The generation of OC2O·+ and OC2O and a study of ionized OC3O and C2O by tandem mass spectrometry

ELSEVIER

International Journal of Mass Spectrometry and Ion Processes 133 (1994) 111 - 119

The generation of OCzO’+ and O&O and a study of ionized OC30 and C20 by tandem mass spectrometry’ Hongwen Department

of Chemistry,

Chen, John L. Holmes* University of Ottawa,

Ottawa, Ont. KIN 6N5, Canada

(Received 3 June 1993; accepted 12 January 1994)

Abstract Three of the less common oxides of carbon, OC*O, OCsO and C20, have been studied by tandem mass spectrometric methods. Neutralization-reionization (NR) mass spectrometry provided evidence for the possible generation of (hitherto unobserved) stable OCZO molecules having a lifetime of at least 1 ps. Ionized OCsO and CZO both produced stable neutral species in their NR mass spectra in keeping with their known chemistry. The heats of formation of OC20’+ and C*O’+ were measured to be 940 f 10 and 1412 f 5 kJmol_‘, the latter in good agreement with earlier work. Key words:

Neutralization-reionization

mass spectrometry;

1. Introduction Linear and quasi-linear cumulenes of the general structure AC,B (A,B=lone pair, HZ, 0, S; for n 2 2) have been the subject of both experimental and theoretical interest for more than a century. Recent interest in these molecules arises from their having been postulated as key intermediates in the formation of interstellar species [l]. They have unusual spectroscopic properties, reactivity and stability, which relate to their unique electronic structures. Unfortunately, some of these molecules have such a high reactivity that they have not been observed, even as transient species. This is especially true for (OS) cumulenes containing an even number of carbon atoms, which are believed *Corresponding author. ’ This work is dedicated to the memory of Jean Louis Roustan, a respected colleague and friend.

C,O,;

C,O,;

Tandem

mass spectrometry

to be considerably less stable than their oddnumbered analogues [2,3] and so have often eluded experimental identification. In general, hydrocumulenes, e.g. allene and ketene, are relatively well-known substances. Recently, neutralization-reionization mass spectrometry (NRMS) [4] has proven itself as a powerful tool for generating elusive molecules in the gas phase. Several cumulenes, including dithiocumulenes SC,S (n = 2,4,6), [5-71 the oxycumulene OC40 [8] and mixed S/O cumulenes SC,0 (n = 2-5) [9,10] have been accessed in the gas phase by Schwarz and co-workers. All efforts to date, beginning as early as 1913, to prepare ethylenedione, O&O, the first member of the evennumbered oxycumulenes, have proven unsuccessful. In this work we provide experimental evidence for the existence of O&O, using NRMS and also related experiments on OCsO, carbon suboxide and the simplest carbonyl carbene, C20.

0168-l 176/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-l 176(94)03962-Y

112

H. Chen, J.L. HolmeslInt.

J. Mass Spectrom. Ion Processes

Since this work was completed for publication, similar experiments on OCzO’+ ions have been reported by Siilzle et al. [l 11; these will be discussed later.

2. Results and discussions 2.1. oc*o Attempts to produce OC20 began in 1913 when Staudinger and Anthes [ 121observed the formation of (COK)2 from the reaction of oxalyl bromide, (COBrL, with potassium metal. However, their study and later pyrolysis and photolysis experiments on bridged a-diketones, such as dibenzobicycle [2,2,2] octadiene-2,3-dione [13,14], provided no direct evidence for the existence of OC20. On the basis of ab initio molecular orbital theory calculations [2], OC20 is expected to be a kinetically extremely stable species. Two reasons [9] have been suggested for the failure to observe OC20. (i) The 0C20 species have not been generated in the bound triplet ground state 3C; but rather in the singlet state ‘Xi, which is predicted to dissociate spontaneously to two ground-state CO molecules. (ii) The lifetime of the stable 3C; OC20 species may be drastically shortened by an efficient spinorbit coupling if a potential-curve crossing is available not too far from its minimum energy. In marked contrast, the ionic counterpart of O&O, OC20’+ is experimentally accessible and has been the object of electron spin resonance (ESR) [15], molecular beam photoionization [ 161, highpressure mass spectrometry [17] and drift tube mass spectrometry [18] experiments. Ab initio molecular orbital theory calculations have been used to investigate the electronic structures of 0C20. Most of the calculations predict [2,3,19-221 that 0C20 has a triplet ground state, 3C;. The heat of formation of this triplet ground state was calculated to be from - 100 to 87 kJ mol-’ by various methods [2,19-221. This triplet ground state was found [2,19] to be linear and to have considerably higher energy than two separated CO molecules in their ground singlet state ‘C. The linear dissociation of OCzO to two ground state CO molecules was predicted [19] to be

133 (1994) Ill-119

symmetry forbidden and accordingly to have a high activation energy. Its lowest spin-allowed dissociation limit was dissociation to a CO molecule in the ground state ‘C plus a CO molecule in the lowest excited state 311.However, the 0C20 (3C,) lay 309 kJ mol-’ below this dissociation limit. Several bent singlet excited states were also found for 0C20. These were at least 80 kJ mol-’ higher in energy than the triplet ground state and could dissociate to two separated CO molecules with little or no activation energy. The ion OC20’+ was predicted by a theoretical calculation [15] to have a trans geometry in its ground state. Its heat of formation was measured to be from 1013 to 1050 kJmol_’ by various techniques [16-181. In this work, the appearance energy (AE) of oc20.+ generated from oxalic acid (COOH)2 and oxalyl chloride (COC1)2 were measured by the MS-902s mass spectrometer (see’Experimenta1 Section). From these two compounds, the values of ArH”[OC20’+] obtained were 939 f 10 and 9424 lOkJmol_‘, respectively. Table 1 summarizes these data for ArH”[OC20’+]. The ArH”[OC20’+] value obtained in this work is lower than those reported in previous work. For oxalic acid, 0C20’+ was believed to be generated by the overall reaction (COOH);+ + 0C20’+

+ H20 + 0’

(1) Because this reaction does not arise from metastable ions, the AE of the m/z 56 peak (4% of the base peak, m/z 45) in the normal electron impact (EI) mass spectrum of oxalic acid was measured (MS-902s mass spectrometer) under high mass resolution. This was necessary to eliminate the common background ions, [C,, Hs] *+ and also present as a weak signal. The P3, &Ol’+, onset of 0C20’+ was well defined even though this AE lies high above the lowest energy dissociation, (metastable) loss of CO2 to produce C(OH);+ at 11.1 eV [24]. The neutrals lost in the lowest energy formation of OC20’+ deserve a brief discussion. The possible species are HzOz; 20H’; H2+20’ and H20+0’. In the H2 + 02; normal EI mass spectrum [25] only one weak peak can be found between m/z 90 and 56, namely m/z 72, C2O3.’ (0.3% base peak, m/z 45;

113

H. Chen. J.L. Holmes/Int J. Mass Spectrom. Ion Processes 133 (1994) 111-119

Table 1 Experimental data for heat of formation of OCzO” Method

Ref.

Results

ArH”[OCzO’+] (kJ mol-t )

Photoionization High pressure MS High pressure MS Drift tube MS AE measurement AE measurement

16 17(a) 17(c) 18(b) This work This work

Binding energy (CO”-CO) = 0.97 f 0.04 eV AE(OCzO’+) = 12.8 f 0.3 eV CO’+ + 2C0 + OCzO’+ + CO, -AH> 106.2 kJmol-’ Binding energy (CO’ +-CO) = 0.8 eV AE(OCr0”) from (COOH)* = 17.4 f 0.1 eV AE(OCzO’+) from (COCl)z = 15.7 f 0.1 eV

1038 f 1013 f c 1025 1054 d 938 f Q 942 f

4 29

10 10

Using A,W[(COCl),] ArH”[CO] = -llO.SkJmol-‘; ArH”[CO’+] = 1242kJmoll’; ArH’[(COOH),] = -723 f 3kJmoll’; -329 f 5kJmol’; ArH”[HzO] = -242 kJmol_‘; ArH”[O’] = 249kJmol-‘; ArH”[Cl’] = 121 kJmolF’, all from Ref. 23.

m/z 90 = 1.2%). Loss of 0’ from m/z 72 would produce m/z 56, and the ArH” value for OCzO’+ given in Table 1, 940 kJ mol-’ , is for this reaction sequence from the molecular ion. Of the other possibilities listed above, the first leads to a ArHo value of 1083 kJ mol-’ (ArH”[HZOZ] = -136 kJ mol-’ [23]), the second and fourth give lower values, 869 and 449 kJmol_‘, respectively, and only the losses of I-I2+OZ give a closer result, 947 kJ mol-’ . Of these alternative fragmentation pathways, the chosen path, Hz0 + 0’ loss, can be related with the postulated H-bonding between the carboxyl groups which leads to CO2 loss [24,25]. A priori, loss of an Hz02 molecule cannot be discounted, but loss of 2H0 * should be accompanied by a peak at m/z 73; however, this is not observed in the mass spectrum of oxalic acid. It is highly unlikely that the loss of the first HO’ is rate determining (i.e. the second HO’ loss is very fast) because that would give an improbably high value for D[HOOCCO-OH]‘+ of about 710 kJ mol-’ (compare D[HCO-OH]‘+ = 151 kJmol_’ [23]). For oxalyl chloride, m/z 56 is an intense peak in its mass spectrum, 17% of the base peak m/z 63 COCl+; there is little choice of neutral loss, Cl2 or 2Cl’. The former gives rise to a high value for ArH”[OCzO ‘+I = 1186 kJ mol-’ . The intermediate peak m/z 91 for sequential chlorine atom loss is clearly present in the normal mass spectrum at 1.1% of the base peak m/z 63 and so the chosen route to m/z 56 is viable. The metastable ion (MI) mass spectrum of OC20’+ ions contains only one peak at m/z 28, CO’+. The kinetic energy release (KER) accompa-

=

nying this dissociation, measured from the half height width of the weak Gaussian peak Ts.s, was very small, about 0.3meV. The addition of traces of collision gas (02) showed that the cross-section for the collision induced dissociation (CID) is relatively large. Under single collision conditions the CID peak is about 100 times as intense as the MI peak and the To.5 value for the former was 167f2meV. The OC20’+ ions generated from oxalic acid, oxalyl chloride, butane-2,3-dione and from the reaction of CO *+ with CO at high pressure in the mass spectrometer ion source, had very closely similar CID mass spectra. These are shown in Table 2. The presence of CO;+ is unexpected but it must result from a rearrangement process. Note that formation of C,O’++ 0’ (ArH’[CzO’+] x 1410 kJmol-‘; A,H”[O’] = 249 kJmol_’ [23]), 1659 kJ mol-‘, is close to that CO;+ + C (ArH’[CO;‘] = 935 kJmol_‘; for ArH”[C] = 717kJ mol-’ [23]), 1652kJmol-‘. It is noteworthy (Table 2) that the kinetic energy releases measured for the base peak, m/z 28, were target gas dependent, indicating that excited states of oc*o’+ must be differently accessed by the three target gases 02, Xe and He. Neutralization-reionization experiments on m/z 56 ions were concentrated on the O&O’+ ions from oxalyl chloride, these being much more abundant than from any other of the above sources. In general, even after extensive baking of the ion source, the m/z 56 peak often contained, under high mass resolution conditions, a contribution from [C,, Hs]‘+ ions. These produce significant

114 Table 2 Collision Precursor

H. Chen, J.L. HolmeslInt.

induced

dissociation Target

co+co

02

(COCW2

02

WOW2

02

Pw2

02

(COC1)2

Xe He

(COC1)2

(CID) mass spectra gas

J. Mass Speclrom. Ion Processes 133 (1994) 111-119

of OCZO”

Fragment

ions from different

precursorsa

ion (m/z)

44

40

28

24

16

12

1.6 2.9 2.0 2.6 0.1 0.7

17.1 17.8 16.3 19.4 5.2 15.2

100 100 100 1OOb 1OOb 1OOb

1.2 1.4 1.3 2.1 0.2 2.0

0.4 < 0.1 < 0.1 0.5 < 0.1 0.3

3.3 2.5 2.0 3.0 1.8 1.0

Collision target gas pressures were adjusted to allow 90% beam transmission. a [Cd,Hs]‘+ ions can give background signals (< 1% of m/z 28) at m/z 55,41 and 39 in these mass spectra. b The kinetic energy releases calculated from the half height width of these peaks were 167 f 2, 86 f 2, and 150 ?Z 2 meV, respectively.

recovery signals in their NR mass spectra as well as hydrocarbon ion peaks at m/z 55, 41 and 39. A series of careful comparison experiments showed that the very small m/z 56 peak in the NR mass spectrum of that ion from oxalyl chloride could always be explained as arising from a trace of [C,, I-M’+ showing that there is no detectable recovery signal in the NR mass spectrum of 0C20’+ ions. Separation of [C,, Hs].+ from O&O’+ requires only modest mass resolution but the intensity loss of the ion beam which results does not compensate for the loss of the interfering ions. Accordingly, the ion source and inlet systems were thoroughly cleaned to reduce [&,Hs]‘+ contamination to a level at which they were undetectable by CID mass spectrometry. Under such conditions any recovered O&O ’ + in the NR mass spectrum comprises less than 1 part in 5000 of the total NR ion yield. Within these limitations, neutral O&O is undetectable. However, the absence of such a signal does not preclude the intermediacy of stable O&O molecules, whose vertical (collision induced) ionization leads to rapid dissociation to co*+ +co. Note the general similarity between the relative abundances of m/z 44,40, (28), 24,16 and 12 in the CID and NR mass spectra. In their recent paper, Stilzle et al. [l l] examined the same properties of the OC20’+ ion produced from squaric acid and by clustering CO ‘+ with CO in the ion source. Even allowing for the lower ion transmissions in their collision experiments, their results are in

good agreement with ours, except for the apparent absence of m/z 44 in their NR mass spectra and the somewhat greater dissociation of the CO’+ ions, i.e. larger m/z 12 and 16. However, in spite of the presence of weak signals in their NR mass spectra at m/z 40 and 24, Stilzle et al. [l l] discounted these as evidence for the possible intermediacy of neutral 0C20. Our view is that this evidence (and the weak but detectable CO;+) cannot lightly be dismissed and so further experiments were performed to more closely compare the CID and NR mass spectra, results which are given in Tables 2 and 3, respectively. First, the KER values for the m/z 28 peaks should be considered. Note that for the CID and He/O2 NR experiments the CO’+ peaks have the same base widths but the T0.s value for the NR peak is larger, 150 : 217 meV, respectively (Tables 2 and 3). Collisions of O&O’+ with He are unlikely to result in electron transfer because of the high ionization energy (IE) of the target gas, 24.6eV [23] and so the He/O* Nr mass spectrum should be dominated by the reionization of neutral species formed by CID of O&O’+. It is suggested that the neutral CO molecules accompanying the large kinetic energy releases in the CID are also internally excited and that the apparent change in kinetic energy release distribution in the He/O* NR mass spectrum arises from such species having a higher cross-section for collision induced ionization than those of lower internal energy. Although m/z 40 is barely discernable, the origin of m/z 24, C;+, in the He/O2 experiment, is

H. Chen. J.L. HolmeslInt Table 3 NR mass spectra Target

X402 02/02 C-C3H6/02

Xc/He He/Q

gas (N/R)

of the OCsO’+

J. Mass Spectrom.

Ion Processes

115

133 (1994) 111-119

ions from oxalyl chloridea Fragment

ion (m/z)a

44

40

28

24

16

12

0.02 0.02 < 0.1 < 0.08 _

0.3 0.2 < 0.5 < 0.5 < 0.1

1OOb 100 100 100 1OOb

0.5 1.2 1.2 1.0 0.8

3.1 6.0 4.0 4.9 7.4

7.8 9.5 1.2 10.5 8.9

<

Neutralization target gas (N) and reionization target gas (R) pressures were adjusted to allow 90% beam transmission. ‘Any very small recovery signal at m/z 56 was always accompanied by m/z 55, 41 and 39 due to interference from background [C,, Hs] ‘+ ions [70]. These were absent in the experiments recorded in this table (see text). b The kinetic energy releases calculated from the half height width of these peaks were 530 f 5 and 217 f 3 meV, respectively.

problematic. The relative cross-sections for the collision events were measured, He(CID) = 105; He/O*(NRMS) = 220; Xe(CID) = 4 x 104; Xe/Oz (NRMS) = 330. Thus Xe is a poor CID target gas relative to He, but (assuming no neutralization by He) some 75% of Xe encounters lead to electron transfer. The change in KER from the Xe CID to Xe/Oz NR is very large, T0.s (m/z 28) rising from 86 to 530meV. This latter KER must result from dissociation of neutral (excited) O&O molecules. However, this does not necessarily mean that all OC20 molecules must dissociate. The sum of the product enthalpies for decomposition of 0C20 to 2C0, CO* + C, C20 + 0’ and C2 + 20’ are -222, 324 629 and 1330 kJmol_’ respectively (data from Ref. 23). The latter lies above the ionization energy of O&O and so seems an unlikely contributor to m/z 24. For the NR mass spectrum to be the result of only reionized fragments from excited OC$O, then the third process above (to CzO + 0.) must be wholly responsible for m/z 40 and 24. This is in keeping with the m/z 16 in the NR mass spectrum, which is significantly larger than in the CID mass spectrum. (See also the CID mass spectrum of C20’+ ions, Table 4.) The final possible origin for CO;+, C20’+ (and C;+) is that they arise from the fragmentation of wholly dissociative O&O ‘+ ions produced from vertical ionization of neutral OCzO having a lifetime greater than that for passage between the collision cells, about 1 ~_ls.

At present, the above interpretations must remain inconclusive with regard to the stability of the O&O molecule. 2.2. OC,O

and C20

Carbon suboxide, OCsO, was first prepared in 1906 by Diels and Wolf [26] by treating diethyl malonate with a large excess of phosphorous pentoxide. The early studies on OCsO have been reviewed by Reyerson and Kobe [27]. Since then it has been the object of numerous IR and Raman [28-311, photochemical [32-351, electron diffraction [36,37], electron spectroscopy (ESCA) [38-401, ultraviolet absorption [41-431, 13C-NMR [44] and high pressure mass spectrometry [45] experiments as well as theoretical calculations [46-511. This compound, also known as “red carbon”, may polymerize to form a sequence of heavier molecules whose colours are graded through pale yellow, orange, reddish-brown, and violet, to nearly black at room temperature. OC30 has been proposed as a possible constituent of the Venusian atmosphere [52] and as the red colour of the Martian surface [53]. As a product of the reactions of carbon suboxide and as an important reaction intermediate, carbonyl carbene, C20, has been studied by a wide variety of experimental techniques, including matrix-isolation IR spectrometry [54,55], photochemical studies [32-34,56,57], electron spin resonance (ESR) spectroscopy [58,59], high pressure mass spectrometry [45]

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J. Mass Spectrom. Ion Processes 133 (1994) 111-119

Table 4 Collision induced dissociation (CID) mass spectra of the OC$O’+ and C20’+ ions from carbon suboxide Parent ion (m/z)

68 40

Fragment ion (m/z) 52

40

36

34a

28

24

16

12

8.4

100 _

0.4 -

5.9 _

10.9 100

5.0 18.1

_ 1.1

2.9 28.7

Collision target gas (02) pressure was adjusted to allow 90% beam transmission. a This is a charge stripping signal, m/z 682+, of the molecular ion.

resonance (ICR) mass and ion cyclotron spectrometry [60] as well as by theoretical calculations [61-641. The heats of formation of OCsO and OCsO’+ were reported to be -94 and 929 kJmol_‘, respectively [23]. Both linear and bent structures were suggested for OCsO on the basis of theoretical calculations [46-5 I] and ESCA experiments [36,37]. The heat of formation of C20 was determined to be 386 f 19 kJmol_’ by experiment [32,33] and was calculated [62] to be 372 kJ mol-’ by an ab initio method. ArH”[C20’+] was estimated to be < 1476 kJmol_’ by an appearance energy measurement [45] of the reaction 0C30-+C20’++CO+e

(2)

and 1421 f 42 kJmol_’ by an ICR mass spectrometry study [60] of the reaction (CO’+)* + co + czo*+ + 0

(3)

where the asterisk designates excited ions. In this work, the appearance energy of C20 ‘+ formed in the ion source from reaction (2) was measured to be 14.7 f 0.1 eV by the MS-902s mass spectrometer and 14.48 f 0.05 eV by the electron energy selector mass spectrometer (see Experimental Section). These results lead to ArH”[C20’+]< 1433 f 10 and < 1412 f 5 kJmol_‘, respectively. These values are in good agreement with the previous results. There were intense signals at m/z 68 and 40 in the normal EI mass spectrum of the pyrolysis products of malonic acid mixed with phosphorous pentoxide (reactions (4a) and (4b)). The remaining significant signals in the spectrum were m/z 44, 32,

28, 24, 16, 14 and 12. The signal at m/z 44 corresponded to the decarboxylation product, COZ, in the pyrolysis reaction (4b). HOOCCH$OOH

- p205 b 0C30 + 2H20 (4a) A CO2 + CH,COOH (4b)

The m/z 68 ion is OCsO’+. It gave a signal at m/z 40 (C,O’+) in its MI mass spectrum with a very small kinetic energy release (Ts.s = 1.8 meV). The CID mass spectrum (Table 4) of the OCsO’+ ion was dominated by this signal and other characteristic fragment ion peaks were observed at m/z 52, CsO’+, 36, C;+, 28, CO’+, 24, C;+ and 12, C’+. The NR mass spectrum (Table 5) of the OCsO’+ ion contains a recovery signal at m/z 68 and the same fragment ions as in the CID mass spectrum. The m/z 40 ion, C20 ‘+, did not fragment metastably. Its CID mass spectrum (Table 4) contained characteristic fragment peaks at m/z 28, CO’+, 24, C;+ and 12,C’+. As with its S and N analogues (C2S ’ + and C2N+) [6,65] the recovery signal of C20’+ is the base peak in its NR mass spectrum (Table 5).

3. Experimental Appearance energies (AE) of fragment ions were obtained with a quadrupole mass spectrometer equipped with an electrostatic electron monochromator and a computer data processing system [66] and a Kratos MS-902s mass spectrometer. For the

H. Chen, J.L. Holmes/Inr J. Mass Spectrom. Ion Processes 133 (1994) Ill-119

117

Table 5 NR mass spectra of the OCsO’+ and CrO’+ ions from carbon suboxide Parent ion (m/z)

68 40

Fragment ion (m/z) 68

52

40

36

34”

28

24

16

12

463 _

31.6 _

100 323

3.0 _

3.0

43.0 100

16.7 19.4

_ 1.6

13.3 24.2

Neutralization target gas (Xe) and reionization target gas (Or) pressures were adjusted to allow 90% beam transmission. a This signal relates to the charge stripping signal m/z 682f of the molecular ion.

latter experiments, the appearance energy was measured for ion-source generated species using the ionization energy of benzene as the standard. The ion abundance versus electron energy data for a small range (3-4 V) above threshold were treated as described in detail elsewhere [67]. All ion dissociation experiments were performed with a modified VG Analytical ZAB-2F double-focusing mass spectrometer of reversed geometry. Ions were generated by 70eV electron impact on precursor compounds and they were accelerated to 8 kV. CID mass spectra were recorded with O2 as target gas unless otherwise stated. NR experiments were performed as described elsewhere [68]. Briefly, target gases were introduced into collision cell 1 for charge exchange neutralization of the massselected ions. The remaining ions were deflected away by a beam deflector electrode and the neutral species were thereafter reionized by collision with target gases in collision cell 2. Gas pressures were adjusted to give a main ion beam transmission of 90% (90% T). All MI, CID, and NR mass spectra were corrected for isotopic contributions from adjacent ions of lower mass. Unless otherwise stated, interference was negligible. In the above experiments all beam defining slits .were fully open to obtain maximum signal strength and to minimize energy resolving effects. Metastable peak shapes were recorded under higher energy resolution conditions, with the main ion beam width at half height being 3 f 1 V at an accelerating voltage of 8 kV. Kinetic energy releases, T,,5, were obtained from the peak width at half height and evaluated using established methods [69]. All compounds, except carbon suboxide, were commercially available and showed no impurities

detectable by mass spectrometry. Carbon suboxide was synthesized by the pyrolysis of malonic acid in phosphorous pentoxide [28]. By-products such as carbon dioxide and acetic acid were also introduced into the ion source of the mass spectrometer along with the carbon suboxide but they did not interfere with the mass spectrometric measurements.

4. Acknowledgements J.L.H. thanks the Natural Sciences and Engineering Research Council for continuing financial support and Dr. F.P. Lossing for the appearance energy measurements and many invaluable discussions.

5. References [l] D. Smith and N.G. Adams, J. Chem. Sot. Faraday Trans. 2, 85 (1989) 1613. [2] G.P. Raine, H.F. Schaefer III and R.C. Haddon, J. Am. Chem. Sot., 105 (1983) 194. [3] L.P. Brown and W.N. Lipscomb, J. Am. Chem. Sot., 99 (1977) 3968. [4] (a) C. Wesdemiotis and F.W. McLafferty, Chem. Rev., 87 (1987) 405. (b) J.K. Terlouw and H. Schwarz, Angew. Chem., Int. Ed. Engl., 26 (1987) 805. (c) J.L. Holmes, Mass Spectrom. Rev., 8 (1989) 513. [5] D. Stilzle and H. Schwarz, Angew. Chem., Int. Ed. Engl., 27 (1988) 1337. [6] D. Stilzle and H. Schwarz, Chem. Ber., 122 (1989) 1803. [7] D. Sillzle, N. Beye, E. Fanghinel and H. Schwarz, Chem. Ber., 123 (1990) 2069. [8] D. Siilzle and H. Schwarz, Angew. Chem., Int. Ed. Engl., 29 (1990) 908.

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