cantaspectof thisstudyis~atonthebasisofthephotoelectronspectrum th~ZSeI.lO optical&-allowed and Fkancmndon accessiblebenzene parent ion states between about 12.8 and 13.7eV [13]_ This raisesinterestingquestions aboutproductionofexcited benzeneionsinthisenergyrange;theycouPdbe formed by autoionization exclusively or by forbidden transitionsin the quartetmanifoldoftbeion.
be
AtiFI’OIONIZATION
AND FXUiGMl3NTATION
IN BEWZEXE
AND 1.5~HBXADIYNB
In~.3and4areshownthephotoionpieldcurvesforparentand~ generation~~ionsinbenzeneandl,~~e.Thebenzenedata were talus at OS-i% interpaZsandthosefor1.5-h~yneat6~ intervals_ approximatelyforscattered ligbt.ThiswasnecesThedatawerecorrected saryinthe1,5-hexadiyne ~ofthewidevaxiationsinthein~of andinthe ~tatctifferentpointsin~ehydrogenmany-linespectrum, benzene,which wasstudied~~theHopfi~dcontinuum.Tbeco~o~ weremadeto~~8swellaspossibletbeen~at~~ion~entationcouldGrstbeobserped_ ThesmaUwavel~SMervakwerecbosen topermito~~ofautoio~onstru~inIbenzene,ifp~~ thereisasuudlbutdefinite In thebenzeneparentphotoionyieldcurve. increase in yieId sta&ing at 90O+WSA (13.7~13.7OeV) which correspondsratWrcloselyfotheonset oftheE =,,bsndinthephotoelectronspectzum [13,29].Abovethisene%ythemostnoteworthyfeatureistheappearante of dktinct autoioCzing stxuckve at 850A and shorter wasmb@b. These comapond tataBydbesg sexies 433nw ta the ionizationlimit at 16_84eV, firstoM by Kl-Sayed &aL[33].'I%evibrationalktzuctarre desctibedq~~~.bytheseautbonr~aitfitbestructareof~e ~o~n~oftlh;elimitingioniccstate[34].~~y,15“‘ etaL[13]give~eIo~energy~atabout16.99~V~~Tbmeret agreeaiefltwiththespec&xxopic aL[34]6howthefir&p&kat1&84eV;in ~~Tbs~pt850-Aw~~~-an~qpanturnnumbern=
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Fig_3_PhotoknyieJdaxvu 750A and ~b)750-600 A Wms~tol6JMeV(736A),rciut~
ofblmzene _ parent and fbgment ion8 in the region (I) 950‘x%e vertxd -idiatupcwitionsofdous Rydberg
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WAVlXENGTH.A
aboveca17.3eV.Obvioualyautoioni2&ionisvexyimportantinpopulating fragmentingionstatesinthisinitiaIenergyrange. Thephotoionyieldcurvesofvariousionsinl,!%hexadiyneshow.anumber ofnotRworthyfeakues.Theparention~eisstron~ypeaked,vergLikely due to contributions from autoionizationofthefourelectrouictrausitions WekctxonbandbetweeulO aud11.5eV 131. thatcomprisethebroadpho TheionpieldsofC~~C~riSeinacloselgpropartionalfashionfrom their o-able at1225A (lO.leV)to neady1150A (10.8eV)following whichtk?yEmain essentialIyflatdownto900A(13.8eV).Thentheygradinyield. uallydecrease asthelikelydaugbterionsC+H;andC~~increase TheeneFgyatwhichtbeCsH:andC~ion~~toincrease(-lO.S eV)maybecompared~~thephotoeledtonspec~,whichintheonset region consistsofagroupoffouroverlappi.ugbands,atartingatjustunder lO.OeV and endiugat about11.3-ll,SeV [35]_ItappearsthatnotaU of theion electronic~~intbe~o~regionco~~~to~entation intheseChWlldl.~,tULtOi~on~Rydbergtezms,conv~to aezA,&a& witb an&iabatkionizationpotentialof-13.0 edim8tqi from Fis,6ofrei,3,alsoappearstobeineffectme.Perhapsfortuitously,~eo~ ~~onsetof~~t~~~toC+H;sndC4H;sfarEsoutvery nespandscl~yinenepggtothi8adiabatic 95OAwhichco ?!Taround inthisregionwilIhaveto lorUzaMnpotentiaLTlx¶?~eutation~ bestudkdmorecIoaeIy~
34+ -; GrossandAemi[36]have~~outanunurrual~o8sofhydrogeaatoms _ e,TheynotedthattheC~/C~ratiointfiemrres iu ionized l,v ~tivetotheresidencetimeiuthemassspectzospectrum==~ew meterionsource,incontragtto~eC~/C~~~in benzene, 2&hexadiyne and f,3-hex&ien*yne which were notvexymuchaffect&bythe resldencetimerrhftnne~m~l06sto-l0~3s.Thereasonforthisis~~ thatinthektterspeciesthereare moreaccesdde ionstateswhicharethermochemi~stableM,~thesameabsolutechan~inextent offragmentationintheenergyrange subjectto~entationinthistimescaleproduces IittIereMivechangeintheparent/daughterratio. Thebehaviourof~eCJI;endC~ionyield~~isquitedifferent;the (-16.5eV)and the latter to -365A (w-14-3 former&es out to -75OA eV),~~,gUtOiO~oaoftheRydbergstatesisvergeffectivehere_Thelack ofob~autoio~structureinthese~entionyield CZlXVEkiSprOb ably duetothenumberrmdbreadttroftheuppersCatesinvolved~inbenzene, tie linear fragment ion increase overalargeenergyrangeposes _ _ tere&ngquestionsof oscilMorskengthdi&ibutionaudtheenergydepenFace ofthe competitionbetween autoionizationandpredkocktionofthe Rydbergstates,Anotherpossibilitp,ofcourse,isthatthelinearfragment ion yieId curves are due to significant departures ftom step function behavior in the partial photoionization crogi-sections. However, theinteusityrektionshipsoftheindividualbandsin benzene [131 make this an unlikeIy possibiliQ. One hopes this is the case for l,!S-hexAiyne as well. The ion :yieId curves of 2&haadiyne do not present any unusual or spe&ic features thatmerit dwxssion or interesting interpretation. AU fragment ion8 show very gradual 0mtS, Theabove dkxssionsuggeststhattheprimarg ~eutation~ in1,5h~emayaZQorepresentasituationinvol~gtwoindependent pairspfcom~reactions,asinbenzene,Studiesemployiagallthediverse techu4ueshithertoappliedtobenzenewiUbevery&zitfuL
ACECNOWe woutd Eke to thank Dr_ ES_ Domalski forhelpfuldiscu&onson organicthermochemistryandtA~ermochemicalestim&~ ‘0nschemeqandDr.J. P.Maier forheIpNinforxnation.
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_>“ . k
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Pople