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charge separation mass spectrometry
International Journal of Mass Spectrometry and Zon Processes, 87 (1989) 135-140 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
135
Short Communication EXPERIMENTAL EVIDENCE FOR THE GENERATION OF CH,COH2+’ FROM CH,CO+. AN APPLICATION OF CHARGE STRIPPING/CHARGE SEPARATION MASS SPECTROMETRY
THOMAS DREWELLO
and HELMUT
SCHWARZ
Znstitut ftir Organische Chemie, Technische Universittit Berlin, D-1000 Berlin I2 (F.R.G.) (First received 12 May 1988; in final form 31 May 1988)
Charge stripping mass spectrometry has turned out to be the method of choice for the generation of doubly charged species, a class of ions having remarkable properties [l]. The combination of energetical data obtained in the charge stripping experiment, i.e. the vertical ionization energy of the monocation (Q,, value), with the corresponding values obtained by ab initio MO calculations of the energy hypersurfaces of both the mono- and the di-cations, permitted the distinction of different dicationic structures [2]. However, the structure (i.e. connectivity) assignment based on the comparison of theoretical and experimental data was, in most of the cases published so far, quite indirect. This was in part due to the limitations of the double focussing instruments commonly employed in these kinds of study. After mass selection of the monocation of interest by B, the charge stripping process occurred in the field-free region between B, and E (see Fig. 1; B stands for magnetic and E for electric sector); further reactions of the generated dication, which might be indicative of its structure, could very often not be studied by using two-sector instruments. With the availability of triple-sector instruments like the ZAB-HF-3F mass spectrometer [3] this experimental limitation no longer exists as the dication is now allowed to undergo charge separation reactions in the third field-free region, i.e. between E and B,. Figure 1 shows the principle of the charge stripping/charge separation technique which, to our knowledge, was first introduced by Beynon and co-workers [4] to probe the unimolecular reactions of CHi+. In this report, we describe charge stripping/charge separation experiments of C2H30+ ions derived from electron impact ionization of acetone; the study is related to earlier work on the corresponding C2H,02+‘dications [5]. In this study it was found that the Qmin values for the dication formation from CH,CO+ and CH2COH+ were 18.8 and 18.7 eV, respec0168-1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
136
2. ffr:
[AS+
+ 02-AB2+.
+ 02 + e-1
charge stripping
3. ffr:
[AB2+.-
A+ + S+.]
charge separation
Fig. 1. Schematic for charge stripping/charge separation experiments performed on a Vacuum Generator ZAB-HF-3F mass spectrometer [3].
tively. The vertical ionization energies of the respective ions were calculated to be 22.6 eV for CH,CO+ and 17.9 eV for CH,COH+ and from the comparison of the data, it was concluded that only C,H,02+’ dications of the CHJOH*+ structure have been generated. This result that the “ vertical” removal of an electron by charge stripping is accompanied or followed by a structural reorganization, i.e. CH,CO+ + CH&-OH2+‘+
e-
has also been observed for other systems. A particularly interesting one is that of ylid ions and their isomers of conventional structure. Both theory and experiment indicate that, irrespective of the structure of the mono-cationic species, only the energetically most favoured dications were formed in the charge stripping processes [6]. The same holds true for the C2H302+’ system as, according to the ab initio results, the global minimum of the dicationic energy hypersurface corresponds to CH,C-0H2? In order to get further experimental support for the assignment of the CZH302+’ structure, the charge stripping/charge separation technique was applied to the C,H,O+ cations generated from acetone in the ion source (electron energy: 70 eV; acceleration voltage: 8 kV). Figure 2(a) shows the resulting spectrum when B, was used for the selection of C2H30+, the electric sector E for the selection of C2H302+; which was generated by charge stripping in the second field-free region (oxygen pressure: 10e5
137
100
(b)
a
e
3 ._ : 2 ? 50 QI .> c 9 iit
Fig. 2. Charge stripping/charge
separation mass spectra of (a) CH$O+
and (b) CD$O+.
mbar), and B, was scanned to record all ions of higher mass-to-charge ratio than C,H,02+, from which they were derived. The signal m/z 43 represents the charge exchange process C,H,02+‘+ C2H30+; m/z 42 and 41 are due to fragmentations caused by either unimolecular or collision-induced reactions due to residual oxygen of the C2H30+ ion thus formed. The peak at m/z 42 is remarkably broad, pointing to the fact that a second process is likely to be involved, most
138
probably the charge separation reaction of C2H302+’ to generate C2H20+’ and H+. The presence of both m/z 43 and 41 signals are, of course, not structure-indicative for the dication of interest. However, in the lower m/z region, dished-topped signals are observed, centered at m/z 29 and (not shown in the spectrum) a weak signal at m/z 14. This points to the fact that the C,H,O*+’ dication undergoes a charge-separation reaction into CH;’ (m/z 14) and COH+ (m/z 29). F or sensitivity reasons, all focussing slits were kept fully open, causing possible interferences from isobaric ions. In order to make sure that the charge separation signal m/z 29 [Fig. 2(a)] is due to pure C2H30+/*+’ ions, ’ the experiment was repeated with acetone-d,. Figure 2(b) represents the corresponding spectrum for the charge stripping/ charge separation processes of C2D30+, leaving no doubt that the observed signals in Fig. 2(a) are indeed due to the C2H30+/*+ system, as all signals in Fig. 2(b) exhibit the expected mass shifts. Therefore, it can be safely concluded that the C,H,O*+’ dications generated from the corresponding cation, using acetone as precursor, correspond to CH,C-OH*+’ and not to the acetyl structure (CH,CO*+). On the other hand, the mono-cationic species used in this study are, as stated in the extensive studies of Burgers et al. [7], > 99% of acetyl structure and < 1% of 1-hydroxyvinyl structure. This conclusion is based on unimolecular reactions and on the charge stripping characteristics of C2H30+ cations generated from a variety of precursor molecules. Our present result shows that the charge stripping process of CH,CO + ion is accompanied or followed by isomerization to the I-hydroxyuinyl structure (Scheme l), which has to occur at an energetic state close to that of the CH,C-OH*+‘geometry. If isomerization occurred in the monocationic state, one would expect a Qmin value in the region of 13.9 eV, which is not the case. CHZ-COH
2*’
chargereparatio~
CH2
l
’ +
COH
l
Scheme 1.
Efforts to perform the charge stripping/charge separation experiment on pure CH2C-OH+ ions, generated according to the procedure described by Burgers et al. [7], using OH’ loss from enolized CH,COOD+’ (Scheme 2) failed on sensitivity grounds. The small amount of dications generated by charge stripping did not permit a successful charge separation experiment due to the poor signal-to-noise ratio *.
* In addition, charge stripping of CH,-&C, the fourth known C,H30+ isomer [8], has so far proved to be unsuccessful as no charge stripping signal around E/2 was observed using oxygen (lo-’ mbar) as target gas [9].
139
0
CHs-C
“a,-
1
+
+. OH CH2 =C
Scheme 2.
/ \
-
OD
-OH
+ CH2=COD
In conclusion, the present result demonstrates that mass-selected CH,CO+ cations produce upon charge stripping, CH,COH2+’ dications. This result supports earlier conclusions. Moreover, charge stripping/charge separation mass spectrometry turns out to be a useful tool to probe further the dication structure, as the decomposition reactions of a dication may provide new insight into its geometry. These findings, combined with Qmin determinations and ab initio MO calculations, may aid in the structure assignment of other organic dications, the actual connectivity of which may be ambiguous. ACKNOWLEDGEMENTS
We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, and the Gesellschaft von Freunden der Technischen Universitat Berlin. REFERENCES 1 (a) W. Koch, F. Maquin, D. Stahl and H. Schwarz, Chimia, 39 (1985) 376. (b) W. Koch and H. Schwarz, in S.G. Lias and P. Ausloos (Eds.), Structure/Reactivity and Thermochemistry of Ions, Reidel, Dordrecht, 1987. (c) T. Ast, Adv. Mass Spectrom., 10A (1986) 471. (d) L. Radom, P.M.W. Gill, M.W. Wong and R.H. Nobes, Pure Appl. Chem., 60 (1988) 183. 2 (a) T. Drewello, W. Koch, C.B. Lebrilla, D. Stahl and H. Schwarz, J. Am. Chem. Sot., 109 (1987) 2922. (b) T. Drewello, C.B. Lebrilla, H. Schwarz and D. Stahl, Int. J. Mass Spectrom. Ion Processes, 77 (1987) R3. 3 VG Analytical Ltd., Wythenshawe, Manchester M23 9LE, Gt. Britain. 4 M.RabrenoviC, A.G. Brenton and J.H. Beynon, Int. J. Mass Spectrom. Ion Phys., 52 (1983) 175. 5 W. Koch, H. Schwarz, F. Maquin and D. Stahl, Int. J. Mass Spectrom. Ion Processes, 67 (1985) 171. 6 (a) W.J. Bouma and L. Radom, J. Am. Chem. Sot., 105 (1983) 5484. (b) F. Maquin, D. Stahl, A. Sawaryn, P. v. R. Schleyer, W. Koch, G. Frenking and H. Schwarz, J. Chem. Sot. Chem. Commun., (1984) 504. (c) L. Radom, W.J. Bouma, R.H. Nobes and B.F. Yates, Pure Appl. Chem., 56 (1984) 1831. (d) S. Hammerum, Mass Spectrom. Rev., 7 (1988) 123. 7 (a) P.C. Burgers, J.L. Holmes, J.E. Szulejko, A.A. Mommers and J.K. Terlouw, Org. Mass Spectrom., 18 (1983) 254. (b) F. Tureiiek and F.W. McLafferty, Org. Mass Spectrom., 18
140 (1983) 608. (c) R.H. Nobes, W.J. Bouma and L. Radom, J. Am. Chem. Sot., 105 (1983) 309. (d) R.H. Nobes and L. Radom, Org. Mass Spectrom., 21 (1986) 407. 8 B. van Baar, P.C. Burgers, J.K. Terlouw and H. Schwarz, J. Chem. Sot. Chem. Commun., (1986) 1607. 9 T. Drewello, C.B. Lebrilla and H. Schwarz, unpublished results.
135
Short Communication EXPERIMENTAL EVIDENCE FOR THE GENERATION OF CH,COH2+’ FROM CH,CO+. AN APPLICATION OF CHARGE STRIPPING/CHARGE SEPARATION MASS SPECTROMETRY
THOMAS DREWELLO
and HELMUT
SCHWARZ
Znstitut ftir Organische Chemie, Technische Universittit Berlin, D-1000 Berlin I2 (F.R.G.) (First received 12 May 1988; in final form 31 May 1988)
Charge stripping mass spectrometry has turned out to be the method of choice for the generation of doubly charged species, a class of ions having remarkable properties [l]. The combination of energetical data obtained in the charge stripping experiment, i.e. the vertical ionization energy of the monocation (Q,, value), with the corresponding values obtained by ab initio MO calculations of the energy hypersurfaces of both the mono- and the di-cations, permitted the distinction of different dicationic structures [2]. However, the structure (i.e. connectivity) assignment based on the comparison of theoretical and experimental data was, in most of the cases published so far, quite indirect. This was in part due to the limitations of the double focussing instruments commonly employed in these kinds of study. After mass selection of the monocation of interest by B, the charge stripping process occurred in the field-free region between B, and E (see Fig. 1; B stands for magnetic and E for electric sector); further reactions of the generated dication, which might be indicative of its structure, could very often not be studied by using two-sector instruments. With the availability of triple-sector instruments like the ZAB-HF-3F mass spectrometer [3] this experimental limitation no longer exists as the dication is now allowed to undergo charge separation reactions in the third field-free region, i.e. between E and B,. Figure 1 shows the principle of the charge stripping/charge separation technique which, to our knowledge, was first introduced by Beynon and co-workers [4] to probe the unimolecular reactions of CHi+. In this report, we describe charge stripping/charge separation experiments of C2H30+ ions derived from electron impact ionization of acetone; the study is related to earlier work on the corresponding C2H,02+‘dications [5]. In this study it was found that the Qmin values for the dication formation from CH,CO+ and CH2COH+ were 18.8 and 18.7 eV, respec0168-1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
136
2. ffr:
[AS+
+ 02-AB2+.
+ 02 + e-1
charge stripping
3. ffr:
[AB2+.-
A+ + S+.]
charge separation
Fig. 1. Schematic for charge stripping/charge separation experiments performed on a Vacuum Generator ZAB-HF-3F mass spectrometer [3].
tively. The vertical ionization energies of the respective ions were calculated to be 22.6 eV for CH,CO+ and 17.9 eV for CH,COH+ and from the comparison of the data, it was concluded that only C,H,02+’ dications of the CHJOH*+ structure have been generated. This result that the “ vertical” removal of an electron by charge stripping is accompanied or followed by a structural reorganization, i.e. CH,CO+ + CH&-OH2+‘+
e-
has also been observed for other systems. A particularly interesting one is that of ylid ions and their isomers of conventional structure. Both theory and experiment indicate that, irrespective of the structure of the mono-cationic species, only the energetically most favoured dications were formed in the charge stripping processes [6]. The same holds true for the C2H302+’ system as, according to the ab initio results, the global minimum of the dicationic energy hypersurface corresponds to CH,C-0H2? In order to get further experimental support for the assignment of the CZH302+’ structure, the charge stripping/charge separation technique was applied to the C,H,O+ cations generated from acetone in the ion source (electron energy: 70 eV; acceleration voltage: 8 kV). Figure 2(a) shows the resulting spectrum when B, was used for the selection of C2H30+, the electric sector E for the selection of C2H302+; which was generated by charge stripping in the second field-free region (oxygen pressure: 10e5
137
100
(b)
a
e
3 ._ : 2 ? 50 QI .> c 9 iit
Fig. 2. Charge stripping/charge
separation mass spectra of (a) CH$O+
and (b) CD$O+.
mbar), and B, was scanned to record all ions of higher mass-to-charge ratio than C,H,02+, from which they were derived. The signal m/z 43 represents the charge exchange process C,H,02+‘+ C2H30+; m/z 42 and 41 are due to fragmentations caused by either unimolecular or collision-induced reactions due to residual oxygen of the C2H30+ ion thus formed. The peak at m/z 42 is remarkably broad, pointing to the fact that a second process is likely to be involved, most
138
probably the charge separation reaction of C2H302+’ to generate C2H20+’ and H+. The presence of both m/z 43 and 41 signals are, of course, not structure-indicative for the dication of interest. However, in the lower m/z region, dished-topped signals are observed, centered at m/z 29 and (not shown in the spectrum) a weak signal at m/z 14. This points to the fact that the C,H,O*+’ dication undergoes a charge-separation reaction into CH;’ (m/z 14) and COH+ (m/z 29). F or sensitivity reasons, all focussing slits were kept fully open, causing possible interferences from isobaric ions. In order to make sure that the charge separation signal m/z 29 [Fig. 2(a)] is due to pure C2H30+/*+’ ions, ’ the experiment was repeated with acetone-d,. Figure 2(b) represents the corresponding spectrum for the charge stripping/ charge separation processes of C2D30+, leaving no doubt that the observed signals in Fig. 2(a) are indeed due to the C2H30+/*+ system, as all signals in Fig. 2(b) exhibit the expected mass shifts. Therefore, it can be safely concluded that the C,H,O*+’ dications generated from the corresponding cation, using acetone as precursor, correspond to CH,C-OH*+’ and not to the acetyl structure (CH,CO*+). On the other hand, the mono-cationic species used in this study are, as stated in the extensive studies of Burgers et al. [7], > 99% of acetyl structure and < 1% of 1-hydroxyvinyl structure. This conclusion is based on unimolecular reactions and on the charge stripping characteristics of C2H30+ cations generated from a variety of precursor molecules. Our present result shows that the charge stripping process of CH,CO + ion is accompanied or followed by isomerization to the I-hydroxyuinyl structure (Scheme l), which has to occur at an energetic state close to that of the CH,C-OH*+‘geometry. If isomerization occurred in the monocationic state, one would expect a Qmin value in the region of 13.9 eV, which is not the case. CHZ-COH
2*’
chargereparatio~
CH2
l
’ +
COH
l
Scheme 1.
Efforts to perform the charge stripping/charge separation experiment on pure CH2C-OH+ ions, generated according to the procedure described by Burgers et al. [7], using OH’ loss from enolized CH,COOD+’ (Scheme 2) failed on sensitivity grounds. The small amount of dications generated by charge stripping did not permit a successful charge separation experiment due to the poor signal-to-noise ratio *.
* In addition, charge stripping of CH,-&C, the fourth known C,H30+ isomer [8], has so far proved to be unsuccessful as no charge stripping signal around E/2 was observed using oxygen (lo-’ mbar) as target gas [9].
139
0
CHs-C
“a,-
1
+
+. OH CH2 =C
Scheme 2.
/ \
-
OD
-OH
+ CH2=COD
In conclusion, the present result demonstrates that mass-selected CH,CO+ cations produce upon charge stripping, CH,COH2+’ dications. This result supports earlier conclusions. Moreover, charge stripping/charge separation mass spectrometry turns out to be a useful tool to probe further the dication structure, as the decomposition reactions of a dication may provide new insight into its geometry. These findings, combined with Qmin determinations and ab initio MO calculations, may aid in the structure assignment of other organic dications, the actual connectivity of which may be ambiguous. ACKNOWLEDGEMENTS
We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, and the Gesellschaft von Freunden der Technischen Universitat Berlin. REFERENCES 1 (a) W. Koch, F. Maquin, D. Stahl and H. Schwarz, Chimia, 39 (1985) 376. (b) W. Koch and H. Schwarz, in S.G. Lias and P. Ausloos (Eds.), Structure/Reactivity and Thermochemistry of Ions, Reidel, Dordrecht, 1987. (c) T. Ast, Adv. Mass Spectrom., 10A (1986) 471. (d) L. Radom, P.M.W. Gill, M.W. Wong and R.H. Nobes, Pure Appl. Chem., 60 (1988) 183. 2 (a) T. Drewello, W. Koch, C.B. Lebrilla, D. Stahl and H. Schwarz, J. Am. Chem. Sot., 109 (1987) 2922. (b) T. Drewello, C.B. Lebrilla, H. Schwarz and D. Stahl, Int. J. Mass Spectrom. Ion Processes, 77 (1987) R3. 3 VG Analytical Ltd., Wythenshawe, Manchester M23 9LE, Gt. Britain. 4 M.RabrenoviC, A.G. Brenton and J.H. Beynon, Int. J. Mass Spectrom. Ion Phys., 52 (1983) 175. 5 W. Koch, H. Schwarz, F. Maquin and D. Stahl, Int. J. Mass Spectrom. Ion Processes, 67 (1985) 171. 6 (a) W.J. Bouma and L. Radom, J. Am. Chem. Sot., 105 (1983) 5484. (b) F. Maquin, D. Stahl, A. Sawaryn, P. v. R. Schleyer, W. Koch, G. Frenking and H. Schwarz, J. Chem. Sot. Chem. Commun., (1984) 504. (c) L. Radom, W.J. Bouma, R.H. Nobes and B.F. Yates, Pure Appl. Chem., 56 (1984) 1831. (d) S. Hammerum, Mass Spectrom. Rev., 7 (1988) 123. 7 (a) P.C. Burgers, J.L. Holmes, J.E. Szulejko, A.A. Mommers and J.K. Terlouw, Org. Mass Spectrom., 18 (1983) 254. (b) F. Tureiiek and F.W. McLafferty, Org. Mass Spectrom., 18
140 (1983) 608. (c) R.H. Nobes, W.J. Bouma and L. Radom, J. Am. Chem. Sot., 105 (1983) 309. (d) R.H. Nobes and L. Radom, Org. Mass Spectrom., 21 (1986) 407. 8 B. van Baar, P.C. Burgers, J.K. Terlouw and H. Schwarz, J. Chem. Sot. Chem. Commun., (1986) 1607. 9 T. Drewello, C.B. Lebrilla and H. Schwarz, unpublished results.