JOURNAL OF MASS SPECTROMETRY, VOL. 31, 55 61 (1995)

Continuing the Search for a Non-classical Ethyl Cation Stabilized by Organic Molecules; the Triethyloxonium Ion

-

D. V. Zagorevskii,* M. Sirois, J. R. Cao, M. George and J. L. Holmes* Chemistry Department, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada

C. W. Ross, IIIt The National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306, USA

Triethyloxonium ions, (C,H,),O+, and their deuterated analogues were produced in a tandem mass spectrometer and a Fourier transform ion cyclotron resonance mass spectrometer by ion-molecule reactions in mixtures of ethyl iodide and diethyl ether. Three ion-molecule reactions were identified as the sources of (C,H,),O+, namely (C,H,),O+' + C,H,I + (C,H,),O+ + I' (l), (C,H,),OH+ + C,H,I + (C,H,),O+ + HI (2) and C,H,I+' + (C2H,),0 -+ (C2H5),0+(*C,H,) + 1- (3). Reactions (1) and (3) produced stable intermediate adducts; (2) did not. Metastable (C5HS),0+ ions formed by all these reactions lost C,H,. Ions from reactions (1) and (3) were pro- posed to have the classical Structure, and this isomer was also generated by the dissociation of the ionized adduct, [(C,H,),O'IC,H,JJ+'. Only C,H, loss from ions produced by reaction (3) showed H-D mixing in the ethyl group originally attached to iodine atom. No firm evidence was found for the participation of a non-classical H-bridged ion.

KEYWORDS: MS/MS; FT-ICR; oxonium ions; unimolecular dissociation; reaction mechanisms

INTRODUCTION

Theoretical calculations have shown that the ground state of the ethyl cation has a symmetrical, hydrogen- bridged ethene configuration.' The classical structure does not occupy a potential well and lies -27 kJ mol- above the ground state.' At this energy level, free exchange of H atoms between the carbon atoms can take place.,

However, when attached to a substrate molecule, the two forms of the ethyl cation can freely and indepen- dently exist and their relative energies are inverted. For example, protonated ethanol has been shown by exten- sive ab initio molecular orbital theory calculations3 to have two significant minima on its potential surface: a classical ion, CH,CH,O+H,, and a non-classical, hydrogen bridged water- ethene structure. The classical structure is about 53 kJ m o l ~ ' lower in energy than the non-classical. These two forms are separated by a sig- nificant barrier, 95 kJ mol-' above the (classical) ground state.

Detailed experimental explorations of these ions4 have agreed with calculations3 and methods for produc- ing the isomers have been described. The ground-state (classical) ion could readily be generated at threshold by the appropriate fragmentation of a larger ion, i.e. the reaction CH,CH(OH)CH(OH)CH, + CH,CH,O+H,

* Authors to whom correspondence should be addressed. Address: Chemistry Department, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5.

t Present address: DuPont Automotive Products, Marshall R&D Laboratories, Philadelphia, PA 19146, USA.

+ CH,CO'. It proved difficult to generate the pure non-classical ion, probably because of the relatively low barrier (40 kJ mol-') for its rearrangement to CH,CH,O+H, . Moreover, the isomers freely intercon- vert at energies below their (common) dissociation limits to C,H5+ + H,O and C,H, + H 3 0 + , which lie -40 kJ mol above their interconversion transition state.

Other studies2*5,6 have also considered the possible existence of analogous substrate-stabilized isomers and recent work from this laboratory has deliberately searched for such ions. Briefly, the classical and non- classical forms of (C,H,),X+ (X = C1, Br, I) and CH31+CzH5 ions have been produced and identified. A key identifying feature of the non-classical form is the loss of positional identity of H/D atoms in the labelled ethyl groups, in ions decomposing by metastable or collisionally induced dissociation pathways. Again, for the cations stabilized by alkyl halides, an ion containing the classical ethyl group was believed to be lower in energy than its non-classical counterpart.

In this paper, the mass spectrometric characteristics of the triethyloxonium ion are described. The tri- ethyloxonium cation is one of the most popular alkyl- ating reagents. The crystal structure of (C,H,),O+ - PF,- has been reported in Ref. 7 and in the solid phase these ions had a classical structure but with the average 0- C bond distance (1.499 A) significantly longer than that in neutral (C2H5j20 (1.433 A), indicating appre- ciable weakening of this bond. Hydrogen atoms were not located, but the structure of one rotamer implied the possibility of intramolecular 0. - .H interaction.

The unimolecular dissociation characteristics of the triethyloxonium cation have not been reported. The aim

CCC 1076-5174/96/010055-07 0 1996 by John Wiley & Sons, Ltd.

Received 1 August 1995 Accepted 26 September 1995

56 D. V. ZAGOREVSKII ET AL.

of the present study was not only to identify the non- classical isomer of (C2H5)30+, but also to investigate the equivalence of the ethyl groups, as a function of their origin from ion-molecule reactions (IMR).

EXPERIMENTAL

Deuterated ethyl iodides were purchased from C/D/N Isotopes (Dorval, PQ, Canada) and CH,'3CH,I from Cambridge Isotope Laboratories (Andover, MA, USA). Partially deuterated diethyl ethers were synthesized by the standard procedure using the corresponding deuter- ated ethyl iodide and C,H,ONa.

Triethyloxonium ions were produced by IMRs in mixtures of ionized vapours of diethyl ether (or its deuterated analogues) with C2H,Br, C,H,I or their labelled derivatives in a quadrupole, quistor, quad- rupole (QQQ) instrument (Extrel Millipore, Madison, WI, USA), Fourier transform ion cyclotron resonance spectrometers, Waters-Extrel FTMS-2000 or FTMS- 2001, operating at 3 T with a standard dual-trap con- figuration or in the chemical ionization source of a modified' VG Analytical (Manchester, UK) ZAB-2F mass spectrometer having BEE geometry.

To produce triethyloxonium ions or their labelled analogues in the ZAB-2F mass spectrometer, mixtures of the appropriate diethyl ether and ethyl iodide were introduced into the chemical ionization ion source to a total pressure (measured by an ion gauge) of 8 x lo->- 1 x Torr (1 Torr = 133.3 Pa). To increase the yield of ion-molecule reaction products the repeller voltage was adjusted to approximately zero.

Metastable ion (MI), collision-induced dissociation (CID) and neutralization-reionization (NR) mass spectra were recorded using the ZABCAT program,' All data listed in the tables are an average of at least ten measurements. Oxygen and helium were used for the collisional activation of ion source-generated and meta- stably produced triethyloxonium ions. For simple CID experiments the pressure of collision gas reduced the main beam by no more than 15%. To separate metasta- ble and collision-induced ethene losses from tri- ethyloxonium ions, two methods were employed. In the first, the intensity of the metastable peak was subtracted from the CID mass spectrum and, in the second, a potential of -1000 V was applied to the collision cell. The results of both experiments were complementary.

In the triple mass spectrometric (MS) experiments, ions formed in the second field-free region (2nd FFR) by the metastable dissociation of mass selected precursor ions (namely ionized ethyl iodide-diethyl ether dimers), were transmitted into the 3rd FFR and then col- lisionally activated. The pressure of collision gas in the 3rd FFR was adjusted to provide 70% transmission of the main ion beam. Kinetic energy release (KER) mea- surements were performed on the same mass spectro- meter at sufficient energy resolution to reduce the main ion beam width at half height to 3 V. The KER values were calculated from the half-height widths of the meta- stable peaks using established methods. The repro- ducibility of To.5 values was better than 20%. In the NR experiments, Me,NH and O2 were used for neutral-

ization and reionization, respectively. Their pressures were adjusted to give a main beam transmission of 50% and 80% respectively.

In QQQ experiments, ions of interest generated by electron impact ionization in the ion source were mass separated by the first quadrupole and transmitted into and trapped in the quistor chamber containing He and a reaction gas. For the detailed description of the instrument and experimental conditions, see Ref. 10.

Experimental control and data interpretation in Fourier transform ion cyclotron resonance (FTICR) experiments were accomplished by use of Extrel- Odyssey software running on a Sun Microsystem (Mountain View, CA, USA) computer station. Samples were leaked into the high-pressure source cell chamber of the FTMS-2000 instrument through 50 pm gold leaks to a total measured pressure between 5 x lo-' and 1 x Torr. For the FTMS-2001, the neutral reagent was introduced by use of a variable leak valve to a constant pressure of 3.5 x Torr and the reagent ions were generated from pulsed gaseous samples at a pressure of 1 x Torr. Note that these source conditions allowed multiple collisions to take place at usual reaction times (hundreds of seconds). Reagent ions were formed with low-energy electron impact between 9 and 12 eV for 20 ms. Ions were collisionally cooled before SWIFT" excitation was used to isolate the ions of interest. Ions reacted with the neutral gas for up to 10 s. If secondary ions were observed and were believed to generate the product ion, those ions were ejected in a quasi-continuous method composed of repeated SWIFT ejections and short reac- tion delays to yield the total desired reaction time.

Twenty co-added direct-mode digitized time-domain transients of 64K data points each were acquired. The data were padded with an equal number of zeros prior to Fourier transformation.

Fifty transients were coadded for sustained off- resonance irradiation CID (SORI-CID) spectra. " The ion to be dissociated was moved to the low-pressure analyser cell where continuous dipolar excitation, -1800 Hz off-resonance, was used for 1 s in com- bination with pulsed N 2 collision gas at 1 x Torr. After a short delay, the CID mass spectra were acquired at a pressure of 2 x lo-' Torr.

RESULTS AND DISCUSSION

Formation of triethyloxonium ions

Before describing tandem mass spectrometric experi- mental results concerning the various dissociation char- acteristics of (C,H,),O+, it is necessary to discuss how they may be formed in a chemical ionization (CI) source. Accordingly, a number of experiments were per- formed in a triple quadrupole mass spectrometer and in an ICR spectrometer.

Reactions of mass-selected ethyl cations with a variety of substrates, e.g. (C2H,)20, C2H,0H and C,H51, showed only proton transfer in ICR experiments and in the quistor at ion translational energies from 0 to 5 eV and independent of buffer gas pressure. Under

MS/MS STUDY OF TRIETHYLOXONIUM IONS 51

such conditions adduct ions should have been stabilized by collisional deactivation. We propose that direct C,H: ion addition to substrate molecules need not be considered further as a significant source of (C,H,),O+.

A study of (C,H,),O alone in an ICR cell showed only one reaction which led to a very small yield of

C2H50CHz+ + (CZH5),0 --f (C,H,),O+ + CH,O A second reaction, observed in the CI source of the tandem mass spectrometer, was the production of the proton-bound diethyl ether dimer [(C,H,),O],H+. This, on collision induced dissociation, produced a minor yield of (C,H,),O+. Under all experimental con- ditions this yield was too small to permit further study. Reactions in mixtures of diethyl ether with a variety of potential ethyl group donors were examined next.

Mixtures of (C,H,),O and *C,H,NO, (* denotes deuterium labelling) were examined. In the ICR cell, reactions of isolated (C,H,),O+ ' and (C,H,),OH + ions with neutral *C,H,NO, did not produce any (C2H5),0+(*C2H5). The latter ions were also not observed in the CI source of the ZAB mass spectro- meter when similar mixtures were studied.

Mixtures of (C,H,),O with *C,H,Br and *C,H,I were introduced into the ICR instrument and reactions of (C2HJ20+', (C,H,)20H+, *C,H,X+' (X = Br, I) and *C2H5XH+ ions were monitored. With the bromide only a small yield of (C,H,),0+(*CZH5) was observed and only from the reaction

*C2H,BrH+ + (C2H5)2O + (C,H,),O+(*C,H,) + HBr In the CI source (tandem mass spectrometer), mix-

tures of (C,H,),O and *C,H,Br produced no tri- ethyloxonium ions, probably because the above reaction is a minor, secondary ion-molecule process (the primary reaction is the protonation of *C,H,Br). In contrast, the (C,H,),O-*C,H,I mixtures proved to be good sources of triethyloxonium ions. Three reac- tions were identified under ICR conditions.

(C2H5)30f:

(C,H,),O+* + *C2H,I

(C2H&0Ht + *C,H,I + (C2H5)20+(*C2H5) + I' (l)

+ (C2H5)20+(*C2H,) + (2) *CZH,I+' + (C,H,),O

+ (C2H5)20+(*C2Hd + I' (3) Yields of triethyloxonium from reactions (1) and (2)

were small. The major source of (C,H,),O+ ions was reaction (3).

Results similar to those of the ICR experiments were found in the quistor, where mass-selected C2H51+' ions reacted readily with (C,H,),O yielding [(CtH5)20]2H+, (C,H,),O" and (C,H,),O+H.$ Reac- tion (3) as well as reaction (1) may involve the interme-

3 The MI mass spectrum of [(C,H,),O . ICD,CH,]+' showed (C,H,)O+D and no (C,H,),O+H ions, whereas only the latter were produced from the dissociation of [(C,H,),O - ICH,CD,]+'. These results indicated that the methylene group of ethyl iodide provides the hydrogen atom migrating to oxygen and a CH,CHI radical as the neutral lost.

diate adduct ion, [(C,H,),O . C,H,I]+', which was produced in the CI source. The MI mass spectrum of this adduct ion showed three principal pro- ducts, *C2H51+' (the most abundant species), (C2H5),0+(*CzH5) and (C,H,),O+(H,D). Kinetic energy releases for these reactions were measured as 6, 55 and 6 meV, respectively. The high To,, value for the I atom loss indicates that the triethyloxonium ions formed by this process may have a substantial internal energy and are not in their ground state. Note also that there was no ethyl group exchange between oxygen and halogen atoms and thus in the bimolecular complex the ethyl group originally attached to the I atom retained its identity.

Fragmentation of (C,H,),O+ ions

First, the AH," value for (C,H5),0L ions needs to be determined. As described elsewhere,6 ethyl cation afin- ities (ECA) can be devised via proton affinities (PA) of appropriate molecules. To estimate ECA of alcohols the PA for a variety of ethyl ethers were used. The ECA values for CH,OH, C,H,OH, (CH,),CHOH and (CH,),COH were calculated to be 209, 230, 238 and 243 kJ rnol-', re~pectively.'~ Now the change in PA value in going from ROH to ROR is - - 52 kJ mol- PA values for ROH being -800 kJ mol-'. Proportion- ally, therefore, the change in ECA in going from ROH to ROR is proposed to be - - 52 x 230/800 x - 15 kJ mol-'. Thus the ECA for diethyl ether is estimated to be -245 kJ mol-'. This, using the established values for AH,"[C,H,+] = 902 kJ mol-I (Ref. 13) and AHf0[(C,H,),~] = -252 kJ moI-I (Ref. 13) gives AH,"[(C,H,), 01 x 406 kJ mol-'.

From thermochemical considerations, three low- energy dissociation pathways exist:

(4) (C~H5)30+ --+ CH,CH,OCH+CH, + C2H,

(C2H5)30f + (C2H5).20'H + C Z H 4 ( 5 )

(C,H,),O+ + CH,CH,OCH,+ + C,H, (6) The sums of the product enthalpies for these reactions are 437, 492 and 489 kJ mol-', respectively, using data from Ref. 13. Relative to (C,H,),O+, all these reactions are endothermic, by 31, 86 and 83 kJ mol-', respec- tively. However, it should be noted that reactions (1) and (3), which involve neutral and ionized diethyl ether and ethyl iodide, are strongly exothermic, e.g. assuming the production of ground-state (C,H,),O + and 2P3,21' atoms, their exothermicities are -144 and -127 kJ mol- ', respectively. Each of these exothermicities exceeds the enthalpy changes of reactions (4), ( 5 ) and (6). Thus, unless all three reactions have an appreciable energy barrier, the ions (C,H5),0+C2H, produced from [(C,H,),O * IC,H,]+* should not be stable inter- mediates.

In the metastable time frame (ions dissociating in the 2nd and 3rd FFR) only one reaction of (CZH5)30+ was observed, namely reaction (5). The kinetic energy release, To.,, was 40 meV, somewhat larger than that typically observed for a simple bond cleavage reaction taking place without a reverse energy barrier. This is in keeping with the above remarks concerning the pro-

58 D. V. ZAGOREVSKII ET AL.

duction of triethyloxonium ions from [(C,H,),O - IC2HS]+* adduct ions and it is proposed that the barrier to reaction (5) lies at least 130 kJ mol-1 above the ground state for (CzHs)30+, i.e. -45 kJ rnol-’ above the thermochemical minimum for reaction (5). Note that neither reaction (1) nor (3) can directly produce ground-state (C,H,)30+ ions and, moreover, reactions (4) and (6), both less energy demanding than (5), were not observed as MI processes: reaction (4) involves formal H atom transfer and reaction (6) involves an appreciable rearrangement.

The other identified source of (C2H5)?0+, reaction (2), is almost thermoneutral and so IS capable of producing low-energy ions. The intermediate adduct, (C2H,),0 - H + IC,H,, was, however, not observed among ion source species.

Under CID conditions other dissociations of (C,H,),O+ became prominent, as shown in Table 1. Note that m/z 75, (C,H,)20tH, is omitted from the table, this peak in the MI mass spectrum is only weakly sensitive to collision gas, a result which will be discussed below.

Labelled ions

Deuterium labelling studies were undertaken in order to examine the mechanism of dissociation of (C,H,),O +

ions, their structures and the dependence of fragmenta- tion behaviour upon energy content.

Labelled triethyloxonium ions can be generated by two routes: using either a labelled ether or a labelled ethyl iodide. In principle, these routes might be physico-

Table 1. Collision-induced dissociations of (C,H,)30+ ions, their relative abundances (I)“ and reaction enthalpies

Reaction products I AH, (kJ mol-’)

C,H,OC+HCH, + C,H, 3 31 C,H,OCH,+ + C,H, 34 83” C,H60+H, +2C,H4 100 205 CH,CH+OH + C,H4 +C,H, 19 255 CH,CO+ + C,H4 + C,H, + H, 6 21 5 HOCH,+ + 2C,H4 + CH, 12 326 C2H6+ + (CZH6)Z0 32 344 C2H3+ + (C2H6)20 + H Z 13 344

CHI+ + (C,H,,O) H,O+ + 3C,H4 4 341

<1 -

* Relative to C,H,O+H, ions; (C,H,),O+H ions are excluded from the table because they were insensitive to collisional activation. bA/-/,[(C2H6)30+] =406 kJ mol-’ was used in all calculations.

164 and 451 kJ mol-’ , respectively. Reaction enthalpies for C2H4 + CH, and C,H,’ + CH,’ losses are

chemically equivalent, producing indistinguishable labelled ions.

Results for (C,H,),Ot ions fragmenting via reaction (5) are shown in Table 2. It is clear that no H exchange between ethyl groups takes place, as shown by the results for (C,H,),O+C,D, ions, which only lose C2H, and C2D,.

For dissociations of labelled (C2H,)30+ ions pro- duced by the dissociation of metastable [(C2H,),0 *

IC,H,]” adducts, the position of the label in the precursor ions was not significant. There were no

Table 2. Deuterium content in ethene lost from labelled triethyloxonium ions’

Ion Ion origin

(C,H,),O+CD,CH, (C2H6)20 + ICDZCH,

[(C,H6),0 ICD,CH,]+’ [ ( C, H ,O C D,CH , . I C, H 6] +

(C,H,),O+CH,CD, C,H,OCH,CD, + IC,H,

(C,H,),O + ICH,CD,

Mode C A

MI lFFR 71 MI 2FFR 69 MI 3FFR 71 CID 3FFR 67 SORI-CIDb 55

MI ZFFR 65 MI BFFR 69 CID BFFR 69 CI D 54 CI D 58

MI 2FFR 76 CID 2FFR 75

MI 1FFR 75 MI 2FFR 75 MI BFFR 78 CID ZFFR 76 CID - 60

MI ZFFR 75 CID BFFR 81 MI 2FFR 65 CID ZFFR 76 CID - 75 CID - 70

C A D *

22 27 25 29 45

35 31 31 46 42

24 25 14 16 17 15 - 40

- - - - - -

‘ The ions were generated in the ion source or from metastable ionized diethyl ether-ethyl iodide dimers. For ions aenerated from reaction (3).

MS/MS STUDY OF TRIETHYLOXONIUM IONS 59

peaks in the CID mass spectra which indicate any loss of positional identity of the label, e.g. the (C,H,),O+(CD,CH,) ions generated from meta- stable [C,H,OCD,CH, - IC,H,]+' or [(C,H,),O *

ICD,CH,] +' behaved identically, showing only the loss of C,H4 and C,H,D, in a ratio of - 1 : 0.8. Thus a small isotope effect favoring the loss of the heavier ethene was operative. The above ions therefore could not be distinguished by their origin, the three ethyl groups behaved identically.

The above results were not reproduced by ion source- generated species. For these ions their origin was important. As can be seen from the results in Table 2, (C2H,),0+(CD2CH3) ions derived from CH,CD,OC,H, and C,H,I and from (C,H,),O and CH,CD,I behave differently. Those for which the only label was in the diethyl ether showed no H -D mixing in either their MI or CID mass spectra. An isotope effect was observed for ethene loss (1 : 0.54), lower than that described above (1 : 0.8). Higher internal energies usually result in reduced isotope effects; the observed change of isotope effect must arise from the different origins of the ions. Note that under single collision conditions the metastable ethene loss peak increased by only -12% (see Experimental for details).

The labelled ions from the ion source reactions of (C,H,),O with CH,CD,I, however, showed clear indi- cations of H-D mixing in the labelled ethyl group (see Table 2). For metastable ions, the degree of H-D mixing decreased at longer times, as shown by the C2H2D2:C2H,D loss ratios in the lst, 2nd and 3rd FFR (3.1, 4.4 and 5.9, respectively). Moreover, the isotope effects now disfauoured the loss of labelled ethene, increasing in going from CH,CD, to CD,CH, and then remaining unchanged for CD,CD, (Table 2).

That the importance of the atom (H or D) migrating to the oxygen atom is greater than the mass of the leaving ethene is well illustrated by the results for I3C- labelled species. (C,H,)z0+13CH2CH3 ions were pro- duced in the mixture of unlabelled diethyl ether with I'3CH,CH,. The ion source generated species showed a statistical (2: 1) C,H,: 13CCH4 loss ratio in all the MI mass spectra recorded in the lst, 2nd and 3rd FFR and in the CID mass spectrum.

Ion structures and fragmentation mechanisms

In previous ~ o r k , ~ , ~ - ~ , ' ~ the observation of H-D mixing in ethyl-containing onium ions was attributed exclusively to the symmetrical non-classical isomer of type 11. Reversible isomerization between I and I1 at energies below or close to the dissociation limit for ethene loss would give rise to H-D mixing within an ethyl group. In the present study, triethyloxonium ions display no mixing unless they originate via the complex [(C2H,),0HIC2H,]+, i.e. via reaction (2). There is, therefore, a priori , no need to propose for ions gener- ated by reactions (1) and (3) the involvement of ion I1 other than as the transition state for ethene loss, or as a species in a shallow potential well, very close to the dis- sociation limit. The ethene loss can thus be described as a simple B-H+ transfer from methyl to oxygen. The dis- sociation of the protonated diethyl ether-ethene ion-

molecule complex follows, allowing the observed small isotope effect favoring the loss of the heavier ethene (e.g. in (C,H5)20+CD2CH3 ions).

f + _.: CH, (C H,CH,),O-C H,CH, (CH,CHJ,O-H .:. 11

CH,

I I1

Therefore, ion source-generated (C,H,),O + ions from reactions (1) and (3) and involving the interme- diate adduct [(C,H,),O * IC,H,]]+', are proposed to have structure I. Note that neither their metastable dis- sociation nor their collisional activation induced H-D mixing prior to the ethene loss. The latter result was confirmed by the study of (C,H,),O+CD,CH, ions generated in an ICR cell by selecting the product from the ion-molecule reaction (3). The collisional activation of these species resulted only in C,H, and C2H,D, losses. No C2H,D loss was observed confirming the absence of H-D mixing. Triethyloxonium ions gener- ated in the ICR spectrometer underwent multiple colli- sions prior to collisional excitation and probably possessed a lower internal energy than their analogues synthesized by the dissociation of the ionized mixed dimer in the tandem mass spectrometer. The lack of H-D mixing in the ions produced (directly or via the adduct) by IMR (3) demonstrated that structure I did not reversibly isomerize to I1 (or any other structure which could be responsible for hydrogen atom mixing) under any experimental condition available to us.

The yield of triethyloxonium ions from IMR (2) under ICR conditions was too small to record the SORI-CID mass spectrum of labelled (C2HJ3O+. However, this reaction remains the only possible source of the species showing hydrogen atom mixing prior to C,H, loss. It is possible that the non-classical ion I1 is only accessible via reaction (2). The barrier between I and I1 would have to lie significantly above the disso- ciation limit for C2H4 loss from I in order that H-D mixing only be available to 11. Alternatively, the inter- mediate adduct [(C,H,),OHIC,H,]+, which was not observed in either the ICR or sector mass spectrometer experiments, is nevertheless the species in which mixing of the proton (from the protonated ether) and H/D atoms from the ethyl iodide takes place. If ions of struc- ture I are produced from the adduct then they would apparently have undergone H-D mixing prior to CzH4 loss.

Kinetic energy releases were identical for all ethene losses and independent of the number of deuterium atoms and the origin of the ethyl group (i.e. from diethyl ether or ethyl iodide), making it highly likely that a single transition state is involved.

C 2 H 5 0 + H , ions, corresponding to two ethene losses, represented the most intense collision-induced disso- ciation of (C2H5),0+ ions, but the competition between single and double C2H4 losses was dependent on the origin of (C,H,)30+. For the species with low internal energy content, i.e. for those produced from a metasta- ble adduct in the tandem mass spectrometer and by IMR followed by collisional stabilization in the ICR

60 D. V. ZAGOREVSKII ET AL.

cell, C2H4 loss was a more significant process than the formation of C , H 5 0 + H , by 2C2H4 loss. The opposite result was observed for higher energy ions originating from the ion source of the tandem mass spectrometer.

The observation of an intense C2H,0CH2+ ion peak in the CID mass spectrum of (C&)30+ deserves addi- tional discussion. Formation of these species may involve C2H4 + CH,, C,H,' + CH,' and C,H, losses, The absence of CH, loss from protonated diethyl ether and ethylated ethanol'' and the non-observation of (C2H5)2O+' ions in the CID mass spectrum of (C,H5)30+ rule out the first two reactions. We could not identify C3H, as a molecule in either CIDI or NR mass spectra, possibly because of a low collisional ion- ization efficiency for this molecule and also the overlap of hydrocarbon ions with isobaric oxygen-containing species, C2H,0+. However, reaction (6) should be con- sidered as that producing C2H,0CH, +. The reaction probably occurs from structure I and must have a very high energy barrier and/or a very low density of states for the reaction intermediate.

NEUTRALIZATION-REIONIZATION

The neutralization-reionization mass spectrum of (C,H'),O+ ions is shown in Fig. 1. No signal for the hypervalent radical was found and the fragment ion of highest mass was m/z 59, C,H,0CH2+. It should be noted that a signal at m/z 84 appeared in the NR mass spectrum produced in the 2nd FFR; it resulted from the transmission of I + with an apparent m/z = 103 [the same as that of (C,HJ30+] from the dissociation of metastable C2H51f'. That it is an artefact is quickly seen by changing the magnetic field slightly, above and below m/z 103. The peaks in the NR mass spectrum rise and fall sharply whereas that from the metastable ethyl iodide ions persists over larger field changes, typical of a metastably generated artefact.

The NR mass spectrum of (C2H5)?O+ ions is remark- ably similar to those of (C,H,),O+ and (C2Hs)20+H. A very weak signal at m/z 74, (C2H5)20+', and no peak

I \ NO C,H,OCH,'

CH,+ I powh qH,O' , RECOVERY

SIGNAL

Figure 1 . Neutralization-reionization [(CH,),NH/O,) mass spec- trum of (C,H,),O+ ions.

for its protonated species at m/z 75 were observed. The neutral from the latter is probably not stable and the extremely low abundance of the m/z 74 ion must result from either the neutral diethyl ether produced by C2H5' loss from (C2HJ30' being so energy rich that it frag- ments rapidly by loss of CH,' (the route to m/z 59) or that its reionization produces no stable molecular ion, e.g. because of geometry differences. The origin of the m/z 45 ion is uncertain; it could result from loss of C2H, from C,H,O'(H)C,H, but a direct bond cleavage in such a radical is more likely. The fragment ions at m/z 43 and 31 can be produced by CH, and C,H, loss, respectively, from C,H,O+, m/z 59, the characteristic dissociation products of these ions.'

CONCLUSION

Triethyloxonium ions were produced by ion-molecule reactions in the gas phase. The most effective route to this species was a reaction between ionized ethyl iodide and neutral diethyl ether. The reaction involved the for- mation of a (stable) intermediate mixed ionized dimer [(C2H5),0 * IC,H,] +.. Similar IMR have been suc- cessfully employed to produce halonium, C,H,It CH, , 2

(C,H,),I+ and (C,H,),Br,6 and diethyloxonium, (C,H5),0+H,17 ions. This type of reaction, involving ethyl iodide as an ethyl group donor, can be used for the generation of other onium ions in the gas phase.

The dissociation of labelled triethyloxonium ions was dependent on their origin. A simple mechanism of ethene loss, involving a P-H atom shift, was operative in most cases. For example, no mixing was identified in ions produced from the ionized metastable dimer or generated by selected ion -molecule reaction (3). However, partial H-D mixing was observed for some ions generated in the ion source of the tandem mass spectrometer. This hydrogen atom mixing did not occur in ethyl groups of diethyl ether under any conditions, a result which was also observed for protonated diethyl ether.17 Only a labelled ethyl group originating from the ethyl iodide showed H-D mixing.

These results could not be rationalized in terms of the participation of the non-classical isomer of (C,H5)30+, but can be explained by the multiple origin of (C2H5),0 + ions in the chemical ionization source of the tandem mass spectrometer. The most likely source for the species showing mixed hydrogen atoms is reaction (1). This reaction must involve the formation of a (unstable) proton-bound dimer in which H- D mixing takes place in the ethyl iodide prior to their dissociation to (C2H5),0+ and HI.

Acknowledgements

J.L.H. thanks the Natural Sciences and Engineering Research Council of Canada for continuing financial support, and also thanks the Uni- versity of Bern and Professor U. P. Schlunegger for an International Exchange Fellowship during the tenure of which the quistor experi- ments were performed. D.V.Z. thanks Alan G. Marshall (National High Magnetic Field Laboratory, Tallahassee, FL, USA) for contin- uous support of the project. The authors (especially C.W.R.) thank Robert R. Matheson and William J. Simonsick, Jr, of DuPont Auto- motive for the use of their FTMS-2001 instrument to complete the ICR studies. We also thank Troy D. Wood (Cornell University, Ithaca, NY, USA) for helpful discussions regarding SORI-CID.

MS/MS STUDY OF TRIETHYLOXONIUM IONS 61

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