International Journa/ of Mass Spectrometry and Ion Processes, 62 (1984) 219-225 Elsevier Science Publishers B.V., Amsterdam - Printed In The Netherlands

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ISOMERIC STRUCTURE DISTINCTION USING ENERGY AND ANGULAR RESOLUTION: C,H,O +.

SUNITA VERMA, JAMES D. CIUPEK and R. GRAHAM COOKS

Department of Chemistry, Purdue University West Lafayette, IN 47907 (U.S.A.j

(Received 8 June 1984)

ABSTRACT

Approximate breakdown curves generated by collision-induced dissociation of C, H, O+. from dioxane show that it does not have the trimethylene oxide structure. Data obtained by varying the collision energy in the low energy range and by varying the scattering angle at kilovolt energy collisions lead to the same conclusion. These methods have more structure- discriminating power than conventional collisional activation spectra. They appear also to be less sensitive to internal energy effects. It is noteworthy that for the reactions studied here, 40 eV collisions deposit approximately the same average internal energy as 3000 eV collisions, perhaps because of different mechanisms of excitation.

INTRODUCTION

Collision-induced dissociation or collision-activated dissociation (CAD) is widely used for characterizing gas-phase ions [l-3]. The experiment is commonly done in the keV range of energies using sector-type mass spec- trometers [4,5], while lo-20 eV is a typical energy when using quadrupole instruments [6,7]. In the latter case, it is a fairly simple matter to vary the collision energy and to obtain energy-resolved mass spectra (ERMS) [8-lo]. This provides a two-dimensional spectrum resembling a breakdown curve, in contrast to the single dimension of information provided by CAD performed at a fixed energy. Another feature of the ERMS technique relative to conventional CAD experiments is that the effect of initial internal energy [ll-131 on fragmentation behavior may be more easily recognized. Similar advantages are provided by experiments in which the relative abundances of fragment ions formed in a high energy collision are measured as a function of scattering angle [14]. Both these techniques have been available for some time but have seen little application to specific ion structural problems [8,15], and none to those which have proven difficult to solve using conven- tional collision-activated dissociation methods. We are, therefore, examining

0168-1176/84,‘$03.00 0 1984 Elsevier Science Publishers B.V.

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ERMS of ISOMERIC IONS

% R.A.

80

“\M (a)

ION KINETIC ENERGY (eV)

X RA.

80 (b)

ION .KINETIC ENERGY [eV)

Fig. 1. Relative abundances of the fragment ions of m/z 43, 30, 29 and 27 from C,H,O+, formed from (a) trimethylene oxide and (b) 1,4-dioxane as a function of laboratory kinetic energy. Argon was employed as the target gas under single collision conditions.

a number of systems for which isomeric ions produce similar or indis- tinguishable CAD spectra in the keV range by obtaining the corresponding data over a range of collision energies (typically lo-100 eV) and scattering angles (O-3 O at 3 keV). The C3H,0+. case is presented here.

RESULTS AND DISCUSSION

Based on close similarities in their CAD spectra, Van de Sande and McLafferty [16] proposed C,H,O+. from 1,4-dioxane to have the same cyclic structure, 1, as ionized trimethylene oxide. Their conclusion was based on comparisons of relative abundances of fragment ions arising from high activation energy processes. Because of their sensitivity to internal energy [17], fragment ions which might have contained contributions from metasta- ble transitions were neglected. Data for the same C,H,O*. ions taken in the low energy regime are more informative. Figure 1 * shows the relative

* Figure 1 is representative of data obtained on several occasions over a span of four months. The average deviation in cross points is + 5 eV.

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abundances of the selected fragment ions generated by low energy CAD of

w%Ol+~ as a function of laboratory collision energy. The parent ions were prepared from trimethylene oxide and 1,4-dioxane in the electron impact source of a hybrid mass spectrometer of BQQ geometry [18]. Other fragment

“277”2

H2C-O+. H$=&-CH$H2

2 I

ions observed (m/z 14,15,16, and 28) have been excluded to emphasize the differences in the energy-resolved spectra of the two parent ions. The reactions presented in this figure include low activation energy reactions [16] leading to the formation of ions of m/z 43 and 30 and higher activation processes leading to the formation of 27+ and 29+.

Because low energy processes are sensitive to small changes in internal energy, they are normally not used to distinguish isomeric structures. In an experiment in which MS/MS spectra are obtained as a function of internal energy, or a parameter related to internal energy, such differences might be expected to manifest themselves in the resulting two-dimensional spectra (“breakdown curves”) as a shift along the x-axis [13]. In the particular case of energy-resolved MS, the x-axis explicitly represents the ion kinetic energy. It appears from Fig. 1, that both the low activation energy processes, and the higher energy reactions, are indicative of structural distinctions in this particular case.

The two ERMS spectra (Fig. l), although similar in nature, are quite distinct. As expected, the relative abundances of fragment ions m/z 43 and 30, associated with processes of low activation energies, decrease as collision energy is increased, while the higher activation energy reactions leading to ions of m/z 27 and 29 increase in relative abundance. The most striking difference between the two spectra is the absence of the 43+ ion above 20 eV in the ERMS spectrum of trimethylene oxide. Other noticeable differences include the positions of “cross points” (energies at which two fragment ions are equal in abundance). In the case of trimethylene oxide, abundances of yn/z 27 and 29 increase relative to 30 with ion kinetic energy, becoming equal at ion energies of 60 and 45 eV, respectively. No significant changes in

the trimethylene oxide spectrum are observed thereafter. By contrast, in the dioxane spectrum, ions of m/z 27 and 29 increase in abundance relatively slowly but constantly over the entire range of energies examined. Only at ion energies of ca. 70 eV do these ions approach m/z 30 in abundance. At energies above 70 eV, m/z 27 is the most abundant fragment in the dioxane spectrum. On the other hand, 29+ is the most abundant ion at all energies above 45 eV in the case of trimethylene oxide.

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The differences between the two spectra can ako be followed by monitor- ing only the high energy products (as has been the practice in conventional collisional activation studies). These data are illustrated in Fig. 2 where the ratio 27 + /29+ is plotted vs. ion kinetic energy. Although the two C3H,0 +. ions have similar thresholds for the formation of 27+, m/z 27 increases in abundance more rapidly in the dioxane case. This difference again points to the presence of two structures. Of the two isobaric ions, CHOf and C,Hc , that might account for the signal at m/z 29, only the latter can fragment further via loss of molecular H, to produce m/z 27. From the curves in Fig. 2, it appears that at low ion kinetic energies and hence at low internal energies of the parent ion, fragmentation results in the formation of CHO+ and/or C,Hz ions that do not have enough energy to undergo further fragmentation. As the internal energy is increased, energetic C,Hc ions are formed, causing an increase in the relative abundance of m/z 27 in both cases. While 27+ and 29+ are equal in abundance at 60 eV in the dioxane spectrum [curve (a), Fig. 21 and the ratio approaches 1.3 at 90 eV, it requires at least a 90 eV collision to produce 27+ and 29+ in equal abundance from trimethylene oxide [curve (b), Fig. 21.

In summary, the ERMS spectrum of the C,H,O+. fragment ion from 1,4-dioxane is different to that of the trimethylene oxide molecular ion and one can conclude that they do not possess the same structure (or mixture of structures). Both low and high energy reactions are useful in characterizing the differences in ERMS spectra and hence in ion structures. Baumann et al. [19], in a study utilizing ion-molecule reactions of C,H60*. derived from labelled 1,4-dioxane, found this ion to have the acyclic structure 2 with a

ION KINETIC ENERGY (eV1

Fig. 2. Fragmentation ratio 27+/29+ as a function of ion kinetic energy for C,H,O+. generated from {a) l+dioxane and (b) trimethylene oxide.

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1 Da 200 300

DEFLECTION POTENTIAL (volts)

Fig. 3. Fragmention ratio 27+/29+ as a function of z-plate deflection potential for 3 keV collisions of C3H60+. generated from (a) 1,4-dioxane and (b) trimethylene oxide. Deflection potentials are approximately related to scattering angle in the following manner: 0.7 o per 100 V of deflection potential [22]. Argon was used as the target gas at 25% beam attenuation.

It is noteworthy that the average energy deposited in keV collisions can be less than that deposited in low energy collisions. The origin of this effect probably lies in different excitation mechanisms (electronic vs. vibrational) [23]. The point can be illustrated by comparing the 3 keV (OO) spectra of C,H,O+- ions (Fig. 3) with the ERMS spectrum at 40 eV (Fig. 2). For both

lifetime of at least 1 ms. Molecular orbital calculations have also been reported which predict this structure to be lower in energy than structure 1

by 10 kJ mol-’ 1201. Breakdown curves were also obtained using angle-resolved mass spectrom-

etry. This experiment utilized another hybrid instrument, one of BEQQ geometry * [21]. Figure 3 shows the relative importance of the two high energy reactions as a function of scattering angle in 3 keV collisions. Variation in the 27+/29+ ratio with scattering angle parallels that observed as a function of collision energy in the ERMS experiment. In this technique also, the two structures can be distinguished by the differences in relative abundances of the 43+ ion. Less than 1% of the total fragment ion current monitored is due to 43+ at all angles studied for the trimethylene oxide molecular ion. On the other hand, C,H,O+. derived from dioxane produces 2-10% of the same ion, depending on the scattering angle.

* Angle-resolved experiments were performed by BE-linked scans for CAD products formed in the first field-free region of the instrument, with the scattering angle being defined by pre-collision electrical deflection of the parent ion beam [22]. The slopes of the two curves are sensitive to angular resolution. This, in turn, depends on source focusing conditions because the focus plates are also used to fix the scattering angle. Spectra shown in Fig. 3 were obtained under identical source conditions with an angular resolution of 0.25 Or

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structures, the 27+/29+ ratios in the high energy, zero angle experiments match those observed at 40 eV, thus they fall in the lower half of the energy scale used for low energy collisions. A consequence of this difference in energy deposition is the fact that keV collisions (conventional zero angle CAD) falls in a region of the breakdown curves where structural distinctions are most difficult to make for C,H,O+.. These facts reinforce the value of obtaining daughter ion spectra as a function of collision energy or scattering angle.

CONCLUSION

We conclude that the ERMS and ARMS techniques can supplement other methods in distinguishing isomeric structures. Because these techniques cover a range of internal energies, difficulties in interpreting fragmentation patterns due to initial state (internal energy) effects can also be alleviated.

ACKNOWLEDGEMENT

This work was supported by the National Science Foundation, CHE80- 11425.

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