The decay of 1-chloropropyne cation studied by photoelectron-photoion coincidence spectroscopy

Chemical Physics 48 (1980) 79-87. 0 North-Holland Pnblishirig Company

THE DECAY OF l-CHLOROPROPYNE CATrOy STLJDiED BY PHOTtiELECTRON-PHOTOION COINCIDENCE SPECTROSCOPY Josef DANNkHER Phpikalisch-chemies

and Jean-Pierre STADELMANN Institut der Unfuersitiic Basel, CH-4056

Basei, S~&erla~~d

Received 29 November 1979

The decay of internal energy selected l-cbloropropyne cations is investigated using.& fixed wavelength (He-I%) photoelectron-photoion coincidence technique. The breakdown curves of the molecular ion and of the C,H,CI’, C,HCl l, CCI’, C,Hz, C,Hz, C,Hf fragment ions are reported. For I-chloropropyne cations initially formed in their AZE state it is found that four fragmentation channels compete with a non-dissociative relaxation pathtiay. The average kinetic energies released on formation of C3H; and C,H; are deduced from the time-of-flight distributions of rhese fragment ions measured at different internal energies of the molecular ion. The coincidence.dafa are supplemented by electron impact appearance energies. The obtained decay pattern of I-chloropropyne cation is compared with the breakdown diagrams reported for the C,H,’ isomers, i.e. allene-, propyne- and cyclopro&e cations.

results reported

1. Introduction The fragmentation behaviour of allene-, propyneand cyclopropene cadons has been studied in detail by photoelectron-photoion coincidence spectroscopy [l-4]. The resuits of these experiments are primarily the breakdown curves, i.e. the branching ratios 6(Zj, nz,j for the molecular and fragment ions of mass rr~,as a function of the ionization ener_qyZi of the molecule. Secondly, values for the kinetic energy released on fragmentation can be deduced from the time-of-flight (TOF) distributions. The almost identical breakdown diagrams found for the three C,H,’ isomers were interpreted as reflecting isomerization to a common stru&ure before fragmentation [4]. Comparing the photoelectron

(PE) spectra

of propyne

l5] and

I-chloropropyne [6] it is evident that replacing the acetylenic H by Cl resuits m&y in a lowering of the first ionization energy and in an additional

PE band. On the other hand, the higher excited electronic states appear to be only slightly affected by this substitution. Therefore, aim of the present study was to relate the fragmentation pattern of excited I-chloropropyne cations to the coincidence

for the corresponding

hydrocarbon

cn

An earlier coincidence study of 3-chloropropyne cation [7] was conliied to the threshold region (z 1 I eV) of the lowest energy fragment (Le. C,Hg); In this energy region there are no states of 1-chloropropyne cation which can be populated by He-Ia

photoionization

and thus the data cannot

be compared directlyI-Chloropropyne cation is one of the very few organic radical cations where the observed radiative decay ‘ii -P *x is in effective competition with one or more dissociative channels [S]. Excited molecular ions which fluoresce to the ionic ground state are detected as undissociated parent ions in the coincidence experiments. Hence,the branching ratio of the molecular ion b(Zj, M ‘) measured for ionization energies Zi corresponding to the first excited electronic state constitut& an upper Iimit.fdr the quantum yield of emission @. Assuming that b(Ij, Mf) z .@, the rates of the-radiative-and-the nonradiative processes depleting the ‘A state can be deduced if the lifetime of the excited state is known. An important purpose of.this work was to quantify the dissociation channels whi&h’compete with the

80

J. Damocher,

J.-P_ Sradel.~nnnjDecayof I-chloropropynecations

radiative decay upon formation of l-chloropropyne cation in its A ‘E state_

Table 1 Mass spectral data of I-chloropropyne

mfz 2. Experimental

The experimental arrangement and the evaluation of the coincidence data have been described in detail in an earlier publication [2] and therefore only a brief review is given here_ The sample zas effusing from an inIet capillary is ionized by a collimated beam of He-la radiation. The charged particles are extracted by a constant electrostatic held of 5 4 V/cm applied to the ionization source_ The photoelectrons are ens-q selected by a conventional 10 cm radius x/,/Z electrostatic analyzer with a resolving power E/AE = 75 under coincidence conditions_ The photoions are accelerated and focused into a quadrupole mass spectrometer which provides unity mass resolution while maintaining a high ion _ _ transmtsston (20-50 yi)/,)The breakdown curves are obtained by appropriately normalizing the counted coincidences between mass selected photoions and energy selected photoelectrons as a function of the electron energy_ Time-of-flight distributions for ions of a given mass originating from molecular ions with defined internal energy are accumulated using a fast buff& register and a minicomputer system serving as multichannel analyzer_ The reported electron impact (EI) appearance energy (A%) values were determined by the method described in ref [9] using a Hitachi-Perkin-Elmer RMU-7D double focussing mass spectrometer_ I-Chloropropyne was synthesized as described in Rf. [IO]. No signilicant amounts of impurities were detected in the mass- and in the photoelectron

3. Results and discussion As a consequence ofthe optimum conditions [2] required by the coincidence experiment, the photoelectron (PE) spectrum or 1-chloropropyne (CT.fig. I) shows a very low electron count rate, a poor signalto-noise ratio and a limited ener,g resolution_ Tlxe

37 4-i 49 72 73 74 75 76

EI”

PI” He-12

Ion

C,H+ C,H; C,%

21 eV

35 eV

7SeV

2

14

15

3

(yC1’

C~‘C1’

C3H-“CIi C&T1 3-

CSH:5Cl*.C,H35CI+

9

17 1.5

21 5.5

21

1

0.5

2

2 6 40 100 17 31

5 35

3 23

100

100

15 33

10 32

CaH:‘Cl+ 7 C3H:‘CI +

15

13 3

6 39 100 16 31

6

al Under coincidenceconditions_ b’ This work, Hitachi-Perkin-Elmer RMU-7D. lo-” Torr. 40 JIA electron current. bands a @and @are assigned to the X’E, A”E and fi’A, states of the 1-chloropropyne radical Qtion adopting the analysis of a high resolution spectrum given in ref. [6]_ The He-La photoionization (Pi) mass spectrum is presented in table 1 together with electron impact (EI) mass spectra measured at various electron energies. The data show that I-chloropropyne cations, formed by He-Ix photoionization, fragment essentially according to the following dissociation reactions: C,H,Cl*+

II

(1)

C,HCI+ C,HCl+

+ Hz + 2H

(2) (2’)

F C,H,Cl+

I

Ccl+

+ C,H,

(3)

C,H;

+ Cl

(4)

C,H; + HCl C,H;+H fC1

(5) (5’)

C,H’+ H + HCl C3H* + Cl+ H,.

(6) (6’)

The heats of formation of the neutrals involved in the above reactions arc accurately known quantities. The values for A,He (C,Hg) cl l] and A,He (CsH+) [3] are rather well established from recent coincidence studies. Using the thermochemical data

TabIe 2 Yalues for the heats of formationused in the thermochemicalcalculations Neu&l

A,H* 298 (g) Ionization

(kl mol-‘)

&He 298 (g)(cation)

energies

(kJ moI-L)

. WI

S-chfortipropyne fexp. 188 kJ,m&- f7], &c. 157 kJ mol-4 [12]), cbioroacetyIeiie (exp. 255 M mol-’ [13], c&z. 198 k.I mol-I. rl2l) and di&bloroacetylene (exp. 197 kJ mol-’ [13], talc. 173 kJ fl2Jb Hence, we accept 210 kJ mol” as a _ reasonable estimate for AtHe of I-chloropropyne

mol-’

and use this value to calculate the thermochemical dissociation limit& for the reactions (4) and (5) listed -.

218I)“’

-923j’

in tabIe 3.

121.P

261. =’

15

1125”’

C3%

1376b’ 1594=’ lO?Y

W53

1176~’

CHs-=-Cl

1

.

l

-

(cyclic)”

(Iinear)” C,H,Cl ‘) Ret [X1_ =J Ref. [16].

ZIO_

9.83d’

b1Ref. [l I]. o See text.

1158 dJ Ref. [6].

ClRet [3].

collected in table 2 and the measurkd appearance energies (AE’s) listed in table 3 one obtains

215 kJ mol-’ (from (S)), 210 kJ mol-’ (from (6)) and 206 M mol-’ (from @‘)),respectively for the heat of formation of I-chloropropyne. These values are about 50 kJ mol-L higher than 158 kJ rnol-‘, calculated by using the bond ener,T terms tabulated in reE [12f. Simiiar differences between experimental and calculated values for the heats of formation of chloroderivatives of acetyIenes are reported for Table 3 Calculateddissociationlimitsand appearanceenergiesof the fragmentions of I-chIoropropynedeterminedby the coincidencetechniqueand electron impact measurement@ mfz

IOll

73

C,H:‘Cl+ _)

Coin&

El=’

=)

13.4g’

dent@

l~i

--;;yE

*

lo

s&q

’

:~~~~~~~~~

,

1 Pas ; Ifuo. g I.0

i -i

C3H$I’

. ,

1

:

f

8

.%k

I

I

L

I

EId’ CalculatiorP 94

72 47

C,H3%Y c?CI’

=) 15.75

I 3s 15.9

39

C,H,’

=t

Il.1

j II o .

(cycIic) 10.19’) 1 (linear)

1127’1

vahies in eY_ ” ihis work, +O.l eV. =’This work, t02eV. ‘) Ref. [15-j, +0.2 eV_

aa All

=’ Between 10.7eV and 13.2eV. fJ 1555 + 0.15 eV for the stepwiseeliminationof H and CL

*) True thresholduncertain. ‘a For the heats of formationused,see table 2. a See text.

Fig. 1. The He-LaPE spectrumof I-chloropropyne

recorded under cdincidenccconditions.The sum curve; the .. breakdown curves of the molecular ion and the C,H,Cl’and C,HCI’ fragmentions. The length of the bars in this

and the foilotiingfigurescorrespondsto plus, minusone standard deviationof the counting statistics.

s2

J_ Dannacher. J_-P. StudelmannfDecay of I-chloropropyne

The AEs of reactions (I)-@‘) have been determined from EI iondation efficiency curves. The thresholds for reactions (3), (5’). (6) and (6’) lie in an ionization energy range where excited states of the molecular ion are populated upon He-Ix photoionization. Thus, these AEs can also be mcasurcd by the tixed wavelength coincidence method (cf. table 3). The breakdown curves of the moIecuIar ion and of the various fragment ions appearing in the ttKtSS spectrum arc shown in figs. I and 2. The large error bars at 14 eV and at 16.5 eV arise from the extremely low intensity of the PE signal found at these energies. Note. that the breakdown curves of the chlorine containing ions are corrected for the contributions of both natural Cl isotopes_ In the following sections the coincidence results are discussed in detail.

carions

3.1. C,H,Cl’(nto!ecular

ion)

I-Chloropropyne cations generated in the electronic ground state (Z’E) by He-I= photoionixation do not dissociate_ ConsequentIy the branching ratio is unity in the corresponding energy range (9.6-10.5 eV) yielding an ion transmission coefficient ofO.2. I-Chloropropyne cations initiaiiy formed in the electronic Aal3 state fragment via four different dissociation channels as wiil be shown. Nevertheless, moIecular ions which are stable on the time scale of our coincidence experiment are also registered in this ener,T range. This is consistent with the emission spectrum [14] assigned to the Ai’E -+ %‘E electronic transition. The following considerations strongly suggest that the branching ratio of the molecular ion measured within the ener_q range of the AizE state can be identified with the quantum yield of emission. The breakdown curve shown in fig 1 was recorded at a molecular ion flight time of z 35 ps. By changing the potential of the mass filter axis the residence time of the ions in the quadrupole mass fiber can be varied. The branching ratio at 13.25 eV (cf. table 4) was not signilicantly altered if the flight time was increased to z 60 ps. This implies a long lifetime of the molecular ions compared with the experimental sampling time_ Moreover, the behaviour of the sum curve (see below) reveals that slowly dissociating moIecular ions are not involved and that the observed parent ions must have a lifetime of the order of 1 ms. The considerable Table 4 The bmncuingratio a(li,

M+) of’the moIecular ion in the energy range of the A ‘E state as a function of the ionization energy Ii

Fig. 2. The breakdown cm-w~ of the Ccl+-, C,H$- and C,H’ fiigment ions.

C,Hf-,

IjEN

b(Ij.MC)=’

1320 1325 ! 3.30 13-4 13.5 13.6

0269 f. 0.245 + 0.146 2 O-044 + 0.039 * 0.007 f

O-026 0_007b’ 0.013 0.011 0.013 0.0 18

a1 Measured at a molecular ion flight time of = 35 Phl Remeasurement of this value at zz 60 ps flight-time yielded 6(Ij, M+) = 0.230 f O-01-

.I_ Dannacher. J--P_ StadeLnonnfDecay excess energy (> 2 eV) with respect tc the lowest fragmentation threshold indicates that the presence of excited parent ions with such long lifetiiues is highly unlikely. Combining the branching ratio of the molecular ion at 13.2 eV (b(Zi = 13.2 eV, M’) = 0.27) (cf. table 4) with the lifetime (t = 18 ns) of the zeroth vibrational level of the A2E state reported in ref. [14], the radiative rate constant is equal to kmi,, = b(Zj = 13.2, M+)/r = 1.5 x loss-‘. The rate constant of the competing nonradiative process, presumably internal conversion, is calculated to be k = 4 x 10’ s-i. The smoothly decreasing bzkdown curve of the parent ion suggests that the lower vibrational levels of the A’E state are also depleted partly radiatively. The non-exponential behaviour of the radiative decay reported in ret [14], does not inlluence the coincidence data since the intensity of the long lifetime component is quoted to be two orders of magnitude smaller than that of the short one. Among the few organic radical cations where competition between emission (‘A + ‘z) and fragmentation is known to occur [S], I-chloropropyne cation (A’E) contains the highest observed excess energy (> 2 eV) above the lowest fragmentation threshold. Therefore it seems reasonable to assume that the internal conversion is the rate determining step of the dissociations followed by much faster randomization of the internal ener,7 and formation of the observed product ions. Molecular ions originally formed in higher excited electronic states are fully dissociated as revealed by the zero branching ratio of the molecular ion above 13.5 eV_

3.2_ C, HJI+ C3H2ClC is the most intense fragment ion in the mass spectra. The measured EI ionization efficiency curve shows a first extremely slowly rising part with an undefinable onsct followed by a distinct change of slope at 13.4 eV. On the other hand, an AE cIearly below 13 eV is inferred from the breakdown curve of the C,H,Cl f ion (cf. fig. 1). In view of tbis very uncertain threshold it is not possible to give a reliable estimate of ArHe (C,H,Cl’). The breakdown curve rises continuously from

of I-chloropropyns

&ions

83

13.2 eV to 15_5.eV-where the internal energy of the C,H,Cl* fragment ions starts to exceed the thresholds for secondary fragmentations_ In addition, a further dissociation channel of the molecular ion (reaction (3)) becomes accessible iu this energy region. These are the reasons for the observed decrease of b(Z, C,H,CI+) from Y 0.7 to z O-05 between 15.5 eV and 17.0 eV. Almost all of the kinetic energy released on formation of C,H,CI+ is carried away by the hydrogen atom. Thus, no distortion of the breakdown curve arising from discrimination against high kinetic energy fragment ions is expected. 3_?_ C,HCL+

C,HCI * fragment ions are already produced at 132 eV as evidenced by the non-zero branching ratio observed at this ener,T (cf. fig. 1). The interpretation of the onset in the experimental EI ionization efliciency curve involved the same . problems’as mentioned for C,H,Cl+. The value quoted for the AE (cf. table 3) is probably related to the increased production of this fragment ion above 14 eV (ct fig. 1). According to thermochemical considerations, the C,HCl* ions are expected to be dominantly formed via reaction (2), i.e. involving the loss of au H2 molecule of the molecular ion_ The nearly constant value of the breakdown clurve between 14 eV and 17 eV suggests that a secondary fragmentation does not contribute appreciably to the C,HCl’ intensity within the considered energy range. Since only z 3 % of the total kinetic energy released during the course of reaction (2) is carried away by the C,HCl’ ion, the breakdown curve is again expected to be practically undistorted. 3.4. C,H: Among all accessible dissociations, the loss of a Cl atom from the molecular ion leading to C3H: fragment ions shows the lowest_threshold energy. Our EI value for the AE is equal within the error limits to the value quoted in ref. [15]_ It is well established that at least two isomeric C,H; are involved in several ionic dissociations and reliable values for the heats of formation of the cycle;.

J. DUFUUZC~~!~_ J--P_ Srad&mnnfDecuy

84

propenium- and the propargyl cation are available r161.In the present case the formation of both isomers is energetically possible for ionization energies close to the measured onset of 11.1 eV (cf. table 3)_ However, the production of the cyclic C,H: isomer would invoIve an activation energy of the order of 1 eV. A comparable result was reported for 3-ehloropropyne [7]_ Coincidence measurements at the high energy end of the electronic ground state (10.2-10.5 eV) prove that within the time scale of our experiment and the error limits of the counting statistics CgH; ions are not formed at these energies. The breakdown curve is strongly affected by the considerable amount of translational ener,T which is released and carried off (up to z 50 %) by the C,H; fragment ions. The true branching ratio is expected to be about three times larger thanthe measured one- This folIows from the observed deviation of the sum curve from unity and the comparison with experimental data on other dissociations involving fragment ions with comparable kinetic energies. The slight rise of the breakdown curve at its beginning is a consequence of the increased number of molecular ions which fragment because the radiative relaxation does no longer compete_ For a few tenth of an eV the curve is flat before it declines and reaches the noise level. Table S

Man kineticenergf’ releasedduring fragmentationof C,H,CI’

Fragment ion

Excess energicsb’ Source of parent ion field (V/cm) W)

C3HC

L3-J’

4.4

kineiic” energy release W)

Totlli

0.40

73&e) CJH; 0.52 7.6 i2.J) C,HJ 4.4 050 4-P’. 4.4 059 C,HT TJ” C,Hi 4-4 0.27 4.4 0.47 3.5” CJHf 5-l.” 0.17 c,tl; 4.4 _ XlDetermined from gaussian distributions fittedto the experimentaltime-of-G@ distributions,see text. b’ 20.1 eV_ =)For the errors involvedsee text. d’ With AE(C,Hz) = 11-l eV (see table 3). c’ 5 02 ev owing to the higher source lie!d used. f’ Relative to the calculated dissociation limit of 1l-1 eV (see table 3).

ofI-chloropropyne cotions

Several time-of-flight distributions were measured in order to study the kinetic energyrelease accompanying the formation of CaHz fragment ions. The results, summarized in table 5, have been inferred by assuming that the distribution of the translational energy is a three-dimensional maxwellian in the center of mass system According to this assumption, the measured time-of-flight distributions must be fitted with a single gaussian. The average fraction of the available excess energy (E,,) which is converted into kinetic energy is nearly constant (16 %). This is in good agreement with the ratio (15 %) of the average kinetic energy relative to E,, expected from the often used empirical formula <&,,>fE,, = (O&W)-r [17], where N denotes the number of internal degrees of freedom. The value of the kinetic energy obtained at 13.2 eV was remeasured using a higher source field strength which tends to decrease the loss of high energetic fragment ions. The enhanced contribution to the time-of-flight signal of fragment ions with larger translational energies accounts for the major part of the observed increase of the kinetic energy value_ Moreover, the use of higher source fields lowers the energy resolution of the spectrometer resulting in a broader internal energy distribution of the molecular ions sampled. The relative standard errors of the average kinetic energies are smaller than 1%. However, the relative deviation of these average kinetic energies from the true mean value is estimated to be one order of magnitude higher (lo_%). The formation of &Hz from two different isotopic precursors (m/z = 76 and mfz = 74) yields slightly different amounts of kinetic ener,7 on the fragment ions. This effect is much smalier than our estimated error limits and thus not taken into account_ 35 c,II: This is the fourth dissociation channel accessible to molecular ions originally formed in the ?i’E state and therefore a competitive pathway to the radiative decay. The AE obtained from the EI measurements is 12.3 eV, i.e_clearly Mow the adiabatic ionization energy of the lirst excited electronic state_ The total available excess energy at 13.2 eV is about 2 eV as

J. Dannacher. J--P_StadelmannjDecay of I-chloropropyne cations the thermochemical dissociation limit is calculated to be 11.3 eV. The breakdown curve for the CsHf fragment ion is again strongly affected by discrimination against high kinetic energy ions which occurs to about the same extent as found for C,Hg_ After a small plateau within the energy range of the w2E state, the breakdown curve declines smoothly until 15.5 eV where a sudden upward break is observed_ This reflects the formation of C3Hl via a different reaction path, which becomes accessible at this energy_ We assign this steep increase at 15.5 eV to the onset of reaction (S), i.e. the sequential loss of H and Cl. This interpretation is strongly supported by the magnitude of the kinetic energy release deduced from time-of-flight distributions taken at 13.4 eV, 14.6 eV and 16.1 eV (cf. table 5) The percentage of the available excess energy released as kinetic energy is about the same for the fit two ionization energies. A much smaller value is obtained at 16.1 eV reflecting the much Iower excess energy of C,H,CI* ions undergoing secondary fragmentation. Although C3H: as well as C,H2Cl+ couId both be the reactant for the corresponding secondary fragmentation, only the latter shows enough intensity between 15.5 eV and 16 eV to account for the considerable increase of the C,Hf breakdown curve. 3.6. C,H+ There are two energetically accessible pathways leading to C,H+ ions according to the reactions (6) and (6’) As the thermochemical dissociation limits are about the same for both reactions the neutrals formed cannot be predicted unambiguously from the

coincidence data. However, from the high intensity of the C,H,Cl+ breakdown curve which decreases simultaneously as C3Hc is formed we conclude that the dominant part of the observed CaH+ fragment ions is generated by HCl loss from C,H,Cl+. The AE found for this process is 15.65 eV (coincidence) and 15.9 eV (EI), respectively. 3.7. CCZ’ In contrast to the dissociations of the CsH,f isomers [l-4] a fragmentation of the carbon

85

skeleton of I-chloropropyne cat&r yielding CCl* is observed. The AE derived for this dissociation is 15.75 eV from the breakdown curve and 159 eVhorn the EI ionization efficiency curve; Taking AfHe (C,HJ = 261 IcJ mol-’ (cf table 2) and the more accurate coincidence value for the AE we obtain A,P (Ccl’) = 1469 kJ mol-‘. This is roughly the average ofthe lowest (1322 kI mol-’ [l&l) and the highest (1649 W mol-i [19]) value given in the literature.

4. Sum curve The theoretical value of the sum curve is unity provided that all dissociation channels are taken into account and that the ion transmission of the coincidence spectrometer is precisely known as a function of the mass of the studied ions, their kinetic energies and the corresponding formation rates. Generally this function is only qualitativeIy known and one usually approximates it by the well defined transmission function for thermal ions. Strong deviations of such an experimentally determined sum curve from unity indicate the presence of high kinetic energy fragment ions and/or sIowIy dissociating molecular ions. Note, that in an energy range where the sum curve is significantly smaller than unity the measured breakdown curves relIect onIy lower limits for the true branching ratios. There is no evidence for the existence of slowly fragmenting molecular ions in the present case within the energy range considered in our study. Hence, the observed deviations of the sum curve from unity (cf. fig. I) originate from the discrimina-. tion of our spectrometer against high kinetic energy fragment ions. The four different dissociations which occur below 15.5 eV can be partitioned into two groups regarding the amount of kinetic energy on the charged fragments. As mentioned above for reactions (I) and (2), aImost the totat kinetic energy is on the undetected neutral fragments, i.e. H and H,, respectively, whereas during the course of reactions (4) and (5) the molecular ion breaks into two parts of nearly the same mass. From the analysis of the time-of-flight distributions follows that C,H: and CsHf carry away translational energy in the order

86

J. Dannacher, J--P_ StadelnrannfDecay ofI-chloropropyne cations

of a few tenth of an eV (cf. table 5). Our qualitative model predicts similar correction factors for the breakdown curves of both fragment ions. If the deviation at 13.2 c-V of the sum curve from unity is ascribed to a reduced collection efficiency equal for both &Hz and CsHg fragment ions, it is found that the measured breakdown curves of the two ions reflect only one third of the true branching ratio_ This is in quahtative agreement with the known discrimination effects of our spectrometer. At higher ionization energies where even more kinetic energy is rek?dsed the correction factor is expected to be even larger. Following these considerations, the behaviour of the experimenta sum curve is easily explained. In the ener,oy range of the electronic ground state no dissociations are observed and the sum curve is necessarily unity_ In the energy gap between the R’E and the A’E state (10.5-132 eV) the PE count rate is zero and hence no coincidence measurements can be carried out_ The value of the sum curve at 13.2 eV is 0.6 including the contribution of the molecular ions (stabilized by emission), which are detected with a high and precisely known collection efficiency. With increasing internal ener*v the dissociative decay is favoured yieiding product ions detected with a lower collection efficiency than the molecular ions. This is the cause for the obvious decrease of the sum curve between 13.2 eV and 13.5 eV. The adjacent increase to about 0.85 (reached around 16 eV) is due to the fact that formation of practically undiscriminated C&Cl’ ions becomes more and more the dominant fragmentation channel. The lowering of the sum curve value close to 17 eV arises from CCI’ formation and the two dominating secondary fragmentations of the CsH$ZI+ fragment ion yielding C,H; and C3H+.

5. Comparison with the coincidence data reported for the three C,H,’ isomers There are two dissociation chanueis accessible for the C,Hz molecuIar ions at lower ionization energies, i.e. loss of H and HZ, respectively. The breakdown curves of the corresponding fragment ions show a plateau for the 2 eV preceding the onset for secondary dissociations of the C,H: fragment

a

13

14

15 16 IONiZAliON ENERGY eV

Fig.. 3. The summed breakdowncurves for loss of H and Cl (a) and loss of HZand HCI (b)_Corrected breakdown curves were used as describedin the text ion with a constant ratio of z 6 : 1 for H versus H, Ioss.

The corrected breakdown curves (correction involves multiplication of the C,H;- and the C,H; breakdown curves by a factor of three, as discussed above) for the loss of a single atom (H, Cl) and of a diatomic (Hz, HCI) of excited l-ckloropropyne cations for ionization energies ranging from 13.2 eV to 15.5 eV are depicted in fig. 3. Apart from the re!atively large scatter of the data a plateau region is observed extending over the same energy range with a similar intensity ratio as found for the CsHz isomers. This is especially surprising regarding the opposite tendencies of the M ‘-H and the Mi-C1 breakdown curves (cf. figs. 1 and 2). A tentative interpretation is that the decay behaviour of the molecular ionic states involved is not or only slightly influenced by the presence of a Cl atom.

Acknowledgement We thank P_ Forster for the preparation of the sample. This work is part C9 of Project No. 2.%X-078 of the Schweizerischer Nationalfonds zur FGrderung der wissenschaftlicken Forschung (for part C8 see ref. [20]). Financial support by CibaGeigy SA, Sandoz SA and E Hoffmann-La Roche & Co. SA, Base1 is gratefully acknowledged_

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J. Dannacher, J--P. Stadelmann/Decay of I-chloropropyne [2] J.

DannacherandJ. Vogt, Helv. Cbim. Acta 61(1978)

cations

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[3] A-C. Pati, A.J. Jason, R, Stockbauer and K.&

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[16-j F-P. Lossin& Can. J. Chem+ 50 (1972) 3973.

361.

[6] G_ Bier&E Heilbronner, V_ Hornun& E. Kloster~ Jensen, J.P. Maier, F. Tbommen and W. von N&sea, Chem. Phys. 36 (1979) t.

[7] B.P. Tsai, AS. Werner and T_ Baer, J. Chem. Phys. 63 (1975) 4384. [S] J.P. Maier, in: Kinetics of ion-mofecuIe reactions, ed. P. Ausloos (Plenum Press, New York, 19?9)_ [9] G.D. Flesch and H.J. Svec, Intern. J. Mass Spectrom. Ion Pbys. 9 (1972) 106. [lo] H-G_ Viehe, Chemistry of acetylenes (Dekker, New York, 1969) p. $70. [I l] J. Dannacher, E. Heilbroriner, L-P. Stadelmann and J. Vogt, Helv. Chim. Acta, 62 (1979) 2186.

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