International Journal of Mass Spectrometry and Zon Processes, 86 (1988) 303-318

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 303

FRAGMENTATION REACTIONS OF THE SINGLY AND DOUBLY CHARGED MOLECULAR IONS OF DIMETHYLANISOLES *

ALEX. G. HARRISON, HAROLD CAMILLERI and ADAM W. McMAHON

Ontario Regional Ion Chemistry Laboratory, Department of Chemistry, University of Toronto, Toronto, Ont. MSS IA1 (Canada)

RAYMOND E. MARCH

Department of Chemistry, Trent University, Peterborough, Ont. K9J 7B8 (Canada)

(First received 4 March 1988; in final form 6 April 1988)

ABSTRACT

The major primary fragmentation reaction of the singly charged molecular ions of the dimethylanisoles involves elimination of a methyl radical. Deuterium labelling shows that, in contrast to the methylanisoles, this fragmentation reaction involves not only the methoxy methyl but also the methyl groups attached to the aromatic ring. The electron impact mass spectra permit limited distinction among the isomeric molecules. The charge exchange (2E) mass spectra of the doubly charged ions produced from the dimethylanisoles permit limited identification of individual isomers. These spectra also show that loss of CH, from the molecular dication is a minor process and involves only the methoxy methyl. The major doubly charged fragment ions observed are C,,Hi+ (n = 5-9), C,,Hi+ (n = 7,8) and C,H$+. The molecular dications of the methoxy-d, dimethylanisoles show two unimolecular charge separation reactions to form CsH,O+ +CD: and C,H,D,O+ +C2H4Dt. The kinetic energy releases associated with these charge separation reactions lead to calculated intercharge distances of 5.3 and 4.7 A, respectively, suggesting acyclic structures for the transition states leading to charge separation. However, the fragmentation reactions of the singly charged molecular ions produced by charge exchange of the doubly charged molecular ions suggest a cyclic structure for the stable molecular dications.

INTRODUCTION

The 70 eV electron impact mass spectra of the methoxybenzenes, anisole and the monomethylanisoles, show abundant fragment ions corresponding to loss of a methyl radical from the molecular ions [l-3]. Isotopic labelling has shown that the methyl group lost originates entirely from the methoxy

* Dedicated to Professor John H. Beynon on the occasion of his 65th birthday.

0168-1176/88/$03.50 0 1988 Elsevier Science Publishers B.V.

304

group, even for the methylanisoles [3]. At the same time the 70 eV mass spectra of the dimethylbenzenes (xylenes) show that the major fragmenta- tion reaction of the molecular ions is loss of a methyl group [4-61, although not necessarily one of the intact methyl groups present in the molecule. The mass spectra of the dimethylanisoles show (vide infra) that the dominant primary fragmentation reaction of the molecular ions is loss of a methyl group. The dimethylanisoles contain not only the methoxybenzene function- ality but also the xylene functionality and it is of interest to explore which functionality determines the fragmentat@ reactions observed. This aspect is addressed in the present study using isotopic labelling to identify the methyl group lost.

In addition, the dimethylanisoles show relatively abundant doubly charged molecular ions in their 70 eV mass spectra, providing the opportunity to explore the chemistry of doubly charged ions using a variety of modern techniques [7]. In particular, the dimethylanisoles provide the opportunity to compare the fragmentation reactions of doubly charged molecular ions with their singly charged counterparts. This aspect is also explored in the present study.

EXPERIMENTAL

All experiments were carried out using a mass spectrometer [S], which has been described previously [9]. Briefly, this instrument is a reversed-geometry (BE) double-focussing mass spectrometer with a third stage consisting of a deceleration lens system, and r.f.-only quadrupole collision cell and a quadrupole mass analyzer. Electron impact mass spectra were obtained using the BE double-focussing mass spectrometer at a resolution of - 2000. The EI/CI source was used in the EI mode with 70 eV ionizing electron energy and a source temperature of - 200 o C.

Fragmentation reactions occurring on the metastable ion time scale were investigated by selecting the relevant ion beam by the magnetic sector and recording the fragment ions formed in the field-free region between the magnetic and electric sectors by scanning the electric sector, the so-called MIKES technique [lo]. Kinetic energy releases associated with the unimolec- ular fragmentation reactions were evaluated from the peak widths at half- height after correction for the main beam width according to the relation

The MIKES technique provides limited mass resolution in the fragment ion spectrum. To improve this resolution, particularly when isotopically

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labelled species were under study, a second approach to the study of metastable ion fragmentation reactions was used. The relevant ion beam was selected by the double-focussing BE stage, decelerated to 50 eV kinetic energy and introduced into the quadrupole cell in the absence of collision gas. The ionic products of unimolecular fragmentation reactions in the cell were analyzed by the final quadrupole mass analyzer. This approach gives unit mass resolution in the daughter ion mass spectrum but at the expense of information on kinetic energy releases. A comparison of the two methods of recording metastable ion mass spectra has been presented recently [12].

The doubly charged ion mass spectra of the dimethylanisoles were re- corded by introducing Xe into the collision cell (collision cell 2 [9]) located between the magnetic and electric sectors and scanning the magnetic sector while keeping the electric sector at 2E, where E is the electric sector voltage at which the main beam of stable source-produced ions is transmitted [13]. In effect, one detects the singly charged products resulting from charge exchange of the doubly charged ions with the Xe collision gas. In these experiments the ionizing electron energy was 100 eV and the ion accelerating voltage was 6 kV with the electric sector set to transmit ions of 12 keV kinetic energy. The Xe pressure was - 2 x low7 torr as read by the ionization gauge attached to the pumping line for the field-free region.

Unimolecular charge separation reactions [7] of the doubly charged molecular ions of the unlabelled and labelled dimethylanisoles were ex- amined by the MIKES technique [lo] with the kinetic-energy releases being determined from the peak widths at the base line of the higher mass charge separation product. In further studies, aimed at obtaining information concerning the structure of the doubly charged molecular ions, the singly charged molecular ions, produced by charge exchange of the molecular dication with Xe in the field-free region between the magnetic and electric sectors, were selected by the electric sector, decelerated and introduced into the rf.-only quadrupole cell where they underwent unimolecular decomposi- tion. The ionic fragmentation products were detected by the final mass- analyzing quadrupole.

In additional experiments, the doubly charged molecular ions, accelerated through 8 kV potential, were selected by the BE stage and introduced into the quadrupole cell where they underwent collision with N, at 50 eV collision energy, in an attempt to examine the charge exchange reactions of the doubly charged molecular ions [14]. The major reaction observed was collision-induced charge separation followed by fragmentation of the singly charged ions so produced.

The dimethylanisoles were either commercial samples of high purity or were prepared by reaction of the appropriate dimethylphenol with methyl iodide in basic solution. The methoxy-d, compounds were prepared simi-

larly by reaction of CD,1 with the appropriate dimethylphenol in basic solution. All chemicals were obtained from the Aldrich Chemical Company

WI.

RESULTS AND DISCUSSION

Singly charged ion experiments

Partial electron impact mass spectra of the six isomeric dimethylanisoles are presented in Table 1. The major fragmentation reaction of the molecular ions (m/z 136) is loss of a methyl radical to form m/z 121. More minor fragmentation reactions involve loss of an H atom and loss of CHO, CH,O and CH,O (to form m/z 107-105), although these latter fragmentation reactions are of lesser importance for the dimethylanisoles than for the monomethylanisoles [l-3]. Metastable ion studies show that the C,H,O+ fragment ion (m/z 121) fragments by at least three reactions [2-41, thus

TABLE 1

70 eV Mass spectra of dimethylanisoles a

m/r Ion Relative abundance formula

136 C,H,,O+* 100 100 100 94.6 100 100 135 CsHi@+ 11.9 17.4 15.4 5.7 37.6 11.3

121 CsH90+ 94.8 95.9 80.4 100 98.1 39.6

107 CsH:, 2.1 1.7 1.9 1.3 1.6 10.9 106 CsH:,’ 3.6 2.3 3.3 1.9 2.4 7.8 105 CsH,+ 18.9 12.5 22.3 14.5 11.5 14.6 104 C,H,+’ 4.3 2.4 6.3 4.5 1.5 1.4 103 CsH: 9.9 6.2 8.2 8.7 5.4 4.6

93 C,H; 11.1 12.9 10.8 13.6 12.2 14.6 92 C,Hs+’ 1.7 1.9 1.6 1.9 1.9 1.4 91 C,H: 49.8 39.9 44.2 48.0 39.1 67.8

79 GH; 7.5 5.9 7.1 6.7 6.7 6.6 78 C,H,+’ 9.0 6.3 7.6 9.2 5.8 5.8 77 GH: 36.4 37.2 33.9 47.9 30.3 30.6

a Corrected for natural abundance 13C.

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accounting for the pathway to the major fragment ions of lower mass-to- charge ratio

C,H90+ -+ C,H,+ + CO (2) + C,H; + CH,O (3) + C,H,+ + C,H,O (4

More detailed information concerning the fragmentation of the molecular ions comes from an examination of the electron impact mass spectra of the

TABLE 2

70 eV Mass spectra of (CH,),C,H,OCD, a

m/z Relative abundance

0033 0CD3 OCD3 OCD3 0CD3 OCD3

& Q-h__b 9-h 139 100 100 100 100 100 100 138 11.1 16.0 14.1 4.5 34.0 11.7

124 47.4 31.5 33.1 17.0 72.7 23.6 123 0.7 0.6 0.7 0.9 0.5 0.5 122 1.8 1.3 1.3 1.3 0.7 1.3 121 39.4 51.8 41.1 83.2 21.3 14.2

110 0.4 0.2 0.3 0.2 0.2 0.5 109 0.3 0.2 0.3 0.2 0.4 3.3 108 0.4 0.3 0.5 0.5 0.3 0.3 107 3.4 2.4 3.1 3.3 1.9 3.3 106 8.9 4.6 9.7 6.2 2.9 5.7 105 8.9 6.7 9.8 7.6 6.9 5.3 104 5.9 3.2 7.6 6.4 2.8 2.0 103 6.6 3.5 4.9 6.1 3.1 2.1

96 1.4 0.7 1.3 1.1 0.9 2.2 95 0.3 0.2 0.2 0.3 0.3 0.3 94 1.4 1.0 1.1 0.8 1.3 1.4 93 8.8 9.2 10.0 9.8 12.3 13.4 92 17.1 8.3 14.5 9.6 11.0 25.6 91 30.0 31.4 29.7 39.8 30.6 27.6

80 2.7 1.9 2.6 2.3 2.2 2.0 79 4.1 3.4 3.5 3.3 3.7 3.2 78 11.1 8.4 9.9 12.1 8.4 7.3 77 31.9 33.0 31.5 45.4 29.6 26.4

a Corrected for natural abundance 13C.

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TABLE 3

Unimolecular fragmentation of (CH,),C,H,OCD: on a metastable ion time frame

Product 4% of total ion signal

m/z OCD3 0CD3 OCD3 0CD3 0CD3 0CD3

&9_&&@d

138 3.0 4.1 4.4 4.6 5.4 5.9 124 67.1 57.9 56.3 40.2 78.4 42.4 123 3.2 1.5 1.8 2.0 0.6 1.9 122 4.6 2.1 4.2 2.5 1.2 4.0 121 16.9 28.2 25.5 57.3 8.9 8.9 110 1.5 109 0.4 0.4 0.6 0.7 0.6 11.6 108 4.2 107 3.3 3.3 4.7 2.9 3.8 17.6 106 0.7 105 1.4 1.8 2.4 1.8 1.2 1.2

methoxy-d, labelled compounds (Table 2) and from the relative metastable ion intensities (quadrupole cell) for fragmentation of the labelled molecular ions (Table 3). Clearly, both CH, and CD, are lost from the labelled molecular ions, both for molecular ions fragmenting in the ion source and for those fragmenting on the metastable ion time scale; the ratio [M - CD,]+/[M - CHJ+ decreases (Table 4) on going from fragmentation reac- tions occurring in the ion source to those occurring in the quadrupole cell (or as the lifetime of the fragmenting molecular ions increases). These results indicate that the dimethylanisoles act both as methoxybenzenes and as xylenes in the fragmentation reactions of the molecular ions. It also is apparent that there is some interchange of the deuterium atoms of the methoxy group with hydrogen atoms of the ring or methyl groups, with the extent of interchange increasing with increasing lifetime of the molecular ions. A similar exchange reaction has been observed in the molecular ion of anisole prior to the elimination of CH,O [3,16]. In anisole the interchange has been shown to involve only the ring hydrogens ortho to the methoxy group; such cannot be the case for the dimethylanisoles since even 2,6-di- methylanisole-d, shows loss of CD,H and CDH, (Table 3) on the metasta- ble ion time scale.

The mass spectra of the dimethylanisoles (Table 1) are very similar with the exception of 3,Sdimethylanisole which shows a decreased abundance of

TABLE 4

(M - CD 3) +/(M - CH 3) + ratio for fragmentation of (CH 3) 2C6 H 3OCD~-

309

Observation mode OCD 3 OCD 3 OCD 3 OCD 3 OCD3 OCD 3

70 eV mass spectra 0.65 1.65 1.24 4.92 0.29 0.60

M* in quad of M ÷ from source 0.25 0.49 0.45 1.43 0.11 0.21

M* in quad of charge-exchanged M 2+ 0.21 0.36 0.41 0.31 0.30 0.19

the [ M - CH3] + fragment and an increased abundance of the [ M - CHO] ÷ [m/ z 107] fragment compared with the other isomers. This difference also is reflected in the increased abundance of the peaks for loss of CDO and CD20 (Table 3) in the metastable ion mass spectra. A similar meta effect favouring CHO and CH20 elimination is noted in the mass spectra of the monomethylanisoles [3] and is particularly pronounced when the sub- stituents are halogens [17]. Further differences among the isomers are revealed in the mass spectra of the labelled dimethylanisoles. As summarized in Table 4, elimination of the methoxy methyl is particularly favoured when both ring methyls are ortho to the methoxy group, and is favoured to a lesser extent when one methyl group is ortho to the methoxy function. On the other hand, 3,4-dimethylanisole shows a pronounced preference for loss of a methyl from the ring, although by analogy with the xylenes [4-6], it is unlikely that the C H 3 lost is an intact ring-methyl group. Rather, it is likely that, in loss of C H 3 from the labelled anisoles, rearrangement, accompanied by hydrogen interchange, has occurred to yield a methoxybenzyl or meth- oxytropylium structure for the fragment ion.

To explore further for possible differences in the behaviour of the isomers we have determined the kinetic energy releases associated with the loss of CH 3 and C D 3 from the labelled molecular ions on the metastable ion time scale. The results are presented in Table 5. The metastable peak for loss o f C H 3 was Gaussian in shape for all isomers and gave kinetic energy releases (T1/2) in the range 0.11 to 0.19 eV, with only that for the 2,3-dimethylan- isole-d 3 appearing to be distinctive. On the whole, these results are con- sistent with isomerization to a common structure prior to loss of C H 3 , as is

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TABLE 5

Kinetic energy releases (T& in fragmentation of molecular ions

Tl/2 WI 139+ -+ 124+

Tl12 (meV) 139+ +121+

0.106 -t_ 0.008

0.182 f 0.014

38,465

76

OCD3

0.159 f 0.013 42,534

OCD3

0.175 * 0.005

OCD3

0.179 f 0.001 84

38,473

observed for the xylenes [4-61. The metastable peak for loss of CD, was clearly composite in three cases and approximate Tl,* values for each component could be evaluated as given in Table 5. In the other three cases the composite nature of the peaks was not readily evident and only a single Tl,2 value is reported in each case. The fact that these values are higher than the lower values calculated from the composite peaks probably reflects a contribution from the process of higher kinetic energy release. The lower T values obtained are in line with the T,,, values reported [17,18] for methyl loss from the molecular ions of a variety of simple substituted

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anisoles. The observation of two kinetic energy releases indicates two pathways for loss of the methoxy methyl groups. Presumably one pathway is simple bond cleavage in an unrearranged molecular ion leading to a dimeth- ylphenoxy cation. The other pathway may involve prior ring expansion to a substituted cycloheptatriene structure followed by fragmentation. The struc- tures of the [M - CH,]+ and [M - CDJ+ fragment ions are still under investigation and the results of this study will be reported separately. Both fragment ions undergo reactions (2)-(4) with considerable H/D mixing in

the [M-CH,]+ ion prior to fragmentation by reactions (3) and (4).

Doubly charged ion experiments

The 70 eV electron impact mass spectra of the labelled dimethylanisoles show a signal at m/z 69.5 for the doubly charged molecular ion; for the 2,6-isomer the M2+ signal was - 0.1% of the M+ signal while for the other isomers the M2+ ion signal was - 1% of the M + ion signal. To explore more completely the doubly charged ion mass spectra of the dimethylanisoles the 2E mass spectra [13] were recorded using Xe as the charge exchange collision gas. These spectra record the singly charged products, A+ formed from the charge exchange reaction

A2++Xe+A++Xe+ (5)

and provide a qualitative picture of the doubly charged ions formed by electron impact on the molecule in question. Quantitative data are precluded because the cross-section for reaction (5) may vary from species to species; however, for isomeric molecules, variations in the 2E mass spectra should reflect structural effects on doubly charged ion abundances.

The 2E mass spectra of the six unlabelled dimethylanisoles are presented in Table 6, where the column entitled mass gives the mass (u) of the singly charged ions detected; the precursor ions have the same mass but two units of charge. In the discussion that follows we shall refer to the spectrum observed as the doubly charged ion spectrum. A rich spectrum is observed in each case but one which is very different from the singly charged spectrum (Table 1). The M2+ ion signal is intense, except for the 2,6-isomer, and forms the base peak in three spectra. This in contrast to the 2E mass spectra of the xylenes [19] where the M2+ ion signal was only - 2% of the base peak and indicates a pronounced stabilizing effect of the methoxy group. This is in line with the observation of Sakurai et al. [20] that electron-donating substituents in aromatic compounds favour formation of stable doubly charged molecular ions. The major doubly charged fragment ions observed are C,Hz+ (n = 5-9; 62, 74, 86, 98), C,Hz+ (n = 7, 8; 90 and 102) and C,Ht+ (91); all of these are common ions in the 2E mass spectra of alkyl

312

TABLE 6

2 E mass spectra of dimethylanisoles

Mass Relative abundance

0CH3 OCH, 0CH3 OCH,

&@&_bq& 137 9 11 10 2 10 10 136 79 100 92 14 100 100 135 6 4 5 1 7 8 134 18 8 14 2 16 19 131 9 6 6 5 2 2 121 31 53 19 11 8 4 120 3 7 2 2 2 3 119 3 6 2 2 2 1 118 8 8 3 3 2 2 104 7 4 7 5 4 5 103 12 8 9 10 9 8 102 57 38 51 43 49 46 99 10 8 8 5 12 10 98 47 28 32 19 41 39 92 11 6 7 7 6 7 91 75 56 70 60 56 56 90 29 22 28 33 27 31 89 17 12 26 16 13 15 88 5 35 5 5 5 3 87 25 18 25 27 22 22 86 100 70 100 100 87 89 85 5 8 12 10 9 10 78 11 7 6 2 6 4 77 19 9 8 3 7 6 76 18 8 11 3 9 12 75 31 18 20 8 19 19 74 32 20 29 14 26 27 65 19 7 5 2 6 3 64 11 7 4 3 10 2 63 20 10 8 5 12 7 62 75 48 34 29 48 31

aromatic compounds. The C, H z’ series are common doubly charged ions in the spectra of aromatic molecules [19-221, alkynes [23] and aliphatic com- pounds [24] and have been shown [25], by theoretical calculations to be linear with hydrogen atoms in the terminal positions. The C,Hi+ ion also is

313

abundant in the spectra of alkylaromatic [19,23] and cyclic [27] hydro- carbons.

The 2E mass spectra of the dimethylanisole-d, isomers were the same, within experimental error, as the spectra of the unlabelled isomers below mass 130, indicating that the methoxy group is lost in forming the lower mass ions without significant interchange of the methoxy hydrogens with other hydrogens. In particular the signal corresponding to (M - CH,)‘+ (121) in Table 6 was observed at the same mass in the spectra of the labelled compounds, indicating that only the methoxy methyl is lost in forming this ion. By contrast, as discussed above, the singly charged molecular ion loses either the methoxy methyl or a methyl derived from the remainder of the molecule.

The 2 E mass spectra permit some isomer distinction. Clearly, 2,6-dimeth- ylanisole is readily identified by the low M*+ ion abundance. In addition, the spectra of the 2,3-, 2,4-, and 2,5-isomers show significant variations in relative fragment ion abundances, however the spectra of the 3,4- and 3,5-isomers are very similar.

A second way of characterizing doubly charged ions is through observa- tion of the unimolecular charge separation reactions [7]. Three charge separation reactions, (6)-(8) were observed for the unlabelled molecular dications.

C,H,,02+ + C,H,O+ + CH,+ (6)

-+ C,H,O+ + C,Hf (7)

+ C,H; + CH30+ (8)

The abundances of the ions for reaction (8) were very low and no measure- ments were made for this reaction. Reaction (6) was favoured over reaction (7) by a factor of 5-6 to 1. Observation of the relative ion signals for charge separation of the natural abundance r3CCsH1,02+ dications showed that reaction (7) occurred as written rather than to form C,HA + CHO+. Ex- amination of the charge separation reactions of the dications of the methoxy-d, compounds showed that reaction (7) involved specific formation of C,HSD20t + C,H,D+. Reaction (6) involved primarily formation of CgH90+ + CDT with some interchange of the deuterium atoms prior to fragmentation. Thus, some formation of C8HSDO+ + CDH: and C,H,D,O+ + CDH,f was observed but only very minor formation of CsHsD30+ + CH; occurred. This latter observation is surprising since the molecular dications of the xylenes undergo charge separation reactions to form CHC [28]. The observation of a charge separation reaction forming C,Hz is, to our knowledge, unique; the fact that this ion specifically incorporates one hydrogen from the methoxy group is intriguing but

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TABLE 7

Kinetic energy releases and intercharge distances for charge separation reactions

1392+ +121+ +18+ 1392+ +109+ +30+

T (ev) R (A> T (ev> R (A>

0CD3

2.71 5.32 3.05 4.72

OCDj

2.69 5.36 3.07 4.69

0CD3

2.71

0CD3

2.09

0CD3

2.72 5.30 3.06 4.71

5.32

5.35

3.07

3.08

4.70

4.68

OCDs

2.71 5.31 3.07 4.69

difficult to rationalize in the absence of knowledge of the structure of the molecular dication.

The kinetic energy releases associated with the charge separation reactions (9) and (10)

CgHgD,02+ + C,H,O+ + CD; (9) + CgH5D20+ + C2H4D+ (10)

in the labelled dications are presented in Table 7 along with the intercharge separations (R) calculated on the assumption that the kinetic energy releases

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arise from the coulombic energy of repulsion between two elementary charges separated by a distance R in the transition state [29]. For each fragmentation reaction the kinetic energy releases are identical, within small experimental error, for all isomers. These kinetic energy releases lead to charge separation distances of - 5.3 A for reaction (9) and - 4.7 A for reaction (10). In addition a weak component wat observed for reaction (10)

with a kinetic energy release of - 2.3 eV (R - 7 A). These charge separation distances appear to be too large to be consistent with fragmentation of a cyclic structure suggesting that the transition state for charge separation is a ring-opened structure. However, there is some evidence that the non-decom- posing molecular dications retain a cyclic structure. The singly charged molecular ions derived by charge exchange of the labelled molecular di- cations with Xe in the second field-free region when transmitted to the quadrupole cell, undergo unimolecular fragmentation by loss of CH, and loss of CD,. As shown by the results presented in Table 4 the [M - CD,]‘/ [M - CHJ+ ratio observed is similar for each isomer indicating that the molecular ions formed in the charge exchange reaction have a common structure and suggesting that the molecular dications may have a common structure. The [M - CD,]+/[M - CH3]+ ratios observed are similar to the ratios observed for fragmentation, in the quadrupole cell, of singly charged molecular ions formed by electron impact in the ion source (Table 4). While this similarity may be coincidental, it does provide some support for the suggestion that the molecular ions formed by charge exchange of the molecular dications and, indeed, the dications themselves have cyclic struc- tures. Thus, it is possible either that the non-decomposing and fragmenting forms of the molecular dications have different structures or that the rationalization of the kinetic energy releases in terms of intercharge dis- tances and coulombic repulsion is not valid.

Finally, the molecular dications of the labelled dimethylanisoles were collided with N, in the quadrupole collision cell in an attempt to observe the products of charge exchange at low collision energies [14]. The resulting spectra (Table 8) are most readily interpreted in terms of collision-induced charge separation of the dications by reactions (9) and (10) accompanied by fragmentation of the m/z 121 ([M - CD,]‘) ion by reactions (2)-(4). The low abundance ion signals at m/z values of 122, 123, 124, 17, 16 and 15 indicate that the charge separation reaction (9) is accompanied by some interchange of the methoxy deuterium atoms with other hydrogen atoms in the molecule. In addition to charge separation there is some loss of H, from the molecular dications leading to m/z 68.5. One should note that, in the absence of discrimination in the quadrupole system, the ion signals for the m/z 109 and m/z 30 products should be equal as should the sum C(H, D),’ and the sum C8H9_,D,0+ where the latter includes the fragment ions

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TABLE 8

Products from collision-induced reactions of M2+ in quadrupole cell

m/z Relative abundance

0CD3 0CD3 0CD3 0CD3 0CD3 0CD3

&@h__b 9-h 124 3 2 3 3 1 2 123 7 2 9 5 3 5 122 17 9 17 12 9 13 121 85 100 92 100 69 100

109 15 10 14 11 12 16

93 16 20 20 12 16 21 91 3 4 4 4 3 5

77 9 12 13 9 8 11

68.5 107 53 81 82 119 77

30 14 8 12 11 10 13

18 100 99 100 86 100 99 17 14 5 11 12 8 8 16 5 2 5 4 2 3 15 2 - 2 - - 1

formed by fragmentation of C8H90f by reactions (2)-(4) (m/z 93, 91 and 77). These identities hold approximately despite the large differences in mass and the relatively large kinetic energy releases in the charge separation reactions. No simple charge exchange products were observed. This is in contrast to the case of the dication of furan where significant charge exchange is observed [14,30] in similar studies but is in agreement with the observation that charge exchange of the molecular dications of polycyclic aromatic hydrocarbons is not observed in low energy collisions [30].

SUMMARY

The singly charged molecular ions of the isomeric dimethylanisoles frag- ment, in part, by loss of the methoxy methyl, characteristic of an anisole and, in part, by loss of a methyl probably originating from the aromatic ring and attached methyl groups, characteristic of a xylene. By contrast the doubly charged molecular ions fragment only to a minor extent by loss of a

317

methyl radical, and this methyl is exclusively the methoxy methyl. The major fragmentation reactions of the molecular dications, as shown by 2 E mass spectra, involve formation of C,H;+, C,Hi+ and C,Hf+ ions which do not incorporate any of the methoxy hydrogen atoms. In charge separation reactions the molecular dications fragment to form C,H,O+ + CH,f , but, again, this involves exclusively the methoxy methyl, albeit with some hydro- gen interchange with the rest of the molecule. Some distinction of the isomeric molecules is possible from the singly charged ion mass spectra and from the doubly charged ion mass spectra.

ACKNOWLEDGEMENTS

The authors are indebted to the Natural Sciences and Engineering Re- search Council of Canada for financial support. A.G.H. gratefully acknowl- edges the award of a Killam Research Fellowship (1985-1987) by the Canada Council.

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