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Prec. Indian Acad. Sd. (Chem. Sei.), Vol. 91, Number 4, August 1982, pp. 359--$69.

Printed in India.

The study of c/s-and tran#-2 butene using m # ~ spectrometry

P ZTAT T M SELIM

Nuclear Physics Department, Atomic Energy Establishment, Cairo, Egypt MS received 22 October 1981

Abstract. Using mass spectrometric technique, the effect of geometrical isomerism on the first and higher appearance energy values for CIH, § C~H7 + and C,H + ions obtained from c/s-2-butene and trans-2-butene is reported. The structure in the ionization efficiency curves (studied for 9 eV above threshold) for the same ions obtained from the two isomers is reported and compared. It is befieved that at threshold C,H7 + fragment is formed from the two isomers as methailyl ion. For CsH + fragment formed from the cis.isomer at threshold the proposed structure is the propargyl ion with A H t equal to 279.4 kcal/mole while for that ion obtained from trans-isomer the proposed structure is the allenyl ion with AH t equal to 296-6 kcaymole.

Keywords. Cis-2-butcne ; trans-2-butcnc ; mass spectrometry ; geometrical iso- merism.

1. latroductio~

The study of the effect of electrons on the ionization and fragmentation of geo- metrical isomers had been carried out by Natalis (1965). He concluded that the ionization energies of cis- and trans-isomers being the same iinply that the same a m o u n t of energy is provided to the two types o f molecules by the impinging particle, electron o r photon. Also, geometrical isomerism disappears by frag- mentation of ionized molecules, the extra energy contev.t of cis-isomers due to sterie conformation is released and used in the fragmentation process.

Cis- and trans-2-butene isomers had been investigated by Meisels et al (1970) using electron impact and by Morrison using the second derivative technique.

They reported the appearance energies at threshold only of C2H5 + and CzH + fragments obtained from the two geometrical isomers.

Recently, Sv, nner (1980) using charge transfer technique reported the appear.

anee energies at threshold only for 16 fragments obtair.ed from cis-2-butcne.

His results also show that it is possible to distinguish isomers by charge transfer mass spectrometry, even if the molecular ions isomerize prior to fragmentation, by exploiting differences in the ionization probabilities.

The purpose of the present article is to report the effect of geometrical isomerism on the value of ionization energies, appearance energies and heats of formation for C4H +, C4H7 + and C3H3 + ions obtained from cis- and trans-2.b~:tene for about 359 P.(A)-- 7

(2)

360 E z z a t T M Selim

9 e V above threshold. The structures in the ionization efficiency (in) curves for the same ions obtained from the two isomers are reported and compared.

7- Experimental teclmiqae

The In data are obtained using the Atlas CH-4 m~ss spectrometer with a normal electron impact ion source (AN-4). The condition of measurements, experi- mental technique and calibration of energy scale had been reportedpreviously (Selim et al 1978 ; Selin 1976). The experimental results are taken in the form o f In curves, i.e. ionization yield against accelerating voltage for the electrons (varied by 0" 1 eV steps). These directly measured In curves are then smoothed by seven-points smoothing m~thod (Savitzky and Oolay 1964). T h e smoothed data are treated by the energy d!stribution difference (EDD) technique (Winter et al 1966) bY re:arts of which the effective electron energy distribution can be r e d ~ e d thus allowing a~ accurate d~term:.nation of appearance energies and fine structure in the curves. The value of the constant b characterizing the ~DD technique is taken to be 0-63 eV. This value of b is chosen so as to g~ve the best results

for Ar ionization energy value.

A computer technique is developed and used for d:termining the exact position o f the breaks in the ion intensity difference curves. The technique is based upon straight line fitting procedure (Selim et al 1978 ~ AUenson and Sedgwick 1968).

In manY cases, however, a step function may occur in the curves and can be detected when the sequence o f increasing slopes is broken. The step ftmetion regions are further examined by manual plotting of the data. The numerical values o f Student t used in the present study are that corresponding for 9 5 ~ confidence limit which is proved to be suitable for the present results.

3. Results and discussion

The ionization effi:iency (In) curves for the ions, studied for 9 eV above threshold a're gNen in fgttres 1-6. The initial portion of each curve is plotted on a magni- fied scale above the inftial one. The ionization and appearance energies obtained from IB curves of the studied ions together with similar results previously reported by different authors using different techniques are reported in table 1. The values g~ven are theaverage of six d~terminations while the errors quoted are the standard d~viafions. Oily reproducible ionization and appearance energies are reported in the table. In table.2 the diff~rep.ces between the ionization energies as well as differences between appearance energies for the same ions o b t a i n e d from the two isomers are reported. I-I~ats of formation corresponding to the first and second appearance energies for the ions studied are also reported in table 2.

3.1, CdHs + (m/z = 56) parent ion

The m~.asured first iov.iz~tion energies for cis- and trans-2-butene are 9.26 and 9-25 eV respectively. The ionization energy for the two isomers is established (Watanabe et al 1962; Dewar and Worley 1969)at 9-13 eV. It is generally accepted that the ionization of butenes is produced by removal of one of the 7r electrons (from C = C bond). We opine that the values 9-26 and 9"25eV obtained presently are not corresponding to adiabatic ionization energies.

(3)

Study of eis. and tranr 361

4

<

~ 3

c

L

~ " o

2

f - r -

c 1

o

1 I ~

,,9

15.55 ,'*~

,s

/

Q

t

10.55,"

e

9 t? -~ i -

9.26 Y . 9

o I a I I I t t

9 11 13 15 17 1 9 .

Elecfron energy volfs lcorrecfed)

Fig~e 1. The ionization efficiency curve for cis-2-butone parent ion (m[z = 56).

Higher energy levels have been detected in the ]~ curves of eis- and trana-2, butene and reported in table 1. The differences between the values of the c orres- pondiug energy levels for the two isomers are within the limits of experimental errors (table 2). The only available similar energy levels in the literature to compare with are those reported by Dewar and Worley 0969) at 11.28, 12.40, (13-74), (14.75), 16"07 and 08-84) eV for c~-2-butene and at 11.46, 12.58 and 13.99 eV for trans-2-butene. 1~o possible correlation can be obtained between the energy levels reported by these authors and that obtained from the prasent

m curves for the two isomers. However, many authors (White et al 1974, L Kimura e t a l 1975) had detected vertical second ionization energy for cis-2- butene at 2-3 eV and for trans-2.butene at 2"542-6 eV above the corresponding vertical first ionization energy.

3"2. C4H7 + (m/z = 55) fragment ion

The IE curve for C~H7 + fragment obtained from cis-2-butene shows a somewhat sharp rise at the threshold while the curve for the same fragment from the trans- isomer exhibits a short tail at the threshold. The values measured for the appee.

arree energy for C4Hr + ion from cis- and trans-2-butene isomers are 11.51 and l l - 5 2 e V r e s p e c t i v e l y . The value l l - 5 1 e V is in good agreement with photo- ionization value (Kramer and Dunbar 1973) 11.43 e V b u t is larger than electron i m p a c t values (Lossing 1972; Omura 1961 ; Dibelar 1947) (table 2). Foe

(4)

~ t a t T M Sellm

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r

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~

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"4--

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1 8 . 9 5 ~ /

.e

ip o

o*

e"

e

i

S

e

o e

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17 19 21

Electron e n e r g y volts (corrected)

Hgm'e 2. The ionization efficiency curve for fragment ion (m/z = 55) from cis.2.~.

butene.

C4~7" fragment from trans-2-butene the only available.value in the literature is the value reported early by Dibelar ( 1 9 4 7 ) ( l l . 2 4 e V ) .

The stmeture in IE curves for cis, and trans-2-butene are similar except for the higher appeara~nee energy 12.50 eV which appears only in tram. isomer curve and appearance ettergy 13.52 eV which appears only in the cis-isomer curve.

The fragment C,H~ + is formed from the two isomers by simple removal of H a t o m from C4Hs + molecular ion. This process requires no reverse activation energy and the kinetic shift associated with the detectable threshold is believed to be small as a result of,using : (a) zero draw-out potential and (b) high sensitivity electron multiplier as detector in the present study. The calculated Z~/-/tvalues for C4H, + fragment obtained at threshold from cis~ and trans.2-isOmer respectively are 211.7 and 210.9 kcal/mole which may indicate that the same ion structure is obtained from the two isomers. These / k t ! t values are corresponding to methallyl s t r ~ t u r e CHz+CHCH=CI-I, ~ith Zkilt value equal to 204 keal/mole as calculated

(5)

Study of eis. and tra.s-butene ]6]

r -

L -

" 0

r"

c - O

5

4

3

2 m

$

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.:

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o

14.121 ,~

3.6,51 i

o J;''" I I I I

14 1 6 18 2 0 2 2

Electron e n e r g y volts {correctedl

I

24

Figure 3. The iomizatiea efl~eacy cUrVe for fragment ion ( m l z .~ 39) from c/s.2- butene.

by Lossiag (1972) from the ionization energy of methallyl rad~cal. The present

~ l ! t value for the ion suggests a kinetic energy shift of about 7 kcal/molc in .the appearance energy o f the ion. Hawever, Lossing (1972) reported the v~hm 206 l~al/mole for methallyl ion obtained from different sources with kinetic shift

of only 2 kcal/mole.

It is possible that at higher energy than the threshold C~H~ § fragment is formed as cyclic CsH~CHs + ion with . ~ H I value (Franklin et al 1960)arotmd 221 kcal/

mole. There is e v i n c e supporting this from the ~ curves o f C ~ - fragments obtained fro~a b o t h c/s,- and trans-is~mers. These :curves reveal second appear.

a n c e energy* at 12-13 and 12.22 eV respectively an4 the corresp(ms r

L~Ht values are 226 anet 227kcal/mole. Taking into account a presumed small kinetic energy shift of about 6-.7 k~al/mole in the a p p e a ~ energy one ca~

* These values have no similars in CsHs + or Cj4a+ IE ~ v e s so, one c ~ n ~ t eom~id~ these valises as the onset of formatioa o f an excited state.

P.)A~...t

(6)

~ Ezzat T M ~el#a

6

5

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.I I ~I I

15 17

E lecfron e n e r g y rOils (.correGfed)

19 84

Flgere 4. The ionization et~eney bmrve for trans.2-buteae parent ion (ml: =~ 56).

Suggest that AHt values-226 and 227" kcal/mole c orrespond well to cyclic' C a H , L - ~ structure with 2~Ht value equal to 221 kcal/mole as reported by Franklin e t a t

(1969).

3.3. C~&+ (m/z •. 39)fragment ion

The~lE curves for the fragment ion obtained=frOm the two isomersexhibit'long tails.at the threshold. The first appearance energy for C~-ls + fragment Obtained from the c/s-isomer is measured a t 13.65eV and is in goodagreement with the pt~Yiously reported values (13.'75, Omura "196i a n 4 13.80,L Dibelar 1947 eV) while t h e i o n obtained from the transqsomer is measured at ,14.44r

in reasona.ble agreement with the onlY previously, reported value 14-20 e V (Dibelai 1947). The difference between the appearance energy values for C~-I8+ fragment obtained.from, the two isomers is equal to 0'79 ~V i n t h e right~ ,direction siiict one expects that fragment ions obtained from e/~-isomers is4ower by-the am0un~ o f

(7)

Study of cis- and trans.butene 365

6

5~

~.,.pi

;J4

k3~

~3

I n E

t- 0 2

I - - I B

i i i

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./

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I-~21'2~ :"

12 14

Electron

. . s

19

8 51~"~"

19"05 L."

e

I I I f f l

16 18 2 0 22

energy volts (corrected}

Figure 5-. The ionization etf~ency curve for fragment ion ~(m/z -~ 55) from trans,

~butene.

energy provided by steric conformation in the c/s-isomer. However, one can argue that the value 0-79 eV is too a large value for the energy o f steric confer- marion in the cis-isomer. Also. one may asi~ why similar energ~y of sterie c onformation does not appear in the fragn~mtation process leading to the formation of:C~I~+ fragment? The plausible explanation for the large difference between appearance energies obi~ined from-the two isomers is t h a t for unknown reason .geom~t~icaI isomerism remains after the fragmentation o f the two isomers-and that'th~ ground state of Calla § fragment ion from tran~-2-butene has a very~low cross-section. As a result Of that the measured appearance energy for C@T-Is + fragment from the trag~.isomer is not the first one but hi,her appearance [eners~.

(8)

366 Ezzat T M Selim

6

5

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C 0 L.

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. 4 - -

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0

J

14 16

t

,o

20"16~. Z

d'

19.251~ /

~ o

oO

I t l I 1 I

18 20 22 2:4

Elecfron energy volfs (corrected)

FiSm~ 6. The ionization efl~iency curve for fragment ion (m/z = 39) from trans 2-butene.

The structure in the m curves of CaI-~ + fragments obtained from the two isomers are given in table 1. Three higher appearance energies appear at 19.57, 2t-63, amt 23,08tV ha t h e l eturve of C ~ * fragment ptodt~er from the cis.

isomer b ~ not in the ~ curve of the fragment from the trans-isomer.

The Carte + fragment ion is believed to be formed at the threshold from both isomers by the following processes having almost eqlml heats of formation :

CjIs*

-~.C,J~+ §

(a)

C , ~ + -~

C d ~ §

+

e ~ . (b)

c l ~ J ~ tr~sfe~ t ~ k n i q ~ Sunncr (1980)~had. &tooted two metastablr peaks a t m/g eqtta~-t~ 27-7 aa&, 3,7,i far the formatioa of Calla + fragment from c/s- 24mY, eae,, The flr qt peak c ~ s p ~ d s to the formati0~ o f C ~ e + fragment from C~I,I,'+ futgnmat while.tlte u:r peak correspo~& t o its formation from C,H~+

f r a ~ t ~ T h e cakalatr gxHt vat u ~ for Calla + form*,t~.or from e/~-2-buteae a t tl~thtv~shot~ are equtl :t0'279.9 keallmole (process a ) and 279.4 k~,al/molr

(9)

Study of cis- and trans-butene 367 Table I. Ie~izatios and appeara~te~ energies for CtHt ~', C4J~l# rout CtHa+ ions eb~dncd from cis, al~[ t~ram~2-bttte~c.

i

Ionization and appearaa~ tutti[it*

Ion cis.b~terro t~am-bctene

Present study Previous work Present stud~ Previous work

9"264-0-06 9"18 t~) , 9 ' 0 0 ('), 9"254-0"05 9" t(ld=O" 15 tt~

9' 134-0"01 (t) 9' 134-0"01 (a}

C4Hs +

On/z =50

C,Jtr

(mlz = 55)

C,I~§

( ~ z = 39)

9" 554-0" 10 9" 574-0" 07 10" Q0 4-0"0,7 9' 1154-0' 05 10- 534-0.06 10" 50::E0" 09 15" 35 4-0"O6 15.304- 0" 10

17.904-0"08 18.054-0"10

11"514-0"08 11.32(1), 11.20 (4) 11"524-0"06 11.I04-0.11 (s)

11.434-0"01 (e)

12-134-0"09 12"22-t-0't0

.. 12"504-0"10

13"524-0"07 ..

t~'95d:~'12 19"@.$~@.'12

19" 95 4-0" 1,() 19" 85:[:@'12 13"65~0"Q9 13"75(0,

13.804-0.3 {s}

14"12+0"10 14- 32 ~:O-OIl

14.64=[=0-05 14.654-=0. ~)

15.184-0.t~9 15" 25 :[:0.06

.. I6- 20+0"06.

19. l~l-l- 0' 0~7 19- 25 4-0~ t~7

19.574-0"07 ..

.. 20" 164-(1" 10

21"63=[;0'08 . ,

23"084-0:08

11" 244-0" 1 (s)

14" 204-0" 3 is)

(i) Logstng I972, (2) Me, Lsel~ etal 1970, (3) Wataxttbr et6l I~2, (4), Otlt~ 1961, (5~ Dtbelar 1947, (t~) Kramer and Daab,~r I973.

~ r o e e s s b). The meatt value is equal t o 279.4 kcalpatole which is in ex~elkr.t agreement with A H t value 281 lmal/mole for t h e propargy! ion HC--c2~CTI~, reported by Lossing (1972). The p r e s e n t ~ reported 2XH~l value 279"4 kcal/~noIe is higher b y a b o u t 23 kent/mole than the h e a t o f f o r m a t i o n o f C ~ a § as r ion reported b y Lossiltg 0972) ar.d Franklht et a2 (1969)which rttles out the formation o f cyclop~ope~yl ion in t h e present stud~ a t least

(10)

368 Ezzat T M Selim

~Table 2. I0,~ization arid appearance energy ]difforcace~ . a n d l~eat.s: 0f formation

f o r C4I'I8 § CoT-I~? a n d CsHa + ions obtained.from cis, a n d t r ~ - 2 - b a t 0 n e .

Ionication and A//" t

appe&ranoo" (koal/mol)

Ion energy differences

(eV) Trans.eis Cis- Trans--

C4H8 § --0'01 211 "9 210"6

(mlz =~ 56) 0 ~ 02 2 1 8 : 6 218" 0

--0"12 . . . .

- - 0 " 0 3 .. ..

--0"05" .. '.:,'

0 " 1 5 . . . .'

C4H~§ 0"01 211 "7 210:9

( m / z ---- 55) 0" 09 226.0 227" 0

CsI-I,+ .. 279" 4. ..

(ra/z -.~ 39) 0" 20 290" .2. 296" 2

at the threshold. While the AHt value for propargyl ion has not been established until now, the excellent agreement between t h e present result for this ion together with that of Lossing 0972) may indicate that AHt for the i.on is about 280 kcal/mole. Although, t h e processes (a and b) for formation' of cycloprogenyl ion from cis-2-butene are m~,dticenter dissociation processes which require reverse activation energy and probably also kinetic shift the presently reported value of A//'I for propargYl ion together with that calcl~late6 by Lossing 0972) m~y indicate that CsI-Ia + is formed presently from cis-2-bv, tcne with almost z,',ro excess energy.

The calculated AHI value for the formation of CaH~ + fragment from trans-2- btRene according to process (a) is equal to 2~,7'2 kcal/mole, while teat value according to process (b) is equal to 296.1 kcal/mole. The mean value is equal to 296"6 kml/mole and probably corresponds to the forma(ion of CsHa § frag.

mint with aUenyl structure (CH,~-C--CH+). The AHI value for the formation of allenyl ion obtai),.ed from i-butene is calculated previously by, Seh'm 0970) ias 287.3 kcal/mole, it appears tha( the format~0n of.C,Ha + fragment fr6m, t/~ans:

2-bl~tene need~ more excess energy than the formation of the same ion from i-butene.

Finally, if one calculates the AHI yahoo for CsHa+ fragment obtained from the c/s-isomer c0rr~sponding to the secorA .appearar.ce: er.r (14.12 eV) the calcu.

lated value will b~e equal to 290.7 kcal/moleaccording t ~ process (a).an~ 289.7 k:al/mo~ ~cr to process (b). The m~an va!ue is. eql:ai to 290. 2 kca)/mole and may indicate that at higher energy than th~ threshold the pcopargyl ion obtained .from cis-2-bqtene (at the ~hroshold) isom~riz;cs to the allcn yl structure,

(11)

Study o f cis- and trans-butene References

369

Ailenson I I O and Sedgwick R D 1968 Advances in mass spectrometry Vol. 4 (ed) E Kendrick The Institute of Petroleum, London, p. 99

Dewar M J S and Worley S D 1969 J. Chem. Phys. (USA) 50 654 Dibelar V H 1947 Y. Res. Natl. Bur. Stand. (USA) 38 329

Franklin J L, Dillard J C, Rosenstoek H M, Herron J T, Draxyl K and Field F H 1969 Ionization potentials, appearance potentials and heats of formation of gaseous positive ions NSRDS-NBS, Document No. 26, US Government Printing Office, Washington D.C.

Kimura K, Katsumata S, Yamazaki T and Wakabayashi H 1975 J. Electron. Spectrosc. 6 41 Kramer J M and Dunbar R C 1973 J. Chem. Phys. (USA) $9 3092

Lossing F P 1972 Can. J. Chem. 50 3973

Meisels G G, Park J Y and Giessner B G 1970 J. Am. Chem. Soc. (USA) 92 254

Natalis P 1965 Mass spectrometry (ed) R I Reed (London and New York : Academic Press) p. 379

Omttra I 1961 Bull. Chem. Soc. Jpn. 34 1227

Savitzky A and Golay M J E 1964 Anal Chem. (USA) 36 1627

Selim E T M 1970 Mass spectrometric investigation of electron impact ionization and dissociation of some aliphatic hydrocarbons and ethers Ph.D. Thesis, Cairo University, Cairo

Selim E T M 1976 Indian J. Pure AppL Phys. 14 547

Selim E T M, EI-Kholy S B and Zahran N F 1978 Optimum conditions for the determination of ionization potentials, appearance potentials and fine structure in ionization efficiency curves using EDD technique AREAEE/Rep. 220

Sunner J 1980 Int. J. Mass Spectrum. Ion Phys. (Netherland) 32 285

Watanabe K, Nakayama T and Mottl J 1962 J. Quant. Spectrose. Radiat. Transfer 2 369 White R M, Carlson T A and Spears D P 1974 J. Electron. Spectrosc. 3 59

Winter R E, Collins J H and Courchene W L 1966 J. Chem. Phys. (USA) 45 1931

References

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