an investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and...

An investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and mesityl oxide

RAYMOND EVANS MARCH Department of Chemistry, Trent Universio, Peterborough, Ont., Canada K9J 7B8

A N D

ALEXANDER BALDWIN YOUNG Ontario Regional Ion Chemistry Laboratory (ORICL), Department of Chemistry, Universio of Toronto,

Toronto, Ont., Canada, M5S IAI

Received May 12, 1987 This paper is dedicated to Professor J . A . Morrison

RAYMOND EVANS MARCH and ALEXANDER BALDWIN YOUNG. An investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and mesityl oxide. Can. J . Chem. 66, 591 (1988).

An investigation of the structures of isomeric ions formed in ion/molecule reactions occurring in 2-propanone (acetone), 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol), and 4-methyl-3-penten-2-one (mesityl oxide) has been performed using a hybrid mass spectrometer of reversed geometry. Ions of mlz 1 17, which have been observed as products of ion/molecule reactions in acetone and in diacetone alcohol, may have a single structure if an aldol type condensation reaction occurs under acidic conditions in the gas phase; however, high energy collision induced dissociation (CID) results proved the two ions to be isomeric only.

Dehydration of the mlz 1 17 ions produced a species of mlz 99 which, in the case of diacetone alcohol, may have the structure of protonated mesityl oxide. That the mlz 99 ion formed was unique to each system was demonstrated by high energy CID and kinetic energy release (KER) spectrometry. In the acetone system, CID results suggested that rnlz 99 was an ion-dipole complex of c~H:(cH~)~co; this hypothesis was tested by clustering C~H: with (CD3)?C0 and comparison of its CID results with those obtained above. In the case of mlz 99 derived from diacetone alcohol or mesityl oxide, the neutral lost via metastable decomposition in the second field-free region was found to be C4Hs, presumably 2-methyl propene. While the ion of mlz 99 may also eliminate methane to yield a species of mlz 83, the dominant metastable elimination from this ion is carbon monoxide rather than ethene as shown by collision induced dissociative ionization (CIDI). The ions of mlz 83 observed in the two systems exhibit very similar CID spectra; however, a small variation in the yield of products of mlz 41 and 43 suggests two structures for the ion derived from diacetone alcohol, one of which is identical to that seen in mesityl oxide.

RAYMOND EVANS MARCH et ALEXANDER BALDWIN YOUNG. An investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and mesityl oxide. Can. J . Chem. 66, 591 (1988).

Utilisant un spectromktre de masse de gComCtrie inversee, on a Ctudie les structures des ions isomkres qui se foment au cours des rkactions molCculaires se produisant dans la propanone-2 (acktone), I'hydroxy-4 methyl-4 pentanone-2 (diacktone alcool) et la mCthyl-4 pentkne-3 one-2 (oxyde de mksityle). I1 est possible que les ions de mlz 117, qui sont observCs comme produits dans les rkactions ionlmolCcules de I'acCtone et du diacCtone alcool, aient une structure unique si une reaction de type aldolique se produit dans des conditions acides, en phase gazeuse; toutefois, il est possible que des resultats de dissociations induites par des collisions B hautes Cnergies (DIC) prouvent que les produits ne sont que des isomkres.

La deshydratation des ions de mlz 1 17 fournit une espkce de mlz 99 qui, dans le cas du diacCtone alcool, pourrait correspondre B un oxyde de mCsityle protonC. Faisant appel B des DIC B hautes Cnergies et B la spectromktrie d'emission d'tnergie cinCtique (EEC), on a dCmontrC que l'ion de mlz 99 est unique B chaque systkme. Dans le systkme de I'acCtone, les rksultats de DIC suggkrent que l'ion de mlz 99 est un complexe ion-dip6le de c~H:(cH~)~co; on a vCrifiC cette hypothkse en provoquant la formation d'un agrCgat entre l'ion C3H: et le (CD3)2C0 et en comparant ses rksultats de DIC avec ceux obtenus plus haut. Dans le cas de l'ion de rnlz 99 qui est dCrivC du diacCtone alcool ou de l'oxyde de mCsityle, on a trouvC que I'espkce neutre - qui est perdue par le biais d'une dCcomposition mCtastable dans le deuxikme champ de la region libre - est du C4H8, probablement du mCthyl-2 propkne. Alors que l'ion de mlz 99 peut aussi Climiner du mCthane pour fournir une espkce de mlz 83, I'ionisation dissociative induite par des collisions (IDIC) permet de dCmontrer que 1'Climination mCtastable la plus importante qui est observCe B partir de cet ion est celle du monoxyde de carbone plut6t que de 1'Cthene. Les ions de mlz 83 qui sont observCs dans les deux systkmes presentent des spectres DIC qui sont trks semblables; toutefois, une faible variation dans le rendement en produits de mlz41 et 43 suggkre qu'il existe deux structures pour l'ion provenant du diacCtone alcool et que l'une d'elle est identique Bcelle provenant de l'oxyde de mCsityle.

[Traduit par la revue]

Introduction Often, the only tractable evidence from gas phase ionimole- [ I 1 2 ,f( -

cule reactions is the variation in intensities of ionic species as a Acetone Diacetone alcohol function of reaction time or pressure, thus it is always a tempta- tion to invoke the reaction mechanisms and product structures observed from solution chemistry. A case in point is the well - known aldol condensation followed by dehydration reaction: Mesityl oxide

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in the gas phase, the condensation product may be fully covalently bonded;

OH O H

121 x+J, +,$J, I (m/z 117)

or proton-bound;

O H 0

131 A + - (>o)zH+

I1 (m/z 117)

The ionic products of these reactions are indistinguishable by conventional measurements of mlz ratio, necessitating further experiments for their differentiation. The ionic species investi-

CID spectra of mass-selected ion species were obtained by pressuriz- ing the second collision cell (CC?) with helium to produce a reduction of ca. 30% in the ion signal intensity. The indicated pressure from the ion gauge situated below the cell was 1 x Pa. CID spectra were corrected for metastable (unimolecular) decomposition by performing a second experiment in the absence of collision gas, and subtracting the metastable decomposition product intensities from those obtained in the CID experiment. Fast neutrals of several keV of kinetic energy produced through metastable decomposition in the 2ndFFR were ex- amined by collision induced dissociation ionization. In this technique, ions are deflected by means of the deflector electrode situated just prior to CC2, and only fast neutral fragments may enter the cell. When an appropriate reagent gas is added to CC2, the neutrals are ionized and dissociated; the charged species produced are subsequently analyzed via the MIKES technique. In the experiments involving m/z 99, OZ was employed as the reionization gas to reduce the fragmentation of the reionized neutral. He was used for reionization, in the other experi-

gated were derived from 2-propanone (acetone), 4-methyl-4- ments. hydro~y-2-~en tanon~ (diacetone alcohol), and 4-methyl-3- Acetone, diacetone alcohol, mesityl oxide, and 3-bromopropene penten-2-one (mesityl oxide), as it had been suggested earlier were supplied by Aldrich Chemical Co. and used without further

purification. Perdeutero-acetone was supplied by Merck Sharp and (1) that the species of mlz 1 17 formed via self-CI of acetone was Dohme Isotopes of Montreal, Quebec, Canada, identical to ~rotonated diacetone alcohol. and that dehvdration of this ion p;oduced mlz 99, tentatively identified as protonated Results and Discussion mesityl oxide.

A recent study (2) employed slow infrared multi-photon dissociation (IRMPD) to demonstrate that the species of mlz 117 obtained from acetone differed from protonated diacetone alcohol (reaction [2], structure I). Unfortunately, the species of mlz 99 and 83 proved unreactive with slow IRMPD and thus no decision could be made as to common or isomeric structures for these ions generated by different precursors. In an attempt to probe the structures, further experiments were performed with a reversed geometry mass spectrometer. The techniques employed were collision induced decomposition (CID) at high (keV) energy, kinetic energy release (KER) from metastable decomposition in the second field-free region (2ndFFR) and dissociative reionization of fast neutrals produced from metastable decomposition in the 2ndFFR. In addition, the exothermicity of the initial protonation reaction was varied and the effect of the subsequent change in internal energy upon metastable decomposition pathways was investigated.

Experimental A mass spectrometer of reversed geometry (3) (Vacuum Generators

ZAB2F) was employed. Ion/molecule reaction products were formed in the source at an estimated pressure of ca. 13 Pa using a conventional Vacuum Generators' combined electron impact/chemical ionization source. The second field-free region of this instrument is slightly more than a meter in length, and has been fitted with two gas collision cells for the study of bimolecular processes. The upstream collision cell, CC,, is permanently grounded and the downstream collision cell, CCZ, which is located at the first focal point of the instrument, is electrically isolated so that a variable voltage in the range +5 kV to -5 kV may be applied to it. In the region between the cells, which are separated by some 56 mm, is interposed an ion deflector electrode which may be grounded or to which a variable voltage to + 5 kV may be applied so as to prevent ions from entering CC2. The reversed geometry instrument has the advantage that a particular ion species may be selected by the magnet and any decomposition products formed in the 2ndFFR between the magnet (B) and the electrostatic sector (E) may be mass analyzed employing the well known MIKES technique (4); such decomposition may be either unimolecular or collision induced. The internal energy of the ions formed in the source was varied by employing either isobutane or methane as the chemical ionization reagent. 'The proton affinities of isobutane and methane are 683 kJ mol-' and 552 kJ mol-', respectively (5).

The experiments to be described were concerned with estab- lishing the structures of ions of identical mlz values formed via ion/molecule reactions occurring in either acetone, diacetone alcohol, or mesityl oxide. The ions investigated were, mlz 117 which is common to acetone and diacetone alcohol; mlz 99 which arises from dehydration of mlz 117 and which may have the structure of protonated mesityl oxide (111); mlz 83 observed in diacetone alcohol and mesityl oxide, and which is presumably formed by methane elimination from excited mlz 99 (reaction [41),

where *. denotes internal excitation. This ion may also have a common structure from its precursors, mlz 1 17.

mlz 117 Ions of mlz 1 17, formed from acetone via isobutane CI in the

source, were mass-selected in the magnetic sector and directed into the 2ndFFR. A fraction of metastable ions of mlz 117 fragmented unimolecularly to produce mlz 59 (99%) and mlz 99 (1 %). Isobaric metastable ions formed similarly from diacetone alcohol dissociated to form the same fragment ions but with the inverse ratio of products. The internal energy of the rnlz 1 17 ions was then increased by employing methane rather than isobutane as the CI agent; the proton affinity of methane is less than that of isobutane. Enhanced internal energy brought about an addition- al reaction channel, that is the loss of HZ. mlz 117 from acetone produced mlz 59 (96%), mlz 99 (1 %) and mlz 1 15 (3%), while isobaric ions from diacetone alcohol produced the same fragment ions but with the composition 25%, 73%, and 2%.

mlz 117 ions, again produced by isobutane CI, were subjected to CID using helium as target gas in CC2. After correction of product ion intensities for unimolecular decomposition, it was found that CID of mlz 1 17 from acetone produced tnlz 4 1 ( 1 %), mlz 43 (5%), mlz 58 (6%), mlz 59 (86%), and mlz 99 (2%). mlz 117 from diacetone alcohol produced mlz 41 (I%), mlz 43 (18%), mlz 59 (5%), mlz 61 (I%), mlz 83 (2%), and mlz 99 (73%).

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TABLE 1. Fragment ion abundances (%IICI) and experimental conditions for mass-selected mlz 99 ions derived from acetone, diacetone alcohol, and mesityl oxide

Metastable decomposition CID

Isobutane CI Methane CI Isobutane CI

Fragment Diacetone Mesityl Diacetone Mesityl Diacetone Mesityl (mlz) Acetone alcohol oxide Acetone alcohol oxide Acetone alcohol oxide

These results demonstrate that the species of mlz 117 derived from acetone is an isomer of that produced in diacetone alcohol. The dominant pathway of the former is loss of a neutral fragment of 58 amu, neutral acetone, and this result is consistent with a proton bridged dimer structure 11. In contrast, the dominant pathway observed in protonated diacetone alcohol is the loss of water, a known reaction of protonated ketones (6,7). The ion of mlz 43 from the CID of this latter ion may be due to loss of a neutral fragment of 74 amu, presumably t-butanol, or via losses of acetone and methane or losses of isobutene and water. Pro- tonated acetone, produced by acetone elimination from the ion of mlz 117, has been shown to eliminate methane to produce an ion of mlz 43 (6).

mlz 99 In Table 1 are shown the corresponding unimolecular frag-

mentations and CID of mlz 99, produced by isobutane CI and methane CI, from acetone, diacetone alcohol, and mesityl oxide. The species produced from mesityl oxide is simply the protonated parent molecule;

0 OH

151 )& + FH+ + + F

98 111 ( m l z 99)

whereas the other species are derived via dehydration of protonated diacetone alcohol;

I ( m l z 117) IV ( m l z 99)

or the proton-bound dimer of acetone;

Structure V has been suggested previously (2).

Inspection of the data in Table 1 shows that the three ion species differ in their metastable decomposition pathways. The species derived from acetone produces predominantly C3H>, mlz 41 and as the exothermicity of the initial protonation reaction is increased by changing from isobutane to methane CI, the rela- tive yield of C3H> is reduced whereas dehydration to rnlz 8 1 is increased. In addition, loss of C3H4, presumably allene to yield rnlz 59, is apparently reduced upon methane CI, however, the concomitant appearance of C2H30+, mlz 43, may be due to methane elimination from mlz 59, protonated acetone. The appearance of the moiety mlz 57 is postulated as the loss of a neutral fragment of 42 amu from mlz 99. Protonated acetone has been shown not to dehydrogenate readily (6), thus the genesis of mlz 57 from rnlz 59 is thought to be unlikely, hence the neutral lost is suggested as propene or ketene. rnlz 99 derived from diacetone alcohol primarily eliminates C4H8 to produce mlz 43 upon isobutane CI. The increase of protonation exothermicity afforded upon methane CI results in an increase in the efficacy of the dehydration pathway, producing mlz 81. In addition, minor amounts of rnlz 57 and 59 are produced. Protonated mesityl oxide, mlz 99, exhibits two major fragmentation routes in the absence of collision gas; dehydration to rnlz 81 and loss of a neutral fragment of 56 amu to produce rnlz 43. As protonation exothermicity is increased, the dehydration channel is dimi- nished, and the genesis of mlz 43 is increased. Minor pathways include methyl radical loss to produce mlz 84; methane loss to afford mlz 83; allene loss to yield rnlz 59 and loss of 42 amu to give mlz 57.

In Table 1 , the CID behaviour of the three ions is depicted, employing helium as the collision gas. Each of the ions exhibited unique behaviour and may be readily distinguished. In acetone, the major fragmentation route produced C3H;, rnlz 41. The species derived from diacetone alcohol predominantly lost a neutral fragment of 56 amu affording mlz 43, whereas the isomeric protonated mesityl oxide exhibited a plethora of products, the three major routes being the production of mlz 43, 8 1 , and 83.

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The species of mlz 99 derived from acetone is intriguing. Although the enthalpy of formation of the ion of structure V is unknown, it is the progenitor of both C3H+j and (CH3)2C+OH:

The observed preponderance of C 3 ~ : indicates that mlz 99 dissociates via kinetic control, not thermodynamic, suggesting a loose ion/molecule complex formed between the 2-propenyl cation (or if a 1,2 H shift occurs, the ally1 cation) and acetone. The appearance of C3H> from the postulated structure V involves only simple bond cleavage, whereas the reaction producing the protonated ketone, mlz 59, involves a proton transfer, hence the former reaction exhibits a more favourable frequency factor than the latter, and predominates despite a greater endothermicity of ca. 40 kJ mol-' .

The structure of V was further investigated by reacting 3- bromopropene with acetone-d6 and mass selecting the C3H: (CD3)2C0 species of rnlz 105. Acetone-d6 was employed to avoid complications arising from the mlz 99 species observed in acetone CI. The CID results obtained with helium in the second collision cell are depicted in Fig. 1, lower trace. For comparison purposes, the upper trace of Fig. 1 shows the CID behaviour of the fully deuterated species of mlz 110. This ion was produced via isobutane CI of perdeutero-acetone, the DOH elimination product ion being mass-selected. In the lower trace, the predominant fragmentation pathway is the loss of acetone-d6 to yield C3H:. The thermodynamically favoured route, producing (CD3)2C+OH, mlz 65, occurs with approximately equal facility to that shown in Table 1. The ion of rnlz 46 is CD3C+0 and is presumed to result through CD3H elimination from mlz 65.

In the upper trace of Fig. 1, the ion of rnlz 46 may have both the C3D: and CD3C+0 structure. The deuteronated d6-acetone product yield (mlz 66) is enhanced in comparison to the lower trace and Table 1, however, due to the isobaric products of rnlz 46, it is not clear as to whether this enhanced yield is due to a lowered activation energy for deuteron transfer or a diminution of the further fragmentation of mlz 66 to yield mlz 46. The cluster ion CID behaviour shown in the lower trace of Fig. 1 supports structure V for the dehydration product of the proton- bound dimer of acetone, mlz 1 17, although it may be argued that the species of mlz 99 observed in acetone is derived by clustering of mlz 41 with the neutral acetone, rather than by dehydration. Structures for the isomeric species of mlz 99 observed in both diacetone alcohol and mesityl oxide were proposed as I11 and IV, respectively:

Mesityl oxide, I11 Diacetone alcohol, IV

Upon methane CI, the metastable fragmentation spectrum of mlz 99 from diacetone alcohol differs significantly from that obtained upon isobutane CI, as shown in Table 1, and approaches the fragmentation spectrum of mlz 99 from mesityl oxide. This change in fragmentation patterns may be due to the formation of rnlz 99 with structures I11 and IV. The metastable decomposition of these ions to yield isobutene and mlz 43 may

0 4 PRODUCT ' &I m/i M 160

FIG. 1. Percentage ion composition of CID products derived via high energy collisions with helium in CC?. Metastable decompositions have been subtracted. In the lower trace, the 2-propenyl cation, C3H:, was reacted with perdeutero acetone, and the cluster ion of rnlz 105 was mass-selected via the magnet for high energy CID. In the upper trace, the fully deuterated analogous species formed in the source from self-CI of perdeutero acetone was mass-selected and collisionally dissociated in CC2.

be written:

It is anticipated that reaction [lo] which iilvolves a proton transfer should require a greater activation energy than reaction [ l I 1. This difference in endothermicity should be reflected in the metastable peak shapes for the two reactions (8), the peaks are shown in Fig. 2. In each of the traces, the energy spread of the mass-selected mlz 99 ion beam was reduced to ca. 0.3 eV which corresponds to approximately 3 eV translational energy. The upper trace was obtained from the metastable decomposition of protonated mesityl oxide (reaction [lo]); the lower trace corresponds to reaction [ l I ] . From the measured widths of the ion kinetic energy peaks at half height, corrected for the energy spread inherent in the precursor ion, the translational energy realease, Tjow, may be calculated (8). The elimination of C4Hs

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I I 1 364 366 37 1

VOLTS, ESA

FIG. 2. Ion kinetic energy peaks of the metastable decomposition of rnlz 99 in the 2ndFFR. The signals correspond to the process 99' + 43' + 56'. In the upper trace, rnlz 99 was generated via isobutane CI of mesityl oxide; in the lower trace, rnlz 99 was formed via isobutane CI of diacetone alcohol.

from protonated mesityl oxide (upper trace) exhibits T50% = 32 meV; the same loss from mlz 99 observed in diacetone alcohol has T50% = 10 meV. The larger ion kinetic energy release seen in the upper trace of Fig. 2 is attributed to a greater activation energy for the process. Such an observation is in agreement with reactions [lo] and [1 11.

That the neutral produced in these reactions was C4H8 was verified by collision induced dissociative ionization (CIDI) (9). The precursor ion (mlz 99) was prepared via isobutane CI, and although the dominant metastable pathway of this ion in the case of mesityl oxide is dehydration, no signals were observed for this H 2 0 species due to the low collection efficiency for reionized neutrals of low kinetic energy (ca. 1.5 keV for H 2 0 from mlz 99 of 8 keV). Oxygen was employed as collision gas in order to enhance the appearance of the molecular ion (lo), and results obtained for the neutral species of reactions [lo] and [ l 11 were similar.

mlz 83 The ion of mlz 83 was observed only in the diacetone alcohol

and mesityl oxide systems and, in the case of the latter, must arise through methane elimination from the protonated molecule. In the diacetone alcohol experiments, mlz 83 was observed in minor abundance in the metastable decomposition of mlz 99 and in somewhat greater abundance in the high energy CID experiment. This ion was also observed in the high energy CID of protonated diacetone alcohol, although its progenitor may be mlz 99.

mlz 83 was generated in the ion source from four separate sets of progenitors, diacetone alcohol and mesityl oxide with each of isobutane and methane as CI agents. The MIKE spectra were virtually indistinguishable for diacetone alcohol and mesityl oxide, and varied only with the CI gas used. With isobutane CI the predominant pathway is the loss of CO (vide infra) to produce mlz 55. Minor abundances, totalling 7% of the base peak and comprised of mlz 43, mlz 56, mlz 8 1, and mlz 82, were observed with diacetone alcohol. With methane CI, enhanced

0 OH ,k.&. : Diocetone olcohol

\ 5 0 U

PRODUCT m/z

)!&, ; Mesityl oxide \

5 0

PRODUCT m/z

FIG. 3. Percentage ion composition of CID products obtained upon high energy collisions with helium in CC2. Isobutane CI was employed to generate the rnlz 83 ions, and contributions to the ion signal intensities due to metastable decompositions have been subtracted.

protonation exothermicity resulted in an enhancement of H' and H2 losses at the expense of the pathway producing mlz 55, such that the intensities of ion signals due to mlz 55 and mlz 82 were equal. Approximately 10% of ion abundance was due to mlz 8 1.

In Fig. 3, the CID products of the two ions are depicted. The ions were generated via isobutane CI and the CID experiments were performed at identical reductions (ca. 30%) in precursor ion beam intensities. The major differences between the two spectra lie in the larger signals due to mlz 41 and 43 obtained from mlz 83 derived from diacetone alcohol, otherwise the spectra are similar.

A possible rationalization of the results may be made by postulating two isomeric ions of mlz 83 in the diacetone alcohol experiments, one of which is identical to mlz 83 observed in mesityl oxide.

The reaction to produce this ion may be written in the case of mesityl oxide;

m l z 99, I11 m l z 55 , V

and with mlz 99 as the precursor in diacetone alcohol;

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FIG. 4. Collisional reionization spectrum of the neutral fragment formed in the reaction 83+ + 55+ + 28'. Isobutane CI was employed to generate mlz 83 from mesityl oxide, and ions were prevented from entering CC2 by the application of ca. 3 kV to the deflector electrode situated prior to CC2. Helium was employed as the reionization gas in CC?; the collision cell was held at a potential of $2.5 kV in order to accelerate the ions formed in CC2, and thus to enhance the signal intensity (1 I).

FIG. 5. Collisional reionization spectrum of ethylene obtained via the McLafferty rearrangement of the molecular ion of 2-pentanone. Helium was employed as the reionization gas and CC2 was biased at $2.5 kV in order to enhance the signal intensities.

The loss of CO may be readily rationalized from structure V. An to C H ~ C + O , rnlz 43 or via a more endothermic pathway to alternative reaction of mlz 99 from diacetone alcohol to produce C3H:, mlz 41. 1n1z 83 may be written: The identity of the neutral produced via metastable decom-

position of mlz 83 to rnlz 55 in the 2ndFFR was investigated by O !I =3+ [Juj] - ,! ....... 11

CIDI. Briefly, rnlz 83 was prepared from mesityl oxide via [I41 isobutane CI and mass-selected by the magnet. With the

deflector electrode potential set at approximately 3 kV and He in m / z 99 m / z 83, VI CC2 at an indicated pressure of 1.2 X Pa, very little signal

where VI is a mixed associative species which may decompose was observed due to,the poor collection efficiency of 28+. at a

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translational energy of 2.7 keV (accelerating voltage for mlz 83 was 8 kV). To enhance the weak signals, CC2 was biased at +2.5 kV, hence any positively charged CIDI products received an acceleration from the cell and were more readily detected (1 1). The results are depicted in Fig. 4 , where the dominant peak is mlz 28. Although the peaks at mlz 12, 13, 14, 15 ,25, and 26 might suggest C2H4 as the neutral lost, the lack of a peak at mlz 27 and the low intensities of mlz 26 and mlz 25 indicate that C2H4 is a minor component only. The CIDI spectrum of C2H4 generated via a McLafferty rearrangement from the molecular ion of 2-pentanone (and hence potentially contaminated with some reionized CH3) is depicted in Fig. 5. The spectrum was obtained under identical conditions to that shown in Fig. 4 , and is clearly different, thus the identity of the neutral lost from mlz 83 is carbon monoxide.

Summary Gas phase ion/molecule reactions of acetone lead to the

formation of an ion of mlz 117 which has the structure of a proton-bound dimer. No evidence was found for an aldol condensation reaction occurring in the gas phase as the ion of mlz 117 formed from diacetone alcohol has been shown to be isomeric. The dehydration products of these isomeric ions yielded isomeric ions of mlz 99. In the case of acetone, CID studies suggested an ion-dipole complex of acetone and C3H< as the structure of mlz 99. This conclusion was substantiated by clustering C3H; with perdeutero-acetone and performing the CID experiment. The dehydration product of protonated diacetone alcohol, mlz 99, differed from protonated mesityl oxide as revealed by CID experiments and KER measurements. Reionization of the neutral produced in the metastable decomposition of mlz 99 from both diacetone alcohol and mesityl oxide revealed that the species lost was C4H8. The latter two ions also eliminate methane yielding an ion of mlz 83. CID studies of these ions showed nearly identical results; the differences observed were ascribed to two ionic structures present in the ion derived from diacetone alcohol, one of which was identical to that seen in mesityl oxide. Reionization of the neutral produced

in the metastable decomposition of these ions showed it to be CO rather than ethylene, C2H4.

Acknowledgements We are indebted to the Natural Sciences and Engineering

Research Council of Canada as the operation of the Ontario Regional Ion Chemistry Laboratory (ORICL) is funded, in part, by a grant from NSERC. R. E. March gratefully acknowledges the financial support of NSERC in the form of an Operating Grant and a Research Development grant, and of Trent Uni- versity. The constructive comments of the referees are much appreciated.

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2. R. E. MARCH, A. KAMAR, and A. B. YOUNG. Can. J. Chem. 64, 1979 (1986).

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an investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and...

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