multiply charged ions in the mass spectra of aromatics

Organic Mass Spectrometry, 1973, Vol. 7, pp. 177 to 183. Heyden & Son Limited. Printed in Northern Ireland

MULTIPLY CHARGED IONS IN THE MASS SPECTRA OF AROMATICS

ROBERT ENGEL, DONALD HALPERN and BETTY-ANNE FUNK Department of Chemistry, Queens College of The City University of New York,

Flushing, NY 11367, USA

(Received 5 May 1972; accepted (revised) 19 July 1972)

Abstract-The prominence of multiply charged molecular and fragment ions upon electron-impact in the mass spectrometer is proposed as an experimental, empirical indication of aromatic character. The effects of electron withdrawing and donating substituents on the production of multiply charged ions are considered and appearance potentials are noted for several species.

. THE PRESENCE of multiply charged molecular and fragment ions in the mass spectra of many aromatic compounds has always been a common observation. Biemannl suggests that the removal of two electrons from a molecule may be aided if a region of high electron density is present. It is further noted that saturated aliphatic hydrocarbons have a very slight tendency to form dipositive ions, resulting not only from the lack of a region of high electron density, but also from the presence of other more favourable processes.

Dipositive molecular and fragment ions for benzene were investigated at a rela- tively early stage of mass spectrometry.2 The species under investigation could be distinguished from a unipositive fragment of nominal mass 39 using the natural abundance of 13C to good advantage; the dipositive ion of 13C12C5H6 was observed at m/e 39.5. Multiply charged molecular and fragment ions for benzene and naphthalene were observed in the mass spectra of the monodeuterated compound^,^ which allowed the dipositive ions to be observed at half-integral m/e values. The appearance potential for the dipositive molecular ion of benzene was determined as 27.2 f 1 eV by Field and Franklin.* Triply charged molecular ions have been noted in the mass spectra of a large number of aromatic compound^.^ In all cases the triply charged ions accounted for less than 0.2 % of the total ion production.

Wacks et aZ.6 have conducted a systematic study of a series of fused ring aromatic hydrocarbons. Appearance potentials were measured for both singly and doubly charged molecular ions. Natalis and Franklin7 published an intensive study of a few aromatic compounds, measuring appearance potentials for molecular and fragment ions. Doubly and triply charged molecular and fragment ions were observed but not investigated in detail. More recently extensive studies on the fragmentation and multiple ion production with a number of fused ring aromatics and polyphenyls have been reported.* Molecular ions with charges up to +3 constituted at least 35 % of the total ion production in many cases. Beynon et ~ 1 . ~ have shown that the dipositive molecular ion from benzene is probably an acyclic species, indicating carbon-carbon bond fission without fragmentation.

From the previous work it would seem possible to use the mass spectrum of a compound as an indicator in the evaluation of its aromatic character. Several other chemical and instrumental criteria have been used in evaluating the aromatic nature of classical and non-classical systems.1° to l5

111

178 R. ENGEL, D. HALPERN and BETTY-ANNE FUNK

In the present work we attempt to show the feasibility (and limitations) of using the production of multiply charged molecular and fragment ions under electron-impact as an empirical experimentally observable indicator for aromatic character.

RESULTS A N D DISCUSSION

Hydrocarbons All of the classical aromatic hydrocarbons examined exhibited doubly charged

molecular ions in significant abundance (greater than 2.5 % of the unipositive molecular ion). Moreover, in all of these cases doubly charged ions were observed accompanied by loss of one, two or more hydrogens from the parent molecule. The ratios of the abundance of dipositive to unipositive ions are given in Table 1, with the abundance as a percentage of the base peak. In all cases except toluene the parent peak was the base peak; for the d,-toluene the base peak was m/e 92, [M - 11. The appearance potentials of the dipositive species are also given. Dipositive fragment ions are also observed with the fused ring compounds; these occur principally from acetylene elision. Several of these ions are noted in Table 2 with dipositive/unipositive ratios and abundances as a percentage of the base peak. The proportions of dipositive to unipositive ions are significantly higher for the fragment species than for the molecular species.

TABLE 1

AP dipositive

[M + 11"/ [M - 1Izt/ [M - 212+/ molecular Compound [M + 1]+ [M]"/[M]+ [M - 1]+ [M - 2]+ ion'

dl-benzene 4-dl-toluene 4-d1-styrene 1 -dl-naphthalene biphenylene biphenyl fluorene anthracene 9-d1-phenanthrene trans-s tilbene fluoranthene pyrene triphenylene

0.141 (1.9%) 0.069 (0.9 %) 0.138 (2.0%) 0.080 (1.2%)

0.111 (1.7%) 0.177 (3.1 %) 0.236 (4.2%) 0.194 (3.8%)

0.081 (8.1%) - 0.176 (3.2%) 0.067 (6.2%) - 0.079 (9.5%) 0.025 (2.5%) - 0.167 (6.5%) 0.081 (8.1%) - 0.320 (7.1 %)

- 0.083 (2.7%) - - 0.095 (4.5%) - - 0.204 (17.8 %) - - 0.221 (2.8%) -

- 0.072 (6.4%) - - 0.292 (4.6%) - - 0.418 (8.7%) -

- 0.468 (7.8%) -

0.145 (14.5%) - 0.500 (15.4%)

26.8 26.5 26.8 28.1 -

- 26.9 -

a Mean value of at least three determinations with maximum deviation h0.2 eV.

TABLE 2

Parent compound Fragment Fragment2+/Fragment+

1-d,-naphthalene C B H D 0.545 (7.7%) biphenylene 13C12C,H, 1-000 (2.3%) biphenyl l3ClzC,H8 1.165 (1.6%) fluorene CiiH, 1.070 (12.9%) anthracene 13C12C1,H, 0.620 (5.2%) 9-dl-phenanthrene Cl,H,D 1.200 (15.4%)

Multiply charged ions in the mass spectra of aromatics 179

The spectra of several non-aromatic hydrocarbons were measured as a basis for comparison. Decahydronaphthalene exhibited no dipositive molecular or fragment ions. 1,3-Cyclooctadiene did not form a dipositive molecular ion, but a species of formula [13C12C,H,]2+ was detected at a low level, presumably representing either the aromatic cyclooctatetraene dication or a styrene system formed with elimination of four hydrogens from the parent molecule. It should be noted that this dipositive (molecular) ion was also observed in the mass spectrum of cyclooctatetraene, although the uncharged species is not aromatic.

d,-Cyclohexane did not exhibit a dipositive molecular ion although dipositive ions could be observed in very low abundance (less than 0.5% of the base peak) accom- panying the loss of two, four and six hydrogens from the parent molecule. This pattern was often noted in the mass spectra of heterocyclic systems. The dipositive molecular ion of isoprene, [13C12C,H5]2+, was present to an extent less than 0.5 % of its unipositive ion. Dipositive ions of abundance less than 1 % of the base peak were noted for the loss of one, three and five hydrogens.

Nitrogen heterocyclic systems A series of aromatic nitrogen heterocyclics was investigated. In all cases they

exhibited a significant abundance of dipositive molecular ions. The dipositive to unipositive ion abundance ratios are given in Table 3, with abundances also indicated as a percentage of the base (parent) peak. Appearance potentials are noted where available.

Quinoline exhibited dipositive ions of fragments left from acetylene elision, in a manner similar to the fused ring hydrocarbons; the C,H5N fragment yielded unipositive and dipositive ions with Fragment2+/Fragment+ of 1.82. Several compounds not considered aromatic were investigated for comparison. Pyrrolidine exhibited no dipositive molecular ion, although as with d,-cyclohexane, dipositive ions were noted with the loss of hydrogen. A similar observation was noted with piperidine, where dipositive ions resulting in loss of hydrogens from the parent molecule were quite significant. No dipositive molecular or fragment ions were noted for either 1- morpholinocyclohexene or triethylamine.

Substituted benzenes A series of substituted benzenes were examined (in addition to the hydrocarbons

mentioned above) in order to observe whether electronic factors affected the production of dipositive molecular ions. The introduction of electron withdrawing groups

TABLE 3

[M - 212+/ [M - 412+/ [M - 6Ic2/ AP dipositive Compound [M]'+/[M]+ [M - 2]+ [M - 4]+ [M - 61.'- Molecular ion'

pyrrole 0.185 (18.5%) 1.705 (5.0%) 0.152 (0.7%) - 28.2 pyridine 0.024 (2.4 %) 1.041 (2.6 %) - - 29-3 cr-picoline 0.067 (6.7%) 0.445 (2.7%) 2.80 (1.2%) 0-200 (06%) - y-picoline 0.020 (2.0%) 0.715 (4.2%) 1.60 (0.8%) 0.50 (0.6%) - 2,6-lutidine 0.045 (4.5 %) 2.28 (8.2 %) - quinolineb 0.431 (14.4%) 0.033 (2.1 %) -

- 29.3 - 27.6

* Mean value of at least three determinations with maximum deviation k0.2 eV. Base peak is [M - 11.


greatly suppressed or eliminated multiply charged molecular ion formation. Electron donating groups enhanced their production.

Multiply charged molecular ion production for p-bromoacetanilide, nitrobenzene, p-nitrotoluene, phthalic anhydride, p-toluenesulfonic acid and trinitrobenzene were below observable limits. For 4-dl-anisole the [MI2+/[M]+ ratio was 0-021 , a significant reduction when compared with benzene (0.081), although the appearance potential (26.0 eV) was close to that for benzene. On the other hand aniline shows an enhanced production of dipositive molecular ion with an [MI2+/[M]f ratio of 0.246. Nitrile substituents seem to be an exception to this general behavior; dipositive molecular

TABLE 4

Compound Fragment mass Fragment formula Fragment2 '-/Fragment+ Base peak

p-bromoacetanilide 91

phthalic anhydride 75

p-toluenesulfonic acid 91

nitrobenzene 77 75

73

89 87

0.243 (5.5%) 0.005 (0.5%) 0.101 (1.5%) 0.425 (21.7%) 0.111 (3.4%) 0.010 (1.0%) 0.031 (1.3%) 0.172 (1.1%)

170 77 77

104 104 91 91 91

ion formation is significant with benzonitrile, as well as with non-aromatic acetonitrile and acrylonitrile. Further consideration of this point will be made in a later report.

It should be noted that multiply charged fragment species are observed in many of the above mentioned molecules containing electron withdrawing groups, once the electron withdrawing substituent has been cleaved from the parent molecule. Several of these ions are listed in Table 4 with their [MI2+/[M]+ ratios and abundances given relative to their base peak.

Cyclopentadiene and cycloheptatriene These molecules provide a basis for some interesting speculations. Both exhibit

dipositive molecular ions of significant intensity. The [M + 1I2+/[M + 1]+ ratio for cycloheptatriene is 0.077 and the [MI2+/ [MI+ ratio for d,-cyclopentadiene is 0.069. Appearance potentials noted for the dipositive molecular ions were 32.8 eV and 28.6 eV for cycloheptatriene and d,-cyclopentadiene, respectively. These values are well within the range observed for classical aromatic hydrocarbons and provide an apparent anomaly. This observation is in agreement with the results of Dauben, Wilson and Laity,12 however, who investigated the diamagnetic susceptibility exaltations of a series of hydrocarbon molecules. They concluded that cycloheptatriene actually possesses aromatic character, a concept previously proposed.l6,l7 It was also rationalized that cyclopentadiene might possess cyclic delocalization in- volving the sigma electrons of the methylene C-H bonds.1sJ9*20 The present results support both of these concepts.

CONCLUSIONS

On the basis of the data presented here a new criterion is proposed for empirically evaluating the aromatic character of a molecular species. The tendency for a molecule


under electron-impact to yield multiply charged molecular ions preferentially to fragmentation is indicative of the presence of an extended region of high electron density, as would be associated with a molecule having a significant measure of aromatic character.

A prediction of this experimental criterion from a strict molecular orbital consideration might not be expected. Using an evaluation based only on molecular orbital energies one would be led to conclude possibly that multiply charged molecular ions would be most prominent in the mass spectra of molecules containing non- bonding orbitals (filled) rather than with aromatics. A detailed evaluation of this discrepancy is not the purpose of this work. It is significant that non-aromatic amines, ethers, alcohols, phosphates and halides do not show prominent percentages of multiply charged molecular ions. This initial investigation points out that this approach is not unequivocal for nitriles.

Another aspect in this vein which may be noted is the availability of additional routes of fragmentation for the non-aromatic molecules; as may be seen from the results with cyclic non-aromatics, this cannot be the complete answer. We expect to obtain further information on this point from work with other (novel) aromatics.21

As noted above, many common classical aromatic compounds (containing electron withdrawing substituents) do not give dipositive molecular ions. The converse, that some compounds whose mass spectra contain dipositive molecular ions need not be aromatic would also be expected and other examples (than acetonitrile and acrylonitrile) will be uncovered in time.

The most prominent cases of this appear to be molecules containing two functional groups, usually at great distances within the molecule (at opposite ends of a long chain). Here there is no destabilization from electrostatic interactions. These difunctional molecules are more prone to form dipositive fragment ions than dipositive molecular ions. Well documented examples of dipositive fragment ions are found in the mass spectra of compounds containing two functional groups which have been trimethyl ~ i l y l a t e d . ~ ~ . ~ ~ If only one of the functional groups has been trimethyl silylated, only unipositive fragment ions are noted.22

A final point of difficulty in the evaluation of aromatic character by the observation of dipositive ions is found with non-aromatic molecules such as cyclooctatetraene, which may lose two electrons to form a six electron dipositive molecular ion which would be predicted to be aromatic. When reviewing data concerning dipositive fragment ions one must consider all possible canonical forms; ring contractions of a substituted cyclic alkene may very well give fragments where one can hypothesize a cyclopropenium as part of a dipositive fragment.24 Moreover, caution is to be used in considering apparent dipositive molecular ions for symmetrical compounds with even mass numbers. A distinction between dipositive molecular ion and unipositive fragment may usually be made by the replacement of one hydrogen atom by a deuterium or by utilizing the natural abundance of 13C.25

This technique appears to serve as a reasonable addition to the experimental empirical criteria for aromatic character.

EXPERIMENTAL All mass spectra were measured using a Varian MAT CH-7 instrument with an ionizing potential

of 70 eV and a trap current of 60 PA. In all cases, except those noted below, the samples were introduced through the heated (200°C) vapor inlet line with a source temperature of 200°C. Solids


introduced in this manner were first dissolved in a minimum amount of a low molecular solvent (e.g. acetone, benzene) which would not interfere with observation of the compound spectrum.

Several compounds (anthracene, p-bromoacetanilide, p-cyanoaniline, phthalic anhydride, p-toluenesulfonic acid, and trinitrobenzene) were introduced using the solid sample probe, thereby vaporizing them directly within the source region.

For appearance potential determinations at least two standards were introduced with the sample. Values for the standards were taken from the work of Morrison.2E Potentials were noted and the instrument calibrated using a high impedance digital voltmeter (Ballantine). Appearance potentials were evaluated from the semi-logarithmic curves of ionization efficiency using the modified critical slope technique described by Morrison.20

4-d1-Anisole, d,-benzene, 4-d1-cumene, d,-cyclohexane, 1-&naphthalene, 9-dl-phenenthrene, 4-dl-styrene and 4-d1-toluene were prepared by deuterium oxide (99.8 atom % D) treatment of the Grignards formed from the corresponding bromides. Solids were purified by repeated sublimation ; liquids were purified by preparative scale gas-liquid-chromatography on a 20 x $ inch 20% SE-30 column. The nuclear magnetic resonance spectra of all compounds indicated greater than 95 % mono-deuterium incorporation.

d,-Cyclopentadiene was prepared by deuterium oxide treatment of the lithium salt formed on the addition of butyl lithium to an ether solution of cyclopentadiene. The pure product was isolated as above with preparative g.1.c.

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multiply charged ions in the mass spectra of aromatics

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