The Distonic Ion ‘CHJH,CH+OH, Keto Ion CH,CH,CH=O”, Enol Ion CH,CH=CHOH +‘, and Related C,H,O +’ Radical Cations. Stabilities and Isomerization Proclivities Studied by Dissociation and Neutralization-Reionization Michael J. Polce and Chrys Wesdemiotis Department of Chemistry, The University of Akron, Akron, Ohio, USA

Metastable ion decompositions, collision-activated dissociation (CAD), and neutralization-reionization mass spcctrometry are utilized to study the unimolecular themistry of distonic ion ‘CH,CH,CH-OH (2+‘) and its enol-keto tautomers CH,CH=CHOH-’ (l+‘) and CH,CH,CH=O+’ (3+‘). The major fragmentation of metastable 1"-3" is H. loss to yield the propanoyl cation, CH,CH,C=O+ . This reaction remains dominant upon collisional activation, although now some isomeric CH,=CH-CH+OH is coproduced from all three precursors. The CAD and neutralization-reionization (+NR+) spectra of keto ion 3+’ are substantially different from those of tautomers 2 +’ and l+‘. Hence, 3 +’ without sufficient energy for decomposition (i.e., “stable” 3 “) does not isomerize to the thermodynamically more stable ions 2 +’ or 1’; and the 1,4-H rearrangement H-CH2CH2CII=Ot’(3+‘) + ‘CH2CH,CH+O-H (2+.) must require an appreciable critical energy. Although the fragment ion abundances in the +NR+ (and CAD) spectra of l+‘ and 2+’ are similar, the relative and absolute intensities of the survivor ions (recovered C3HhO+’ ions in the +NR+ spectra) are markedly distinct and independent of the internal energy of l+. and 2+: Furthermore, l+’ and 2+ show different MI spectra. Based on these data, distonic ion 2+. does not spontaneously rearrange to enol ion l+’ (which is the most stable C3H60+’ of CCC0 connectivity) and, therefore, is separated from it by an appreciable barrier. In contrast, the molecular ions of cyclopropanol (4 ‘.> and ally1 alcohol (5 ‘.) isomerize readily to 2 +‘, via ring opening and 1,2-H- shift, respectively. The sample found to generate the purest 2 +’ is cu-hydroxy-y-butyrolactone. Several other precursors that would yield 2 +. by a least-motion reaction cogcneratc dctcctablc quantities of enol ion 1’; or the enol ion of acetone (CH,=C(CH,)OH+‘, 6 “), or methyl vinyl ether ion (CH,~CH=CH;‘, 7+.). Ion 6+’ is coproduced from samples that contain the - CH,CH(OH) - CH, substructure, whereas 7 +’ is coproduced from compounds combined with with methoxy substituents. Compared to CAD, metastable ion characteristics neutralization-reionization allow for a superior differentiation of the ions studied. (J Am Sac Mass Spectronz 1996, 7, 573-589)

R

adical cations C3H,0+. with the CCC0 frame appear in the mass spectra of many organic compounds [l] and, thcrcforc, have been investigated extensively both theoretically [2-61 and experimentally [7-141. It is now established that enol ion CH&H=CHOH+’ (l*‘) is the most stable isomer of CCC0 connectivity 1151 and that the threshold dissociation of this cation yields the propanoyl ion,

Address reprint requests Chemistry, The University

to Dr. Chrys Wcsdemiotis, Department of Akron, Akron, OH 44325.3601.

Q lYY6 American Society for Mass Spectrometry 1044-0305/96/$15,00 PI1 51044.0305(96)00053-O

by H’ loss [9-121. Computational CH,CH,C=O+, studies have shown that the lowest energy path for this fragmentation proceeds by an enol to keto tautomerization via the P-distonic ion ‘CH2CH2CHtOH (2+‘), namely, eq 1 [5, 61. n+

of

Reccivcd Novcmbcr 9, 1995 Rcviscd January 11, 1996 Accepted February 2, 1996

574

POLCE

AND

J Am Sot Mass Spectrom

WESDEMIOTIS

OH

Four relevant

(2)

O\11

(3bI

According to theory, distonic ion 2 +’ is a distinct, bound species that resides in a deep potential energy well and is thermodynamically more stable than the keto ion 3 +’ [5, 61. Although often postulated to arise upon the rearrangement fragmentation of molecular cations that contain the C,H,O substructure [7-10, 12-14, 161, 2+’ has not been thoroughly characterized yet and, considering the attention devoted to similar distonic ions [17-191, direct experimental data on this cation are scarce. McAdoo and Hudson [9, lo] first proposed that 2+’ may be formed upon the dissociative ionization of cycloalkanols (eq 2) [9], possibly together with l+’ and/or the enol ion of acetone [lo]. These authors later reported that the C,H,O +’ ion generated by CH,=O elimination from ionized 3methoxypropanal (eq 3a) most likely is 2 +‘; the corresponding collision-activated dissociation (CAD) spectrum was very similar to that of enol ion 1’; but included a unique feature, namely, a more abundant m/z 43 fragment (‘CH, loss) [12]. Unfortunately, such a fragment is not characteristic for structure 2 +. which bears no methyl substituent. The appearance energy of the C3HhO” ion from reaction 3a, determined by photoionization [12], led to a heat of formation of 757 kJ mall’. This value matches well the value for 2+’ theoretically predicted by the same authors (760 kJ mall’) [6], but also is very similar to the AHfO of the isomeric methyl vinyl ether ion (761 kJ mall’) [15a] that can be accessed from the 3-methoxypropanal precursor (eq 3b). In fact, an admixture of the latter ion, which is known to lose ‘CH, upon collisional activation [20, 211, would explain the aforementioned increased intensity of m/z 43 in the CAD spectrum of 2+‘. Most recently, Gu and Turecek [ 131 suggested that 2” is produced from ionized cis and trans 4-methylcyclohexanol, albeit contaminated with varying amounts of en01 ion 1+’ and the isobaric C,H,; cation. In light of the confusing and limited information available about distonic ion 2 +’ and its importance in the decomposition of other C,H,O” ions with the CCC0 frame, we present here a detailed study of the unimolecular chemistry of 2 +‘. (Preliminary results of this study were presented at the 1993 and 1994 ASMS conferences [22].) The stability and fragmentations of 2+’ are probed by metastable ion (MI) and collisionactivated dissociation [23] as well as by neutralization-reionization mass spectrometry (NRMS) [24-261.

issues are addressed

1996,7,573-589

in this paper.

The isomerization extent of the molecular ion of propanal (3 “I to the more stable distonic ion 2 +’ is appraised. Many similar keto ions have been found to completely or extensively rearrange to more stable distonic isomers [27-291. The composition of CZH60+’ from more than 10 linear and cyclic compounds is examined to determine which structural features favor the production of distonic ion 2 “, and which favor the production of en01 ion 1 + or other pertinent C3Hb0+’ isomers. The relationship is determined between 2 +. and the molecular ions of cyclopropanol(4 “) and ally1 alcohol (5 +.), which represent the ring-closed and a tautomeric form of 2+‘, respectively [22a]. Ions 4 + and 5 +’ have been studied previously [7, 11,30-331, although mostly not together and never in conjunction with isomer 2 +‘. Photoionization breakdown experiments found 4 +. and 5 +. to be indistinguishable [32], whereas their CAD spectra were shown to be very similar to those of enol ion l+‘, which suggests equilibration to the enol ion [31]. Finally, this investigation briefly questions the stability of neutralized 2+‘, that is, of the elusive 1,3diradical ‘CH,CH,CH ‘OH, which has been implicated as an intermediate in the pyrolysis and photolysis of C,H,O and homologous compounds 134, 351.

Experimental All experiments were conducted with a modified VG AutoSpec tandem mass spectrometer (VG Analytical Ltd., Manchester, UK) that has been described in detail previously [27]. This trisector E,BE, instrument has one collision cell (Cls-1) in the field-free region following the ion source (FFR-1) and two more collision cells (Cls-2 and Cls-31, with an intermediate deflector, in the region between E,B and E, (FFR-3). Tandem mass spectra (MI, CAD, or NRMS type) of source-generated precursor ions were acquired in FFR3 by using E,B as MS-1 and the second electric sector as MS-2. The structures of selected products in the MI and CAD spectra were further probed by MS” [23], where the fragment under investigation was formed in FFR-1 (via spontaneous or collision-activated decomposition of the C,H,O+’ precursor1 and then transmitted through appropriate setting of E,B to FFR3 for the acquisition of its own CAD spectrum. CAD spectra were measured by using 0, as the collision target (in Cls-3). Neutralization-reionization (+NR+I mass spectra [24-261 were acquired by neutralization of the C,H,O” precursor ion in Cls-2 with Xe and reionization of the emerging neutrals to cations in Cls-3 with OZ. Neutral fragment-reionization (+N,R+I spectra [36] were obtained similarly by replacing the neutralization target (Xe) with He, which,

J Am Sot Mass Spectrom

1996, 7, 573-589

owing to its poor neutralization efficiency, mainly causes CAD, not charge exchange of the precursor ion [37]. Note, that CAD liberates many neutral fragments simultaneously and that the +NrR+ spectrum observed represents the convolution of the collision-induced dissociative ionization (CIDI) spectra [38] of the individual neutral losses. For +NR+ or +N,R+, the pressure of each collision gas was adjusted to attenuate the precursor ion (main beam) by 20% (overall transmittance 64%). CAD was effected at 60% transmittance. The samples were ionized by electron impact (EI) at 70 eV (or as stated) and all ions that exited the ion source were accelerated to 8 keV. Kinetic energy releases were calculated from peak widths at half height (T,,) [39] and are corrected for the main beam width by using established procedures [40]. The +NR’ yield (i.e., absolute abundance) of the survivor ion or another +NR+ product was calculated by dividing its flux in the +NR+ spectrum by the flux of the unattenuated main beam. The spectra shown represent multiscan summations and the reproducibilities of relative and absolute abundances are better than plus or minus 10 and 30%, respectively. For several of the samples investigated, the $H,O+’ precursor (m/z 58) is contaminated by the C satellite of C,H,O+ (m/z 57). For example, the m/z 58 peak of ally1 alcohol contains 8.5% i3C’*C,H,0t. The MI, CAD, and +NR+ spectra of C3HsOt from ally1 alcohol reveal however that, at such a proportion, 13C satellites do not contribute appreciably to the MI, CAD, or +NR+ spectra of C3H60? Specifically, based on the +NR’ yield of the survivor ion of CaHsO+ from ally1 alcohol, which is over 10 times smaller than that of the ally1 alcohol ion itself, the survivor ion of ally1 alcohol (m/z 58) contains much less than 8.5% i3Ci2C,H,0+. A few C3H60t’ precursors contain admixtures of the isobaric hydrocarbon ions C,H& and/or i3Ci2C3Hi; these impurities are readily detected by their abundant m/z 42 fragments in MI spectra as well as by the appearance of m/z 48-51 fragments (C,H&,) in CAD and +NR+ spectra. The +NR+ yields of m/z 57 and 58 from C,H& and i3Ci2C,H: are extremely low (again, more than 10 times smaller than those of m/z 57 and 58 from C,H,O”); therefore, the hydrocarbon cations do not affect noticeably the [m/z 58]:[ m/z 571 ratio, which was used to deduce the structure of C3H60? Cyclopropanol was synthesized from epichlorohydrin, magnesium bromide, ferric chloride, and ethylmagnesium bromide by the procedure of Cottle [41] as modified by Roberts and Chambers [42]. Methyl vinyl ether and 3-methoxypropan-l-01 were purchased from Matheson Gas Products (Secaucus, NJ) and Karl Industries Inc. (Aurora, OH), respectively. All other chemicals were acquired from Aldrich Chemical Co. (Milwaukee, WI). Cyclopropanol was purified by repeated distillation, whereas all other samples were introduced into the mass spectrometer as received from the manufacturer.

DISTONIC

ION

‘CH2CH2CH+OH

AND

RELATED

C,H60+’

575

Results and Discussion Unimolecular Chemistry of Enol Ion CH,CH=CHOH +’ (2 +‘), Distonic Ion ‘CH,CH,CH + OH (2 +‘), and Keto lon CH,CH,CH=O +. (3 “) From the various potential precursors used for the synthesis of distonic ion 2 +’ in this study, only CXhydroxy-y-butyrolactone (HBL) produces this ion with an adequate intensity and essentially pure from isomerit and isobaric contaminants (vide infra). Dissociative EI of this compound leads to the incipient ‘CH2CH2CH+OH via ring opening and elimination of a small stable molecule (eq 4), that is, a reaction sequence that is commonly encountered in the preparation of gaseous distonic ions [ 18, 19,21,43]. First, the unimolecular reactivity of 2 +. (from HBL, eq 4) is compared to those of its well known enol-keto tautomers l+’ and 3+’ [7, 11, 441. The two latter radical cations are formed by the McLafferty rearrangement of 2-methylpentanal (eq 5) and direct ionization of propanal, respectively.

IHBL I

2+

Metasfable Ion Dissociations and Theiv lmpovfance After Collisional Activation. Metastable 2 +. that decomposes in FFR3 has an average lifetime of 10.5 ps and primarily loses H’ to form C3Hs0+ (m/z 571, see Table 1. is the only For isomers If’ and 3 +‘, H’ elimination reaction that takes place in this time window, whereas minor dissociation that 2 +’ undergoes an additional, produces m/z 30 (Figure 1). MS/MS/MS on the CaHsO+ fragment formed from metastable l+-3+’ in FFR-1 shows that it basically is the propanoyl cation + in all three cases (Figure 21, in line CH,CH,C=O with the previous findings of McAdoo and co-workers [9,12] and the theoretical prediction that CCCO-framed C3H60t‘ ions with threshold energies isomerize to propanal ions before H-atom cleavage (eq 1) [5, 61. The latter mechanism is consistent with earlier reported labeling studies, based on which keto ion 3 +’ only loses the carbonyl-bonded hydrogen [31], whereas eno121) lit l+’ and distonic 2+’ (from 3-methoxypropanal [ mainly (> 90%) eliminate one of their carbon-bonded H-atoms [9, 11, 12, 17, 30, 311. The m/z 57 signals from all three precursors have the same gaussian peak shape (Figure 1) and the corre-

576

POLCE

Table 1.

AND

WESDEMIOTIS

MI spectra of C,H,O”

J Am Sot Mass Spectrom

isomers Mass-to-charge

Precursor

30b

ratioa 43

42’

(HBL)

1.2 (ng)

Propanal Methoxyacetone Methyl vinyl Cyclopentanol

ether (CP)

3-Methoxypropanal

0.29

4-Methylcyclohexanol

(MCH)

3-Hydroxytetrahydrofuran 3-Methoxypropan-l-01

0.41' 0.38' 0.61 (ng)

(MEP) (HTF)

2.5

0.51'

(MP)

2-Methylbutanal

100

I+’

100

2+’

100

3+’

100

(g)

6+’

100

(ng)

7+.

2.2 (9)

fng)

Assigned structure

57d

2-Methylpentanal a-Hydroxy-y-butyrolactone

1996, 7, 573-589

2+’ & G+‘”

100

2+.

& 7+.e

5.7 (ng)

100

f (g) 0.67 (9)

100

l+' & 2+' & 6+' 2+‘ & 6+'

1.2 fng)

100

21' & 7+.

100

I+.

100

11

2-Methylundecanal

1.2

100

l+'

Pentanal

9.8

100

I+'

8.5

Hexanal Cyclopropanol

0.33 (9)

Ally1 alcohol

0.16

fg)

6 Relative abundances I” percent of base peak (from The letters “ng” and “g” I” parentheses designate i This fragment fC,H;‘) is due to Isobaric C,H:;, Gaussian signal for all precursors. eSome ‘Weak

I+’ also intensity.

could be present, see collisional background,

0.15 (g)

peak areas). a nongaussian

or gaussian

peak shape,

100

I+'

100

2+' & ci+.

100

2+'

respectwely.

text. and/or

adiacent

artefact

peaks

make

It

impossible

to

determine

the

peak

shape

and/or

its

abundance

a 100%

57 0

80: 601 40: 20: 0; 53514 b 100% 100%

O53

J

I

54

55

56

5l5

56

m/z

/ i; IV

I

m/z

c 1

Figure 1. MI spectra of (a) enol ion CH,CH=CHOH+’ cl+‘; from Z-methylpentanal, eq 5), (b) distonic ion ‘CH,CH,CH’ OH (Z+‘; from a-hydroxy-y-butyrolactone, eq 4), and (cl keto ion CH,CHZCH=O+’ (3+‘; from propanal). Unimolecular dissociation takes place in FFR-3. There is no m/z 30 peak above collisional background in the spectra of 1+ and 3 + ‘.

sponding kinetic energy releases are very similar (T&S = 200 meV). Our T,,, values are in good agreement with those reported for tautomers l*’ and 3 +’ by Holmes et al. [ 311, Hoppilliard et al. [ 31, and Turecek et al. [ll]. The large kinetic energy releases observed upon CH,CH,C=O+ formation from 1+‘3+’ indicate that there is a significant reverse activation energy for this reaction. For precursors I+’ and 2 ‘; which must first rearrange to yield the propanoyl fragment (eq l), a large T,,, is not unusual [23, 39, 40, 451. The large T, ‘r value for the propanal ion, from which H is lost by ‘a direct bond cleavage (see preceding paragraph [31]), is however surprising. Evidently, the reac=O+ + H is not continuously tion 3 +.+ CH,CH,C endothermic and has a sizable reverse barrier. The same conclusion was reached earlier by Turecek et al. [ll] and is in accord with preliminary ab initio computations by Bouchoux [Bouchoux, G., private communication; ab initio calculations at the MP2/ 6-31*//HF/6-31G*/ZPE level find the reaction CH,CH,CH=O-‘4 CH,CH,CO++ H. to have a reverse activation energy of 30 i 10 kJ mol-‘. This theoretical study, which updates ref 5, also provides new values for the heat of formation of distonic ion .CH,CH,CH+OH and for several isomerization barriers relevant to this ion, which include those that separate it from tautomers CH,CH,CH=O+’ and CH&H=CHOH+ , as well as those needed for cyclopropanol and ally1 alcohol ions to rearrange to ‘CH,CH,CHAOH (see Figure 3 or text for the exact

IAm Sac Mass

DISTONIC

Spectrom 1YY6, 7, 57%58Y

1'0 b 1003 803 603

1’0

AND RELATED C&O+

577

nn

1'5

2'0

2'5

3'0

3'5

4'0

4'5

---+-d m/z

1;

40; 20; o- ,e,, 10

ION ‘CH2CHZCH+OH

/, 15

, , 2lo

( 25

30

35

40

, 45

, ,

/ ,fy, m/z

,li

Figure 2. MS/MS/MS spectra (acquired in FFR-3) of C,H,O+ produced in FFR-1 from metastable (a) CH,CH=CHOH+’ cl+‘), (b) ‘CH,CHZCH+OH (2+‘), and (c) CH,CH,CH=O-‘ (3+‘). C,H50+ from metastable 3+‘, which has the propanoyl structure [9, 121, provides a reference spectrum for the CH,CH,CGO+ cation.

values).] All these results are reconciled by the potential energy diagram illustrated in Figure 3. The elimination of H’ remains the major decomposition channel of l+‘-3 +’ even after CAD (Figure 4). However now, the MS/MS/MS spectrum of the emerging C,H,O+ product (Figure 5a1 contains more tn/z 31, 36-39, and 55 than the spectrum of propanoyl (Figure 2~). These extra fragments are indicative of the isomeric 1-hydroxy-2-propenium cation CH,= CHCH L OH (protonated acroleinl; compare Figure 5b [46]. It is noteworthy that CH2=CHCH+OH is observed not only from enolic l+’ and distonic 2 +‘, which can yield this carbenium ion by direct cleavages of an H-atom, but also from keto ion 3 ‘1 which must undergo prior H rearrangement. Such behavior attests that collisionally activated 1+-3+' can readily interconvert before unimolecular decomposition. In contrast to collisionally excited lf’-3 +‘, metastable l+-3 +’ produce barely any CH2=CHCH’OH as shown, for example, by the negligible abundance of nz/z 31 in the MS/MS/MS spectra of Figure 2. Hence, the appearance energy of the CH,=CHCH+OH fragment must be larger than that of the propanoyl fragment (cf. Figure 3). As previously mentioned, a minor dissociation of metastable 2 +’ gives rise to a fragment of nr/z 30; the corresponding peak shape is nongaussian, with T,,, = 84 meV (Figure lb). The MS/MS/MS spectrum of m/z 30 (Figure 61 indicates the composition C,Hl’ (CO loss). The C,Hl’ fragment is unique for 2 +’

(Table 1); nevertheless it does not diagnose any spedistonic ion cific structural element in the ‘CH2CH2CHtOH. If collision gas is slowly admitted in FFR3, the peak shape of m/z 30 gradually changes to gaussian (cf. Figure lb versus Figure 4b). When the collision gas is introduced in FFR-1, two signals appear at m/z 30, namely, approximately 10% [C, H,, O]+’ and approximately 90% C,Hl’ (separable owing to the superior product ion resolution of linked scanning [23]). The MS/MS/MS spectrum of the combined m/z 30 peak remains similar to Figure 6. Apparently, the amount of [C, H,, O]+’ formed is too small to produce visible fragments at y1z/z 16618, which would allow one to determine the structure of this fragment (CHOH+’ or CH,=O-‘) [16, 471. The simplest dissociation route to [C, H,, O]+’ proceeds via the direct cleavage ‘CH&H,CH+OH

(2+‘) + CH,=CH,

+ CHOH+’

and leads to the carbenic isomer. The coproduction of approximately 10% [C, H,, O]+’ cannot be solely responsible for the complete change of the peak shape of m/z 30 upon admission of collision gas in FFR3. Most probably, collisional activation also opens a new pathway(s) to the major C,Hh+’ component, that mechanistically differ from the pathway taking place at threshold [40, 451. The critical energies required to form CH,CH,CEO’ (m/z 57) and C,Hz. (m/z 301 differ

578

I’OLCE

AND

WESDEMIOTIS

J Am Sot Mass Spectrom 1996, 7, 573-589

C2Hs+, + CO

E in kJ moK’

h

860 HO+

834e +

H

809 d

HO

.2 +. 665

Figure 3. Potential energy diagram for the reactions of metastable l’.-3+‘. Solid lines designate experimental data and broken lines designate theoretical data. Unless noted otherwise, heat5 of formation are from ref 15. “Bouchoux, G., private communication. hRefcrence 5. ‘Reference 12. “Based on lHy(CH,CHLC~Oi ) = 5Yl kj mol-’ (Traeger, J. C. 0~s. Mass S;wctvo!~f. 1985, 20, 223~227). ‘Using AH;(CH,CHZC~O’) = 616 kJ mom’, predicted by ah initio computations at the G2 level (Scott, A. I’.; Radom, L., private communication). ‘Obtained by adding to IAH: (CH-CHICsO’ + ‘H) the reverse activation energv calculated by Bouchoux (private communic&on). The presence of such a reverse barrier is &ealed by this~ and other [3, 17, 311 studies. ‘According to the predicted transition state levels shown in this diagram, the dissociations I+‘-, C;H,O’ and 2+‘+ C,H,O’ require the same critical energy (because l+‘+ 2+. costs less energy than 2*‘-* 3+’ 1. Experimentally (from appearance energies), the transition state energies of the latter two reactions are indeed found to be similar; they lie at 854 [II] and 865 kJ molt ’ [ 121, respectively. “The transition state for l+. + CH,=CHCH+OH + ‘H has been estimated to lie about Y20 kJ mol ’ [ 11 I. ‘The peak shape and T,, s value observed for this reaction (Figure lb) point out that its transition state l&s above’Y21 kJ &l ‘.

by over 50 kJ mol- ‘, compare Figure 3. The much greater amount needed for the latter rearrangement explains the very small yield of ethane ion in the MI spectrum of 2+ (Table 1). Metastable l+’ and 3+’ reacting in FFR-3 form no detectable amount of nr/z 30, even though they can interconvert to 2+. at the energy level that leads to this fragment (Figure 3). Presumably, the overall process (i.e., isomerization of l+' or 3+’ to 2+’ and subsequent loss of CO) becomes too slow to produce an observable metastable signal. In addition, the population of l+’ and 3 +’ with sufficient internal energy for spontaneous dissociation to C,H,‘+ CO may be negligibly small. For ion 3 ‘., this assumption is verified by the photoelectron (PE) spectrum of propanal. PE spectra are believed to adequately describe the internal energy ranges deposited on EI [48]; that of propanal displays a large gap ( - 100 kJ molt ‘1 about the energy necessary for C,H,+’ formation [49], explaining the lack of CO loss from 3 +‘. As expected, collisionally activated l+’ and 3 +‘, which are supplied the missing excitation, do form fragments at VZ/Z 30 (Figure 4). As with isomer 2+‘, CAD of l+‘

and 3+’ generates mainly C,Hi’ plus some [C, H?, 01’ ‘. The parallel reactivity to rn/: 30 of collisionally excited 1 +‘- 3 +. further documents the high propensity of these tautomers for dissociation through common intermediates. Because metastable 2+’ cnfl form C,Hl’, it should also be energetically capable of producing some CH,=CHCH+OH (viz. Figure 3). This theory is corroborated by the MS/MS/MS spectra of metastably generated CiH,O ’ (Figure 2); the spectrum of C,H,O’ from 2+’ (Figure 2b) contains a trace of IJI/Z 31, which is diagnostic of CH,=CHCH ‘OH (vide sup&, thereby verifying the formation of a small amount of CH,=CHCH+OH from precursor 2+‘. Theoretical data on the mechanism(s) of C,Hi. formation from l+‘p 3 +’ are not available. This fragment could arise via ion-molecule complexes, for example, through the sequence ‘CH ,CH ,CH ’ OH + C,H,,KHOH”C,H,:./CO + C,H;.+ CO 1501. Alternatively, C,H,‘+ CO may be formed after isomerization of the precursor ion to the carbenic species CH,CX-C-OH ” [5, 511. A third possible path-

n A >1'0 b 100~x10.00~

1'5

II ,~~T~,II'I,"11,"~'/~'1'/""," 25 30 35

2'0

40

45

50

m/z

*

+’ (I+‘), (b) d&tonic ion ‘CH,CH,CH+OH Figure 4. CAD spectra of (a) en01 ion CH ,CH =CHOH (2+‘), and (c) keto ion CH;CH,CH=O-’ (3”). Collisional activation takes place in FFR-3. Relative to [I>//; 291, the abundance of~ru,/z 57 is 17OG in (a), SOPi in (b) and 142% in cc). The ii//r: 2Y-30 region has been expanded in the insets to better show the presence of C;H,O’+ and C 3H,0” dications (marked by ’ 1 in the spectra of l+’ and 2+‘.

way could involve the keto ion CH?CH,CH=O-’ the reacting configuration for CO rupture.

the N/Z

30 and 57 fragments are more abundant for because m/z 30 and 57 also are present in the MI spectrum (vide supra), their relative intensities may be sensitive to internal energy effects and cannot be used for structural distinction. Compared to ions l+’ and 2 ‘; keto isomer 3 +’ gives rise to a unique

as

2 +: However,

Dissociations. The CAD spectrum of superimposable on that of enol ion 4). Only one minor difference is evident:

Collision-Activrrt~c 2 +. is practically

1 +’ (Figure

/““Ill -. 31

”

10

b 100 80 60 4od

,

15 x15.009

2’0

2'5

1

3'5

4'0

4'5

40

45

’ m/z

’

n

.2,5 ‘T”y II II II

:

20: o-,&,,, 10

3'0

,,, 15

20

,Y!;\,

,

30

Figure 5. (a) MS/MS/MS spectrum (acquired in FFR-3) of C,H,O+ produced in FFR-1 from collisionally activated CH,CH=CHOH+’ cl+‘). Precursors .CH2CHZCHfOH (2+.) and CH,CH,CH=O’ (3 *‘I give rise to quite similar spectra. (b) Reference CAD spectrum (MS/MS) of CH?=CH-CH’ OH formed in the mn source by self-protonation of acrolein (CH,=CH-CHO).

580

100% ‘.;(,,,,,,,,;-,$ I’OLCE

AND

J Am Sot Mass Spectrom

WESDEMIOTIS

28

x15.00-

80 60 40 20 0

26

15

10

Figure 6. MS/MS/MS 30 fragment generated in FFR-1.

2'0

m/z

spectrum (acquired in FFR-3) of the m/z from metastable ‘CH,CH,CH+OH (2+‘)

and structurally indicative CAD spectrum. Note that m/z 31 (‘CH,OH) and 17 (OH+), which are characteristic for OH-bearing structures (such as If’ and 2”) 1481, are markedly smaller for the keto ion (Figure 4~). Moreover, there are no detectable charge stripping products in the CAD spectrum of 3 +‘. In sharp contrast, the enol and distonic ions, which can better support two separated charges [231, produce clearly visible C3H602+ (m/z 29) and C,H,02+’ (m/z 28.5) (viz. Figure 4a and b). Neutral Fragment-Reiotzization. CID1 [25, 26, 381 of the neutral losses released from collisionally activated l +‘-3 +. gives rise to the neutral fragment-reionization (+N,R+) spectra of Figure 7. Helium was used for CAD of the mass-selected reactant ions (m/z 581. Due to its high ionization energy [15], this target minimizes

17

10

the extent of concomitant charge exchange [37]. Still, some neutralization takes place, as shown by the appearance of a recovered, that is, neutralized-reionized, WZ/Z 58 (Figure 7). This ion and the weak signals above m/z 29 are attributed to neutralization-reionization (+NR+) of l+‘-3-‘ (vide infra). On the other hand, the major source of m/z < 29 should be the neutral fragments eliminated upon CAD of l+‘-3 +‘. Enol ion l+’ and distonic ion 2 +’ yield the same fragment ions on CAD (viz. Figure 4a and b). It is, therefore, not surprising that the corresponding +NiR+ spectra (i.e., the complementary neutral losses) also are essentially identical (Figure 7a and b). In contrast, the ‘N,R+ spectrum of keto ion 3+’ (Figure 7c) is dominated by VI/Z 28 and displays several distinctive features, inter alia less abundant nr/z 17-18 and 24-27. The larger Hi-,O+’ signals for 1 +’ and 2 +’ are consistent with their OH-carrying structures, from which water can be eliminated more readily than from keto structure 3 +’ [48]. Comparison of the CAD and +N,Rdata (Figures 4 and 7) further points out that most water is cleaved upon the decomposition C,H,O+‘+ C,Hl (m/z 39) + H,O + ‘H. Similarly, the dissociation C,H,O+‘+ +CH20H (nz/z 31) + ‘C,H,, which is much more pronounced for l+’ and 2 +’ than for 3 +. (viz. Figure 4), must be the major source of the enhanced relative intensities of C,Hi-, (m/z 24-27) in the +NfRt spectra of l+’ and 2+’ (Figure 7). The most abundant peak in all three +NiR+ spectra appears at m/z 28. A fraction of this product origi-

28

x5.00-----w

20: o-

18 Jq

,

15

, , ,

19~6, 7,57x-589

20

, ,

I””

25

30

b 100%+---x5.00-

3'5

““l”“l”“l”‘~I”“I

40

r-%-A

45

50

F

m/z

!

80:

1

60: 40:

17

10

15 c lOO%le---x5.00-

18 20

I I/I'

25

Ii I II '1 I I / '0 I I( ' I II'< 28

30

35

40

45

I'I'I

50

-AA I I II "I m/z

c

60:

01 10

I

15

20

25

30

/I "'I"

35

40

1'11

45

1 I I I ( I IPI 50

m/z

*1

(+NrR+J spectra of (a) enol ion CH,CH=CHOH-’ cl+‘), Figure 7. Neutral fragment-reionization (b) distonic ion ‘CHzCHzCH+OH (2*‘), and (c) keto ion CH,CH,CH=O+’ (3+‘). These spectra contain the postionized neutral fragments eliminated upon CAD of l+ ’-3 +‘. The ionic fragments coproduced upon CAD of l+‘-3+’ are shown in the spectra of Figure 4.

J Am SW Mass Spectrom lYY6, 7, 573 58Y

DISTONIC

nates from the reaction C3HhO+’ (l+-3+‘) ---) C,Hl’+ CO (28 u), which is observed from all three tautomers (vide supra). Two other neutral CAD losses that can contribute (after postionization) to m/z 28 are the isobaric radicals ‘CHO and ‘C1H5, cogenerated with C,Hl and +CHO (m/z 29 in the CAD spectra).

--

.CHzCHzCH

OH

AND

RELATED

H-CH,CH,

‘CH2CHZCH2CH+O-H vis 2 vis that of the 1,4-migration H-CH,CH,CH=O+’

(3”)

--)

‘CH,CH,CH’O-H(2+‘)

Distinguishing between tautomers l+. and 2 +’ is a much more difficult task. As previously mentioned, their CAD (Figure 4a and b) and +N,R+ (Figure 7a and b) spectra are practically identical. Their +NR’ spectra are similar too (Figure 8a and b), but include three discrete characteristics: 1. The absolute intensity of neutralized-reionized C3HhOf’ at m/z 58 (“recovery peak” or “survivor

29 " n

/JWlL n

17

o- &fJ+L 15 10

[17].

58

60 7 80 404

2'0

39

31

rr j-1

I (I

25

30

I I 35

40

, / , , , I , , ,': &Jyj 50 m/z

,

45

b loo%-x5.00+ 80: 26

60: 401

28

58 : 39 t

10 15 c 100~x5.00-----w

Figure 8. (b) d&tonic abundances for (b) and experimental

/II'

20

I""

'25

3b

581

CH,CH=O+‘+

x5.00-+.

20:

C?H,O+’

propanal ions that have been collisionally activated above their dissociation threshold do inferconzwf with 1 +. and/or 2+’ to some extent, as attested by the coformation of CH2=CHCH’ OH (W/Z 57) and the formation of some +CHZOH (311, H,O+’ (18), and OH+ (17) upon CAD (vide supra). The behavior of 3 L’ sharply contrasts to that of the butanal ion, CH ,CH ,CH ,CH= 0’ , homologous which to a large extent rearranges spontaneously to the distonic ion .CH,CH,CH,CH+OH [27, 281. This difference most likely arises from the lower critical energy of the 1,5-H migration

Neutrnlizntiorz-Reioni=afiolz. The -NR+ spectrum of 3 +. also is markedly different from the spectra of l+’ and 2 +’ (Figure 8). Again, m/z 31 and 17 are much smaller for the keto ion 3 +’ than for the OH-carrying ions I+' and 2 +‘. Other features of 3+’ that stand out are the high abundance of M/Z 29 (C,H,c and +CHO) and the relatively low yield of C,H: (nz/z 39) and C2HRO+ (VZ/Z 43), as also seen on CAD. The unique CAD, +N,R+, and -NR+ characteristics of 3 +. clearly indicate that propanal ions with insufficient energy for unimolecular decay (which are those sampled in CAD, ‘NfR+, and -NR+ experiments [23-261) do not readily isomerize to more stable l+’ and/or 2 +‘. Based upon the [W/Z 18]:[ nz/z 281 ratio in the ‘NfR+ spectra (Figure 7) as well as the [W/Z 31]:[ nz/z 291 ratio in the ’ NR - spectra (Figure 81, less than 15% of nondecomposing propanal ions (lifetime > 10 ps) have interconverted to If’ and/or 2 +‘. In fact, the absence of molecular dications in the CAD spectrum of 3 +’ demonstrates that no detectable amount of stable CH,CH,CH=O ” (zuhich cmnof dissociate) rearranges to distonic or enolic structures. On the other hand,

a 100”

ION

""1""l""l""I""l"" 35

40

45

50

m/z

Neutralization-reionization (+NR+) spectra of (a) enol ion CH,CH=CHOH+’ Cl+'), ion ‘CH,CH,CH+OH (2+.), and (c) keto ion CH,CH,CH=O+’ (3*‘). The absolute (i.e., +NR+ yields) of the recovery peaks (survivor ions) are 9 X lo-” for (a) 9 X lo-’ 7 X lo-’ for cc). The spectra were obtained with 70-eV EI and do not change outside error if the electron energy is reduced to 15 eV.

582

POLCE

AND

WESDEMIOTIS

J Am Sot Mass Spectrom

ion”) is significantly higher for the enol ion than for the distonic ion. 2. The ratio [C3HbOt’]:[CaHsOf] is much greater for l+’ (1.4) than for 2+’ (0.41; peak heights used in both cases). 3. The abundance pattern of m/z 26-29 from +NR+ of 2 +; namely, [m/z 261 > [m/z 271 < [m/z 281 > [m/z 291, is slightly, yet reproducibly, different from the pattern observed from l+‘, namely, [m/z 261 > [m/z 271 < [m/z 281 < [m/z 291. Changing the internal energy of precursor ion 1 +‘, by forming it from different compounds, does not alter appreciably the described features. For example, +NR+ of CaHhO+’ from 2-methylbutanal, 2-methylpentanal (Figure Ba), 2-methylundecanal, n-pentanal, and nhexanal (which have been shown to yield isomerically pure l+’ 17, 11, 12, 17, 441) gives rise to [CsH60+‘]:[C3H50+] ratios of 2.0, 1.4, 1.5, 2.4, and 1.6, respectively. These ratios are markedly larger than the value obtained for 2 +’ (0.41). Obviously, the differences between Figure 8a and b are not caused by internal energy effects but rather originate from the different structures of ions l+’ and 2+‘. The smaller survivor ion for 2+’ most probably results from the less favorable Franck-Condon factor of the redox sequence 2 +‘--) 2 (diradical) --) 2 +’ vis a vis the sequence l”+ 1 (closed-shell enol) 4 l+’ [23-261. Overall, our combined tandem mass spectrometry experiments provide sufficient information to determine the fate of the incipient distonic ion 2+’ that

Table 2.

Partial CAD

and +NR+

spectra of C3H60+’

431 [m/z 391

Precursor 2-Methylpentanal

arises from HBL (eq 4). According to the CAD spectra (Figure 4), this ion either maintains its original structure or rearranges to l+‘; contamination by other, known C3H60t’ isomer(s) can be excluded [7-14, 20, 21, 31, 43, 511. The MI data (Figure 1) further point out that the [ HBL-CO,]*’ beam cannot be (entirely) l+‘. This is confirmed by the +NR+ spectrum of [ HBL-CO,] +’ (Figure Bb), which is characteristic of structure 2+’ (vide supra). In addition, the unique MI and +NR+ features assigned to ion 2+’ are reproduced when this ion is generated from other logical precursors (see next section). Based on all these facts, we conclude that distonic ion 2+’ is a distinct C,H,O+’ isomer, as proposed by theory [5, 61 and suggested by the previous studies of McAdoo and co-workers [B-lo, 121. Consequently, the similarity of the CAD, ‘NrR+, and (to a lesser degree) +NR+ spectra of l+’ and 2+’ must result from the dissociation of these two ions through common intermediates. It is interesting that the biggest spectral difference between l+’ and 2 +’ is observed in the relative and absolute abundances of the survivor ion (Figure 8a and b), whose generation involves gas-phase redox reactions but no dissociation.

Other Precursors for Distonic Ion ‘CH,CH,CH + OH (2 “) In the search for the best precursor for distonic ion 2 ‘; several linear and cyclic compounds were investigated (Tables 1 and 2). With the exception of HBL (eq 4), all other compounds considered were found to coproduce

isomers

CAD”

[m/z

1996, 7,573-589

+NRta

[m/z 431 [m/z 391

[m/z

581

[m/z

571

Assigned Base

peak

structure

0.18

0.12

1.4

58

I+'

0.21

0.15

0.41

57

2+’

Propanal

0.16

0.10

1.4

Methoxyacetone

2.2

1.7

59 0.52

o-Hydroxy-y-butyrolactone

Methyl vinyl Cyclopentanol

(HBL)

ether (CP)

3-Methoxypropanal 4-Methylcyclohexanol

(MEP) (MCHY

3-Hydroxytetrahydrofuran 3-Methoxypropan-l-01

1.2

(HTF) (MP)

58

3+’

13

58

6+.

29 0.26

33 1.2

58 58

2t.

7+’ & 6t.b

0.71

1.2

58

2f’

& 7t.b

0.18

0.11

1.1

58

0.24

0.15

0.57

57

l+'

2+’

& 2+'

2+’

0.24

0.14

0.61

57

2-Methylbutanalc

0.21

0.1 1

2.0d

58

2-Methylundecanalc

0.19

0.14

1.5d

58

I+’

n-Pentanalc

0.20

0.13

2.4d

58

l+’

n-HexanalC

0.31

0.14

1 .6d

58

Cyclopropanol

0.26

0.20

0.50

57

Ally1 alcohol

0.22

0.13

0.25

57

a From peak heights. b Some l+' also could be present; see text. ’ These precursors coproduce measurable amounts of the isobaric C,H& cation (viz. Table 1). Based on its CAD mzy affect [m/z 431 and [m/z 391, but does not contribute appreciably to the +NR+ peaks at m/z 58 and 57. The abundance pattern for m/z 26 -29 is identical to that in Figure 8a. that is, [m/z 261 > [m/z 271 < [m/z

& 6+'

& 6” & 7f’ I+'

I+’ 2+’

& 6+’ 2+’

and

+NR+

281


and ally1 alcohol (5 “1 are found to spontaneously rearrange to 2 ‘., which, as the more stable isomer, should be the common structure; further isomerization to enol l+’ does not take place.

Acknowledgments We thank Fred A. Wiedmann and Dr. Michael J. Taschner for the synthesis of cyclopropanol and Dr. John C. Traeger for useful comments. We are indebted to Dr. David J. McAdoo and Dr. Charles E. Hudson for a sample of 3-methoxypropanal as well as helpful suggestions, Dr. Guy Bouchoux for providing us his newest theoretical data on the C,HI,O+‘ system, and Dr. Leo Radom for the G2 value of the propanoyl cation. This work was supported by the Petroleum Research Fund, which is Administered by the American Chemical Society, and the University of Akron.

16. 17. 18. 19. 20.

S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levm, R. D.; Mallard, W. G. I. F%ys. Chem. Ref. Datn 1988, Suppl. 1. Wesdemiotis, C.; McLafferty, F. W. Tetmhedro~ Mt. 1981, 22, 3479~3480. Bouchoux, G. Mass Spectrom. Rev. 1988, 7, l-39. Hammerurn, S. Mnss Spectronz. Rcu. 1988, 7, 123-202. Stirk, K. M.; Kiminkinen, M.; Kenttamaa, H. I. Chern. Rev. 1992, 92, 1649~1665. Turecek, F.; McLafferty, F. W. 1. Am. Gem. Sot. 1984, 106,

2528-2531. 21. Polce, M. J,; Wesdemiotis, C. J. Am. Sot. Mass Spectrom. 1995, 6, 1030@1036. 22. (a) Polce, M. J.; Wiedmann, F. A.; Wesdemiotis, C. Proceedings of tke 4Zst ASMS Cqference, San Francisco, CA, May 31pJune 4, 1993, pp 283a-283b. (b) Polce, M. J.; Wesdemiotis, C. Proceedings of the 42nd ASMS Conference, Chicago, IL, May 29-June 3, 1994, p 206. 23. Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spectrometry/Mass Spectromctry; VCH Publishers, Inc.: New York, 1988. 24. Wesdemiotis, C.; McLafferty, F. W. C1zem. Rev. 1987, 87, 485~500. 25. Terlouw, J. K.; Schwarz, H. Angw. Ckem. Int. Ed. En@. 1987, 26, 8055815. 26. Holmes, J. L. Mass Spectrom. Reu. 1989, 8, 513-539. 27. Polce, M. J.; Cordero, M. M.; Wesdemiotis, C.; Bott, I’. A. Int. 1. Mass Spectrom. Ion Processes 1992, 113, 35-58. 28. Bouchoux, G.; Alcaraz, C.; Dutuit, 0.; Nguyen, M. T. Itit. 1. Mass Spectrom. Ion Processes 1994, 137, 93-106. 29. Zeller, L.; Farrell, J., Jr.; Vainiotalo, I’.; Kenttgmaa, H. I. 1. Am. Ckem. Sot. 1992, 114, 1205-1214. 30. Kurland, J. J.; Lutz, R. P. I. Chem. Sot. Ckem. Commute. 1968, 1097-1098. 31. Holmes, J. L.; Burgers, I’. C.; Mollah, Y. A. Org. Mass Spectram.

References 1. McLafferty,

F. W.; Stauffer, D. B. Wiley/NBS Registry of Mnss Spectral Data; Wiley: New York, 1989. 2. Bouma, W. J.; MacLeod, J. K.; Radom, L. 1. Am. Ckern. SK.

1980, 102,2246-2252. 3. Hoppilliard, Y.; Bouchoux, G.; Jaudon, I’. Nova. I. Chinz. 1982, 6, 43~52. 4. Bouchoux, G.; Hoppilliard, Y. Int. r. Mass Spectrom. Ion Processes 1983/1984, 55, 47-53. 5. Bouchoux, G.; Tortajada, J. Rapid Commun. Mass Spectram. 1987, 2, 86-87. 6. Hudson, C. E.; McAdoo, D. J. Tetrahedron 1990, 46, 331-334. 7. Van de Sande, C. C.; McLafferty, F. W. \. Am. Ckem. Sot. 1975, 97,4617-4620. 8. McAdoo, D. J.; Witiak, D. N. I. Ckem. Sot., Perkin Trans. 2

1981, 770-773. 9. Hudson, C. E.; McAdoo, D. J. Org. Mass Spectrom. 1982, 17, 366-368. 10. Hudson, C. E.; McAdoo, D. J. 0~s. Mass Spectrom. 1984, 19, l-6. 11. Turecek, F.; Hanus, V.; Gaumann, T. Int. 1. Mass Spectrom. [on Processes 1986, 69, 217-231. 12. McAdoo, D. J.; Hudson, C. E.; Traeger, J. C. Org. Mass Spectrom. 1988, 23, 760-764. 13 Gu, M.; Turecek, F. Org. Mass Spectrom. 1994, 29, 85-89. 14. McAdoo, D. J.; Hudson, C. E. J. Mass Spectrom. 1995, 30, 492-496. 15 (a) Lias, S. G.; Liebman, J. F.; Levin, R. D.; Kafafi, S. A. NZST Standard Reference Database 25, version 2.02, 1994; (b) Lias,

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1982, 27,127-130.

32. Bombach, R.; Dannacher, J.; Honegger, E.; Stadelmann, J.-P.; Neier, R. Chem. Phys. 1983, 82, 459-470. 33. Bouchoux, G.; Flammang, R.; Maquestiau, A. Org. Mass Spectrom. 1985, 20, 154~155. 34. Borden, W. T., Ed. Diradicals; Wiley: New York, 1982. 35. Wentrup, C. Reactive Molecules: Tke Neuhal Reactive lntermediate in Organic Chemistry; Wiley-Interscience: New York, 1984; pp 128~161. 36. Cordero, M. M.; Houser, J. J.; Wesdemiotis, C. Anal. Ckcm. 1993, 65, 1594&1601. 37. (a) Danis, I’. 0.; Feng, R.; McLafferty, F. W. Anal. Ckem. 1985, 58, 348-354; (b) Hop, C. I?. C. A.; Holmes, J. L. Org. Mass Spectrom. 1991, 26, 476-480. 38. Burgers, I’. C.; Holmes, J. L.; Mommers, A. A.; Terlouw, J. K. Gem. Pkys. Lett. 1983, 102, l-3. 39. Cooks, R. G.; Beynon, J, H.; Caprioli, R. M.; Lester, G. R. Met&able Ions; Elsevier: Amsterdam, 1973. 40. Holmes, J. L. Org. Mass Spectrorn. 1985, 20, 169-183. 41. (a) Magrane, J. K.; Cottle, D. L. I. Am. Ckem. Sot. 1942, 64, 484-487; (b) Stahl, C. W.; Cottle, D. L. I. Am. Ckem. Sot. 1943,

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V. C. 1. Am. Ckem. Sot. 1951, 73,

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Soc.1983, 705,5976-5979.

] Am Sot Mass Spectrom lY%, i’, X-589

48. McLafferty,

F. W.; Turecek, F. Irltcqwtotiotl cf Mass Sp~trn, 4th ed.; University Science Books: Mill Valley, CA, 1993. 49. Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Hrzudhook of He1Photoclrctro~~ Sprctrrz of Furrdn~rxwtnl Orptlic M&w/es; Halsted Press: New York, 1981. Rw. 1988, 7, 363-393. 30. McAdoo, D. J. Mnss Spbm. 51. McAdoo, D. J.; Hudson, C. E.; Traeger, J. C. Int. \. Mass

Sp~ctrm. hi Proc~~ssc~s 1987, 79, 183~187. 52. (a) Lifshitz, C.; Tzidony, E. 11/t. 1. Mr7ss Sprctrom. loti Phys. 1981, 39, 181~195; (b) Turecek, F.; &Lafferty, F. W. 1. Am. Clxvu. Sue. 1984, 106, 25252528; (c) McAdoo, 0. J,; Hudson, C. E. //It. /. Moss Spcctrofu. Ion Prowws 1984, 59, 77-83. 53. (a) Turecek, F.; McLafferty, F. W. 1. Am. Chum. SK. 1984, 106, 2528-2531; (b) Lifshitz, C. Org. Mnss Spcctvonf.1988, 23,

303-306.

DISTONIC

ION

.CH2CH,CH

OH

AND

RELATED

C,H,O+’

589

54. (a) Danis, P. 0.; Wesdemiotis, C.; McLaffcrty, F. W. 1. Anl. UICIII. Sot. 1983, 205, 7454~7456; (b) Terlouw, J. K.; Kieskamp, W. M.; Holmes, J. L.; Mommers, A. A.; Burgers, I’. C. Int. 1, Mass Spuctrom. Ion Processes 1985, 64, 2455250. 55. Gilpin, J. A.; McLafferty, F. W. Am/. Cirm. 1957, 29, 990~994. 56. Drewello, T.; Heinrich, N.; Maas, W. P. M.; Nibbering, N. M. M.; Weiske, T.; Schwarz, H. I. Am. Chcrn. Sm. 1987, 109,

4810~4818. 57. Goldberg, N.; Schwarz, H. AK. Chrr~~. Rrs. 1994, 27, 347-352. 58. Pedersen, S.; Herek, J. L.; Zewail, A. H. Science 1994, 266, 1359-1364. 59. Zagorevskii, D. V.; Holmes, J, L. Mass Spcactrom. Rev. 1994, 73, 133-154. 60. Cohen, N.; Benson, S. W. Clrrm. RPU. 1993, 93, 2419~2438.