Gas phase ion-molecule reactions of dimethylsulphoxide

JOHN EDWARD FULFORD, JOSEPH WAYNE DUPUIS, A N D RAYMOND EVANS MARCH Deprrrtttzetzt of C/zenzistty, Tretlt Utliversity, Peterboro~rg/z, Ot~t. , Cntzrrda K9J 7B8

Received December 3, 1976'

JOHN EDWARD FULFORD, JOSEPH WAYNE DUPUIS, and RAYMOND EVANS MARCH. Can. J. Chem. 56,2324 (1978).

The gas phase ion-chemistry of dimethylsulphoxide (DMSO) and deuterated dimethyl- sulphoxide (DMSO-d6) has been examined using a quadrupole ion store (QUISTOR) a s an ion-molecule reaction chamber. The QUISTOR results are compared with those obtained by ion trapping and high pressure mass spectrometry as reported by other workers. The per- formance of the QUISTOR demonstrates the versatility of the technique for ion-molecule reaction studies with variation of ambient pressure and duration of ion storage.

JOHN EDWARD FULFORD, JOSEPH WAYNE DUPUIS et RAYMOND EVANS MARCH. Can. J. Chem. 56,2324 (1978).

On a Ctudie, faisant appel a un rkservoire d'ions quadrupolaires (QUISTOR) sous forme de chambre de reaction ion-molecule, la chimie ionique en phase gazeuse du dimCthylsul- phoxyde (DMSO) et du dimCthylsulphoxyde deutere (DMSO-cf). O n compare les resultats QUISTOR avec ceux obtenus par spectrometrie de masse avec piegeage ionique et haute pression tels que rapportes par d'autres auteurs. Le bon succes de QUISTOR dkmontre la versatilite de la technique pour des etudes de reactions ion-molCcule avec variation d e la pression ambiante et de la duree de l'entreposage des ions.

[Traduit par le journal]

Introduction mass s~ectrometrv which were carried out in order

The ion-chemistry described here has been ob- served with a novel system in which ions may be created and stored in a three-dimensional quad- rupole ion store QUISTOR (1) and mass analysed by subsequent ejection into a quadrupole mass filter. When the QUISTOR is used as a storage device and reactor for the study of chemical processes (e.g. ion-molecule and charge transfer reactions) a knowledge of ion energies is essential.

In consideration of ion kinetic energies within the QUISTOR, a distinction must be made between primary ions which are formed initially by electron impact throughout the effective reactor volume and secondary ions formed in subsequent ion-molecule reactions. Attempts have been made to calculate primary ion kinetic energies in collision-free systems (2-4) and secondary ion kinetic energies pertaining after several collisions (5-7). There is good agree- ment among the estimates of mean primary ion energies in a collision-free system. Recent simulation studies (7) indicate that although a fraction of secondary ions may have several eV of kinetic energy, rapid energy dissipation is effected in the first 6-10 collisions so that after 15 collisions the mean ion energy is < 1 eV. A brief account is given below of the examinations of several well charac- terized chemical systems by quadrupole ion storage

to asceitain effectt've ion energies in the QUISTOR. The ammonia system has been investigated with

this technique (8) as a means of estimating the pri- mary ion mean kinetic energies. The experimentally determined rate constants of the principal ion- molecule reactions occurring in ammonia were com- pared with literature values: from this comparison it was concluded that the primary ion kinetic energies are of the order of 1-3 eV.

The thermal energy reactions of methane have been studied extensively; more than twenty refer- ences are cited by Henchman (9) for the production of CH,' which is an agent for chemical ionization. The occurrence of endothermic reactions requiring a primary ion kinetic energy threshold of 1.5 eV for the ground state ion provides a n opportunity for assessment of primary ion energies in the QUISTOR. Anders (10) had reported that the reaction producing C2H3+ is endothermic by 1.6 eV which is in good agreement with the observations of Harrison et al. (1 l), who found that the C2H3+ intensity exceeded that of C2H5+ for reactant ion energies in excess of 3 eV. Clow and Futrell (12) have reported however that the C2H3' peak intensity was less than the C2H5' peak intensity for ion energies up to 10 eV. Rate constants for the methane system have been obtained with the QUISTOR (8) and are in good agreement with literature values. Comparison was

'Revision received May 5, 1978. made also of the ratio of C2H3+ and C2H5' inten-

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FULFORD ET AL. 2325

sities as a function of ion energy, the primary ion kinetic energy being varied within the QUISTOR by operation with rf potentials between 800 and 1600 (peak-to-peak).

The results of a QUISTOR study of the water system (8) show very good agreement with those collected by Huntress and Pinnizzotto (1 3). Com- parison of the observed ratio of 1.6 for K,,,,+/ K,,?+ with Ryan's tabulation (14) of this ratio agalnst ion energy yielded a primary ion kinetic energy of between 1 and 2 eV.

The observation of CH,' in the QUISTOR study of the methane system2 indicated that chemical ionization should be realizable at extended storage times. This was demonstrated (16) with a methane- methanol (400: 1) mixture a t a pressure of Torr. Chemical ionization in the QUISTOR has been observed in this laboratory for a number of alcohols (17), amines, mercaptans, explosives (18), and a miscellany of compounds including ketene (19).

The simulation studies mentioned earlier indicate that the kinetic energy of secondary ions is less than 1 eV. This is supported strongly by the chemical ionization observations in that little, if any, frag- mentation of the protonated species occurs in near thermoneutral reactions, hence there has been no enhancement of the exothermicity of the proton transfer reaction by reactant ion kinetic energy. An example may serve to demonstrate further that energies of ions stored in the QUISTOR for periods in excess of ca. 0.5 ms are near thermal energy. The proton affinity (PA) of ketene was determined by the 'bracketing' technique wherein proton transfer equilibrium was approached from both directions. Dimethyl ether and 2-propanol were employed, wherein the proton affinities are less than and in excess of that of ketene, respectively. As our value for the PA of ketene has since been confirmed by the ICR technique (20) and as no fragmentation of the equilibrated (M + I)' species was observed, it is concluded that the protonated species were thermal- ized. At long storage times, multiply-solvated protons have been observed together with dissocia- tion products of these adducts. Consequently, it is possible to obtain information on collisional stabili- zation of excited addition ions and relative rates of dissociation and deactivation.

While CH,' is the most widely applied chemical ionization agent and has been studied in the QUIS- TOR, the presence of such an agent is not necessary for the formation of protonated parent molecules,

ZA similar observation has been made recently with a cylindrical device (15).

per se. Ionic species of this type are produced in ion confinement devices by auto-chemical ionization through mechanisms which involve the reaction of parent molecules with primary ions formed initially by electron impact. Thus the ion-chemistry of a given compound may be described in terms of the concurrent mechanisms of this type.

Previous workers have shown that the ion- chemistry of simple aliphatic alcohols (21) is charac- terized by associative complexes leading to the formation of solvated protons and multiply-solvated protons. In the aliphatic alcohols, dissociation of the complex may result in the elimination of water or alkene. This mode of reaction requires an electro- negative atom (oxygen) to which is attached a labile hydrogen atom. Recent reports on the ion-chemistry of sulphur (H2S, CH3SH, (CH3),S (21) and CH,SH, CD3SH (23, 24)) and nitrogen (25-27) analogues show that similar reactions, at least i n part, occur in these systems also.

The basic features of the quadrupole ion storage technique are virtually identical with those of the ICR ion trapping technique and the data reported here are similar t o those obtained with ICR ion trapping devices. The basic difference arises in the mode of trapping, a s Lawson and Todd (28) have explained: in the QUISTOR, a net restoring force experienced by the ions attracts them to the centre of the device due to their motion in the inhomoge- neous oscillating electric fields, while in the ICR cell, confinement occurs through the combination of a magnetic field with a n inhomogeneous static electric field. In both techniques the space charge potential within the ion cloud determines the maximum density of trapped ions, and this appears to b e some two or three orders of magnitude greater in the QUISTOR.

The data reported here are similar also to those obtained from high pressure mass spectrometry but extended to a much longer time scale. I t is customary in presenting high pressure mass spectrometry data to display plots of ion intensities a s a function of pressure and from such plots conclusions are drawn concerning ion-molecule mechanisms. These con- clusions are based o n observations o f the decay of primary ion intensities as a function o f pressure and the concurrent appearance of secondary and higher order ions. Similar conclusions may be drawn from plots of ion intensities as a function of time a t a fixed pressure in ion trapping devices such as are used in ICR mass spectrometry (29), beam trapping (24), and QUISTOR studies (8). Furthermore, the temporal dependences of ion intensities obtained with ion confinement techniques may be observed over a range of pressures. Brupbacher et al. (25)

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2326 CAN. J. CHEM. VOL. 56, 1978

have shown that as a result of ionization of a sub- strate material, R, several different primary ions, Pi+ and neutrals, N,, may result from the process

Once formed, each of these primary ions may under- go ion-molecule reactions with the substrate. The reactions of one particular primary ion can be sum- marized as follows:

where S,' represents the various secondary ions formed. For this reaction scheme, the rate of loss of a specific primary ion, P i f , is given by

L

[l] d[Pif]/dt = -[R:IIPif] C k j j= 1

L where C k j represents the total rate constant,

j = 1 k,, for the loss of P i f by reaction with R. Integration of eq. [l] over reaction time, t, gives the expression

which describes the real kinetic surface for the reac- tion of P i f with substrate R. High pressure mass spectrometry data yield ion intensity vs. pressure profiles of the kinetic surface for a given reaction time, while ion trapping data yield ion intensity vs. time profiles of the surface for a given pressure.

In the identification of a chemical system, on which the application of quadrupole ion storage devices for the study of ion-chemistry may be demonstrated, the following factors were considered: (i) As the identification of ions is facilitated by comparison of the ion-chemistry of a given substance and the fully deuterated analogue, the ready availability of the compounds was essential. (ii) The existence of pre- vious studies of the ion-chemistry carried out by ICR technique and high pressure mass spectrometry. (iii) The system should exhibit an associative ion- chemistry. (iv) The existence of some degree of ambiguity in the studies reported.

Thus dimethyl sulphoxide was selected for this demonstration.

Experimental The apparatus, which is depicted schematically in Fig. 1,

consists of a three-dimensional quadrupole ion store (QUIS- TOR) mounted in place of the ion source of a conventional quadrupole mass filter (Vacuum Generators Q7B).

Filament .....h d-?q

qr.lFp Generalor

FIG. 1. Schematic diagram of apparatus.

The geometry of the QUISTOR is that of a hyperboloid of one sheet combined with a hyperboloid o f two sheets forming the ring and end-cap electrodes, respectively; these are fabri- cated from stainless steel with r., = 1 cm and are separated by lava spacers, the whole assembly being machined and mounted with a tolerance of 0.001 in. A simple electron gun is employed comprising a spiral filament made of 0.008 in. tungsten wire and a perforated stainless steel plate, which allows the electron beam to be gated as desired. The QUISTOR/quadrupole corn- bination is mounted vertically inside a stainless steel vacuum system, pbmped by a 4-in. oil-diffusion pump, which is con- nected to a diffusion-pumped, unheated glass inlet system through a needle valve. An rf voltage supply, capable of pro- viding up to 800 V at 1.6 o r 2.5 MHz, is connected to the ring electrode, the end-caps being earthed except when ion ejection pulses are applied to the lower one (Fig. I). In operation the electron gate is made a few volts more negative than the filament, so that the electron beam is turned off, and a positive pulse periodically applied to permit passage of the beam through the QUISTOR with consequent formation of ions. The ionizing beam in this work is directed along the symmetry axis o f the device, rather than radially (30), since in this manner the electrons are not influenced by the rf voltage until after they have entered the device, with correspondingly better control of the electron energy. The ions, having been created inside the device, are stored under the influence of the field for a variable time before extraction by a negative pulse applied to the lower end-cap (Fig. 1) and subsequent mass analysis. This is represented in Fig. 2 which shows the typical pulse train; a trigger pulse from the internally triggered ejection generator (Hewlett Packard 214A), triggers the Boxcar Detector (Brookdeal Ortec Scan Delay Generator 9425A (SDG) and Linear Gate 9415 (LG)) and the creation pulse. The dead time is adjusted by the pulse delay circuitry of the creation generator and the storage time may be adjusted up t o 100 ms by the pulse repetition control o n the ejection gener- ator.

The pulsed ion signal from the electron multiplier is passed to an amplifier which is an integral part of the quadrupole control unit, the Boxcar Detector and thence to the Y-drive of an X-Y recorder (Hewlett-Packard Model 7044A), the X-drive of which is furnished by the quadrupole control unit. The major purpose of the amplifier is to serve as an impedence matching unit between the high (ca. 100 MR) output impe- dence of the boxcar detector, although additional gain can be introduced at this point. Inclusion of the linear gate achieves two results; firstly, since it may be triggered when desired, only stored ions (i.e. those which appear coincidentally with the extraction pulse) give rise to a signal at the recorder and any ions which might appear coincident with the creation pulsed (owing to their creation in unstable positions, etc.) are ignored;

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FULFOR :D ET AL.

Ejectmoo

sour storage lime

C r e o t l o n

- - dead 5 0 ~ . time

FIG. 2. Pulse train employed under typical operating con- ditions.

secondly the level recorded during the time that the gate is open is maintained at the output until the next pulse is due, so ensuring that any averagingeffect, caused by applying a pulsed signal to the pen recorder, is avoided.

In order to study the ionic reactions occurring in a par- ticular system, the pressure of the compound in the vacuum system is increased to ca. Torr, and a series of mass spectra observed at several storage times. For each storage time the intensity of each peak is normalized by expressing it as a percentage of the total intensity for that time; this enables comparison between spectra by allowing for fluctuations in ion currents owing to ion-loss processes, etc. It must be empha- sized that ion intensities have not been corrected for mass dis- crimination either in the QUISTOR or in the quadrupole mass filter. The QUISTOR is operated with zero dc bias on the electrodes, hence ions of all mass-to-charge ratios should be stored. Furthermore there is substantial agreement between the ion profiles observed here and those reported by Nixon et al. (31) after allowance has been made for the difference in operating pressures. Mass discrimination can be brought about in the QUISTOR so that the device may be operated in a mass spectrometric mode (32); mass spectrometric opera- tion has been effected in the cylindrical storage device also (33).

Electrons with a nominal energy of 55 eV were admitted axially to the QUlSTOR in pulses 50 ps wide, and ions ejected after a variable storage time by application of a pulse of -20 V magnitude to the end-cap adjacent to the quadrupole mass filter. It can be shown from the theory of operation of the QUlSTOR (34, 35) that when a radio-frequency voltage only is applied to the ring electrode the m/e values stable within the trap are given by

where Vo is the amplitude of the oscillating potential (radial frequency 2nf radians s-') applied between the end-cap electrodes and the ring, zo is half the closest distance between the end-caps, and qz has values between 0 and 0.9. The value offwas fixed at 1.6 MHz and Vo at 600 V . During individual experiments the source pressure was kept constant and during most experiments this pressure was 2.45 x Torr, as measured by the M.K.S. Baratron. At constant pressure, ion- molecule reactions may be studied in the QUlSTOR as a function of storage time only.

The DMSO was obtained from the Fischer Scientific Company, dried over molecular sieves, and dissolved gases were removed by freeze-pump-thaw cycles. The DMSO-d, was obtained from Stohler Isotope, Montreal and contained traces of mixed deuterated and hydrogenated molecules; the sample was used as obtained.

The progress of the ion-molecule reactions occurring in each sample is illustrated graphically as a plot of the logarithm

FIG. 3. Variation in the logarithm of normalized ion abun- dances in DMSO with storage time; 0-8 ms, 1.67 MHz, 600 V(p-p), 2.45 x Torr.

of relative ion abundance vs. storage time for each of the reaction products. Some experimental points have been omitted to prevent confusion.

Results and Discussion

While the uses of DMSO vary widely and occur in many fields (36), there are but four studies of the gas phase ion-chemistry: Blais et al. (37) studied the formation of positive and negative ions in DMSO; Mackenzie Peers and Muller (38) measured the dis- appearance rate constants of some primary ions in DMSO; more recently, McAllister (39) and Nixon et al. (31) have reported on ICR studies of DMSO.

The DMSO mass spectrum obtained under condi- tions of continuous ionization and zero storage is in excellent agreement with the electron impact mass spectrum at Torr reported earlier (37). The ion profiles for DMSO are shown in Fig. 3 for the storage time range 0.2-8 ms, and in Fig. 4 for 0.2-70 ms. Ions of m/e 12, 13, 14, 15, 16, 17, 18, 26, 28, 32, 33, and 46 which are not shown in Fig. 3 and Fig. 4 reacted completely in less than 400 ps by charge o r proton exchange reactions to produce the parent (M+) o r protonated parent ((M + I )+ ) ions. The rapid relative increase in M + (mle 78) and (M + I)+ (mle 79) is shown in Fig. 3. Other fragment or pri-

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CAN. J . CHEM. VOL. 56, 1978

FIG. 4. Variation in the logarithm of normalized ion abundances in DMSO with storage time; 0-70 ms, 1.67 MHz, 600 V (peak to peak), 2.45 x Torr.

TABLE 1. Ions observed in DMSO and DMSO-c/, together with structure assignment

nr/e in

DMSO DMSO-c/, Structures Reference

mary ions of m/e 27, 29, 31, 34, 35, 45,47, 62, and 63 react more slowly as shown in Fig. 3 and involve proton or charge exchange with parent ~iiolecules. The ion-chemistry of a given conipound is charac- terized by the reactivity, with parent ~iiolecules, of secondary ions and those primary ions which show an increase in relative concentration due to ion- molecule reactions. In DMSO, the ion-chemistry determining ions are of IH/LJ 61, 63, 78, 79, 93, 139, 141, and 157.

The mle 61 ion, which is predominantly C2H5S+ (31, 37), was observed to become one of the inore abundant species at the longest storage time used in this study, and the observation ofm/e 66 in DMSO-d6 supports the assignment of Table 1 . In the sole reference to the role of mle 61 (39) it is indicated, from ICR double resonance experiments, that n7le 61 is a product ion froiii a reaction of t?l/e 78.

In this study, growth of the parent ion (m/e 78) during the initial 800 ps of storage time was due to charge exchange from fragment ions. Reactions of the parent ion during the subsequent 2 ins (i.e. until the ion intensity could no longer be determined with precision due to the intensity increase of m/e 79) are given in Table 2.

The mechanisms for the formation ofthe (M + 1)' ion, of mle 79, have been proposed 011 the basis of ICR double resonance experiments (31, 39) or appearance potential measurements (37). Observa- tion of the (M + 15)' ion of n1/e 93 is of interest in this system. It is produced by methyl cation transfer from parent ions (37, 39), a reaction which is not observed in acetone. The t77le 102 ion was observed in DMSO-4. No reactions have been proposed for n7/e 93, [(CH,),SO]+, which persists at long storage time and it is concluded from a comparison with the persistence of (CH,),O+ and (C,H,),O+ (26) that it is a stable and unreactive ion.

In the four studies reported on DMSO, Mackenzie Peers and Muller (38) observe only the disappearance of reacting ions of m/e 78 and below without speci- fying products; Blais et al. (37) observe only those reactions which produce ions of mle greater than 78; in the concurrent ICR studies, McAllister (39) reports on double resonance experiments and gives three mass spectra, while Nixon et al. (3 1) report on double resonance experiments, give an ion profile a t unspecified pressure, a mass spectrum at unspecified time, and some rate data. Within the four studies the principal inconsistency arises from the observations of high mle ions.

The mle 139 ion, (M + 61)+, was reported by Blais et al. (37) and was observed in this work but was not observed in either of the ICR studies. The appearance potential method (37) indicated the formation of mle 139 from parent ion. However, in

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FULFORD ET AL

TABLE 2. Ion-molecule reactions occurring in DMSO. The parent molecule (CH3),S0 is understood as a reactant in each reaction

I ) I / E Ionic reactants t~z /e Products Reference

141 [(CH3)zSOOSCH3] + 157 [((CH3)2SO)zHI+, CHzSO 31, 39 79 [(CH3)2SOHl+ 157 [((CH3)zSO)zHl+ 31, 39 78 [(CH3)2SOl+. 155* [C4H,,SzOzI+, Hz 3 7 78 [(CH3)zSOl+ . 141* [(CH3)zSOOSCH3]+, CH3. 31, 39 78 I(CH~)ZSOI+ . 139 [c313,szozl+, (CHs.1 3 7 78 [(CH3)zSOl+ . 93 [(CH3),SOCH3] +, CH3SO. 37, 39 78 [(CH3)zSOl+. 79 [(CH3)2SOH]+, [CH3SOCH2]. 31, 37, 39 78 [(CH3)zSOIt . 61 [CzHSI+ 3 9 63 [CH3SO]+. 79 [(CH3)2SOH] +, CHzSO. 31, 37, 39 63 [CH3SO]+. 78 [(CH3)zSOl+' 3 9 62 [(CH3)2Sl+. 79 [(CH3)zSOHl+, [CHZSCH~I. 31, 39 61 [CH3SCH2] +. 139 [ C ~ H I ~ S ~ O I + ~ 47 [CH,S]+. 79 [(CH3),SOH] +, CHZS. 39 45 [CHS] +. 79 [(CH3)2SOH]+, CS. 3 9

'Not observed in this work.

this study, the initial appearance of the tn/e 139 ion is coincident with the maximum intensity of the 177/e 61 ion. The ion profiles suggest that in a high pressure domain the m/e 139 ion may arise as a collisionally stabilized product of an associative reaction of m/e 61 with parent molecule. Blais et al. (37) found that the ion intensity of the m/e 139 ion could be due to C,H, ,S20+ and C3H7S202+. From a comparison of the experimental and theoretical ratios for the mle 139 to mle 140 peaks they con- cluded that the ion must be C3H7S20,+. However in this study, the observation of the mle 150 ion in DMSO-(I, indicates that the ion is C,H, ,S,O+.. As mentioned previously, there is little commentary on the mle 61 ion in the ICR studies.

By way of compensation, the mle 141 ion (M + 63)+ was observed only in the ICR studies and was confirmed (39) by adding xenon with iso- topes 177/e 129-1 36 for mass calibration.

Double resonance experiments indicated that m/e 141 is a product of a reaction involving parent ions

and reacts to form protonated dimer

As an unsuccessful search for mle 141 was made in this study, it is suggested that reaction [4] may be endothermic for thermal ions. The explanations offered for the above inconsistencies are not incom- patible with a consideration of the operative condi- tions pertinent to each technique.

The observation of the m/e 155 ion (37) has not been confirmed in any subsequent study. The proton bound dimer, m/e 157 was observed in the ICR studies and in this study, and is the dominant ion at long storage times. Double resonance experiments indicated that it was a product of reactions involving mle 78 and m/e 79. Pressure variation in the QUIS- TOR indicated unit increase in the order of reaction with respect to DMSO concentration for m/e 157 relative to mle 79.

The ion-chemistry of DMSO is understood to be characterized by rapid simple proton transfer and by other processes which are relatively slow. At long reaction times, the thermodynamically favoured species appears to be the associative ion, the proton- bound dimer. While some inconsistency remains to be resolved concerning principally the ions of m/e 139, 141, and 155, there is considerable agreement among the results obtained by high pressure mass spectrometry, in cyclotron resonance and quad- rupole ion storage techniques. The measure of agreement serves to demonstrate the applicability of quadrupole ion storage mass spectrometry to the study of ion-molecule reactions though with some reservation concerning the definition of ion energy. Previous workers (4 1, 42) indicate that the energy of a small fraction (<0.1%) of ions in a QUlSTOR may be enhanced and oven additional endothermic reaction channels.

Acknowledgements

The authors acknowledge gratefully the financial assistance of Trent University and the National Research Council of Canada.

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2330 C A N . J . C H E M . VOL. 56, 1978

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