RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 6 , 249-253 (1992)

Electron Impact and Chemical Ionization Mass Spectrometry of Heterocumulenes Produced by Flash Vacuum Pyrolysis Jeff Brown,+ Robert Flammang,+* Yasmine Govaert,* Michel Plisnier,$ Curt Wentrup$ and Yves Van Haverbeke* ' VG Analytical Ltd, Wythenshawe, Manchester, UK

' Department of Chemistry, University of Queensland, Brisbane, Queensland 4072, Australia Organic Chemistry Department, University of Mons-Hainaut, 19 Avenue Maistriau, B7000 Mons, Belgium

Heterocumulenes such as ketenes, propadienones and cumulogs of isocyanates are produced by flash vacuum pyrolysis of Meldrum's acids. Real-time monitoring of the pyrolysis products is performed by methods of tandem mass spectrometry. In particular, chemical ionization is shown, for the first time, to generate protonated molecular ions of these kinetically unstable neutral molecules which can be identified by collisional activation experiments.

Short-contact gas-phase pyrolysis is now widely ac- cepted for the preparation and study of reactive organic species.' Besides the use of matrix isolation techniques,* real time analysis of the pyrolysis products has been performed by microwave spectroscopy ,3 pho- toelectron spectroscopy4 and mass spectrometry.' More recently, tandem mass spectrometry has improved the quality of the data.' Most of the work described in this field has been realized by using conventional electron ionization. One group has modified a reversed- geometry mass spectrometer fitted with a field- ionization ion source by the coupling of an inductively heated reactor.'

We report here some preliminary experiments des- cribing on-line preparation and ionization (electron impact (EI) and chemical ionization (CI)) of kinetically unstable compounds using a pyrolysis device installed on the ion source of a new tandem mass spectrometer, an AutoSpec 6F instrument (VG Analytical Ltd, Manchester, UK).X The target compounds chosen to test the pyrolysis device were Meldrum's acids which are well-known precursors of ketenes. The use of chemical ionization to characterize the products and sites of protonation, which is, to our knowledge reported for the first time, will be described with some selected examples.

EXPERIMENTAL The flash vacuum pyrolysis (FVP) device is shown in Fig. 1. It consists of a quartz tube (3 mm ID, 50 mm length) equipped with a tantalum wire heater and radiation shields. Temperature (up to 1000 "C) is data- system controlled. Samples are introduced in the entrance of the pyrolysis tube with a conventional solids probe. All the system shown in Fig. 1 is installed in the source housing of the spectrometer where the pressure is < lo-' Torr in EI and about 5 x Torr in the CI mode (isobutane or methanol reagent gases).

The collisional activation (CA) spectra and neutralization/reionization (NRMS) spectra have been recorded on a spectrometer of EIB1E2E3B2E4 geometry (E =electric sector, B = magnetic sector) at an acceler-

Author to whom correspondence should be addressed.

PYROLYSIS TUBE SOURCE EI/CI

2

5 1

Figure 1. Schematic diagram of the flash vacuum pyrolysis unit adapted to the VG AutoSpec 6F source unit. 1 =quartz tube; 2 = machineable glass; 3 = stainless-steel radiation shields; 4 =tantalum wiring; 5 =supports (supply floating at source potential).

ating voltage of 8 kV. Details of the spectrometer will be described elsewhere.' In the CA experiments, a beam of ions is mass selected by the combination of the first three sectors (E,B,E2), submitted to collisional activation in a cell pressurized with oxygen (80% trans- mission), unreacted ions being eliminated by floating an intermediate calibration ion source inserted between the two cells at 9 kV. The two kinds of spectra were recorded by scanning E3 and collecting the ions with an off-axis photomultiplier detector located between E3 and B2 (4th field-free region). Spectra shown in the Figures result from the averaging of 20 (CA) or 100 (NRMS) scans corresponding to 0.5 and 2.5min acquisition times, respectively.

RESULTS 2,2-Dimethyl-1,3-dioxane-4,6-diones (Meldrum's acids) decompose in a melt to give many products, but when flash-vacuum pyrolysed at about 500 "C, they fragment cleanly to give acetone, carbon dioxide and ketenes."' This is illustrated by the behaviour of the parent dioxane dione 1 when pyrolysed using FVP then ionized by electron impact (Table 1). Although the molecular ion peak is not observed under these EI conditions, the intensity modifications of the peaks at mlz 44 (CO?+'). 58 (CH,COCH;') indicate that the

095 1-4 198/92/040249-05 $05.00 @ 1992 by John Wiley & Sons, Ltd.

Receioed I January 1992 Acrepted I I February I992

250 EI AND CI MS OF HETEROCUMULENES

Table 1. FVPlEI mass spectra of the parent Meldrum’s acid 1 mlr

144 12Y 100 59 44 43 42 41 200°C - 6 7 5 15 100 51 7 600°C - 1 1 5 45 100 70 22

pyrolysis is almost complete at 600°C with the gene- ration of ionized ketene (mlz 42). Chemical ionization mass spectrometry (methanol reagent gas) facilitates the monitoring of the pyrolysis as the parent compound is now clearly identified by the peak due to protonated molecules at mlz 145 (Table 2). At 600 “C, products are observed at rnlz 59 (protonated acetone) and mlz 43 (protonated ketene).

The CA spectrum of these [C2H30]+ ions is com- pared in Fig. 2 to the CA spectrum of acetylium ions, CHsCOC, generated by dissociative ionization of acetone.’ The similarity of the spectra is interpreted in terms of carbon protonation instead of oxygen protona- tion. A b initio molecular orbital calculations have indeed shown that carbon protonation is favoured over oxygen protonation by ca 35 kcal.mo1-I.“’ The small differences observed between the spectra are thus ascribed to internal energy differences which mainly affect the relative intensity of the unimolecular frag- mentation, the loss of CO (to give rnlz 15) in the present case.

42

29

(b) 15 I 29

Figure 2. CA spectra of the [C2H30]’ ions produced (a) by FVP/CI of Meldrum’s acid 1 and (b) by dissociative ionization of acetone. Charge stripping region around rnlz 21.5 expanded by a factor of 10.

Table2. FVP/CI (methanol) mass spectra of the parent Meldrum’s acid 1

mlz 14s (MH+) 59 (ilcctonc + H’) 43 (ketcne+H+)

200 “C 75 20 15 600 “C 1 61 38

Methyleneketenes, RR’C=C=C=O, can also be formed in the gas phase by pyrolysis of methylene or substituted methylene derivatives of Meldrum’s acid. However, if the exocyclic double bond bears a hydrogen-containing substituent, tautomerization occurs and the first products observed are vinyl-, imidoyl- or acylketenes, 3a-c, as shown in Scheme 1.

0 H H 7Ln

X=CH2 48 NH 4b 0 4c

2a 38 2b 3b 2c 3c

Scheme 1

Upon electron impact, dissociative ionization of the Meldrum’s acids 2a-c yields the corresponding ionized propadienones 4a-c" . Ionized methyl-,” amino-I2 and hydroxypropadienones’” have been prepared in that way and identified by tandem mass spectrometry. l 4

Structure characteristic fragmentations (a-cleavages relative to the cumulene function) induced by CA strongly supported the assignments.

Very recently, FVP of a series of (alkylamino) methylene derivatives of Meldrum’s acid has been described.lS Based on low temperature IR experiments, it has been shown that the N-t-butyl compound 5 produces at > 540 “C cyanoacetaldehyde, 6, the ulti- mate product of tautomerization of aminopropadienone 4b (Scheme 2). It has thus been shown possible to

Scheme 2

generate in the gas phase the three isomers presenting the NCCCO sequence by an appropriate choice of the experimental conditions.

Figure 3 shows the neutralization/reionization mass spectra (NRMS)16 of [C,H,NO]+’ ions produced by (a) dissociative ionization of 2b or 5, (b) FVP of 2b fol- lowed by ionization and (c) FVP of 5 followed by ionization. In these three cases, an intense signal cor- responding to ‘survivor’ ions at rnlz 69 indicate the gas phase stability of the corresponding neutrals. Moreover, the overall good correspondence between the NR mass spectra and the CA spectrai7 demon- strates the actual structures of the C3H,N0 ions and neutrals. For example, the rnlz 53 fragments (loss of NHJ characterize the aminopropadienone structure 4b. They are replaced by rnlz 42 fragments (loss of HCN) for imidoylketene 3b. Finally, cyanoacetalde- hyde ions 6+’ present a strong signal at m/z 29 (formyl cations) and lose a hydrogen atom more easily as would be expected.

EI AND CI MS OF HETEROCUMULENES

63.5

46

25 1

I

111 97

69

Figure 3. NRMS spectra using ammonia for neutralization and oxygen for re-ionization (NH,/02) of mlz 69 ions produced by (a) dissociative ionization of Zb, (b) FVP and ionization of 2b and (c) FVP and ionization of 5 .

42

Figure4. CA spectra of (a) protonated imidoylketene a and (b) protonated cyanoacetaldehyde b.

82

72

Figure 5. CA spectrum of the mlz 127 ions produced by protonation of the pyrolysis product (400 "C) of the Meldrum's acid 8.

252 EI AND CI MS OF HETEROCUMULENES

After FVP of compounds 2b and 2c, isobutane che- mical ionization allows the preparation of the proto- nated forms of 3b and 6 (mlz 70). In order to avoid interference with background peaks of the isobutane plasma, the mass resolution of the first three sectors (MS1) was increased up to 5000 and the CA spectra of the m/z 70 ions recorded by scanning the third (EJ electric sector field (Fig. 4). The CA spectrum of protonated imidoylketene, 3bH+, is characterized by an intense charge stripping peak at m/z 35 and the overall fragmentation is best explained by nitrogen protonation yielding a push-pull stabilized aminocarbe- nium ion a. Pyrolysis of 5 followed by protonation affords an isomeric structure b identified by CA as N-protonated cyanoacetaldehyde (see Scheme 3).

41

43

(4

Scheme 3

CI protonation of aminopropadienone 4b is not poss- ible as the neutral species cannot be studied in the ion source. The application of the recent technique of neutralization/chemical ionization/reionization" should be particularly useful in this case. A small peak

I 67

61

Figure 6. FVP of 8 at 600°C: (a) CI mass spectrum, (b) CA spectrum of the m/z 82 ions and (c) CA spectrum of the mlz 46 ions.

EI AND CI MS OF HETEROCUMULENES 253

MeN

H S - mfz 127 400'

H NMe2 8 7

Scheme 4

is however observed at m / z 70 in the mass spectrum of 2b; its CA spectrum indicates the occurrence of structure a which is the expected protonation product of 4b.

Among the various aminomethylene derivatives of Meldrum's acid studied so far, the bis(alky1amino) derivatives show behaviour worthy of comment. FVP results's have been discussed in terms of facile 1'3-shifts of the alkylamino group in imidoylketenes intercon- verting imidoylketenes and acylketeneimines. Tandem mass spectrometry supports these proposals. Indeed, Meldrum's acid 7 pyrolyses cleanly at 400 "C by loss of acetone and COz. After isobutane chemical ionization, the product ions at rnlz 127 were submitted to CA (Fig. 5 ) and the observation of an intense structurally signifi- cant peak at mlz 72, [NMe,CO+], strongly supports the formation of the ketoketeneimine structure 8 (Scheme

Interestingly, at elevated temperatures (about 600 "C), the intermediate 8 decays by loss of dimethyl- amine. That is confirmed by the observation of peaks at mlz 82 and 46 (protonated dimethylamine) in the CI mass spectrum of the pyrolysate (Fig. 6(a)). The struc- ture of the mlz 82 ions can be deduced from the CA spectrum (Fig. 6(b)) showing a loss of CH; (to give rnlz 67), CO (to give mlz 54) and an intense charge strip- ping peak at mlz 41 (usually intense for cumulene ions). Structure c is thus tentatively proposed for these mlz 82 ions indicating that Meldrum's acids such as 8 are potential precursors of cumulog~ '~ of isocyanates, RN=C=[C=C]=O, molecules which to our know- ledge have not yet been described in the literature.2n Figure 6(c) which describes the CA spectrum of the mlz 46 ions clearly identifies protonated dimethyl- amine.

4) *

CONCLUSION The results of this paper are intended to show that chemical ionization can be applied to characterize pro- ducts formed by flash-vacuum pyrolysis. The results indeed show that CI can interestingly supplement EI to identify molecular ions in pyrolysis mixtures and also, with the help of tandem mass spectrometric methods, be used to infer preferential protonation sites of kineti- cally unstable molecules. The possibility of preparing unconventional reference ions by this method is also important in the general field of organic-ion gas-phase chemistry.

Acknowledgements We thank the 'Fonds National de la Recherche Scientifique' for its contribution in the acquisition of a new tandem mass spectrometer, VG AutoSpec6F. Y.G. thanks the IRSIA for the award of a Fellowship.

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