Charge separation mass spectrometry

International Journal of Mass Spectrometry and Zon Processes, 97 (1990) 175-201

175

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

CHARGE SEPARATION MASS SPECTROMETRY Part 2*. Methyl compounds

E. RUHL**, S.D. PRICE, S. LEACH*** and J.H.D. ELAND+ Physical Chemistry Laboratory, South Parks Road, Oxford (Gt. Britain)

(Received 5 October 1989)

ABSTRACT Charge separation following double ionization of organic methyl compounds has been studied using the photoelectron-photoion-photoion triple coincidence technique. Two- and three-body dissociation pathways are observed with intensities related to the thermochemical stability of the products and to their odd/even electron numbers. The decay mechanisms show some evidence for rearrangement processes via ylide dications prior to charge separation. The kinetic energy releases in the charge separation reactions can be rationalized in terms of a maximum charge separation within the molecular structure of the dications. The possibility and implications of dissociative double ionization of methyl compounds in the interstellar medium are discussed. INTRODUCTION

Interest in the formation, dissociation and isomerization of dications has increased in recent years, partly as a result of theoretical work, such as ab initio calculations on the structure and stability of small dications and their potential energy surfaces [l-5]. Ylide dications of organic methyl compounds, CH2XH2+, have been the subject of specific investigations [l]. Most experimental work on the formation and decay of dications has been done using mass spectrometric methods; charge stripping has been used to determine ionization potentials of cations [6], and charge transfer collisions of dications with neutrals have been used to distinguish dications from singly charged ions in mass spectra [7]. Other techniques, such as double charge transfer also provide double ionization energies [8]. *For Part 1 see ref. 12. **Permanent address: Institut fur Physikalische und Theoretische Chemie, Freie UniversitSit Berlin, Takustrasse 3, D-1000 Berlin 33, F.R.G. ***Permanent address: Laboratoire de Photophysique Moleculaire du CNRS, Universite de Paris-Sud, 91405 Orsay, France and DAMAp, Observatoire de Paris-Meudon, 92195 Meudon, France. ‘Author to whom correspondence should be addressed. 0168-l 176/90/$03.50 0 1990 Elsevier Science Publishers B.V.

176

In photoionization, dissociation of dications has been studied by the ionion coincidence technique (PIPICO) [9], where the difference in time of flight of the singly charged products of charge separation is measured. The electrostatic repulsion in charge separation of dications gives fragments with a considerable amount of kinetic energy, whose magnitude can be determined by analysis of the peak shapes of the coincidence signals. However, this technique cannot be used where a variety of dissociation channels have a similar difference of flight time. Congested PIPICO spectra are obtained for organic molecules containing many hydrogen sites as the resolution does not allow distinction between the individual reaction channels. This drawback is overcome in the photoelectron-photoion-photoion (PEPIPICO) triple coincidence technique which has been developed recently [lo-121. In this type of experiment the flight times of both of the two singly charged fragments produced in dissociative double ionization are measured in coincidence with a photoelectron. A systematic analysis of peak shapes in the three-dimensional spectra has been given [l 11,taking account of a variety of model mechanisms for three-body dissociations such as spontaneous, sequential and deferred charge separations. The new technique has been applied recently to organic perfluoro compounds [12], aromatic hydrocarbons and related heterocyclic species [ 131. In this paper we focus our interest on methyl compounds as the first members of homologous series carrying different functional groups. The compounds studied here are of the type CH3X, where the functional groups X comprise Cl, OH, SH, NH, and CHO. Major topics of interest to us were the influence of functional group on dication decay, and the possibility of isomerization processes prior to charge separation. Deuterium labelled compounds were used in some cases to resolve close masses. We discuss the observed spectra in relation to the structure of dication precursors and report in particular on evidence for rearrangement processes involving ylide dications. The relevance of reaction thresholds and of the odd/even electron rules known from mass spectrometry [14] to reaction probabilities is also examined, since the breakdown of such small species might not be statistical in the sense of depending on energy content alone. Astrophysical implications of charge separation in small organic dications are also addressed in the discussion. EXPERIMENTAL

The PEPIPICO apparatus has been described elsewhere [lo]. Ionizing radiation was provided by a d.c. capillary discharge in helium at low pressure followed by a toroidal-grating monochromator, with which the 30.4nm line

177

was selected for all the experiments reported here. The photoelectrons, accelerated in a continuous field were detected with a channeltron multiplier after passing through a circular aperture. The effect of this aperture is that under the usual experimental conditions electrons of up to 5 eV energy are detected with high efficiency, whilst more energetic electrons, such as those originating in single ionization, are discriminated against. Cations are accelerated in the opposite direction into a time-of-flight (TOF) mass spectrometer operated under Wiley-McLaren time focussing conditions [ 151. The electron signal is used to start a multihit time-to-digital converter (TDC) which receives stop signals from cations arriving at the channel plate detectors of the TOF spectrometer. Single, double and higher multiple ion arrival events can be recorded simultaneously, and of these data, single ion arrivals and ion pair arrivals are transferred to a microcomputer where they are analysed. The samples were of commercial quality. Liquid samples were degassed by multiple freeze-thaw cycles and for studies of deuterium labelled compounds the gas inlet system was treated for several hours with D20 to avoid H/D exchange reactions. RESULTS

AND DISCUSSION

Figures l-3 give sample PEPIPICO spectra both as three-dimensional stick diagrams and as contour plots showing peak shapes; the spectra of the compounds are compiled in Tables l-5. The masses of the two ions in each pair are obtained from the two flight times, and from the two-parameter spectra of t, versus t, (Figs. lb, 2b, 3b) the slopes of the coincidence peaks and their lengths are deduced. The slopes give information on the mechanisms leading to the observed products and the lengths contain information on the kinetic energy releases. For all two-body dissociations (that is, reactions in which only two fragment ions are formed), a slope of - 1 is expected because the initial momenta of the ions must be equal and opposite in order to conserve linear momentum [l 11.Three-body dissociations can be classified into the following types. (i) Simultaneous charge separation, where all the fragments fly apart simultaneously. The neutral fragment may be a “spectator” getting no impulse, in which case the slope for this type of reaction is - 1 as for two body dissociations. (ii) Deferred charge separation, where a neutral loss precedes charge separation. Since a slope of - 1 is expected, it is not distinguishable from simultaneous charge separations without detailed analysis of the peak shape [I 11. (iii) Sequential reaction, in which one or more of the monocations formed in the initial charge separation later undergoes loss of a neutral. The slope of the peak is often equal to the ratio of the masses of the singly charged

178

0

0

20

30

Mass of first

40

ion

so

Fig. 1. PEPIPICO spectrum of CD30D ionized at 30.4nm. The upper part gives relative intensities of the ion pair products. The lower part shows the peak shapes by contours at 2/3 and l/3 peak height within each peak. Although mass numbers are given, the scales in the lower part are time, not mass linear. No contours are shown for equal mass pairs such as 18,18 because of distortion at equal times in the two channels.

intermediate and the singly charged final product of this secondary decay. However, the exact mass ratio is found only if the second dissociation step happens both without any energy release and outside the Coulomb field of the other ion, which requires a time delay after charge separation of at least several tens of picoseconds. Mass ratios calculated for sequential reactions following different pathways have been compared with experimental slopes for the three-body

179

10 15 20 5 Mass of Firstion

0

12

25

3 13 Mass OF Firstion

Fig. 2. PEPIPICO spectrum of CH,NH, of the peak for 13,16.

Ii

15

at 30.4nm, as Fig. 1 Note the clearly non-unit slope

dissociations listed in Tables l-5 to identify the reaction mechanisms. It must be admitted, however, that the observed slopes do not always suffice to select a mechanism unambiguously, and some intuition is involved. Four-body dissociations are also possible in principle, but as they must be even more energy-costly than three-body reactions we have only rarely included them in the analysis. The photoionization mass spectra of the compounds at 30.4nm, taken simultaneously with the PEPIPICO spectra, contain few doubly charged

180

2,42

6

Ib

io jo

Mass of first

40 ion

Mass of first Fig. 3. PEPIPICO

spectrum

io

ion

of CD3CN

at 30.4nm,

as Fig. 1.

parent ions. The most abundant dications observed are those of methanethiol (2.5% base peak) and perdeuteroacetonitrile (2% base peak). The undeuterated acetonitrile gives no stable dications on our - 1 ,UStime scale. Most dications initially formed from all the compounds are therefore dissociated in much less time than this by charge separation. Methyl chloride (Table I)

The main charge separation channel in CH3C12+ is cleavage of the C-Cl bond. 70% of the products observed are CH: + Cl+ and their further

181 TABLE 1

Charge separation reactions in methyl chloride dications Products

Intensity (%

CH: + Cl+ CH;+Cl++H H; + Ccl+ H; + CHCl+ H+ + CH2C1+ H+ + CHCl+ + H H+ + Ccl+ + Hz H++CH;+Cl

Slope

pairs)

Obs.

61 9 6 2 13b 4 3 2

0.98( 1) 1.06(2) 0.97(6) 1.1(2) 1.06(7) 1.1(l) 1.1(l) 3.0(14)

Calc. 1 1.07’ 1 1 1

36d

KER (ev)

AH,

5.4(10) 4.3(10) 3.4( 10) 2.8( 10) 5.2(20) 3.6(20) 3.6(20) 3.6(20)

590 714 545 662 595 722 646 728

Types

(kcal mall’) EE OEO EE 00 EE EOO EEE EOO

E and 0 signify even-electron and odd-electron products in the first column of this and later tables. b Intensities are given for 3SC1-containing species except in this case where there is some contribution from H+ + CHz3’Clt ’ Secondary reaction: CH: + CH: + H. d Secondary reaction: HCI+ + HC + Cl.

dissociation products such as CHZ + Cl+ + H, for which a coincidence peak slope of - 1.06(2) is found. This is significantly different from - 1 and close to the calculated slope of - 1.07 derived from the mass ratio 15/ 14. In this case we can be sure that the H atom is formed by decay of the methyl cation rather than by loss of H from an HCl+ precursor (calculated slope 0.97) or by deferred charge separation (slope - 1). A second main channel is the cleavage of one C-H bond by charge separation giving H+ + CH,Cl’ (13%) with possible sequential fragmentation pathways to H+ + Ccl+ + H, (3%) and H+ + CHCl+ + H (4%). The experimental slopes in these cases do not differ significantly from - 1, however, so we cannot identify the mechanism positively. Other two-body dissociation channels lead to H: + Ccl+ and H: + CHCl+ formation. The formation of H: is quite remarkable since three C-H bonds have to be broken and three new H-H bonds have to be formed. By comparison with CH,C12+, the fragmentation of CH,12+, studied recently by PEPIPICO [l l] and PIPICO [9], shows a somewhat different pattern. As in the case of methyl chloride the main charge separation reactions occur via C-I bond cleavage. However, sequential fragmentation of the methyl cation leading to C+ and CH+ is observed only in CH,I. This difference may be rationalized in terms of higher internal energy deposited in the methyl cation after charge separation, because of the lower double ionization potential of methyl iodide (26.7 eV for CH, I and 3 1.7 eV for CH, Cl according

(%)

[a]

be measured

Isotope

[c] [d]

[b]

Calc. slope 1.12 1.5 2 2 3 3 4 accurately.

570 694 568 459 580 683 537 641 560

6 2 11 4 13 17 6 40

4.2(7) 4.2(16) 2.9( 16) 3.6(10) 3.4(14) 3.7(11) 4.4( 14) 3.3(11) 4.5(10)

mall’) 2-body Secondary 2-body 2-body 2-body Secondary 2-body Secondary Secondary

(ev)

KER

_b

Mechanism” d4

dications

d4 d4 d4 d, 1.35(7) [d]H:+H++H2 d4 2.4(5) d, H,D+ +H+ + HD 3.5(6) d, bFor methanol-d, these products are of equal mass and cannot

Obs. slope 1.16(5) 1.10(5) 1.34(14) 1.5(3)

Secondary reaction [a] CH: + CH: + H [b]H:+H:+H [c]H:+H++H

h,

Intensity

in methanol

17 7 2 I1 3 10 10 4 36

reactions

CH; + OH+ CH:+OH++H CH; + OH: H; + CHO+ H: + CH*O+ H:+CHO++H H+ + CH30+ H+ + CH,O+ + H H+ + CHO+ + H,

Products

Charge separation

TABLE 2

EE OEO 00 EE 00 OEO EE EOO EEE

Type

w

4 3 12 3
3.3(16)

2 -

4.0( 10) 3.0(12) 4.4(9) 2.7(7)

38 11 14 11 _

Calc. slope 3 2 1.07 1.15 3 2

655 566 612 [a] 660 [b] 733 694

511

536 619 660 571 4.9(8) 2.0(5) 2.0(5) 3.6(6) 3.3(7) 3.6(6) 3.1(11) 4.5(11) 3.0(10)

7 3 13 3 13 8 29 5

1

2.1(4)

(ev)

KER

(Y = NH)

19 -

%

AH, (kcal mol-‘)

KER

% (ev)

Methylamine

(Y = S)

dications

Methanethiol

and methylamine

Obs. slope 1.5(3) 3.0(15) 1.03(2) 1.15(6) 1.25(5) 1.13(9)

EE EOO OEO 00 EEE EE 00 OEO EE EEE EOO EOO EEE

CH; + YH+ CH:+Y++H CH:+YH++H CH; + YH; CH+ + YH+ + Hz H; + CHY+ H; + CHYH+ H;+CHY++H H+ + CH*YH+ H+ + CHY+ + H, H+ + CH2Y+ + H H++CH;+YH H++CH:+Y

Secondary reaction [a]Hl+Hf+H, [b]H;+H++H [c] CH: + CH: + H [d] CH; + CH+ + H, [e]H:-tH++H, [QH;+H++H

Type

in methanethiol

Products

Charge separation

TABLE 3

mol-‘)

596 M

688 [c] 559 689 [d] 490 534 634 543 591 [e]

564

21

11 10 1 16 1 2 1 3 1 36 9 4 6

4b lob -

43 12 6 6

lob 1 2 1 4 -

d,

h,

Intensity (%)

Obs. slope “Secondary reaction 1.15(10) [a] CD: --*CD: + D [b] HCNH+ --*HCN+ + H 0.77(10) 1.07(5), 1.2(l) [CID:-D’ +D 1.2(l) [d] D: --) D+ + D2 1.30(15) [e]CD++D++C b These intensities are approximate because of overlap.

CD; + CN+ CD; + CND+ CD:+CN++D CD+ + DCND+ CD* + CND+ + D C+ + DCND+ + D D; f C2N+ D; + CDCN+ D:fC*N++D D+ + CD$N+ D* + CDCN+ + D D+ -k C2N+ + D, D+ + D%ND+ + C

Products

Charge separation reactions in acetonitrile dications

TABLE 4

[c] [d] [e]

[b]

[a]

Calc. slope 1.13 (4 isotope) 0.96 (h3 isotope) 2 (both) 3 (4 isotope) 7 (4 isotope)

2-body 2-body Secondary 2-body Secondary ? 2-body 2-body ? 2-body Secondary Secondary Secondary

Mechanism”

4.0(14) 4.6(15) 2.5(17) 4.6(15) 2.5(11) 2.4( 11) 1.9(l) 3.3(7) 1.3(8) 3.3(8) 3.3(8) 3.3( 10) 2.8(11)

KER (ev) 691 680 815 613 785 708 651 727 795 657 788 753 762

21

mol-‘) EE 00 OEO EE EOO OEO EE 00 OEO EE EOO EEE EEE

Type

5

Lii1.4(2) Ul 1.8(6) [kl 2.2(7)

[i] 2.8(4)



kl

Calc. slope 1.07 0.96 1.15 0.7 0.33 0.66 2 17 29 15

3.1(9) [kl 4.4( 11) _

4.4(11) 4.5(11) 4.5(11)

3.6(17)

- M

2.7(16) 2.6( 11) [e]

1 1

[al

2.9( 10) [c]

4.3(9) -

21 _ _ 4

4.9(7)

54 -

6.0(40) [1] -

3.6(28) [h] 3.3( 16) [i] 4.2(19) b] -

2 4 8 3 1


5.0( 10) 5.1(10) 4.6(6) 4.8(11) 4.1(10) [b] 5.7( 14)

KER (eV)

- WI 2.3(11) _

oxide

14 11 42 3 3 4
%

%

KER (eV)

Ethylene

Acetaldehyde

oxide

kl 1.20(15),[hl 1.7(5)

Id1 0.4(3) H 0.5(2) M 0.4(3)

Obs. slope [a] 1.04(2) [b] 0.88(3) [c] 1.1 O(3)

“Secondary reaction CH:+CH;+H C,H: + C2H: + H CHTO+ + CHO+ + C,OH+ -+ CHO+ + CHzCO+ + CH; + C*H,O+ -, C2H: + Hf-+H++H OH++H+ +O HCO+ + H+ + CO CH+H+ +CH, Mechanism unknown

H, C CO 0

EE 00 OEO EE EOO EEE EEE 00 OOE OOE OEO EE EOO EEE EEE EOO EEE EEE

CH: + CHO+ CH; + CH*O+ CH; + CHO+ + H OH+ + C2H; OH+ + C,H; + H CH+ + CHO+ + H, H:+CHO++C H: + C*H,O+ H:+CH:+CO H:+C,H:+O H: + CHO+ + CH H+ + CH,CO+ H+ + CH*CO+ + H H++C,H:+O H++CH:+CO H+ + CH: + CHO H+ + CHO+ + CH, H+ + CCHO+ + H,

and ethylene Type

in acetaldehyde

Products

Charge separation

TABLE

457 537 581 575 677 582 630 564 662 732 693 516 625 692 601 708 654 628

AH,

186

to the “rule of thumb” [ 161 or 3 1.46 eV from double charge transfer experiments [8e]). The weak C-I bond (D(C-I) = 56.3 kcalmol-‘; D(CCl) = 8 1 kcal mall’ [ 171)also favours reaction with C-I bond cleavage leaving a large excess energy in the methyl ion. The C-H bond dissociation energies are nearly identical for methyl chloride and iodide (D(C-H) = 103.2 kcalmol-’ and 100.9 kcal mol-’ respectively [17]) and are much higher than those of the C-X bonds. The bond.dissociation energies may not be the same in the dications as in the neutral molecules, however. If on double ionization the HOMO is emptied a conjugational or hyperconjugational interaction will stabilize the C-X bond, but weaken the C-H bonds. The experimental result is that in the dications the C-X bonds break more readily than C-H bonds, which may reflect the bond strengths but might also be determined by kinetic factors. The fact that no H: formation occurs in methyl iodide [lo] may be seen as another consequence of the lower C-X bond energy since in this dissociation the C-I bond has to survive charge separation. Methanol (Table 2 and Fig. 1) Methanol-h,, methanol-d, and methanol-d, have all been studied, the isotopic species serving both to improve resolution of individual peaks and to help clarify the mechanisms. Some kinetic isotope effects could also be expected, as recently observed for dications of other small molecules [18]. Because all peaks are fully resolved in its PEPIPICO spectrum, we concentrate on methanol-d, in discussing the main reaction pathways. Four distinct charge separation reactions are observed. (i) C-O bond cleavage (16%). The primary products are CD: + OD+ and the main secondary products are CD: + D + OD+ , for which the observed slope of - 1.16(5) agrees well with the calculated mass ratio of 1.12. This C-O cleavage is weak by comparison with C-X cleavage in the halomethane dications. (ii) Two-body dissociation into CD: + OD $. This fragmentation is quite weak (2%) but is direct evidence for rearrangement into an ylide dication prior to charge separation. There is no detectable secondary decay from these products. (iii) Dissociation into Dl + CDOD+ and subsequent fragmentation of the Dz ion (8.5%). It is hard to understand how the D: ion can be formed efficiently in states with a long enough lifetime ( > 20 fs) to undergo a distinct secondary reaction, but the experimental evidence seems clear, and no alternative reaction which could explain the observed slope is stoichiometrically possible. The same reaction is found as a secondary process in several other PEPIPICO spectra reported here.

187

(iv) Formation of DC + CDO+ and further decay products. The two-body reaction is relatively weak (1 l%), but if the main coincidence line (D+ + CDO+ + D2) is included as a secondary product this is the most important decay channel in methanol. The slope of the relevant peak in CD,OD is - 1.35(7), to be compared with a calculated mass ratio of 3 if the D+ is formed by D: decay. Alternative mechanisms would require a slope of - 2 if the D+ comes from D: or - 0.88 if CDO+ comes from secondary decay of CD*OD+ . The slopes of peaks involving very low mass ions are often affected by instrumental factors because the low mass partner carries most of the kinetic energy but only a little momentum, giving narrow peaks [l l] with too low slopes. The measured slope is between the values predicted for the different mechanisms, so a mixed mechanism cannot be excluded. If a single pathway is dominant, however, as we prefer to believe, it must involve either Dl or DC to give a slope greater than unity. In the spectrum of the d, isotopic species secondary decays from both H: (observed slope 2.4(5), calculated slope 3) and H2D+ (observed slope 3.5(6), calculated slope 4) are observed, and seem to point unambiguously to triatomic hydrogen ions as the precursor. On the basis that the mechanism should not change on deuteration, we adopt this as our preferred pathway. The low value of the slope observed for the d4 species may then be an instrumental effect, or may reflect the actual lifetime of the D: . From the observed slope and the mass ratio of 3 we can calculate [ 1l] a lifetime of 14(5) fs. The analogous reaction in methanol-h, corresponds to a rather longer lifetime for H: of about 35fs, but may be even more severely affected by instrumental effects than the deuterated species. The relative intensities of different channels in the PEPIPICO spectra of methanol-d, and methanol-h, show only weak isotope effects. We have therefore used the total intensities from the spectra of the pure isotopic species to unscramble the fates of the single D atom in CH,OD. This process is needed because many peaks in the spectrum of CH,OD can include contributions from isobaric species containing either H, or D moieties. For example, in the pure isotopic compounds the reaction CD,OD*+ + D+ + CD20D+ has an intensity of 10% of the total pair formation in the hydrogenic compound and 17% in the perdeutero compound. This is the only reaction for which a kinetic isotope effect, of the same magnitude as those seen in recent work on other compounds [18], is unambiguously observed. After the unscrambling process it transpires that in CH,OD the two reactions CH30D2+ + H+ + CH20Di and CH, OD2+ + D+ + CH30+

have intensities of 12% and 1.5%, respectively, showing that the positional integrity of the deuterium label is retained in this dissociation. In the raw PEPIPICO spectrum the products D+ + CH30+ could not be distinguished from H: + CHDO+. As in the reaction above, the two-body charge separation reactions of CH30D+ show no significant exchange or scrambling of the D atom label. The C-O cleavage yields OD+ exclusively, and the formation of stable H: species gives ten times more H: than H2D+ . The existence of some atom migration product in the two-body reactions may be significant, however, because in the three-body reactions almost complete hydrogen mixing seems to take place. According to our interpretation the major process is initial formation of Hz + CHO+ followed by secondary decays of the triatomic hydrogen ion. When all the secondary products are added up it turns out that in the charge separation stage almost exactly three times more H2D+ than Hc is formed, i.e. the statistical ratio. Other minor three-body reactions also show substantial label mixing. The reason for this dramatic difference between the two-body and three-body reactions must be the higher internal energy in the doubly charged precursor, which forces the final split into three products. Before charge separation the high energy level must promote internal rearrangements, which may very plausibly include ylide formation. The intermediate H,D+ product is found to decay preferentially into H+ + DH. This trend has also been observed in predissociation of H,D+ and HD: [19] and may be related to the higher density of states in the diatomic product. Methanethiol (Table 3) Methanethiol decays mainly by C-S bond breaking in two- and three-body dissociations. In contrast to the case of methanol, single C-H or S-H bond cleavages are not observed as two-body reactions, nor are the products H: + CH, Sf detected. Stable H: formation is weaker (2%) than in methanol (14%), and the secondary dissociation channel H+ + CHS+ + H,, which amounts to 12% of total pair production, is also relatively weaker than in methanol. It can be safely considered to be a consequence of the fragmentation of H:, because of the relatively high slope of the coincidence signal (- 1.5(3)) despite the unfavourably low mass of the proton. A weak (4%) set of products which could arise from a competing decay of H: into H: + H is observed, but because the slope of the coincidence peak does not differ significantly from - 1 no clear attribution is possible.

189

Methylamine (Table 3) In methylamine four main classes of charge separation reactions are observed. (i) Cleavage of the C-N bond yielding CH: + NH: and CH: + NH: together with the secondary dissociation products CH: + NH: + H and CH+ + NH: + H, (30%). Formation of CH: + NH: again directly demonstrates the occurrence of ylide dications. (ii) Cleavage of one C-H or N-H bond forming CNH: + H+ . (iii) Formation of Hz + CNHC . This is quite strong (13%) as a two-body process, and there is also a strong peak which may come from secondary fragmentation of H: to Hf. The slope of this coincidence signal is much lower (- 1.25(3)) than the expected mass ratio of - 3.0, however, so various alternative explanations are also plausible as discussed in the case of methanol. If the observed slope is the result of a limited lifetime of Hz an extremely short lifetime of 7fs is required. (iv) Formation of Hz + CHNH: (3%) and subsequent dissociation of the H:. Acetonitrile (Table 4 and Fig. 3) The PEPIPICO spectrum of acetonitrile illustrates the use of deuteration to clarify congested reaction pathways. In the region of CHC + CN+ the spectrum of acetonitrile-h, contains one long coincidence signal. In the spectrum of the perdeuterated isotopomer three peaks are well resolved showing the processes CD+ + CD2CN+, CD: + CNDf and CD: + CN+ (see Fig. 3). In double ionization the bond order of the C-N bond is formally decreased from 3 to 2 and the dication might undergo rearrangement to a +CH2CH =N’ structure. Products would then be expected to be formed by cleavage of the C-C bond. In fact C-C cleavage reactions produce 40% of cation pairs, while 60% arise from C-H cleavage. The most abundant twobody products come from cleavage of one C-H or C-D bond upon charge separation. Further dissociation of the resulting CH2CN+ might be expected to yield CHCN+ + H, but the slopes of the coincidence peaks are numerically larger than - 1, rather than smaller. Because the slopes are close to unity deferred charge separation or a mixture of pathways cannot be excluded, but the slope indicates that the products CHCN+ + H+ arise at least in part from secondary dissociation of primary Hl, as in several other spectra.

190

Acetaldehyde and ethylene oxide (Table 5) The main charge separation reaction in acetaldehyde is simple cleavage of the C-C bond (54%) producing CH: + CHO+ . The origin of the significant (21%) three-body dissociation product CH: + CHOf + H is again rather uncertain since the experimental slope of - 1.04(2) is close to - 1, but formation by secondary decay of the methyl ion (calculated slope - 1.07) seems most likely. Charge separation into H+ + CH,CO+ is a very weak reaction in acetaldehyde (1 Oh) by comparison with the corresponding reaction in acetonitrile. A variety of low intensity products involving C-C and C-O cleavage such as H+ + CH: + CO, H+ + CH: + CHO, H+ + C,H3+ + 0, and H+ + CHO+ + CH, are observed, but H: formation is extremely weak and no coincidences involving H: are observed. The PEPIPICO spectrum of ethylene oxide was examined partly to see whether isomeric molecules with similar mass spectra could be distinguished more easily by this technique, and also because simple ring-opening in ethylene oxide might yield the ylide dication of acetaldehyde. The main dissociation channel is the three-body dissociation into CH: + CHO+ + H (42%), which occurs in acetaldehyde as well, but at lower intensity (24%). The two-body dissociation channel CH$ + CH,O+ , where 11% of the total intensity is found, may possibly be the precursor of the major reaction. No matching two-body reaction is observed for acetaldehyde. Another two-body dissociation yielding CHZ + CHO+ is found with much smaller intensity than in acetaldehyde, so that overall the intensity ratio of the two principal pathways is inverted in the two compounds. The difference shows that complete isomerization of both isomers via a common precursor does not occur. A small amount of the characteristic reaction of the other isomer does occur in each case, however, and this may be due to a partial rearrangement of the dications to a common structure prior to charge separation. Stability of dications and their products Heats of formation of methyl compound dications are available from theoretical results agree well [8], while the charge stripping experiments often double charge transfer [8]. It has been found that double charge transfer and theoretical results agree well [8], while the charge stripping experiments often give lower values. Therefore it has been suggested that charge stripping produces ylide dications, which are expected to be more stable than ions with the same structure as the neutral molecules. Another means of determining dication formation energies is by the “rule of thumb”, according to which the

191

double ionization energy of a molecule can be approximated as the first (single) ionization energy multiplied by the factor 2.8(2) [16]. The “rule of thumb” usually agrees with data from charge stripping or double charge transfer within the error limits. In order to interpret PEPIPICO spectra we should like to know the bond strengths in dications, especially for bonds which are broken in charge separation. Some information is available from ab initio calculations (e.g. refs. 1, 5) where potential energy surfaces of dications have been calculated. To estimate bond dissociation energies in dications empirically it is important to know which molecular orbitals are involved in the double ionization process. The lowest double ionization energy should correspond to two-electron ejection from the highest occupied molecular orbital (HOMO) of the neutral closed shell molecule. The compounds studied (except acetonitrile) have non-bonding orbitals on Cl, 0, S, or N which provide the HOMO. The effect of the positive holes created by double ionization has been studied recently for methylene dications [20], where the HOMO is located at the carbon centre. In the case of monocations it is known that n-electron donor substituents interact with the formally vacant 2p hole of carbon, which leads to a stronger and shorter C-X bond [21]. The same effect occurs in dications, where extremely short C-F and C-Cl bonds have been calculated by ab initio methods for substituted methylene dications [20]. In the case of the methyl compounds an equivalent effect is expected, but here the vacancy is not at the central carbon centre but at the functional group. Therefore the central carbon is the donor in the hyperconjugative interaction and the C-X bond order is expected to be increased. As a consequence the C-X bond should be shortened and strengthened in the dication. In acetaldehyde this effect is expected to occur within the C = 0 moiety, and consequently the C-C bond should be weaker in the dication than in the neutral. These stabilization effects are expected to be smaller in methyl dications than in substituted methylene dications, since carbon should be a weaker electron donor than substituents like Cl or F with non-bonding electrons. The C-X binding energies are therefore expected to be somewhat increased, because of a bond length shortening or increase of the C-X bond order. The C-H bond energies should be decreased within this model. This might help explain the high probability of the formation of H+ , H: , and Hz for all methyl compounds. Because bond energy data for most of the dications are lacking we have tried to correlate their fragmentation behaviour with the bond dissociation energies of the neutral molecules. In methanol the C-H and C-O bonds are of nearly the same strength (94 and 92.3 kcal mol-’ [17,22]), whereas the hydrogen at the hydroxyl group is more strongly bound. Cleavages of both weaker bonds are observed but the main product channel (H+ + CHO+ + H2) leaves the

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C-O bond intact, possibly due to a lower C-H bond strength in the dication consistent with the hyperconjugation model mentioned above. The C-S bond in methanethiol is weaker than the C-O bond in methanol (74 kcal mol-’ [22]) and in accordance with this most of the fragmentation in the dication (74%) involves C-S cleavage. In methylamine the bond strength of the C-H bond is 88 kcal mol-’ whereas the C-N bond is somewhat stronger (94.6 kcal mol-’ [17,23]), and as in the case of methanol most of the products are formed by C-H bond breaking. In acetonitrile the situation is different since the HOMO is not a non-bonding orbital, but a n bond. Therefore the bond strength of the triple bond should be decreased and the strong C-C bond (123.9 kcal mol-’ ) might be weakened. The weakest bond is the C-H bond, and in agreement with this 60% of charge separation reactions break it, whereas 40% of the products come from C-C bond cleavage. In acetaldehyde the C-C bond is the weakest (79 kcal mall’) whilst the C-H bonds are stronger: 92 kcalmol-’ (methyl group), 84 kcal mol-’ (functional group). If the C-C bond is even weaker in the dication, it is consistent with the result that 80% of the reactions show this bond rupture. Fragmentation reactions where firstly a C-H bond is broken are accompanied by subsequent C-C bond cleavages, so that altogether 92% of the reactions include C-C bond breakage. From these observations it is apparent that trends in bond cleavages on charge separation show strong correlation with the bond strengths in the neutral molecules. The fragmentation patterns of singly charged cations are closely connected with the stability of the products [23]. For the dications, Tables l-5 contain heats of formation for ion pairs formed in the ground states of their most stable isomers at 298 K for each reaction for which data are available. It has been assumed that a minimum number of products are formed, and no distinction has been made between deuterated species and the equivalent hydrogenated systems. The reference data needed were obtained from standard compilations [22,24]. Overall there is a tendency for products with low heats of formation to be formed more readily than those requiring larger energies, but the correlation is not at all exact. A much clearer correlation is found when the summed abundances of all primary and secondary products from each initial ion pair are compared with the order of initial pair energies, as in Table 6. In compiling the table we have had to assume that analogous reactions in different methyl compound dications follow the same mechanism. It is notable that whilst pairs which can be formed without any rearrangement are relatively abundant, highly rearranged pairs are also seen when their stability is high. It may possibly be relevant that the product pairs D and F in Table 6, whose abundances are often less than their stabilities would predict, are expected to correlate mainly with spin-singlet states of the parent dications, whereas the dication ground states are most likely to be triplets.

Cl OH SH NH, CN CHO

x

70 24 63 26 12 82

A CH: + X+

Channel

13 10 3 8 36 1

B CH2X+ + H+ 2 2 11 3 10

C CH: + XH+ 9 57 18 55 6 1

D CX+ + H: 6 7 6 8 12 5

E CHX+ + H;

Intensities of charge separation channels in methyl compound dications and thermodynamic

TABLE 6

17 4

-

F CH+ + XH, D>A>F oB>C>E D>F>B>CxA>E D>A>B>C>F>E D>E>F>B>C>A F>D>B>C>A>E A>B zD>C>F>E

Product stability order

stabilities of the initial pairs

194

Another well known property of all mass spectra is the prominence of characteristic “key fragments”. The same or similar key fragment ions are seen in PEPIPICO spectra. The key fragment in PEPIPICO spectra of oxygencontaining compounds is CHO+ , which occurs in charge separation of CD30D2+ (64% CDO+), acetaldehyde (80%) and ethylene oxide (61 “A). We assume that the formyl cation is produced rather than the isoformyl cation, because of the lower stability of the latter [25]. The equivalent in methanethiol charge separation is CHS+ , the thioformyl cation, (18% abundance), which is known to be formed by fragmentation of a variety of sulphur-containing cations [26]. The key fragment from nitrogen-containing dications is HCNH+ , which is formed in charge separation of methylamine (55%) and acetonitrile-d, (24% DCND’). Its isomer, CNH: , is less stable [27]. CNH+ is observed only in the case of acetonitrile (11% CDN+ in CD,CN). The isomer, HCN+ , is less stable and is unlikely to contribute to the ion intensity. In the paragraphs above we have demonstrated some correlation of ion pair intensities both with bond energies and with the heats of formation of the products. If the energetic requirements of different channels were found to be the sole factor determining branching into those channels, that would imply that the dications decompose in their ground states by a purely statistical unimolecular decay mechanism. The degree of correlation observed here is not sufficient to support any such proposition, but it does suggest that the behaviour of these small dications is at least partly statistical. The widespread occurrence of rearrangement reactions also tends to confirm this view. When a neutral molecule is irradiated with He II radiation, approximately 940 kcalmol-’ are offered to the system. The internal energy actually deposited in the dication equals this energy diminished by the double ionization energy and by the kinetic energy carried away by the two electrons ejected. The excess energy available for partition between the degrees of freedom of any particular set of products ranges from a minimum equal to the double ionization energy of the parent minus the reaction threshold up to a maximum value of the photon energy minus the same threshold. From a consideration of the threshold law for double electron ejection we estimate that the most probable excess energies will be several electronvolts above the minimum but well below the maximum value. The minimum excess energies for charge separation with C-X cleavage range from 58 kcalmol-’ in methylamine to 205 kcal mol-’ in acetaldehyde. Part of such excess energy is released as coulomb repulsion (between 50 and 100 kcalmol-‘), but the remaining energy must go into internal degrees of freedom of the singlycharged fragments and is often enough to break more chemical bonds [12] causing secondary reactions. It is because of the wide range of internal energies deposited in the dications that no simple quantitative correlation is found between product abundance and product stability.

195

Precursor and product energies

Both PIPICO and PEPIPICO measurements on small molecular species allow estimates of double ionization energies to be made. A lower limit on the energy of the Franck-Condon zone of the rn2+ surface on which dissociation takes place is obtained by adding the kinetic energy release to the minimum asymptotic energy of the products, including the kinetic energies of any neutral species. The usefulness of this procedure for determining double ionization energies of polyatomic molecules is rather questionable, however, because the probability of forming products in their ground states is low and the products are not necessarily even in their most stable isomeric form. On the other hand comparisons of the apparent precursor energies with independent estimates of the double ionization potential, from the rule of thumb [ 161 for instance, may reveal something about the degree of excitation of precursor or products. For methyl chloride the major two-body reactions apparently arise from a precursor of 31 eV energy, which agrees with the double ionization energy of 31.6 eV estimated from the rule of thumb. It seems that these products are formed in their ground states from the lowest state of CH3C12+. By contrast the products Ccl+ + Hc seem to come from a precursor of only 27 eV energy, which is impossibly low. The conclusion must be that the products are highly excited, and in view of the large change in interatomic distance between the CH,- moiety and H: it is probably the triatomic hydrogen that is the most excited. Precisely the same conclusion is reached by similar reasoning in the cases of methanol, methanethiol, methylamine and acetonitrile. It accords well with the high reactivity of the Hz produced, as demonstrated elsewhere in this paper by its secondary reactions. In the case of acetonitrile it seems that the reaction leading to CH+ + HCNH+ also leaves substantial excitation energy in the products, but here it may be that an excited isomer of CH2Nt is formed. Odd/even electron rules

Double ionization produces cations with an even number of electrons if the neutral molecule has a closed electron shell. Singly charged protonated molecules have been studied before [ 141as examples of even-electron systems. In contrast to odd-electron cations, which produce one odd- and one evenelectron product on dissociation, even-electron species should dissociate predominantly to two even-electron products (odd/even electron rule [23]). Violations of this propensity rule are known [28], but it generally indicates the principal trends in product formation. In the case of naphthalene dications, for example, 66% of the two-body reactions have even-electron numbers at

196

34.8 eV photon energy [ 131.This fraction is slightly decreased at higher photon

energies, and at 40.8 eV only 57% of the products have an even-electron number. The fraction is even higher in the case of benzene dications, where 91% of two-body decays give even-electron-number products. The methyl compounds show a similar pattern, as two-body reactions yielding both products with an odd electron number (odd/odd (00)) reactions are of very low intensity (l-4%), while even/even (EE) reactions make up the vast majority (9699%, Tables l-5). The much higher proportion of EE reactions in the methyl compound dications compared with naphthalene may be related to the much lower double ionization potential (21.5 eV [29]) of the latter compound. As a result the energy available for naphthalene dication dissociation is higher than in the methyl compounds favouring higher energy reactions [29]. One exception from this general trend is observed in acetonitrile. Two-body decay into CH: + CNH+ has a relatively high intensity of 10% (Table 4). In this decay, where a rearrangement precedes charge separation, the especially favourable energetics of this fragmentation pathway may explain the violation of the EE propensity rule. Another two-body dissociation with odd electron products is the formation of CH: + CH,O+ in ethylene oxide, where a similar explanation may apply. The applicability of the even-electron rule to three-body dissociations is less clear, because the very occurrence of these reactions signals the presence of a large amount of excess energy in the doubly charged precursor. In fact quite abundant products with EOO electron numbers are observed, many involving H atoms as one constituent. Kinetic energy release

The kinetic energies released in charge separation have been determined by analysis of the peak shapes of the coincidence signals and are listed in Tables l-5. In an (over)simplilied model of pure electrostatic interaction, the observed kinetic energy releases can be used as a measure of the intercharge distance before charge separation. We can then attempt to relate the apparent intercharge distances to the geometries of the neutral molecules, or to see whether rearrangements may have taken place in the dications before charge separation. In methyl chloride, for instance, the kinetic energy releases of 5.4(10) and 5.2(lO)eV in the two main reactions producing CH,f + Cl’ and H+ + CH2Cl’ correspond to charge distances of 2.65 A and 2.75 A, respectively. These are approximately equal to the distance between hydrogen and chlorine atoms in the molecules (2.8 A). The charges are apparently separated as much as possible before the dication dissociates. In ylide dications a lower kinetic energy release (KER) would be expected because of the longer C-Cl bond calculated in ab initio work [l]. Most of the remaining reactions show

197

relatively lower KERs, which might indicate ylide or other rearrangements before these minor fragmentations. In methanol-d, a KER of ca. 6.5 eV is observed for the main C-O cleavage. This value corresponds to the distance in the neutral molecule between charge locations at the hydrogen centres of the methyl group and the oxygen atom, respectively. For the decay products of the rearranged ylide dication (CD: + OD:) a substantially lower KER is found (3.4( 18) eV). Ab initio calculations indicate that the C-O bond is longer than in the neutral and that the charges might be separated by 3.5 A, corresponding to a KER of 4.1 eV. This agrees within the wide error limits with the measured KER for this reaction. The higher KER for the main product (D+ + CDO+ + D2) suggests decay from the original dication without rearrangements into the ylide form. For methanethiol a KER of 6 eV would be predicted for charge localization at the methyl hydrogen and the sulphur atom, which agrees with the measured 4.0(10)eV for the main fragments. In methylamine, acetonitrile-d,, acetaldehyde, and ethylene oxide rough agreement of the experimental KERs of 4-5.5 eV is found with values calculated from the maximum possible charge separation within the molecular geometry. The various rearrangement products in acetonitrile-d, which decay by cleavage of the C-C bond show nearly the same KER (4-4.6eV), which indicates that the charges have the same separation in these cations (ca. 3.2A). From these simple considerations it is concluded that the positive charges in the dication are separated between the heteroatom of the functional group and the hydrogen sites of the methyl group, and that the majority of charge separation reactions take place from this configuration. Fragmentation of ylide dications

Recent ab initio calculations on methyl compound dications have pointed out the possible importance of ylide forms because of their low heats of formation [l]. These isomers are stabilized in relatively deep potential wells against fragmentation by the decay reactions studied in the theoretical work [l], i.e. C-X and C-H bond cleavages. The predicted products are CHZ + XH+ and H+ + CH2X+, which are both of only minor importance in the fragmentation patterns in our PEPIPICO spectra. A problem in the comparison of the ab initio calculations and experimental photofragmentation is that direct double photoionization is a vertical process which would populate a dication potential energy surface at the geometry of the neutral molecule, not at that of the ylide. Therefore some dications may react by “exploding” from the original structure and others by slower rearrangement processes. Because the ylide is a lower energy isomer, the proportion of charge separations involving ylides might vary with the photon

198

energy, but as there may also be energy barriers between the two structures the nature of the variation is not easy to predict. Recent results suggest, furthermore, that indirect double ionization via superexcited monocation states, which populates dication levels outside the normal Franck-Condon region, may play a major role [30]. Clear experimental evidence for the occurrence of some ylide dications is found here for all the molecules studied except methyl chloride. In most cases the ylide dications undergo both two-body reactions, where the evidence for their existence is direct and three-body dissociations, where the slope of the coincidence signals provides indirect evidence. On balance the theoretical case for the importance of these species is partially supported by our experimental results. Astrophysical implications

The suggestion that doubly charged molecular ions could play a role in astrophysics [31) has been followed up recently by model [32] and laboratory [13] studies on dications of polycyclic aromatic hydrocarbons. In discussing here the astrophysical implications of the present study of methyl dications, we address the following questions: (i) Can doubly charged cations of CH,X be formed in the interstellar medium? (ii) Can Coulomb dissociation processes of CH3X2+ be a means of destruction of interstellar CH,X molecules? (iii) Are the ionic fragments of CH,X2+ dissociation liable to play a role in interstellar chemistry? The neutral forms of five of the methyl compounds studied here have been observed by radioastronomical spectroscopy in interstellar clouds, and are listed in Table 7. Methyl chloride and ethylene oxide have not yet been observed, but they may well exist as interstellar molecules in view of observation of chlorine compounds [33,34] and cyclic molecules [35,36], respectively. Formation of CH,X dications requires excitation energies of the order of 30 eV. Photon energies of this order of magnitude are not normally available within dense clouds because absorption by the abundant hydrogen species (and possibly by interstellar grains) in the outer layers screens the hard photons from penetrating deep into the cloud. However, it is recognized that stars can be buried within dense clouds and can act as local high energy sources [37]. Furthermore, the H-atom absorption cross section at wavelengths less than 100 A is small enough to allow soft X-rays to penetrate great depths. Other high energy sources can be active within dense clouds, such as cosmic rays, or stellar wind particles. Thus we expect dication formation, followed by spontaneous Coulomb dissociation to be a means of CH,X

199 TABLE I Methyl compounds in the interstellar medium (ISM) and related fragment ions Species

Observed in ISM

CH,Cl CH,OH CH,SH CH3 NH, CH, CN

No Yes Yes Yes Yes

CH3CH0

Yes

Fragment ions in ISM

Fragment ions potentially in ISM” -

HCO+ , HOC+

HCS+ HCNH+ HCNH+ HCO+, HOC+

OH+ H2CS+ HZCNH+ CN+, HCN+, HNC+, CHzCN+ CHICO+, H2CO+

a Only those for which the neutral equivalents have been observed in the interstellar medium are noted. destruction in the outer portions of dense molecular clouds, as well as in the neighbourhood of embedded stars and of cosmic ray interactions with matter. For these relatively small molecular species, the CH,X dication formation cross sections and relative yields are probably small with respect to the corresponding cross sections for dissociation, by photons or by charged particles, of neutral and singly charged CH,X species. Therefore formation of CH,X2+ in dark clouds is expected to be only a minor general pathway for destruction of these methyl compounds. Nevertheless, it could have specific interest in particular situations. Anticorrelations in spatial distributions of these parent/ion fragment species could provide evidence for a possible role of interstellar dication formation and dissociation. Table 7 lists the CH,X molecules studied and their interstellar observation. Also listed are cations observed in the interstellar medium which appear as fragment ions produced in our experiments by dissociation of the parent dications. Another column gives a further series of fragment ions which are potential interstellar ions and whose neutral species have been observed by radioastronomy. Furthermore these ions are generally considered to be intermediates in cosmochemical reaction schemes [38]. In Table 7 we have not included the series of hydrocarbon ions CH,+ , nor H+ , H: and H: also formed in our experiments for each compound studied, and which play an important role as intermediates in cosmochemical reactions. CH,X dication formation and fragmentation have not been considered as yet in interstellar chemistry models. We cannot firmly assert that this will indeed become necessary to create models which fully explain radioastronomical observations, but we consider that our experimental results should be borne in mind in those particular circumstances where sources of high energy excitation of extant CH,X could exist.

200 CONCLUSIONS

Fragmentation of dicationic methyl compounds, studied using the PEPIPICO triple coincidence technique is characterized mainly by formation of ion pairs including H+ , H$ , or H: as one partner and by charge separation with cleavage of the C-X bond. Further decay of the singly charged fragments having sufficient internal energy occurs by extrusion of neutrals. The fragmentation patterns show some correlation to bond dissociation energies, stabilities of products, and odd/even electron numbers, and contain characteristic key fragments. The kinetic energy releases suggest that the charges are located at the maximum distance apart within the unrearranged molecules before the fragmentation reactions take place. The species are possibly significant in astrophysical contexts where their destruction by double ionization, studied here, may be important. ACKNOWLEDGEMENTS

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