ELECTRON IMPACT STUDIES By J. H. BOWIE~ and T. K. ...

ELECTRON IMPACT STUDIES LV1.T THE FLUORENYL AND PHENALENYL CATIONS. THE APPLICATION OF METASTABLE CHARACTERISTICS

By J. H. B O W I Eand ~ T. K. BEADSHAWS [Manuscript received February 20, 19701 Abstract A study of the abundances of certain metastable peaks formed by the decomposition of ions in the first and second field-free regions of a double-focusing mass spectrometer has shown that the M- 1 ions (mle 165) in the mass spectra of fluorene and phenalene have different structures. Correlations based on these structures have been obtained for C13H9+ species formed from different systems.

We have previously studied the formation of C13Hgf (mle 165) in the spectra of a variety of aromatic and heterocyclic species (see Table 1, p. 1437, for references). This ion has been represented as the fluorenyl cation ( a ) ,but we have notedl-3 that more extensive rearrangement to the phenalenyl cation ( b ) may occur. The purpose of this study was to determine whether the two ions have the same or different structures, and to attempt to correlate these structure(s) with CI3Hg+ fragments produced by various mechanisms from other species.

The mass spectra (Figs. 1 and 2) of fluorene (1) and phenalene (2) are similar but not identical. The major difference is that the M/M-1 ratios for fluorene and phenalene are 1 : 0 ~ 9 and 5 0.48 : 1 respectively at 70 eV. The spectra of both [9,9-D2]fluorene (3) and [l,l-Dz]phenalene (4) show M-1 and M-2 peaks in the ratios (corrected for 13C isotopes and for the small amount of the Dl component in (3)) 3.7 : 1 and 3.8 : 1 (calculated 4 : 1) respectively at 25 eV. These ratios are consistent

t Part LV, Org. M a s s Spectrom., in press. $ Department of Organic Chemistry, University of Adelaide, P.O. Box 498D, Adelaide, S.A. 5001. 1 Bowie, J. H., Donaghue, P. F., Rodda, H. J., Cooks, R. G., and Williams, D. H., Org. M a s s Spectrom., 1968, 1, 13. 2 Donaghue, P. F., White, P . Y., Bowie, J. H., Roney, B. D., and Rodda, H. J., Org. Mass Spectrom., 1969, 2, 1016. 3 Nussey, B., and Bowie, J. H., Org. M a s s Spectrom., in press. Aust. J . Chern., 1970, 23, 1431-7

J. H. BOWIE AND T. K. BRADSHAW Fig. 1

Fig. 2

with complete hydrogen scrambling ( C ~ . ~ in I ~both ) molecular ions prior to elimination of a hydrogen radical. There is only one "metastable ion"T in both spectra formed from the de-

?. I n order to avoid the confusion involving the use of the term metastable ion to describe those ions produced in the two field-free regions of a double-focusing mass spectrometer, we will use metastable ion for an ion formed in the first, and "metastable ion" for an ion formed in the second field-free region of the mass spectrometer. Two recent articles which consider aspects of metastable ions are available.677 4 Grubb, H. M., and Meyerson, S., in "Mass Spectrometry of Organic Ions." (Ed. F. W. McLafferty.) Ch. 10. (Academic Press: New York 1963.) 5 Howe, I., Williams, D. H., and Cooks, R. G., Org. Mass Spectrom., 1969, 2, 137. 6 Tajima, E., and Seibl, J., J. Mass Spectrom. Iolz Phys., 1969, 3, 245. 7 Beynon, J. H., in "Advances in Mass Spectrometry." (Ed. E . Kendrick.) Vol. 4, p. 123. (Institute of Petroleum and Elsevier: London 1968.)

ELECTRON IMPACT STUDIES. LVI

1433

composition of mle 165 in the second field-free region of the mass spectrometer. This is formed by the process C13H9+-+ CllH7++C2H2 (m/e 165 -+ 139, m* at mle 117.1). The ratios of the relative abundances of mle 139 : mle 117.1 show no correlation between 70 and 25 eV for (1) and (2). Ions of identical structure and energy must fragment in an identical manner,538 but in this case the lack of correlation of the ratios indicates only that the structures and/or energy distributions of the two ions are different. A "metastable ion" is also observed for the decomposition CI3Hg2+-+ CllH72++C2H2 (mle 82.5 -t 69.5, m* a t mle 58.5). Plots (Fig. 3) of the ratios of mle 69.5/58.5 a t different beam energies, and a t different source pressures, show remarkable correspondence, and it is possible that the two doubly charged species may have the same structure. Fig. 3

lo6 x source pressure (torr) of (mle 5 8 . 5 ) / ( m / e6 9 . 5 ) against source pressure. 0 For compound ( 1 ) ; compound ( 2 ) . Fig. 4.-Values of ml* and mz* for ( a ) compound ( 1 ) ; (b)/compound ( 2 ) .

Fig. 3.-Plot

for

A much superior method for comparing metastable characteristics is to utilize decompositions which occur between the source and electric sector (the first field-free region) of the mass spectrometer. The "metastable defocusing" techniqueg-14 allows the accurate detection of metastable ions, gives very sharp signals, and enables the unequivocal determination of the decomposition which produces the metastable ion. Our method is based on that described by Struck and Afajor,l4 and is briefly described in the Experimental section. The decompositions of mle 165 [from (1) and (2)] in the first field-free region of the mass spectrometer produce three metastable ions, and these are summarized in Bursey, M. M., and McLafferty, F. W., J. A m . chem. Sac., 1966, 88, 529, and subsequent papers in this series. 9 Barber, M., and Elliott, R. M., 12th Annual Conference on Mass Spectrometry and Allied Topics, Committee E 14, ASTM, Montreal, 1964. 10 Jennings, K . R., J. chem. Phys., 1965, 43, 4176. 11 Futtrell, J. H . , Ryan, K. R., and Sieck, I,. W., J. chem. Phys., 1965, 43, 1832. 1 2 Jennings, K . R . , Chem. Commulz., 1966, 283. 13 Barber, M . , Wolstenholme, W. A., and Jennings, K. R., Nature, 1967, 43, 664. 14 Struck, A. H., and Major, H. W., Jr., Paper presented a t ASTM E 14 Meeting, May 18-23, 1969, Dallas, Texas. 8

J. H. BOWIE AND T. K. BRADSHAW

1434

Scheme 1. The metastable ions resulting from the loss of acetylene (ml*) and C4H2 (mz*) are of comparable intensity, but that for the loss of bemyne (m3*) is much smaller. We chose to use the parameter m2*/ml* (cf. refs.5Al5) to compare the species C13H9+produced from fluorene (I), ~henalene(2),and the compounds (5)-(21). The results are listed in Table 1. These values (see also Pig. 4) show that the two ions C13H9+from fluorene and phenalene do not have the same properties. Although the actual structures of these ions cannot be determined, they will subsequently be represented as a and b respectively.

Scheme 1.-Metastable ions from the decompositions of mle 165. (For C11H7+,E/Eo= 1391165, ml* = 13921165, etc.)

The ions at mle 165 in the spectra of 9-methylfluorene, diphenylmethane, bromodiphenylmethane, stilbene, and 9,lO-dihydrophenanthrenecorrespond to the fluorenyl cation ( a ) . I t is gratifying that a is produced from both stilbene and 9,lOdihydrophenanthrene, as we have previously suggested2 that the stilbene and 9,lOdihydrophenanthrene molecular ions rearrange to a common ion prior to the formation of a. I t has been suggestedl6J7 that the intermediacy of a 9-methylfluorene radical ion may account for the rearrangement. This is certainly possible, but the formation of any such ion is preceded by carbon and hydrogen scrambling.2

G., and Williams, D. H., Chem. Commuw,., 1969, 784. R. A. W., and Millard, B. J., 2. Naturf. (A), 1966, 21, 604; Johnstone, R. A. W., and Ward, S. D., J. chem. Soc. (C), 1968, 1805. 1 7 Johnstone, R. A. W., and Ward, S. D., J. chem. Soc. (C), 1968, 2540.

15 Cole, W.

18 Johnstone,


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The production of both a and b from different heterocyclic systems was unexpected, but the results are consistent for particular series of compounds. Both diphenyloxazoles and diphenylisoxazoles give abundant ions a t mle 165. I t has been proposedl8-21 that the fragmentations of isoxazoles proceed through acylazirine intermediates, and we suggested18 that the proposed isoxazole -+oxazole interconversion19 (e.g. c + e), which has photochemical analogies,22 does not account for the formation of mle 165 in the spectra of diphenylisoxazoles. The present study supports this proposal, as the rearrangement ions in the spectra of 2,5-diphenyloxazole (10) and 3,5-diphenylisoxazole (18) correspond to a and b respectively. This seems to be general for such compounds, i.e. [diphenyloxazoles]+ + a , and [diphenylisoxazoles]f -+ b. Two isoxazoles which do not give a or b specifically are 4,5diphenylisoxazole (21) and triphenylisoxazole (20). The values of mzw/ml*for (20) and (21) do not correspond to those expected for a or b, but they are very similar to each other (see Table 1). This is an interesting observation, as the formation of mle 165 from (20) involves mainly the 4- and 5-phenyl groups.18 I n these two cases rnle 165 may either correspond to a mixture of a and b, each produced by a specific mechanism, or the ion may have a different structure altogether.

Deuterium labelling18323 has shown that the formation of mle 165 from the 4,5-diphenylimidazole and 3,4-diphenylpyrazole molecular ions involves a hydrogen transfer from a phenyl ring to nitrogen followed by transfer of either hydrogen or Simons, B. K., Kallury, R. K. M. R., and Bowie, J. H., Org. MassSpectrom., 1969, 2, 739. H., Sakurai, H., Yoshizumi, H., Tatematsu, A., Org. MassSpectrom., 1968,1, 199. 20 Ohashi, M., Hamachi, H., Tatematsu, A,, Yoshizumi, H., and Nakata, H., Tetrahedron Lett., 1968, 379. 21 Bowie, J. H., Kallury, R. K. M. R., and Cooks, R. G., Aust. J. Chem., 1969, 22, 563. 22 Singh, B., and Ullman, E . F., J. A m . chem. Soc., 1967, 89, 6911. 23Bowi0, J. H., Donaghue, P. F., Rodda, H. J., and Simons, B. K., Tetrahedron, 1968, 24, 3965. 18

19 Nakata,

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J. H. BOWIE AND T. K. BRADSHAW

nitrogen back to the phenyl groups. We assumedlsJ3 that the initial hydrogen transfer allowed cyclization and that the second transfer ultimately formed f from (16) and g from (17). These species could then eliminate C2H3N2. to form a. The ion formed by the rearrangements has now been shown to be 6, and although we still favour a cyclization reaction, such a reaction must be more complex than that represented by f and g. We were previously unable to decide whether the rearrangement which occurs for 4,5-diphenyloxazole (11) parallelled that of 4,5-diphenylimidazole (16).23 The present data suggest that two different pathways are involved, as a is formed from ( l l ) , while b is formed from (16). Certain diphenyl-oxazoles, -isoxazoles, and -pyrazoles are now being labelled with 13C in order to further investigate these complex rearrangements, and the results will be reported in due course.

The elimination of water from the benzyl phenyl ketoxime molecular ion produces an ion which has the properties of the "2,3-diphenj lazirine" molecular ion (h).24 This ion eliminates H2CK. to produce mle 165. Deuterium and 13C labelling demonstrates3 that the carbon involved in this elimination originates equally from each of the two azirine ring carbons, and that the azirine hydrogen is largely lost during the decomposition, with the remaining hydrogen(s) coming randomly from the whole molecule. On the assumption that a was the rearrangement ion, we proposed an ion i to account for the transformation. As the ion from both (14) and (15) is now shown to correspond to b, a much more complex cyclization must occur.

All metastable peaks were measured with an Hitachi Perkin-Elmer RMU 6D doublefocusing mass spectrometer modified? to incorporate a metastable defocusing device.14 If we consider a process A -t B occurring in both the ion source and the field-free region between the ion-source and the electric sector, then the mass of B formed in the first field-free region equals that formed in the ion source. The velocity of the ion formed in the field-free region is much less than that formed in the source, and under normal operating conditions the former ion will be

t The defocusing device was designed and constructed by Mr R. Dowdell, 11 George St., Vale Park, S.A. 5081. 24 Bowie, J. H., Simons, B. K., Donaghue, P. F., and Kallury, R. K. M. R., Tetrahedron, 1969, 25, 3969.


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unable to pass the electric sector. If the accelerating potential is kept constant, and the electric sector voltage is lowered to a particular value, then the metastable ion may pass through the electric sector. The ratio of the lower voltage to the normal voltage which allows passage of the ion through the sector (EIEo) is equal to m ~ l m * . For our measurements, the value of m ~ / m *is preset using a five-figure digital voltmeter, and then a magnetic scan is carried out. The peak then occurs a t the same m l e value as the "metastable peak" for this transition in a normal spectrum, i.e. m* = m B 2 / m ~and E/EO = mB/m* = m * / m ~ . For more detailed accounts of this technique, see refs.6~14

Each value is an average of ten measurements and is correct to within &2%. The values are reproducible and were unaltered when remeasured two weeks after the initial measurements were made I

Compound Fluorene (1) 9-Methylfluorene (5) Diphenylmethane (6) Bromodiphenylmethane (7) Stilbene (8) 9,lO-Dihydrophenanthrene(9) 2,5-Diphenyloxazole (10) 4,5-Diphenyloxazole (11) 2,5-Diphenyl-1,3,4oxadiazole (12) 4,5-Diphenylthiazole (13)

Ref.

70 eV

35 eV

1.15 1.14 1.13 1.I5 1.13 1.15 1.13 1.14

1.11

2 16,25-27 16,25-27 2,16,17 2,28,29 23,30 23,30

1. I 3 1.12 1.I2 1.I2 1.11 1. I 1 1.12

31 23

1.12 1.14

1.11 1.12

I

Compound phenalene (2) 2,3-diphenylazirine (14) benzyl phenyl ketoxime (15) 4,5-diphenylimidazole (16) 3,4-diphenylpyrazole (17) 3,5-diphenylisoxazole (18) 3,4-diphenylisoxazole (19) triphenylisoxazole (20) 4,5-diphenylisoxazole (21)

Ref.

70 eV

0.81 3,24 0.80 3,24 0.79 23 0.81 18,32 0.80 18J9 0.82 18 0.83 18 0.98 18 1.02

35 eV 0.82 0.80 0.81 0.82 0.80 0.83 0.83 1.00& 1.04

The intensities of the metastable peaks in these two spectra decreased from 70 to 35 eV. In all other cases the intensity of each metastable ion is greater at 35 eV than at 70 eV.

Compounds (I), (2), and (5)-(21) were available from previous studies (see Table 1). [9,9-DzIFluorene (3) was produced by treating fluorene with deuterium oxide a t 180' for one week (Dl = 11, D2 = 89%). [l,l-DzlPhenalene (4) was prepared in 12% yield by the reduction 33 of phenalen-1-one with lithium aluminium deuteride (Dz greater than 99%).

ACKNOWLEDGMENT The Hitachi Perkin-Elmer RMU 6D mass spectrometer was purchased with the aid of a grant from the Australian Research Grants Committee.

Williams, D. H., Ward, R. S., and Cooks, R. G., J. chem. Soc. ( B ) , 1968, 522. Meyerson, S., Hart, H., and Leitch, L. C., J. A m . chem. Soc., 1968, 90, 3419. 27 Bowie, J. H., Bradshaw, T. K., and White, P. Y., Chem. Commun., in press. 28 Dynesen, E., Lawesson, S.-O., Schroll, G., Bowie, J. H., and Cooks, R. G., A r k . K e m i , 1967, 26, 379. z9 Maquestiau, A., Haverbeke, Y. van, Delalieu, F., B u l l . Soc. chim. Belg., 1968, 77, 355; 1969, 78, 589. 30 Crow, W. D., Hodgkin, J. H., and Shannon, J. S., Aust. J. Chem., 1965, 18, 1433. 31 Cotter, J. L., J. chem. Soo., 1964, 5491. 32 Krasnoshchek, A. P., Khmel'nitskii, R. A., Polyakova, A. E., and Grandberg, A. A., Z h . org. K h i m . , 1968, 4, 689. 33 Boekelheide, V., and Larrabee, C. L., J. A m . ohem. Soc., 1950, 72, 1245. 25 26