Endothermic ion-molecule reactions: the reactions of H3O+ and H3S+ with isobutane

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I n t e r n a t i o n a l -Journal o f M a s s S p e c t r o m e t r y _ a n d - I o n P h y s i c . s , - 2 7 ( i 9 7 8 - ) i 3 9 ~ 1 ~ 7 ~ : ~ i-: ~1 3 9 -= ~) b-3.~ev,.]er ~ . i e n t i ~ c P u b ! ~ h ! n g C o m p a n y , .~m~r, ez~s~m ~ . ~ - P r J n t e d i n T h e N e t h ~ - r | ~ d s : ~:-

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ENDOTHR.RMIc ION--MOLECULE-REACTIONS: T H E . I ~ - ~ . ~ C T I O N S _ O F :~-~ H a O + A N D H a S + W I T H ISOBUTAIN-E-~.! = :-!:=::-- -.-- _ --~: - ~- - - -~_~ -~= _- - -

Facuity of Engineering, Yamanashi

~-00 (Ja'p'an~)

U n i v e r s i t y , T a k e d a 4 . J~of~,

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(Received 7 October 1977)

ABSTR_~C~

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S.doth - io ol ¢ e H3S . . . . . Suta-ew - - St.m d , i ~ . g a - p u l s e d e l e c t r o n b e a m m a s s s p e c t ~ o m e t e - r - ~ v i t h -a higll p ~ ~ion s o u r c e . A n Aa~h e n i u s p l o t o f r a t e - c o n s t a n t s v ~ s u s i n v e r s e t e m p e r a t u ~ - f 0 r - t h e r e a c t i o n Of H a D ÷ w i t h i ~ 4 H 1 o l e d t o t h e z ~ i a t i o n ~ h i p : k----1.8 × 1 0 -9 ( c m z m o l e c u l e - I s - I ) e x p ( - - 3 . 9 k c a l m o l - l I R ~ r ~ . T h e pre~-~cponential f a c t o r is v e r y clo6e t o t h e Langevizz c a p t u r e raise consfaznt (1.75 X 1 0 - 9 ) w h i c h is u s u a l l y o b s e r v e d f o r t h e p r o t o n t L ' ~ n s f e ~ r e a c t i o n s . For the reaction of H3S + with i-C4Hlo , an _Arrhenius plot led to the relationship: k = 1.6 x 10 -x° (~m 3- m0lecuIe -x s-l) - exp(--7.7 kcal-mol-X-/RW).I n thi_~ c a s e t h e p r e exponentialf a c t o r i s o n l y - 1 0 ~ o f t h e - L a n g e v i n c a p t u r e rathe e o n s t a n t ( 1 ; 4 x 1 0 - 9 ) . T h e activation energies of these two reactions lead to lower ]~mi~s for the proton dimityof

isobutane of 165 and 164.3 kcal mol: I, z~eetively.:

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INTRODUCTION -The pentacoozxlinated methoni~mion--CI~sv~as ~mongst the-=f~rst io~1-molecule reaction products -observed with mass spe~ometers[l]; The next higher analogue C~I~v was als0 observed l~_]_~tively e a r l y [ 2 ] . - H o w e v e x : u n t i l recently no clear experimental evidence for the existence and stability of higher protona~l alkanes was available. -- : : -= - ::~ = Great inf~es~ in pen~acooz~in~f~l carboni~Im ions in solution was created by the work of Olah et al. [3]. It was ]m~ely_~ais Work-that increased in-retest in pentacoo~d!n~f~l ca~b0nil]m ions-nbt only~f~Sm-~lie standpoint-of bonding-and-~Uc~-the~ry but als0 as intermediates in alipha~ic- substitutionzeactions of- alpines suchasacid~talyzed ~mentati0n-~.isomerLzafion and

cyelization.::_

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~- " - e n t a 1 i n f _0 ; , , , a t i 0 n : o n. _ . . .t i l e h e a l m 0 f f 0 r ~ n a t i o n ~ e n e r g e i ~ c s = a n d m t~,,ity o f s u c h Carboniumi o n s ( i i n t h e : ~ a s p h a s e , - -id- C l i ~ a r . l y . o f - c o 6 . - ~ d e m b l e inte-z~t.--!~ntly , Studi~s-bf-sever~. " ibn~-molecule equilibr~a--iny01ving t~rC~ cen~o~r-b0nded"-cvzl~ni~m:i0ns ~ i n ~ h e -g~_ 4 ~ h ~ L ~ - w ~ e : : r n ~ d e - b y i the-:-~dberf~: gro-x~p -[4:~10] .--~k-=-st-kxC~y : o f - f 3 a e " t ~ b m p e r a t t u ' e dependence~f me-e~l~bi-i n- f o r •

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Y

140

gaveinformation

ontbeprotonafEni~oftheC~bondofi.sobutane[lO]_ The proton~affinity-for the C-C bond ofisobutane calcul&ed from the enthalpy of formation of C4s, is 164.0 kcal mol-'. This value is-only several kcal mol-' lower than the proton affinityof water (168.9 kcal mol-1)orhydrogensulfide(172kcalmol-')[11]_ In this experiment,the temperature dependence oftherate constants of theendothermicreactionsof HSO* +i-C4H,,

(2)

and H3S+i-C9H10 (31 was examin ed, The activationenergies derived from the Arrhenius plots wereexpected~gFvethedifferer;cesinprotonRffinitvwithsomeadditional activationenergyfortheprotontrausferprocessifanyexists.

Themeasurementsweremadewiththepulsedelectronbeamhigh-pressure ion source mass spectrometer at the UniversityofAlbertawhichhasbeen describedpmviously[12]_ The HSO~+i-CJ&, experimentswereperformed usingH, gascontaining known small quantitiesofH,O andi_C4HI0. TheH,gas(LindeUHP)was purified by passing it throughaLiquid-nitrogencooledp containingmolecularsieves(5A)_ The H3S++i-C&H10 experiments were done with CH, gas containing known small quantitiesofHzSandi-C&EIO_ TheC~gas(LindeUHP)and H~Sgas(MathesonResearchGrade)werepurifiedbypassingthemtbrough dry-iceacetone-cooled&aps con~molecularsieves (5A), RESULTS

AND

DISCUSSION

The results obtained from a typical run at412,5"C withhydrogen containFngsmallamountsofHzOandi_C4H10areshowninFig.i.Themajor~on producedby electron impactwithhydrogenis~_Atthehighpresrmreused in the experiment,this ion reacts very rapidlywithhydrogentoyiel~~. H; then furtherreacts with Hz0 audi-C~~Obyprotontransferrea.ctious~ Themajoriousobservediu Fig.1 ares-C&,H~O~and%C~~withamiuor contzibu~onfromC&~ Attheendoft.heionizingpulse(f=O),their&nsitgofs~,~suchllarger~t-C~_This~~thatthemajorproduction from the reaction G+i-C&X ,,iss-C~_Thiswasconfirmedbya Separate &e&in which avery small quanti~ofi-C&tIO(0.3mtqr)

TIME (psec)

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Fig. 1, Concentration changes of major ions observ&_ in 5-ton Hz containing 5.6-mtorr Hz0 and 25-mtorr i-C4HIo at 412.5% after the electron pulse (durati& of ~ectron p&se=10 CLS). -.

was introduced iuto the hydrogen carrierfas (5torr)- The onlymajorion gp~(t=0)wass-C3~,andtheintenobserved at the end of&he ionizin sityoft-C~wasonly=40%ofs-C&l$. The H&* signaldecreasedexponentiallywithtieviatheprotontransfer reaction with 1-C4Hr0_ The intensity of s-C,W, showed au initial small increase whkh was followed by an exponentialdecay_Thedecay ofthese ionscorrespondsto thegrow& oft-C~_Th&initiazgrowkh oftheC&I& canbeexplainedbyreaction(2a),

HjO*+i-C4Hlo =s-c;W,+cH,+H,O However,anot,her path for the reaction ofH30" reaction(2b). H30++i-CJ&

=&C~W9+H,+Hz0

(2a) with i-C&II0 is apparentin

(2b)

Reaction(2b)shouldbethermoche.@Aly_~~refavorable&mrez@ion(2a) becausethereportedprotonaffini~o~&he~-~bond(whereC&represents the tertiary carbon) ishigherthzu_thatofC-C bondof-isobutaneby-about -3 k+mol-f fICQ._.Theprqtqqation of_theC~bondorC,-Hbond~oirld ~,_l_~.~,~e~d~o~p~~tionofp~~~~_isobutane~yields~~~and.~~ ort-C&andH~,respectively.

I!42

The sa and t-C& reactions (4) and (5)_

ions at f = 0 in Fig,1 are mainly produced

from

These reactions are so exothermic that reaction (5) may be statistically more Savored than reaction (5) because there are three C-C bonds which may be protonated and only one C;-H bond. Owing to the contribution of reactions (4) and (51, it was dSficult to detemxine the major product ions from the rear&ion of X&O’ v&h i-G,H,, jn this system- In order to eliminate the proton transfer from WS to i-C&HIO, a large amount of Hz0 gas (480 mtorr) was introduced into the Hz carrier gas with a small amount of i-CaHIO (10 mtorr) in a separate expeziment, Under these experimental conditions, -Q8% of H; shouId be converted to the H,O’ ion which reacts further with isobutane, The ion f&ne profiles are shown in Fig, 2. Approximately 3% of s-C*, and no C.& were observed at the end of the electron pulse. The amount of s-C&W~ observed at t = 0 is so small that it is not clear whether this ion was produced fkom reaction (Za) or (4). Therefore it is difficult to determine the relative contributions of reactions (2a) and (2b) under such experimental conditions_ However, the initial increase of s-C& with time clearly shows that reaction (2a) does occur although reaction (2b) is thermochemically more favorable

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TIME (psec)

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FZg- 2, Concentration changes of major ions observed in 4.5torr Hz con?ai&ng 482mtoxr Hz0 and 10.3-mtorr i64Hx0 at 429_4% after the electron phe~(dur+ion of eIectroll p&X? 50 crs)_

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TIME(prec)

lOOC?/T(-K)

Fig_ 3. Logarithmic plots of the decay of H30’ with time hecau of reaction (2), (6) T = 375%, P(H2) = 5 toti, P(H,O) = 2.9 mtorr, P(iQHib) = 29 mtorr; (8) T = 412.5%,

p(Hz) = B-ton, P
P(HzO)

= 5-l

Fig. 4, Arrhenius Straight

line

plot

obtained

mtorr,

mtorr, P(iCeH&

q(GCeH&

of temperaturedependetit leads

to

mol-l/RT).

= 24.8

mtorr;

(m)

T=

430.4’%&

P(H*)

=

= 71 mtorr.

kz = 1.8 x lo*

rate con&a& (un3

Kz fok the reaction

molecule-1

s-1)

exp(-3.9

(2)_ kcal

reaction (2a) [lo]. The temperature dependence of k2 was exa&ined. Figure 3 shows the plots of log (H30*) versus time which indicate good first-order decay kinetics for H30*_ Determination of k2 based on the decay of (H30’) at four different temperatures was used to obtain the Arrhenius plot shown -ti ~Fig- 4, To suppress the formation of higher protonated water cluster ions H’(F20)n (n Z 2) it was necessary to increase the ion source temperature which limitid the temperature range for the Arrhenius plot. The least squares fit straight line obtained defines relationship (6)_ than

k2 = 1.8 X low9 (cm3 molecule-’

s-l) exp (3.9 kcal mol-‘/RT)

(6)

factor of kz isvery clczseto#eLangevinrati conStant The pre-ex ponenl-75 x 1O-g which is usually observed for excthermic proton tr&sfer reactiOSlS_ It is interesting to note that the activation energy ftir reactiori (2 j isonly 3.9 kcal mol-1 although reaction (2a) is &doth&!mic by-8 kcal tiol-‘;Thjs is expected simply because the rate control step is the-proton transfer process 1 -for- the .formation .qf &he. C&& ion_ -ad, af@r ~$he Iprotona@on,_ C&I (+.&kcal m&‘l)~_to $ield ~decompbses by-~ surmounting an exiergy -barrier s-C& and CI& under the present experi&ental conditions [lo] _r : .- ~~

144

Hz0 is a good reagent gas to investigate the endothermic proton transfer reaction (2) since it has only a slightly higher proton affinie than iC&tIo and the rate of the proton transfer is reasonably fast (see Fig. 4). However, as poin’ted out earlier, the temperature range covered by the plot (Fig. 4) was limited due to the formation of larger water clusters at low temperatures_ On the other hand I-&S is known as a weaker chxstering agent than Hz0 [13], while its proton affinity is only 3.1 kcal mol-’ higher than that of Hz0 [II]_ Thus, H2S was considered to be a better candidate for the investigation of an endothermic proton transfer reaction, and the kinetics of the reaction of H&F with X&HI0 were studied further, Results from a typical run at 324°C with 4-3~ton C&, 0_4ton i-C4HIo and 0_48_torr H,S are given in Fig_ 5 which shows the time dependence of the normalized ion intensities after the short ionizing electron pulse. The major product ions are H&F and t-C&_ At all temperatures measured, no H’(H,S), was observed. The slow decay of H$T is completely accounted for by the growth of t-&H& The two major ions produced by electron impact with methane are CH: and CE3$ which qtickIy react with methane to produce C2s and a and

flME[-)

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Fig_ 5, Concentration changes of major ions observed in 4.3~torr C& containing 475after the electron pulse (duration of elecmton HzS and 406-mtorr i-C& 1~ at 324% tron pulse =lO m)_

they.fi&ier

react with-i~~io.--The-proton~transfer &&i*n~CE.+i&H~~ .-: willproc&d.hy the Iangevin capturerate---l0~~cm~~~~lec~e~~s~'-~~g~lifetime for- aof ca. -lo-' s~underlthese condi~ons~ The-temperature -. dependence.-of the hydride -ion transfer reaction-:C&+ i-C&?rO.was reported.-by Meat-Ner.and Field [14].:The r&e:corWant:9 Xr+O:"?cm3. .molecule-1 s-t ~obtained by them~aIso gives a Jifetime.for the.C2Hf ion of:.. about lo-' s. Consequently..C~ and C!&l,_were.not~observed in this:sys~ :: .r_ :_..I. tern,-The major product-ion from &sample of-methane(5torr) conijaining a~smahquantityofi-C~H,0(lmtorr)wasfoundtobes-C3H7.Theinitial(t=.O) idfmsi~ Oft-CSg wasonly. ==10%ofs-C3H7.However,no s-Ca7 was ob-.' served when-H,S wasadded with 0.4torr ofi-C~H,, (see3ig~S)_ Since the rate constant of the-hydride transfer reaction s-C3H;+i-C4H1~=t-C~~+ C&m~m~l~e_~ s~~tainecK by Meot-Ner and- Field: [14]: is 14X10-!" the Wetime~of sGH, in the presence of 014torr ofi-&HI0 islessthan~~.Consequentlytheidenti~~oftbeproduction(either s-C&H7 or.t-Cs) of the relativeIy much slower reaction (H3S?+i-C4HI-o) . couldnotbedetermined, ~~ ~ A study of the temperature dependence ofthe-H3S+ decay-&c (Fig_6) gave a good.Arrhenius plot.(Fig_?) for k3 and deIinedrelationship(7). k3 = l-5 X10-f0

(cm3 m_olecuJe-* s?)exp(-7-7

kdalmol-'l&F)

x7)

do 30

0

2

1 TIME

(+ec)

3

-

*L5

12

1.9

23

1000/T(%)

Fig_ 6, Logarithmic plot oith&deca$ of k,S+with tinme be&& of&&ion (3)_(o)T= = 414 mt0x-r; (A) T = 312_8%, 259°C,Z'(C&)=4.3torr, p(H2S) = 525 mtorr, P(i-CiHxo) p(CH,) = 4.3 ton, p
-

146

Intbiscase,thepre-is pongntiaZfac~risonlylO%oftheLangevincapture ratel-4X10-g;thereforetherem~besomedifferencebetweenthereaction mechanisms ofreactions(2)and(3).Ahnl~fandWahlgrenhaveexaminedthe e~~~o~cstructureofH30*byablnitioMOc~culations[151.Theydeduced and netpositivecharges onHand of0.328.and an HOH angie ofllp

0.015 re.spectively_Thusnearlyallthepositivechargein H,O"isonthehydrogens_Thenear pIanari~ofthisionmaybeaconsequenceoftheH+repulsion. Comparable calculations for H# by Yamabe etal. [lS] predict that much momchargeresidesonthesulfuratom(H,0.150andS,0.550);conseguentiy smallerhydrogen repulsions result in a pyramidal structure(HSH bond angle 94"). The above difference may be considered to be largely ponentialfactoroftherateconstants reIatedtothedifferencesiuthepre-ex of~tions(2)and(3)_Duetothelownetpositivechargeonthehydrogens in HsS',the strength of the bond of theweak Langevin complex H,S'--iC&Era might be much weaker than thatforPI,O* -i-C.J&O_ThisdZference may lead to the appreciable difference in the statisticalvibronic energy re&stribution in the complexes, Forexample,the weakercentrifugal force of the complex H,S* -i-C&Iro might assist a bond-breakingreactionwhich probably leads to the smaller pre-exponential factor, Inaddition,reaction (3) has larger activation energy than reaction (2) According to RRKM theory [17],the activated complex hasmuchlessnon-fixedenergythanthe energized complex due to the existence of some activation energy, Thus, th~arefewerquantumstatesfortheactivatedcomplex~thegivenenergg range, Therefore,for similar reaction system.s,the smaller preexponential factor w-illbe expected for the reactionwiththehigheractivationenergy_ This might explain the srnaherpreexponential factor of the presentreaction_Unfortunately,thestructureoftheactivatedcompIexwhichisessential forRRKManaIysis cannot be established on thebssisofthepresentexpcrimentaI results; therefore,it would not be profitable to speculate further, However, the marked differences between the pm-exponential factors of reactions (2) and(3)areinterestingandmightbeusefulinconsideringthe reactionmechanisms_ Theactivation energyoftheendothermicprotontransferreactionmaybe consideredto givethedifferencebetweentheprotonaffinityandtheactivationenergyforthereverseprocessofthereaction_Thereforethevahxeofprotonaffinityestimated fromtheactivationenergyoftheendothermicproton transferreactionwouldgivethelowerlimitfortheprotonaffinity.Thusthe proton affinitiesofHzO (168_9kcal mol-') and HzS (1'72kcal mol-') lead to lower limits ofproton affinie for i-C&ilo of 165 and 164.3 kczl mol-', respectZvely_Thereported protonaffinitiesoftheC--Cand~~bondsof i-C.$T,, are 164 and166~8kcalm0l-~,respecGvely [lOJ,andthedifference betweenthesevaluesandthelowerlinitofprotonnfFirritVobtaIned~omthe activation energy,F.e,theacti~tionenerggforthe reverseprocessofreactions (2)and(3),can be estimated tobeoftheorderofafewkcalmol-'or less_Since the mezsured activation energies-only reflect the energetics of

147

product formation in addition to any activation energy for the reverse process,ao informationconc erningtheenergeticsofC~lformationcanbe derived from these experiments_ However thegoodagreementofthepresently obtained proton affillitcr of i-C&HI0 withthevahesobkdnedbyadifferentkindofexperiment [lO] suggeststhat it wouldbepossibletoestimate at least the lower Emits of the proton affinitiesof some high&r alkanes whicharelabileagainstprotonationbyusingthekineticapproachdiscussed inthispaper.

The author is greatly indebted toProfessorP_KebarleforvaluabIediscussions andformakingthebighpressuremassspectrometeravailab~etohim. _ gforstimulatingdiscussionsonthis HeaIsowishestothankDr_J_(=L subject_ REFERENCES 1 V_L_TaIrozeandA_L_Lyubimova,Dokl_Akad_NaukSSSR,86(1952)509. 2 S_WexlerandN_Jesse,J_Am_Chem_Soc_,84(1962)3425_ A_HengleinandG_A_Muccini,Z,Naturforsch_A,17(1962)452_ F_H_ Field,J_L.FrankEnandM.S_B_Munson,J_Am_ Chem_ S0c.,85(1963)3575_ P_KebarleandE.W_GodboIe,J_ Chem_Phys.,39(1963)1131. M_S_B.MunsonandF_H_ Field,J_Am_ Chem, Soc_,87(1965)3294_ 3 G_k0lah,G_glopmanandR_H-~osberg,J_Am_Chem_Soc.,91(1961)3261_ G-A_ G~,Carbocation.s and ElectropbilicReactions,VedagCbemie,JohnWiley&d Sons,NewYork,l974_ 4 K_HiraokaandP_Kebarie,J_Cbem_Phys.,62(1975)2267_ 5 K_HiraokaandP_Kebarle,J_Am_ Cbem_ Soc_,97(1975)4179. 6 H Hiaoka and P_ Kebarle, Can_ J_ Cbem_. 53 (1975) 970_ 7 K_ Hiraoka 8 9

10 11 12 13 14 15 16 17

and P_ Kebarle,

J_ Chem- Phys., 63 (1975)

394,1689_

M_FrexnzbandP_ Kebarle,Can_J_Chem_,53(1975)2268_ K_HiraokaandP_Kebar1e,Can.J.Chem_,54(1976)1739. BHiraokaandP_KebarIe,J_Am_Chem.Soc.,98(l976)6119. R_YamdagniandP_Kebar1e,J_Am_~em_Soc_,98(1976)1320_ kJ_Cmmingham,J_D_PayzantandY.Kebarle,J_Am_ Chem_Soc_,94(1972)7627_ K_HiraokaandP_Kebarle,Can_J_Chem_,55(1977)24_ M_Meot-NerandF_H_Field,J_Chem_Phys_,64(1976)277. J_Ah&~fandV_WahIgren,Theor_Cbim_Acta,28(1973)161. T_ Yamabe,T_ Aoyagi, S. Nagata, H_ Sakai and K_ Fukui, Cbes Phys. L&t_, 28 (1974)182_ P_J_RobinsonandILk HoIbrook,UnimoIecuIarReactions,Wiiey,NewYork,l972.