- Email: [email protected]
Mixed-site vs. charge-site-remote fragmentation reactions of long-chain quaternary ammonium ions
E LSEVI E R
International Journal of Mass Spectrometryand Ion Processes 160 (1997) 223-240
Mixed-site vs. charge-site-remote fragmentation reactions of longchain quaternary ammonium ions I Kevin Whalen a, J. Stuart Grossert a, Jonathan M. Curtis b, Robert K. Boyd b'* aDepartment of Chemistry, Dalhousie University, Halifax, N.S. B3H 4J3, Canada blnstitute for Marine Biosciences, National Research Council, 1411 Oxford Street, Halifax, N.S. B3H 3Z1, Canada
Received 13 May 1996; revised 9 July 1996; accepted 9 July 1996
Abstract The mixed-site fragmentation reactions discovered by Tuinman, Cook and Magid (J. Am. Soc. Mass Spectrom., 1 (1989) 85), as reaction channels competitive with charge-site-remote fragmentations, have been further investigated. Two complementary deuterium-labelled tetra-alkylammonium ions, (CH3)3N+C14D29and (CD3)3N+C14H29,were investigated as precursor ions for MS/MS/MS experiments using a sector/time-of-flight hybrid instrument employing orthogonal acceleration into the time-offlight analyzer. By comparing the second-generation fragment ion spectra originating from the two precursors, it was possible unambiguously to assign atomic compositions and also to make plausible structural assignments. The fragment spectra thus obtained for the first-generation product ions of the charge-site-remote reactions are consistent with previous conclusions concerning their structures. The first-generation product ions from the mixed-site fragmentations, for example (CH 3)2N+(C D 2)m,are competitive with their charge-site-remote counterparts for m = 4, 5 and 6 in a manner consistent with the anticipated stabilities of cyclic transition states. However, interpretation of the second-generation fragment spectra of the (CH3)2N+(CD2)m ions and their deuterium-labelled complements was best accomplished in terms of a postulated diradical ion intermediate. Crown copyright © 1997 Elsevier Science B.V. Keywords: Fragmentation reactions; Long-chain quaternary ammonium ions
I. Introduction
Mass spectral fragmentation patterns represent the outcome of an often complex network of parallel and sequential unimolecular dissociation reactions, integrated over an observation time window defined by the instrumental configuration. The interpretation of such integrated chemical kinetics data (the mass spectrum), in terms of the molecular structure of the pre-ionization species, seems likely to remain as much an art as a science for the foreseeable future. Only the continuing efforts to * Correspondingauthor. i NRCC No. 39734
systematize the vast amount of empirical information in terms of a few fundamental mechanistic principles make possible any rational scientific interpretation at all. The present work is dedicated to Professor Fred McLafferty, one of the pioneers and still a leading exponent both of the interpretative art and also of the science of understanding how and why energized isolated organic ions dissociate as they do. His early contributions in this context can be found in monographs and book chapters published in 1963 [1,2], and in the first 1966 edition [3] of his ground-breaking text "Interpretation of Mass Spectra", now happily in its fourth (and updated) edition [4].
0168-1176/97/$17.00 Crown Copyright © 1997 Elsevier Science B.V. PII S0168-1176(96)04490-4
224
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
One of the important systematizing classifications of ion fragmentation mechanisms [1-4] is the division into radical-site-initiated reactions and charge-site-initiated reactions. Even-electron ions, such as the protonated molecules formed in chemical ionization, fast atom bombardment (FAB), and electrospray ionization, are naturally dominated by charge-site-initiated fragmentations. More recently, however, Gross and his collaborators [5,6] discovered and characterized a new class of fragmentation mechanisms for closed-shell even-electron ions, which they termed charge-site-remote (CSR) fragmentations. These CSR dissociation reactions, as described in recent reviews [7,8], generally involve portions of the molecular structure which are well removed from a site of fixed charge. The charge site itself does not appear to be involved in the reaction mechanism, as indicated by the fact that the sign of the fixed charge (positive or negative) does not affect the nature of the observed CSR reactions, although the extent to which they compete with other reaction channels does vary. The CSR fragmentations involve characteristic expulsion of the elements of alkanes, and are most commonly observed in
MS/MS experiments employing high-energy collisional activation (CA), although reports exist [8] of such reactions induced by much lower collision energies. Indeed, the best estimates [8] of the critical energies for CSR reactions are of the order of only 2 eV. The accumulated evidence [6-8] appears to favor a mechanism involving 1,4-elimination of H2 to form two w-unsaturated fragments, as illustrated in Scheme 1 for long-chain tetra-alkylammonium ions, the example of interest in the present work. Upon collisional activation it is supposed that a number of gauche conformations in the hydrocarbon chain become accessible, permitting some hydrogen (or deuterium) atoms to come into sufficiently close proximity for 1,4-elimination of H 2 (O2) , via an electrocyclic (4n + 2) transition state, to become feasible (Scheme 1). However, Wysocki and Ross [9] have proposed an alternative mechanism involving homolytic cleavage remote from the charge site, to form a neutral free radical plus a distonic radical cation (radical and charge sites are remote from one another) which expels a t3-hydrogen atom to form the same functionalized 1-alkene as in Scheme 1. This alternative mechanism has also been discussed
CD3 D3C\ ~dcD3H Hu +N'
H,,.[
~.,,n
m/z 265 AH Ha
Hfi
CD3 H 13C+x~''CD3H._-H H",.~
N~_:/" m/z 123
C
-
I- --H
Ii
HR
HmH
Scheme 1. Mechanism for CSR fragmentations involving 1,4-elimination of H2, as proposed in [6].
K. Whalen et aL/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
by Gross [8], and more recently has received additional support from the work of Claeys et al. [10]. Wesdemiotis and co-workers [11,12] have investigated the structures of the neutral fragments formed in CSR fragmentations using collision-induced dissociative ionization of fast neutrals. The neutral fragments from CSR reactions of linear fatty acid ions, both (M - H)- and (M - H + 2Li) ÷, were shown [11] to indeed be 1alkenes. A similar conclusion was reached for metalated linear polyglycols [12], but their cyclic counterparts (crown ethers) preferentially expelled saturated free radicals to form distonic radical cations in accord with the mechanistic proposals of Ross and Wysocki [9]. As summarized by Gross [8], it is unclear how the compromise between a homolytic mechanism and the concerted 1,4-elimination mechanism for CSR reactions is established for any particular reactant ion. Tuinman, Cook and Magid [13,14] discovered an entirely different channel competitive with CSR reactions (Scheme 1) of trimethylalkylammonium ions [(CH3)3N+CnH2n+I].The crucial
finding
[13,14]
was
that
CA
of
[(CH3)3N+CnD2n+I] reactant ions, with a perdeuterated alkyl chain, resulted in a second series of fragment ions accompanying the predicted CSR fragments, but at 3 Da higher mass. This observation seems explicable only in terms of incorporation of one of the methyl groups into the neutral fragments, clearly incompatible with a CSR mechanism. It was proposed [14] that the fixed charge site on the nitrogen atom interacted in some way with the backbone a-bonds, resulting in an assumed cyclic fragment ion plus a saturated alkane (Schemes 2 and 3). In view of the participation of the fixed charge site in a midchain fragmentation, Tuinman and Cook [14] coined the name "mixed-site-fragmentation" (MSF) reaction to describe this competitive mechanism. Consistent with Schemes 2 and 3, labelled reactant ions of the type [(CD3)3N+Cn_ID2n_2CH3] were shown both by Wysocki and Ross [9] and by Tuinman and coworkers [13,14] not to incorporate the w-methyl group into the fragment ion. Whalen et al. [15]
2 m/z 265
°3ci°'
225
" "
,
m/z 120 Scheme 2. Proposed mechanism for MSF reaction of 2 in which a diradical ion 9' is the initial product ion.
226
K. Whalen et aL/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
a HII '' ~
1I
9,
H
D_
-
-
+ N--H
.,/
""
-/
H'
o,
o
27
m/z 120 C rJ m/z 18 D3
D3(~_CD3 H HH
+7
D3CI~.,~',\~ m/z 120
:~4 I'[ 6' m/z 120
m/z 29 3 m/z77
Scheme 3. Proposed mechanism whereby the diradical ion 9' forms closed-shell structures which may be the stable forms detected in the MS/ MS spectrum of 2 (Fig. 2 and Fig. 3), and which contain structural elements corresponding to some of the second-generation fragment ions of 2 via the MSF ion at m/z 120 (Table 2). Note that, for reasons of clarity, not all of the single-electron movements in 9', which lead to pathways a, b and c, are shown explicitly. Also, pathways a and b require that 9' adopt conformations different from that illustrated here.
used the competition between the MSF and CSR reactions to probe the nature of the collisional activation process in a new high-pressure quadrupole collision cell. These experiments [15] confirmed the earlier work of Tuinman and co-workers [13,14], although with some differences in relative intensities which were attributed [15] to differences in pre-collision internal energies corresponding to different ionization methods (FAB and electrospray), laboratory-frame collision energies, average numbers of collisions and reaction times within the collision cell, etc. More recently, Bambagiotti-Alberti et al. [16] described FAB mass spectra of [(CH3)3N+CnH2n+I] bromides for n -- 12, 14 and 16, for which they had shown previously [17] that the CSR fragments were clearly observed but that the smallest such fragment observable was at m/z 114 in
each case, corresponding to [(CH3)3N÷C4H7], i.e. the CSR fragmentation did not extend to below C-5. In order to examine this phenomenon further, the N,N,N-trimethyl-d9 analogs were synthesized and studied by FAB mass spectrometry and by mass analyzed ion kinetic energy spectroscopy (MIKES) [16,17]. Incorporation of the methyl groups into the neutral fragments was again observed via appropriate mass shifts, and cyclic intermediates and fragment ions were proposed, although these authors [16,17] did not appear to be aware of the earlier work [13-151 . The objective of the present work was to investigate further the MSF mechanism [13,14] through the use of complementarily d-labelled precursors, [(CH3)3N+C 14D29] and [(CD3)3N÷C14H29],together with modern techniques of tandem mass spectrometry.
227
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
acceleration time-of-flight analyzer for MS/MS experiments, as described previously [18]. FAB ionization used a glycerol matrix and a primary beam of cesium ions of effective incident energy 17 keV. At the normal ion source potential of 8 kV the laboratory frame collision energy in the collision cell, located between the collector slit of the double-focusing stage and the push-out region of the oaTOF analyzer, was 800 eV for singly charged ions [18]. In all MS/MS experiments xenon collision gas was used, at a pressure inside the cell estimated to be 2 x 10 -5 mbar. For MS/MS/MS experiments the precursor ion, together with the first-generation fragment ion formed in the first field-free region (FFR1),
2. Experimental The bromide salt of [(CH3)3N÷C14D29] (1) was synthesized as described previously [14]. The iodide salt of the complementary labelled ion [(CD3)3N+C14H29] (2) w a s synthesized by heating 1-aminotetradecane (Aldrich Chemical Co.) with an excess of trideuteroiodomethane (Aldrich). The identity of each synthesized product was checked by 1H NMR and by FAB mass spectrometry. All experiments reported here were conducted using an AutoSpec-oaTOF instrument comprising a double-focusing stage of EBE configuration, coupled to an orthogonal
I<
100%-
x5.00
>I
CH 3
90 ¸
I+ CH3--N--(CD2) IsCD3
CH3
80 ¸
70.
59
4r
124 60 ¸ 285 50.
121
40. •
•
30 20.
•
lO.
• 20
60
100
140
•
* 180
220
• 260
I
LJ 300
m/z
Fig. 1. MS/MS spectrum of (CH3)3N*C14D29(1), obtained using the AutoSpec-oaTOF instrument, with conditions described in Section 2 (Experimental). O, CSR ions; -k, MSF ions.
228
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
were selected by setting the analyzer fields of the double-focusing stage to static values determined by the appropriate linked-scan law at constant B/ E [19]. Argon collision gas was used in the collision cell in FFR1, at a pressure resulting in 50% attenuation of the precursor ion intensity. A laboratory-frame collision energy of 8 keV was used in this first MS/MS stage. The collision energy in the oaTOF collision cell, and also the TOF accelerating potential, are linked to the ion energy determined by the field in the second electric sector of the EBE analyzer in order to accommodate the full fragment ion mass range on the channel plate detector of the oaTOF [18]. Therefore, the collision energy for the first-generation
I00%.
fragment ions transmitted to the oaTOF collision cell was automatically reduced from 800 eV by the mass ratio of the first-generation fragment ion to the precursor. This reduction invalidated the m/z calibration of the oaTOF, but a simple correction could be applied using the known m/z value of the selected first-generation fragment ion which in turn acted as the precursor ion entering the oaTOF in the MS/MS/MS experiment.
3. Results and discussion
In accord with previous findings, the FAB
~
xlO.O0
~1 265 CD3 I+ CD3--N~(CH2) lsCH3
90"
CD3
80"
70" 60
120
50
123 68
40' 30"
20"
L.
10"
I
0
..
,
20
[
',
40
....I1. I
6O
•
i
80
,
I1" ,
i
.
100
.
.
.
120
-
i
'140" m/z
,I
I
.
160
.
.
.
-,
180
li lLit ,
i-
200
,
,
*. *. .1 220 240
260
Fig. 2. MS/MS spectrum of (CD3)3N*C14H29(2), obtained using the AutoSpec-oaTOF instrument, with conditions described in Section 2 (Experimental). 0, CSR ions; -k, MSF ions.
229
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
corresponding to CSR losses of the elements of perdeuteroalkanes (Ck, Dzk÷2), are accompanied by a less intense series 2 Da higher which presumably corresponds to losses of the elements of free radicals (Ck, Dzk+l). (The notation in which the elements are separated by commas is here taken to indicate that no assumptions concerning the structure(s) of the fragment(s) are made). This interpretation is supported by the corresponding peaks in the fragment ion spectrum of 2, which appear at only 1 Da higher than the intense peaks arising from CSR expulsion of the elements of (Ck, Hzk+2) at m/z 123, 137, 151, etc. Such radical expulsions are also considered [7-9] to be CSR processes, and indeed
mass spectra (not shown) of the halides of 1 and 2 were found to contain abundant fragment ions attributable to both CSR and MSF reactions, in addition to intense signals corresponding to 1 and 2 themselves. The pattern of relative intensities of the two families of fragment ions was very similar to that reported previously [13,14,16,17]. Figs 1 and 2 show fragment ion spectra of 1 and 2 obtained by MS/MS experiments using the oaTOF. In addition to the intense peaks corresponding to these two competing mechanisms [13-17], low intensity satellite peaks are also observed which were not readily apparent in the FAB mass spectra. In the case of 1, the series of peaks at m/z 121, 137, 153 etc.,
,ell3
124
100%-
CH3"N+--(CD2)13CD3
(A)
CH 3 80121
153
60-
4,
137
169
40-
20, , ,*^ .
.
.
.
.
i
120
I00%-
(B)
.
.
_JL ; ^ ^ - * .
i
.
.
.
.
i
.
.
.
.
.
.
.
.
,CD3
* 123
CD3"N,+(CH2)IsCH3 CD 3
80•
151
137
60-
165
40-
20-
124
o
134
.
120
125
.
.
.
.
.
130
.
i
*
.
135
.
.
.
l
140
.
.
.
.
I
145 m/z
.
.
.
*
.
.
.
.
150
.
I
155
I
.
160
.
.
.
.
.
.
165
.
l
.
.
.
.
170
Fig. 3. (A) Expanded view of the MS/MS spectrum of (CH3)3N+CI4D29 (1) shown in Fig. 1; (B) expanded view of the MS/MS spectrum of (CD3)3N+C]4H29 (2) shown in Fig. 2.
K. Whalen et al./International Journal of Mass Spectrometry and lon Processes 160 (1997) 223-240
230
have been cited [9] in support of CSR mechanisms involving diradical intermediates. Other less intense satellite peaks are more clearly seen in the expanded middle m/z range versions of Figs 1 and 2, shown in Fig. 3. In order to investigate the MSF mechanism, it was necessary to obtain structural information for the fragment ions. Some MS/MS experiments were conducted on precursors present as fragment ions in the FAB mass spectra, and the resulting spectra were satisfactory for the more intense of these precursor ions. However, for those cases in which the intensities of the selected precursor ions were not much greater than that of the "peak-at-every-mass" background characteristic of static FAB spectra, it became problematic to determine the contributions of the glycerol-derived ions. An example
l<
100 % 811
>1 188
+,I,I1,4 IL 105.
4O
'50
i134
2o
100%
x50
(A)
60
o
of this ambiguity is shown in Fig. 4, which compares the MS/MS spectrum (Fig. 4(A)) of a fragment ion present in the FAB mass spectrum with that obtained by a MS/MS/MS experiment (Fig. 4(B)) in which the first-generation fragment was that selected as precursor in the MS/MS experiment (Fig. 4(A)) and the primary precursor ion was 1. Peaks in Fig. 4(A) labelled with an asterisk are absent from Fig. 4(B), and are presumably derived from the FAB background. The sensitivity and duty cycle of the oaTOF analyzer are comparable with those of an array detector fitted to a double-focusing analyzer, and the FAB matrix artifacts in the MS/MS spectra are similar to those described previously by Falick et al. [20], who used a foursector instrument. Accordingly, in order to avoid such ambiguities, only the MS/MS/MS
.' .' . '. .' .I. .' . '. .' .' . I. .'. .' '
,....-
' I ' + ' ' I
66 I
, ....
88
+ I,'I J,,+ I''
''I'''~I
(B)
....
152"
I. ,,i..l+ ,.,..,,,I L., k.,,.b,-,
I''''I''''I
74
.
....
I ....
I ....
I ....
.,,--..,,.
I ' ' ' ' I ' ' ' ' I ' ' ' ' I
xlO0
....
I ''%.
I''''I''''I'
>l 1T8
80 5O
60
60
40 20
0
. . . . . . . .
20
40
60
J+L -
80
1O0
m/z
J ,.
120
140
160
180
'"1'"'1' 200
Fig. 4. (A) MS/MS spectrum of fragment ion at m/z 188 observed in the FAB mass spectrum of the bromide of 1. Peaks marked with an asterisk are believed to correspond to ions that are formed from precursors of m/z 188, but are derived at least in part from the glycerol matrix; (B) MS/ MS spectrum of the first-generation fragment at m/z 188, formed in the first field-free region from 1. See Section 2 (Experimental) for details of this MS/MS/MS experiment.
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
spectra will be referred to in the following discussion. However, there is a limitation of the MS/MS/ MS technique employed in the present work, which arises from use of the B/E linked scan relationship to define both the precursor ion and the first-generation fragment formed in FFR1. It is well known [19] that, although adequate resolving power for the fragment ions is provided by this technique, the effects of kinetic energy release are reflected in poor resolution of the precursor. In the present case, the intensity of the first 13C isotope peak of the precursor ion (C17) is about 18% of that of the all- 12Cprecursor nominally selected, and undoubtedly contributes lowintensity interference artifact peaks [19] to the linked-scan spectra. This effect will be important in cases where the selected intermediate ions in MS/MS/MS spectra are of low intensity, and are accompanied in the first-generation fragment spectrum by intense peaks 1 Da lower. An example where such B/E artifacts [19] may be important is discussed below.
3.1. Interpretation of second-generation fragments formed from first-generation fragment ions in the range m/z 120-125 Figs. 5 and 6 show fragment ion spectra, obtained in MS/MS/MS experiments, for firstgeneration fragment ions in the range m/z 120125 formed in FFR1 from 1 and 2, respectively. It is convenient to discuss corresponding first-generation fragments of 1 and 2 together, as the comparison assists the interpretation. The CSR fragment ions in this range are those at m/z 121 and 123 in Fig. 5(A) and Fig. 6(C), respectively, and on this basis [7,8] are assigned the following structures: m/z 121, (CH3)3N÷CDzCDzCD=CD2 (3): m/z 123, (CD3)3N+CHzCH2CH=CH2 (4) The proposed interpretation of the fragment ion spectra of 3 and 4, shown in Table 1, is consistent with the suggested open-chain structures for these even-electron ions. The ISOMABScomputer program (available from the authors) was
231
used to generate all possible formulae containing (C, N, H, D) for each m/z value observed. Unique assignments were possible through restrictions arising from the formulae of the precursors 1 and 2, together with the requirement that all fragments derived from 1 must also be formed from 2 with appropriate H/D exchanges, and vice versa. The relative intensities of corresponding fragments (Table 1) are also in gratifying agreement considering that the two spectra (Fig. 5(A) and Fig. 6(C)) were obtained on successive days, and this agreement provides some additional reassurance that the interpretations are correct. Most of the fragmentations of 3 and 4 appear to be charge-site initiated, and the only contentious interpretation appears to be that of the corresponding low-intensity ions at m/z 59 and 68 as the odd-electron molecular ions of the trimethylamines. The MSF first-generation products of 1 and 2 in this m/z range are m/z 124 and 120, respectively. The molecular formulae of these ions are unambiguous, given the known structures of the respective precursors 1 and 2 and also the reasonable assumption that they are even-electron ions. The fragment ion spectra of these MSF ions are shown in Fig. 5(D) and Fig. 6(A) and summarized in Table 2, together with proposed structures for the second-generation fragment ions. By far the most striking feature of this interpretation is the assignment as odd-electron species of the intense ions at m/z 77 and 74, derived respectively from 1 (via m/z 124) and 2 (via m/z 120). However, there appears to be no other interpretation which can account for all of the available information. The less intense ions at m/z 61 (from 1) and 60 (from 2) also require assignment as odd-electron species (Table 2). Although the production of odd-electron fragments from even-electron precursors must be regarded as unusual [3,4], it is established [79] that CSR expulsions of radicals can compete with expulsions of the elements of alkanes, as discussed above with reference to Figs. 1-3. Again, the degree of agreement between relative
232
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
I.
x~
~ 121
(A)
CSR 62
............. I_ ....
I ' ' ' ' l ' ' ' ' i
i ....
"ill
........ .ll . ~
I ' ' ' ' i ' ' ' ' l
....
I ....
I''"'1'';'1
....
I . . . . . . . .
~ 122
62
....
I ....
I'=''I''''I
....
I ....
I''''I''''I''''I''''I''''I''''I
(C)
62
. . . . . . . . . . . . . . ....
J<
I ....
I ' ' ' ' 1 ' ' ' ' 1
I ....... ....
(
•,
I I ....
. . . . . I ....
a~
4r
''''I''''I''''I
:
'''
....
. . . . . . I ....
| .
.
.
I ....
.
.
I ....
~
....
I''''I
....
I ....
I ....
~
I
I
''
124
MSF
~1
43
.
I ....
,f
I''''I''''I''''I
(E)
L.I I ....
innn
I''
125 1:
75
h dl Jl
~..:. ,.,.:; :,..~.,a.,:.!j,.~,..!L:. c',!, !':k.;;. !LJ,~: .~.::'z~,", t.',",.,,, 20
40
60
80
100
120
in/z Fig. 5. Second-generation fragment spectra obtained by MS/MS/MS experiments on 1. The first-generation fragments were formed in FFR1 of the AutoSpec-oaTOF instrument and transmitted to the collision cell preceding the oaTOF analyzer. (A) m/z 121 from CSR expulsion of the elements of (C m, D22) from 1; (B) m/z 122 from CSR expulsion of the elements of (C10, HI, D21) from 1; (C) m/z 123 from CSR expulsion of the elements of (C ]0, D21) from 1; (D) m/z 124 from MSF expulsion of the elements of (C 10, H3, D Jg) from 1; (E) m/z 125 from MSF expulsion of the elements of (Cl0, H2, D19) from 1.
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
~ (A)
233
>4120 M S F
45
64
77
2,,,I s,I J
' ' ' ' I ' ' ' ' I
....
I''''I
....
I ....
I''''
....
''''
''''
, i l l
'''~/
122 i
x5O
(a)
>(
55 65
.............. ....
I ....
i ,.J. . . .,. . .l . . .1. . .2. . . .9. . .
I ....
i ....
I ....
I ....
............................ I ....
i ....
I ....
t ....
I ....
, I ....
~so
I
'''
~ 123
(C)
CSR 55
{
xl,' 6
'Jr ~. . . . . .
. .-.-,
' . . . . . . . . . . . I .... I ....
l I ....
(D)
I
I ....
. . . . . . . . . . . . . . . . . . . . . . . . . I .... I .... I ' " ' ' l
24
66
56 . I ....
.
. I'
. . . . . . . .
.
.
' 'I'll
. '''
.
. I i ....
96
,l_ I ....
i
I ....
_,
I,_
I ....
. . . . . .
I ....
I ....
J_ I ....
106 _ _L I ....
~3o (E)
__ I ....
I
~
"";
125
69
°°1
~:: :~ :.:,,!L~::,.:,7:,!:;I:'~r :!; ~'~-:,.,~:~:: .: ~, .... 20
40
60
I
80
100
I
120
m/z Fig. 6. Second-generation fragment spectra obtained by MS/MS/MS experiments on 2. The first-generation fragments were formed in FFR1 of the AutoSpec-oaTOF instrument and transmitted to the collision cell preceding the oaTOF analyzer. (A) m/z 120 from MSF expulsion of the elements of (C [0, D 3, H 19) from 2; (B) m/z 122 from CSR expulsion of the elements of (C 10, D i, H21) from 2; (C) m/z 123 from CSR expulsion of the elements of (C 10, H 22) from 2; (D) m/z 124 from CSR expulsion of the elements of (C ]0, H 21)' from 2; (E) m/z 125 from CSR expulsion of the elements of (CIo, H20 ) from 2.
234
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
Table 1 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on 1 and 2, with 3 (m/z 121) and 4 (m/z 123) respectively as the firstgeneration fragments presumed to have been formed in CSR reactions. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(A) and Fig. 6(C)
1 ~ 3 [(CH3)3N+CD2CD2CD=CD2, m/z 121] -* X
2 ~ 4 [(CD3)3N+CH2CH2CH-CH2, m/z 123] --~ Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.I.)
Interpretation of Y
34 58 59 60 61 62
C2D~ (CH3)zN+=CH2 (CH3)3N + (CH 3)2N +=CD 2 (CH3)3N+D C4D~
29 (12) 66 (44) 68 (21) 64 (21) 69 (75) 55 (100)
CzH~ (CD3)2N+-CD2 (CD3)3N + (CD 3)2N +=CH 2 (CD3)3N+H C4H~
(14) (39) (19) (22) (65) (100)
intensities of corresponding fragments in the two spectra permits some increased confidence in the proposed interpretations. Formation of the MSF product ions, such as those whose fragment spectra are shown in Fig. 5(D) and Fig. 6(A) and interpreted in Table 2, has been assumed [13-16] to require a cyclic transition state (Scheme 2). However, this assumption does not necessarily imply that the stable MSF ions are cyclic (e.g. 5, below), and various linear closed-shell structures (e.g. 6-8) are possible for the stable forms of the MSF ion at m/z 124 formed from 1:(CH3)2 N+ < [cyclo-CsD10] (5);
(CH3)2N+=CDC4D9 (6); (CH3)zND+CD: CDC3D7 (7); ( C H 3 ) z N D + C D z C 4 D 7 ( 8 ) w h e r e 8 includes three possible isomers with different positions for the double bond. (Branched-chain
structures have been excluded from consideration here as being less likely to arise from the unbranched precursor). The cyclic structure 5 is that originally proposed for the MSF fragments [13,14]. The complementary structures arising from 2, in which the H and D atoms are exchanged, will be referred to as 5'-8'. Even if the most stable structures are indeed cyclic, production of second-generation fragment ions requires cleavage of at least two bonds, so that any of the linear structures 6 - 8 could in principle represent intermediate reactive forms. In addition, the participation of diradical forms as reactive intermediates in the fragmentation of the first-generation MSF ions cannot be ruled out. In fact, only a few of the ions listed in Table 2 can be readily derived by plausible mechanisms
Table 2 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on 1 and 2, with m/z 124 and m/z 120 respectively as the firstgeneration fragments presumed to have been formed in MSF reactions, and with molecular formulae [(CH3)zN(CsD10)] + and [(CD3)zN(CsH10)] +, respectively. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(D) and Fig. 6(A)
1 --. [(CH3)2N(CsDI0), m/z 124] +--. X
2 ~ [(CD3)2N(CsH10), m/z 120] + ---- Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.1.) Interpretation of Y
15 (15) 30 (21) 34 (35) 43 (100) 46 (42) 50 (23) 59 (46) 60 (75) 61 (33) 74 (92) 76 (21)
CH~ C 2D~ CzD~ CH3N+-=CD C3D~ C3D.~ CH3N+=CCD3 (CH 3)2N+-CD 2 CH3N+=CDCD3 or cyclic isomer (CH3)2N+CD=CD2 (CH3)2ND+CD=CD2or cyclic isomer
18 (24) 27 (24) 29 (39) 45 (100) 41 (46) 43 (33) 59 (56) 64 (95) 60 (43) 77 (95) 78 (24)
CD; C2H~ CEH; CD 3N+=CH
C3H~
C3H~ CD 3N +~CCH 3 (CD3)2N+-CH2 CD3N+=CHCH30r cyclic isomer (CD3)2N+CH=CH2 (CD3)2NH+CH=CH2or cyclic isomer
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
directly from the cyclic structures 5 or 5'. Instead, it is proposed that the diradical ion 9' (Scheme 2) is the initial MSF product ion from 2, with an analogous structure 9 initially formed from 1. This proposal clearly carries implications for the mechanism of the MSF reaction itself. It is not apparent whether or not such diradical structures represent local minima on the potential energy surface, but they must be species of higher energy than the closed-shell structures 5 - 8 (or 5'-8'). Accordingly, the diradical ions, e.g., 9', may be expected to rearrange to more stable closed-shell structures such as 5', 6' and 10' as illustrated in Scheme 3 (note that 10' is a special case of 8'). In turn these proposed intermediates contain structural elements which correspond to structures of several secondgeneration fragment ions listed in Table 2, although the sequence of events leading to these ions is not obvious in all cases (Schemes 3 and 4). Alternatively, diradical ion structures such as 9' can fragment directly to produce the remaining fragment ions listed in Table 2, as illustrated in Scheme 4. Of course, as for most mass spectrometric fragmentation mechanisms, the proposals summarized in Schemes 2 - 4 are
D3~. _CD3 -/-.~ at, •
235
largely speculative. However, they do have the merit of accounting for the experimental observations (Table 2), based on a minimal number of assumptions which do not violate chemical intuition derived from more substantive experimental evidence from physical organic studies of reactions in solution. The less intense first-generation fragment ions at m/z 123 (from 1) and 124 (from 2) arise from CSR expulsion of C10X21, where X is D or H for precursors 1 and 2 respectively. The secondgeneration fragment spectra (Fig. 5(C), Fig. 6(D), and Table 3) are broadly similar to those of their even-electron CSR counterparts (Fig. 5(A) and Fig. 6(C)). However, these are probably distonic ions, e.g. (CH3)3N+(CaD8) ', and their odd-electron character might be expected to facilitate rearrangement reactions. Indeed, Table 3 is appreciably more complicated than either Table 1 or Table 2, and the proposed interpretation is correspondingly less secure. It is now more difficult to match up relative intensities of corresponding fragments because of multiple possibilities of interpretation. For example, the relative intensity of the fragment at m/z 64 from 1 (Table 3) is a good match for that of the
D3C_ i D3
HH
-.
H
H
+N"
H"[
X"~ t/t~ m/z 120
m/z 120
at
.j ÷
D3C--N----C--CH 3 m/z 59
/\ m/z 78
>
D3Q -. N+
/\
m/z 60
Scheme 4. Proposed mechanisms for formation of some second-generation fragment ions of 2 via the MSF ion at m/z 120 (Table 2).
236
K. Whalen et aL/lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
Table 3 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on ! and 2, with m/z 123 and m/z 124 respectively as the firstgeneration fragments presumed to have been formed in CSR expulsions of Ct0X21 radicals, where X is D or H for precursors 1 and 2 respectively. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(C) and Fig. 6(D) 1 ~ [(CH3)3N+(C4D8), m/z 123] ~ X
2 ~ [(CD3)3N+(C4Hs) , m/z 124] ~ Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.I.)
Interpretation of Y
58 (13)
(CH3)2N+=CH2
66 (100)
(CD3)2N+=CD2 and (CD3)2N+H(CH3) and (CD3)zN÷CH2D
59 60 61 62
(CH3)3N + (CH3)zN÷-CD2 (CH3)3N+D and (CH3)2N* CD2H C4D~ and DN+=-C-CzD5 and (CH3)zN+-CD3 (CH3)zNH+CD3 C4D~ and (CH3)2N+D(CD3) and CH3NH+C2D5 (CH3)3N+(C2D4) (CH3)2-N+=CDC3D7
68 (30) 64 (26) 69 (26) 55 (20) 65 (76) 67 (35) 56 (30) 63 (7) 96 (24) 106 (22)
(CD3)3N÷
(39) (48) (87) (100)
63 (22) 64 (6) 91 (11) 108 ( < 5)
ion at m/z 63 from 2, suggesting that the possible interpretations of the m/z 64 ion other than CH3NH+C2D5 do not make significant contributions. If correct, this conclusion implies that the main contribution to m/z 56 from 2 is that of HN+=CCzHs, for which the corresponding ion from 1 is at m/z 62, the base peak in Fig. 5(C). Also, the proposal that (CH3)zN+D(CD3) does not contribute significantly to the relative intensity of m/z 64 from 1 (Table 3) implies that its counterpart (CD3)zN+H(CH3) does not contribute much to m/z 66 from 2, which must therefore correspond mostly to (CD3)2N+=CD2 and (CD3)zN+CH2 D. In turn, the corresponding ion (CH3)2N+CDzH from 1 must make a significant contribution to the intensity at m/z 61, and so on. In view of these complicated relationships it is not surprising that the matching of relative intensities for corresponding fragments is not as satisfactory for Table 3 as for Tables 1 and 2. Nonetheless, the proposed interpretations of Table 3 do have a measure of self-consistency, and do indeed indicate an increased extent of (presumably radical-site initiated) rearrangements. The very low-intensity peaks at m/z 122, apparent in Fig. 3(A) and 3(B), appear to
(CD3)2N+=CH2 (CD3) 3N+H C4H~ (CD3)2N+'CH3 (CD3) 2ND+CH3 C4H~" and HN+=CC2Hs CD3ND+C2H 5 (CD3) 3N+(C2H4) (CD3)2-N+=CHC3H 7
correspond to isotopic variants of the CSR fragments at m/z 121 and 123, respectively. There appear to be two possibilities for isotopic substitution to occur. The first possibility involves incomplete deuteration of the precursor. In the case of l, the sequence summarized in Eq. (1) could account for formation of a CSR fragment at m/z 122 containing one 13C atom (CH3)3 N÷ C4DsC10D20 H [13C~2C16, m/z 285] --~ (CH3)3 N÷ C4D7 [13C~2C6, m/z 122] + (C10, H, D21)
(1)
In the case of 2, the incomplete deuteration would be retained in the CSR ion at m/z 122, which now contains no 13C atom, as described in Eq. (2) (CD3)z(CD2H)N + C14H29 [13C~2C16,
m/z
265]
(CD3)z(CDzH)N + C4H7 [13C~2C7, m/z 122] + (C10, H22) [13C~2C9] (2) The alternative possibility invokes H/D scrambling in the precursor ion prior to CSR fragmentation with no need to consider 13C
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
C4D~,arising
variants, as shown in Eq. (3) for 1 (CH3)3 N+ C14D29 [1, m/z 285] ---* (CH3)2(CH2D)N + C4DsC10D20 H [m/z 285] ---* (CH3)z(CHzD)N + C4D 7 [m/z 122] + (C10, H, D21 )
(3)
and correspondingly for 2, as in Eq. (4) (CD3)3N + C14H29 [2,
m[z
265]
(CD3)2(CD2H)N + C4HsC10H20 D [m/z 265] ---* (CD3)z(CD2H)N + C4H 7 [m/z 122] + (C10 , D, H21 )
237
(4)
Another possibility is that the first-generation fragment at m/z 122, formed from 1 in FFR1, represents an artifact of the B/E technique [19] used to define both the precursor ion and the FFR1 fragment ion. As discussed above, the effects of kinetic energy release will result in a sufficiently poor resolving power for the precursor such that the 13C1 version of the 1 ----, 3 CSR reaction (i.e. m/z 286 ~ m/z 122) will be observed to some extent. Any second-generation fragments at m/z 122 produced thus from 1 as B/E artifacts [19] will be indistinguishable from those produced via the sequence described by Eq. (1). However, such considerations are unlikely to apply to the ion at m/z 122 formed from 2, since the CSR ion is now at m/z 123 (i.e. already at higher m/z) and the intense MSF ion is at m/z 120 (Fig. 3(B)), too far away to make a significant contribution via a B/E artifact from the second 13C isotope peak of 2 as FFR1 precursor. Comparison of the fragment ion spectrum of m/z 122 from 1 (Fig. 5(B)) with that of its unmodified CSR counterpart 3 (m/z 121, Fig. 5(A)) shows that the only qualitative difference is the appearance in Fig. 5(B) of a fragment ion at m/z 63 which is not observable in Fig. 5(A). If the interpretations summarized in Table 1 are valid, the base peak at m/z 62 is C4D~. The secondgeneration fragment at m/z 63 in Fig. 5(B) is then readily interpreted as the ~3C1 version of
either via the sequence described in Eq. (1) or as a B/E artifact as described above. The alternative sequence of Eq. (3) cannot give rise to this species. However, the intensity of m/z 63 is only 12% of that of m/z 62 in Fig. 5(B), whereas random distribution of the 13C atom in m/z 122 ions of this type predicts that the intensity ratio of these two ions should be 4:3. Therefore, while sequence Eq. (1) and/or the B/E artifact route do occur, they appear to be appreciably less important for 1 than sequence Eq. (3). In the case of 2, comparison of Fig. 6(B) and Fig. 6(C) shows that the C4H~ions at rn/z 55 are unaccompanied by any observable intensity at m/z 56, as predicted both by Eqs. (2) and (4) and by the impossibility of a B/E artifact of the required type, so that no discrimination amongst these mechanisms is possible on this basis. The most striking difference between Fig. 6(B) and Fig. 6(C) is the shift of the second most intense peak at m/z 69 in Fig. 6(C) to m/z 68 in Fig. 6(B). This observation is consistent with a change from (CD3)3N÷H (m/z 69, Table 1) to (CD3)2(CD2H)N+H (m/z 68) as predicted by both Eqs. (2) and (4). The corresponding predictions for 1, from both Eqs. (1) and (3), are that the (CH3)3N+D ion (m/z 61, Fig. 5(A) and Table 1) should shift to m/z 62 in Fig. 5(B), but the latter is the base peak in the spectrum and the predicted shift is observable only via changes in relative intensities. The overall conclusion drawn from these observations on the ions at m/z 122 is that the effects of incomplete deuteration and/or B/E artifacts are indeed observable, but are not as important as H/D scrambling in the precursors prior to CSR fragmentation. However, it should be emphasized that these effects make very minor contributions, as indicated by the relative intensities at m/z 122 in Fig. 3(A) and Fig. 3(B).
3.2. Other CSR fragment ions The discussion thus far has been restricted to the intense sets of peaks in the range m/z
238
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
correspond to CSR reactions, though in this case the mechanism presumably involves either 1,2elimination of CX4 with X being D or H, or successive expulsions of CX3 plus X [9], rather than the 1,4-elimination reaction [5,6] shown in Scheme 1. In contrast, loss of CH3D from 1 and of CD3H from 2 are most likely chargesite initiated reactions forming structures similar to 6 and 6', but with the entire C14 backbone intact. Fragment ion spectra of the first-generation fragment ions at m/z 249 and 246 from 2 are shown in Fig. 7. The spectrum of m/z 249 (Fig. 7(A)) is very similar to that of 2 itself (Fig. 2) with both CSR and MSF fragments, exactly as predicted for the proposed structure
120-125, as it was hoped that the structural interpretation of these smaller ions would be relatively straightforward. The fragment ion spectra of the larger CSR product ions (not shown) contained the CSR and MSF fragment ions predicted for precursor ions with a terminal double bond (Scheme 1). These spectra contained no surprises. However, Figs. 1 and 2 show that both 1 and 2 can also lose methane neutrals, by which losses of CD4 and of C H 3 D from 1 occur with roughly equal probabilities to form m/z 265 and 268, and correspondingly losses of CH4 and of C D 3 H from 2 to form m/z 249 and 246. Expulsion of C D 4 from 1 and of CH4 from 2 might be expected to x2o
lOO % -
x200
÷
,~ 2 4 9
69 (A)
80 'k
120
60
55 123
40
20 ,,,
J.,,l
L-i i j ..,.
, ''. . . . ,.
,
. . ;
+
x20
100 %
•
(B)
! ki,,.p
.J+,L]. •
'
'
--/JL"
~
I
,d
246
x200
90
80 64 60 77 40
104
20
0
, ,.,,,.
.,It, .J. . 'lI 20
40
+!l,+J,,L, 60
80
I
"'1, -
118
-,..:-,: .: ..,..+ ~. :-. , . . . . - . . - + . . - , 120
140
160
180
'i. 200
..~. ++,-,-.. +...+-, 220
m/z Fig. 7. (A) Fragment ion spectrum of (2-CH4), m/z 249; (B) fragment ion spectrum of (2-CD3H), m/z 246.
240
, ,
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
(CD3)3N+(CH2)12CH=CH2 . Similarly, the structure (CD3)2N+=CHCa3H27 proposed for m/z 246 from 2 is expected to form CSR fragments but no MSF ions, owing to the inflexibility introduced by the double bond, and indeed this is observed in Fig. 7(B). The low-intensity ions (Figs 1 and 2) at m/z 269 and 245, corresponding to losses of C H 4 f r o m 1 and of C D 4 f r o m 2 , correspond to e.g. a structure CD3N+(=CDz)C14H29 in the case o f ( 2 - CD4).
3.3. Other MSF product ions Examination of Figs. 1 and 2 shows that the MSF ions, e.g. the fragment ions of formula (CH3)2N+(CD2)m from 1, are prominent only for m -- 4, 5 and 6 (m/z 108, 124 and 140 for 1 in Fig. 1, and m/z 106, 120 and 134 for 2 in Fig. 2). The MSF ions for other values of m are appreciably less intense than the products of the competing CSR reaction channels, and this trend presumably reflects the probabilities of formation and stabilities of different ring sizes in cyclic transition states. Note, however, that the MSF ions with m -- 9 (m/z 188 and 176 in Figs 1 and 2, respectively) are more competitive relative to their CSR counterparts than for any value of m other than 4, 5 and 6. The nature of these MSF reactions will require further study.
4. Conclusions The present study of the competition between CSR and MSF reactions of long-chain tetraalkylammonium ions upon collisional activation has confirmed that the product ions of these two types are very different from one another. The MS/MS/MS experiments used to probe the structures of the CSR product ions produced no major surprises. The MSF reaction channels are competitive with their CSR counterparts when the assumed cyclic transition states involve rings incorporating five, six or seven atoms (including the nitrogen). This observation fits in well with
239
established principles of organic chemistry. The fragment ion spectra of these MSF intermediate ions, however, are difficult to rationalize as arising directly from cyclic structures, and at least the activated forms of these ions are best considered to arise from a diradical ion intermediate. On this basis it was possible to postulate rearrangement reactions to forms which include structural elements corresponding to all of the observed second-generation fragment ions. The notion of a diradical intermediate is consistent with the earlier proposals of Wysocki and Ross [9] for CSR reactions. Although these mechanistic proposals (Schemes 2-4) are necessarily speculative, they do account for the MS/MS/ MS spectra in a reasonable way. Clarification of these MSF reactions will require additional experiments.
Acknowledgements This paper benefited greatly from the perceptive comments of an anonymous referee.
References [1] F.W. McLafferty, Mass Spectral Correlations, Advances in Chemistry Series 40, American Chemical Society, Washington, DC, 1963. [2] F.W. McLafferty (Ed.), in Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p. 309. [3] F.W. McLafferty, Interpretation of Mass Spectra, W.A. Benjamin, New York, 1966. [4] F.W. McLafferty and F. Ture6ek, Interpretation of Mass Spectra, 4th edn., University Science Books, Mill Valley, CA, 1993. [5] K.B. Tomer, F.W. Crow and M.L. Gross, J. Am. Chem. Soc., 105 (1983) 5487. [6] N.J. Jensen, K.B. Tomer and M.L. Gross, J. Am. Chem. Soc., 107 (1985) 1863. [7] J. Adams, Mass Spectrom. Rev., 9 (1990) 141. [8] M.L. Gross, Int. J. Mass Spectrom. Ion Processes, 118/119 (1992) 137. [9] V.H. Wysocki and M.M. Ross, Int. J. Mass Spectrom. Ion Processes, 104 (1991) 179. [10] M. Claeys, L. Nizigiyimana, H. Van den Heuvel and P.J. Derrick, Rapid Commun. Mass Spectrom., 10 (1996) 770. [11] M.M. Cordero and C. Wesdemiotis, Anal. Chem., 66 (1994) 861.
240
K. Whalen et aL/lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
[12] T.L. Selby, C. Wesdemiotis and R.P. Lattimer, J. Am. Soc. Mass Spectrom., 5 (1994) 1081. [13] A.A. Tuinman, K.D. Cook and L.J. Magid, J. Am. Soc. Mass Spectrom., 1 (1989) 85. [14] A.A. Tuinman and K.D. Cook, Proc. 38th ASMS Conf. Mass Spectrom. Allied Topics, Tucson, AZ, 1990, p. 938. [15] K. Whalen, J.S. Grossert and R.K. Boyd, Rapid Commun. Mass Spectrom., 9 (1995) 1366. [16] M. Bambagiotti-Alberti, S.A. Coran, F. Benvenuti, P.
[17] [18] [19] [20]
LoNostro, S. Catinella, D. Favretto and P. Traldi, J. Mass Spectrom., 30 (1995) 1742. M. Bambagiotti-Alberti, S.A. Coran, V. Giannellini, D. Favretto and P. Traldi, Rapid Commun. Mass Spectrom., 8 (1994) 439. R.H. Bateman, M.R. Green, G. Scott and E. Clayton, Rapid Commun. Mass Spectrom., 9 (1995) 1227. R.K. Boyd, Mass Spectrom. Rev., 13 (1995) 359. A.M. Falick, K.F. Medzihradszky and F.C. Walls, Rapid Commun. Mass Spectrom., 4 (1990) 318.
International Journal of Mass Spectrometryand Ion Processes 160 (1997) 223-240
Mixed-site vs. charge-site-remote fragmentation reactions of longchain quaternary ammonium ions I Kevin Whalen a, J. Stuart Grossert a, Jonathan M. Curtis b, Robert K. Boyd b'* aDepartment of Chemistry, Dalhousie University, Halifax, N.S. B3H 4J3, Canada blnstitute for Marine Biosciences, National Research Council, 1411 Oxford Street, Halifax, N.S. B3H 3Z1, Canada
Received 13 May 1996; revised 9 July 1996; accepted 9 July 1996
Abstract The mixed-site fragmentation reactions discovered by Tuinman, Cook and Magid (J. Am. Soc. Mass Spectrom., 1 (1989) 85), as reaction channels competitive with charge-site-remote fragmentations, have been further investigated. Two complementary deuterium-labelled tetra-alkylammonium ions, (CH3)3N+C14D29and (CD3)3N+C14H29,were investigated as precursor ions for MS/MS/MS experiments using a sector/time-of-flight hybrid instrument employing orthogonal acceleration into the time-offlight analyzer. By comparing the second-generation fragment ion spectra originating from the two precursors, it was possible unambiguously to assign atomic compositions and also to make plausible structural assignments. The fragment spectra thus obtained for the first-generation product ions of the charge-site-remote reactions are consistent with previous conclusions concerning their structures. The first-generation product ions from the mixed-site fragmentations, for example (CH 3)2N+(C D 2)m,are competitive with their charge-site-remote counterparts for m = 4, 5 and 6 in a manner consistent with the anticipated stabilities of cyclic transition states. However, interpretation of the second-generation fragment spectra of the (CH3)2N+(CD2)m ions and their deuterium-labelled complements was best accomplished in terms of a postulated diradical ion intermediate. Crown copyright © 1997 Elsevier Science B.V. Keywords: Fragmentation reactions; Long-chain quaternary ammonium ions
I. Introduction
Mass spectral fragmentation patterns represent the outcome of an often complex network of parallel and sequential unimolecular dissociation reactions, integrated over an observation time window defined by the instrumental configuration. The interpretation of such integrated chemical kinetics data (the mass spectrum), in terms of the molecular structure of the pre-ionization species, seems likely to remain as much an art as a science for the foreseeable future. Only the continuing efforts to * Correspondingauthor. i NRCC No. 39734
systematize the vast amount of empirical information in terms of a few fundamental mechanistic principles make possible any rational scientific interpretation at all. The present work is dedicated to Professor Fred McLafferty, one of the pioneers and still a leading exponent both of the interpretative art and also of the science of understanding how and why energized isolated organic ions dissociate as they do. His early contributions in this context can be found in monographs and book chapters published in 1963 [1,2], and in the first 1966 edition [3] of his ground-breaking text "Interpretation of Mass Spectra", now happily in its fourth (and updated) edition [4].
0168-1176/97/$17.00 Crown Copyright © 1997 Elsevier Science B.V. PII S0168-1176(96)04490-4
224
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
One of the important systematizing classifications of ion fragmentation mechanisms [1-4] is the division into radical-site-initiated reactions and charge-site-initiated reactions. Even-electron ions, such as the protonated molecules formed in chemical ionization, fast atom bombardment (FAB), and electrospray ionization, are naturally dominated by charge-site-initiated fragmentations. More recently, however, Gross and his collaborators [5,6] discovered and characterized a new class of fragmentation mechanisms for closed-shell even-electron ions, which they termed charge-site-remote (CSR) fragmentations. These CSR dissociation reactions, as described in recent reviews [7,8], generally involve portions of the molecular structure which are well removed from a site of fixed charge. The charge site itself does not appear to be involved in the reaction mechanism, as indicated by the fact that the sign of the fixed charge (positive or negative) does not affect the nature of the observed CSR reactions, although the extent to which they compete with other reaction channels does vary. The CSR fragmentations involve characteristic expulsion of the elements of alkanes, and are most commonly observed in
MS/MS experiments employing high-energy collisional activation (CA), although reports exist [8] of such reactions induced by much lower collision energies. Indeed, the best estimates [8] of the critical energies for CSR reactions are of the order of only 2 eV. The accumulated evidence [6-8] appears to favor a mechanism involving 1,4-elimination of H2 to form two w-unsaturated fragments, as illustrated in Scheme 1 for long-chain tetra-alkylammonium ions, the example of interest in the present work. Upon collisional activation it is supposed that a number of gauche conformations in the hydrocarbon chain become accessible, permitting some hydrogen (or deuterium) atoms to come into sufficiently close proximity for 1,4-elimination of H 2 (O2) , via an electrocyclic (4n + 2) transition state, to become feasible (Scheme 1). However, Wysocki and Ross [9] have proposed an alternative mechanism involving homolytic cleavage remote from the charge site, to form a neutral free radical plus a distonic radical cation (radical and charge sites are remote from one another) which expels a t3-hydrogen atom to form the same functionalized 1-alkene as in Scheme 1. This alternative mechanism has also been discussed
CD3 D3C\ ~dcD3H Hu +N'
H,,.[
~.,,n
m/z 265 AH Ha
Hfi
CD3 H 13C+x~''CD3H._-H H",.~
N~_:/" m/z 123
C
-
I- --H
Ii
HR
HmH
Scheme 1. Mechanism for CSR fragmentations involving 1,4-elimination of H2, as proposed in [6].
K. Whalen et aL/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
by Gross [8], and more recently has received additional support from the work of Claeys et al. [10]. Wesdemiotis and co-workers [11,12] have investigated the structures of the neutral fragments formed in CSR fragmentations using collision-induced dissociative ionization of fast neutrals. The neutral fragments from CSR reactions of linear fatty acid ions, both (M - H)- and (M - H + 2Li) ÷, were shown [11] to indeed be 1alkenes. A similar conclusion was reached for metalated linear polyglycols [12], but their cyclic counterparts (crown ethers) preferentially expelled saturated free radicals to form distonic radical cations in accord with the mechanistic proposals of Ross and Wysocki [9]. As summarized by Gross [8], it is unclear how the compromise between a homolytic mechanism and the concerted 1,4-elimination mechanism for CSR reactions is established for any particular reactant ion. Tuinman, Cook and Magid [13,14] discovered an entirely different channel competitive with CSR reactions (Scheme 1) of trimethylalkylammonium ions [(CH3)3N+CnH2n+I].The crucial
finding
[13,14]
was
that
CA
of
[(CH3)3N+CnD2n+I] reactant ions, with a perdeuterated alkyl chain, resulted in a second series of fragment ions accompanying the predicted CSR fragments, but at 3 Da higher mass. This observation seems explicable only in terms of incorporation of one of the methyl groups into the neutral fragments, clearly incompatible with a CSR mechanism. It was proposed [14] that the fixed charge site on the nitrogen atom interacted in some way with the backbone a-bonds, resulting in an assumed cyclic fragment ion plus a saturated alkane (Schemes 2 and 3). In view of the participation of the fixed charge site in a midchain fragmentation, Tuinman and Cook [14] coined the name "mixed-site-fragmentation" (MSF) reaction to describe this competitive mechanism. Consistent with Schemes 2 and 3, labelled reactant ions of the type [(CD3)3N+Cn_ID2n_2CH3] were shown both by Wysocki and Ross [9] and by Tuinman and coworkers [13,14] not to incorporate the w-methyl group into the fragment ion. Whalen et al. [15]
2 m/z 265
°3ci°'
225
" "
,
m/z 120 Scheme 2. Proposed mechanism for MSF reaction of 2 in which a diradical ion 9' is the initial product ion.
226
K. Whalen et aL/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
a HII '' ~
1I
9,
H
D_
-
-
+ N--H
.,/
""
-/
H'
o,
o
27
m/z 120 C rJ m/z 18 D3
D3(~_CD3 H HH
+7
D3CI~.,~',\~ m/z 120
:~4 I'[ 6' m/z 120
m/z 29 3 m/z77
Scheme 3. Proposed mechanism whereby the diradical ion 9' forms closed-shell structures which may be the stable forms detected in the MS/ MS spectrum of 2 (Fig. 2 and Fig. 3), and which contain structural elements corresponding to some of the second-generation fragment ions of 2 via the MSF ion at m/z 120 (Table 2). Note that, for reasons of clarity, not all of the single-electron movements in 9', which lead to pathways a, b and c, are shown explicitly. Also, pathways a and b require that 9' adopt conformations different from that illustrated here.
used the competition between the MSF and CSR reactions to probe the nature of the collisional activation process in a new high-pressure quadrupole collision cell. These experiments [15] confirmed the earlier work of Tuinman and co-workers [13,14], although with some differences in relative intensities which were attributed [15] to differences in pre-collision internal energies corresponding to different ionization methods (FAB and electrospray), laboratory-frame collision energies, average numbers of collisions and reaction times within the collision cell, etc. More recently, Bambagiotti-Alberti et al. [16] described FAB mass spectra of [(CH3)3N+CnH2n+I] bromides for n -- 12, 14 and 16, for which they had shown previously [17] that the CSR fragments were clearly observed but that the smallest such fragment observable was at m/z 114 in
each case, corresponding to [(CH3)3N÷C4H7], i.e. the CSR fragmentation did not extend to below C-5. In order to examine this phenomenon further, the N,N,N-trimethyl-d9 analogs were synthesized and studied by FAB mass spectrometry and by mass analyzed ion kinetic energy spectroscopy (MIKES) [16,17]. Incorporation of the methyl groups into the neutral fragments was again observed via appropriate mass shifts, and cyclic intermediates and fragment ions were proposed, although these authors [16,17] did not appear to be aware of the earlier work [13-151 . The objective of the present work was to investigate further the MSF mechanism [13,14] through the use of complementarily d-labelled precursors, [(CH3)3N+C 14D29] and [(CD3)3N÷C14H29],together with modern techniques of tandem mass spectrometry.
227
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
acceleration time-of-flight analyzer for MS/MS experiments, as described previously [18]. FAB ionization used a glycerol matrix and a primary beam of cesium ions of effective incident energy 17 keV. At the normal ion source potential of 8 kV the laboratory frame collision energy in the collision cell, located between the collector slit of the double-focusing stage and the push-out region of the oaTOF analyzer, was 800 eV for singly charged ions [18]. In all MS/MS experiments xenon collision gas was used, at a pressure inside the cell estimated to be 2 x 10 -5 mbar. For MS/MS/MS experiments the precursor ion, together with the first-generation fragment ion formed in the first field-free region (FFR1),
2. Experimental The bromide salt of [(CH3)3N÷C14D29] (1) was synthesized as described previously [14]. The iodide salt of the complementary labelled ion [(CD3)3N+C14H29] (2) w a s synthesized by heating 1-aminotetradecane (Aldrich Chemical Co.) with an excess of trideuteroiodomethane (Aldrich). The identity of each synthesized product was checked by 1H NMR and by FAB mass spectrometry. All experiments reported here were conducted using an AutoSpec-oaTOF instrument comprising a double-focusing stage of EBE configuration, coupled to an orthogonal
I<
100%-
x5.00
>I
CH 3
90 ¸
I+ CH3--N--(CD2) IsCD3
CH3
80 ¸
70.
59
4r
124 60 ¸ 285 50.
121
40. •
•
30 20.
•
lO.
• 20
60
100
140
•
* 180
220
• 260
I
LJ 300
m/z
Fig. 1. MS/MS spectrum of (CH3)3N*C14D29(1), obtained using the AutoSpec-oaTOF instrument, with conditions described in Section 2 (Experimental). O, CSR ions; -k, MSF ions.
228
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
were selected by setting the analyzer fields of the double-focusing stage to static values determined by the appropriate linked-scan law at constant B/ E [19]. Argon collision gas was used in the collision cell in FFR1, at a pressure resulting in 50% attenuation of the precursor ion intensity. A laboratory-frame collision energy of 8 keV was used in this first MS/MS stage. The collision energy in the oaTOF collision cell, and also the TOF accelerating potential, are linked to the ion energy determined by the field in the second electric sector of the EBE analyzer in order to accommodate the full fragment ion mass range on the channel plate detector of the oaTOF [18]. Therefore, the collision energy for the first-generation
I00%.
fragment ions transmitted to the oaTOF collision cell was automatically reduced from 800 eV by the mass ratio of the first-generation fragment ion to the precursor. This reduction invalidated the m/z calibration of the oaTOF, but a simple correction could be applied using the known m/z value of the selected first-generation fragment ion which in turn acted as the precursor ion entering the oaTOF in the MS/MS/MS experiment.
3. Results and discussion
In accord with previous findings, the FAB
~
xlO.O0
~1 265 CD3 I+ CD3--N~(CH2) lsCH3
90"
CD3
80"
70" 60
120
50
123 68
40' 30"
20"
L.
10"
I
0
..
,
20
[
',
40
....I1. I
6O
•
i
80
,
I1" ,
i
.
100
.
.
.
120
-
i
'140" m/z
,I
I
.
160
.
.
.
-,
180
li lLit ,
i-
200
,
,
*. *. .1 220 240
260
Fig. 2. MS/MS spectrum of (CD3)3N*C14H29(2), obtained using the AutoSpec-oaTOF instrument, with conditions described in Section 2 (Experimental). 0, CSR ions; -k, MSF ions.
229
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
corresponding to CSR losses of the elements of perdeuteroalkanes (Ck, Dzk÷2), are accompanied by a less intense series 2 Da higher which presumably corresponds to losses of the elements of free radicals (Ck, Dzk+l). (The notation in which the elements are separated by commas is here taken to indicate that no assumptions concerning the structure(s) of the fragment(s) are made). This interpretation is supported by the corresponding peaks in the fragment ion spectrum of 2, which appear at only 1 Da higher than the intense peaks arising from CSR expulsion of the elements of (Ck, Hzk+2) at m/z 123, 137, 151, etc. Such radical expulsions are also considered [7-9] to be CSR processes, and indeed
mass spectra (not shown) of the halides of 1 and 2 were found to contain abundant fragment ions attributable to both CSR and MSF reactions, in addition to intense signals corresponding to 1 and 2 themselves. The pattern of relative intensities of the two families of fragment ions was very similar to that reported previously [13,14,16,17]. Figs 1 and 2 show fragment ion spectra of 1 and 2 obtained by MS/MS experiments using the oaTOF. In addition to the intense peaks corresponding to these two competing mechanisms [13-17], low intensity satellite peaks are also observed which were not readily apparent in the FAB mass spectra. In the case of 1, the series of peaks at m/z 121, 137, 153 etc.,
,ell3
124
100%-
CH3"N+--(CD2)13CD3
(A)
CH 3 80121
153
60-
4,
137
169
40-
20, , ,*^ .
.
.
.
.
i
120
I00%-
(B)
.
.
_JL ; ^ ^ - * .
i
.
.
.
.
i
.
.
.
.
.
.
.
.
,CD3
* 123
CD3"N,+(CH2)IsCH3 CD 3
80•
151
137
60-
165
40-
20-
124
o
134
.
120
125
.
.
.
.
.
130
.
i
*
.
135
.
.
.
l
140
.
.
.
.
I
145 m/z
.
.
.
*
.
.
.
.
150
.
I
155
I
.
160
.
.
.
.
.
.
165
.
l
.
.
.
.
170
Fig. 3. (A) Expanded view of the MS/MS spectrum of (CH3)3N+CI4D29 (1) shown in Fig. 1; (B) expanded view of the MS/MS spectrum of (CD3)3N+C]4H29 (2) shown in Fig. 2.
K. Whalen et al./International Journal of Mass Spectrometry and lon Processes 160 (1997) 223-240
230
have been cited [9] in support of CSR mechanisms involving diradical intermediates. Other less intense satellite peaks are more clearly seen in the expanded middle m/z range versions of Figs 1 and 2, shown in Fig. 3. In order to investigate the MSF mechanism, it was necessary to obtain structural information for the fragment ions. Some MS/MS experiments were conducted on precursors present as fragment ions in the FAB mass spectra, and the resulting spectra were satisfactory for the more intense of these precursor ions. However, for those cases in which the intensities of the selected precursor ions were not much greater than that of the "peak-at-every-mass" background characteristic of static FAB spectra, it became problematic to determine the contributions of the glycerol-derived ions. An example
l<
100 % 811
>1 188
+,I,I1,4 IL 105.
4O
'50
i134
2o
100%
x50
(A)
60
o
of this ambiguity is shown in Fig. 4, which compares the MS/MS spectrum (Fig. 4(A)) of a fragment ion present in the FAB mass spectrum with that obtained by a MS/MS/MS experiment (Fig. 4(B)) in which the first-generation fragment was that selected as precursor in the MS/MS experiment (Fig. 4(A)) and the primary precursor ion was 1. Peaks in Fig. 4(A) labelled with an asterisk are absent from Fig. 4(B), and are presumably derived from the FAB background. The sensitivity and duty cycle of the oaTOF analyzer are comparable with those of an array detector fitted to a double-focusing analyzer, and the FAB matrix artifacts in the MS/MS spectra are similar to those described previously by Falick et al. [20], who used a foursector instrument. Accordingly, in order to avoid such ambiguities, only the MS/MS/MS
.' .' . '. .' .I. .' . '. .' .' . I. .'. .' '
,....-
' I ' + ' ' I
66 I
, ....
88
+ I,'I J,,+ I''
''I'''~I
(B)
....
152"
I. ,,i..l+ ,.,..,,,I L., k.,,.b,-,
I''''I''''I
74
.
....
I ....
I ....
I ....
.,,--..,,.
I ' ' ' ' I ' ' ' ' I ' ' ' ' I
xlO0
....
I ''%.
I''''I''''I'
>l 1T8
80 5O
60
60
40 20
0
. . . . . . . .
20
40
60
J+L -
80
1O0
m/z
J ,.
120
140
160
180
'"1'"'1' 200
Fig. 4. (A) MS/MS spectrum of fragment ion at m/z 188 observed in the FAB mass spectrum of the bromide of 1. Peaks marked with an asterisk are believed to correspond to ions that are formed from precursors of m/z 188, but are derived at least in part from the glycerol matrix; (B) MS/ MS spectrum of the first-generation fragment at m/z 188, formed in the first field-free region from 1. See Section 2 (Experimental) for details of this MS/MS/MS experiment.
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
spectra will be referred to in the following discussion. However, there is a limitation of the MS/MS/ MS technique employed in the present work, which arises from use of the B/E linked scan relationship to define both the precursor ion and the first-generation fragment formed in FFR1. It is well known [19] that, although adequate resolving power for the fragment ions is provided by this technique, the effects of kinetic energy release are reflected in poor resolution of the precursor. In the present case, the intensity of the first 13C isotope peak of the precursor ion (C17) is about 18% of that of the all- 12Cprecursor nominally selected, and undoubtedly contributes lowintensity interference artifact peaks [19] to the linked-scan spectra. This effect will be important in cases where the selected intermediate ions in MS/MS/MS spectra are of low intensity, and are accompanied in the first-generation fragment spectrum by intense peaks 1 Da lower. An example where such B/E artifacts [19] may be important is discussed below.
3.1. Interpretation of second-generation fragments formed from first-generation fragment ions in the range m/z 120-125 Figs. 5 and 6 show fragment ion spectra, obtained in MS/MS/MS experiments, for firstgeneration fragment ions in the range m/z 120125 formed in FFR1 from 1 and 2, respectively. It is convenient to discuss corresponding first-generation fragments of 1 and 2 together, as the comparison assists the interpretation. The CSR fragment ions in this range are those at m/z 121 and 123 in Fig. 5(A) and Fig. 6(C), respectively, and on this basis [7,8] are assigned the following structures: m/z 121, (CH3)3N÷CDzCDzCD=CD2 (3): m/z 123, (CD3)3N+CHzCH2CH=CH2 (4) The proposed interpretation of the fragment ion spectra of 3 and 4, shown in Table 1, is consistent with the suggested open-chain structures for these even-electron ions. The ISOMABScomputer program (available from the authors) was
231
used to generate all possible formulae containing (C, N, H, D) for each m/z value observed. Unique assignments were possible through restrictions arising from the formulae of the precursors 1 and 2, together with the requirement that all fragments derived from 1 must also be formed from 2 with appropriate H/D exchanges, and vice versa. The relative intensities of corresponding fragments (Table 1) are also in gratifying agreement considering that the two spectra (Fig. 5(A) and Fig. 6(C)) were obtained on successive days, and this agreement provides some additional reassurance that the interpretations are correct. Most of the fragmentations of 3 and 4 appear to be charge-site initiated, and the only contentious interpretation appears to be that of the corresponding low-intensity ions at m/z 59 and 68 as the odd-electron molecular ions of the trimethylamines. The MSF first-generation products of 1 and 2 in this m/z range are m/z 124 and 120, respectively. The molecular formulae of these ions are unambiguous, given the known structures of the respective precursors 1 and 2 and also the reasonable assumption that they are even-electron ions. The fragment ion spectra of these MSF ions are shown in Fig. 5(D) and Fig. 6(A) and summarized in Table 2, together with proposed structures for the second-generation fragment ions. By far the most striking feature of this interpretation is the assignment as odd-electron species of the intense ions at m/z 77 and 74, derived respectively from 1 (via m/z 124) and 2 (via m/z 120). However, there appears to be no other interpretation which can account for all of the available information. The less intense ions at m/z 61 (from 1) and 60 (from 2) also require assignment as odd-electron species (Table 2). Although the production of odd-electron fragments from even-electron precursors must be regarded as unusual [3,4], it is established [79] that CSR expulsions of radicals can compete with expulsions of the elements of alkanes, as discussed above with reference to Figs. 1-3. Again, the degree of agreement between relative
232
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
I.
x~
~ 121
(A)
CSR 62
............. I_ ....
I ' ' ' ' l ' ' ' ' i
i ....
"ill
........ .ll . ~
I ' ' ' ' i ' ' ' ' l
....
I ....
I''"'1'';'1
....
I . . . . . . . .
~ 122
62
....
I ....
I'=''I''''I
....
I ....
I''''I''''I''''I''''I''''I''''I
(C)
62
. . . . . . . . . . . . . . ....
J<
I ....
I ' ' ' ' 1 ' ' ' ' 1
I ....... ....
(
•,
I I ....
. . . . . I ....
a~
4r
''''I''''I''''I
:
'''
....
. . . . . . I ....
| .
.
.
I ....
.
.
I ....
~
....
I''''I
....
I ....
I ....
~
I
I
''
124
MSF
~1
43
.
I ....
,f
I''''I''''I''''I
(E)
L.I I ....
innn
I''
125 1:
75
h dl Jl
~..:. ,.,.:; :,..~.,a.,:.!j,.~,..!L:. c',!, !':k.;;. !LJ,~: .~.::'z~,", t.',",.,,, 20
40
60
80
100
120
in/z Fig. 5. Second-generation fragment spectra obtained by MS/MS/MS experiments on 1. The first-generation fragments were formed in FFR1 of the AutoSpec-oaTOF instrument and transmitted to the collision cell preceding the oaTOF analyzer. (A) m/z 121 from CSR expulsion of the elements of (C m, D22) from 1; (B) m/z 122 from CSR expulsion of the elements of (C10, HI, D21) from 1; (C) m/z 123 from CSR expulsion of the elements of (C ]0, D21) from 1; (D) m/z 124 from MSF expulsion of the elements of (C 10, H3, D Jg) from 1; (E) m/z 125 from MSF expulsion of the elements of (Cl0, H2, D19) from 1.
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
~ (A)
233
>4120 M S F
45
64
77
2,,,I s,I J
' ' ' ' I ' ' ' ' I
....
I''''I
....
I ....
I''''
....
''''
''''
, i l l
'''~/
122 i
x5O
(a)
>(
55 65
.............. ....
I ....
i ,.J. . . .,. . .l . . .1. . .2. . . .9. . .
I ....
i ....
I ....
I ....
............................ I ....
i ....
I ....
t ....
I ....
, I ....
~so
I
'''
~ 123
(C)
CSR 55
{
xl,' 6
'Jr ~. . . . . .
. .-.-,
' . . . . . . . . . . . I .... I ....
l I ....
(D)
I
I ....
. . . . . . . . . . . . . . . . . . . . . . . . . I .... I .... I ' " ' ' l
24
66
56 . I ....
.
. I'
. . . . . . . .
.
.
' 'I'll
. '''
.
. I i ....
96
,l_ I ....
i
I ....
_,
I,_
I ....
. . . . . .
I ....
I ....
J_ I ....
106 _ _L I ....
~3o (E)
__ I ....
I
~
"";
125
69
°°1
~:: :~ :.:,,!L~::,.:,7:,!:;I:'~r :!; ~'~-:,.,~:~:: .: ~, .... 20
40
60
I
80
100
I
120
m/z Fig. 6. Second-generation fragment spectra obtained by MS/MS/MS experiments on 2. The first-generation fragments were formed in FFR1 of the AutoSpec-oaTOF instrument and transmitted to the collision cell preceding the oaTOF analyzer. (A) m/z 120 from MSF expulsion of the elements of (C [0, D 3, H 19) from 2; (B) m/z 122 from CSR expulsion of the elements of (C 10, D i, H21) from 2; (C) m/z 123 from CSR expulsion of the elements of (C 10, H 22) from 2; (D) m/z 124 from CSR expulsion of the elements of (C ]0, H 21)' from 2; (E) m/z 125 from CSR expulsion of the elements of (CIo, H20 ) from 2.
234
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
Table 1 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on 1 and 2, with 3 (m/z 121) and 4 (m/z 123) respectively as the firstgeneration fragments presumed to have been formed in CSR reactions. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(A) and Fig. 6(C)
1 ~ 3 [(CH3)3N+CD2CD2CD=CD2, m/z 121] -* X
2 ~ 4 [(CD3)3N+CH2CH2CH-CH2, m/z 123] --~ Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.I.)
Interpretation of Y
34 58 59 60 61 62
C2D~ (CH3)zN+=CH2 (CH3)3N + (CH 3)2N +=CD 2 (CH3)3N+D C4D~
29 (12) 66 (44) 68 (21) 64 (21) 69 (75) 55 (100)
CzH~ (CD3)2N+-CD2 (CD3)3N + (CD 3)2N +=CH 2 (CD3)3N+H C4H~
(14) (39) (19) (22) (65) (100)
intensities of corresponding fragments in the two spectra permits some increased confidence in the proposed interpretations. Formation of the MSF product ions, such as those whose fragment spectra are shown in Fig. 5(D) and Fig. 6(A) and interpreted in Table 2, has been assumed [13-16] to require a cyclic transition state (Scheme 2). However, this assumption does not necessarily imply that the stable MSF ions are cyclic (e.g. 5, below), and various linear closed-shell structures (e.g. 6-8) are possible for the stable forms of the MSF ion at m/z 124 formed from 1:(CH3)2 N+ < [cyclo-CsD10] (5);
(CH3)2N+=CDC4D9 (6); (CH3)zND+CD: CDC3D7 (7); ( C H 3 ) z N D + C D z C 4 D 7 ( 8 ) w h e r e 8 includes three possible isomers with different positions for the double bond. (Branched-chain
structures have been excluded from consideration here as being less likely to arise from the unbranched precursor). The cyclic structure 5 is that originally proposed for the MSF fragments [13,14]. The complementary structures arising from 2, in which the H and D atoms are exchanged, will be referred to as 5'-8'. Even if the most stable structures are indeed cyclic, production of second-generation fragment ions requires cleavage of at least two bonds, so that any of the linear structures 6 - 8 could in principle represent intermediate reactive forms. In addition, the participation of diradical forms as reactive intermediates in the fragmentation of the first-generation MSF ions cannot be ruled out. In fact, only a few of the ions listed in Table 2 can be readily derived by plausible mechanisms
Table 2 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on 1 and 2, with m/z 124 and m/z 120 respectively as the firstgeneration fragments presumed to have been formed in MSF reactions, and with molecular formulae [(CH3)zN(CsD10)] + and [(CD3)zN(CsH10)] +, respectively. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(D) and Fig. 6(A)
1 --. [(CH3)2N(CsDI0), m/z 124] +--. X
2 ~ [(CD3)2N(CsH10), m/z 120] + ---- Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.1.) Interpretation of Y
15 (15) 30 (21) 34 (35) 43 (100) 46 (42) 50 (23) 59 (46) 60 (75) 61 (33) 74 (92) 76 (21)
CH~ C 2D~ CzD~ CH3N+-=CD C3D~ C3D.~ CH3N+=CCD3 (CH 3)2N+-CD 2 CH3N+=CDCD3 or cyclic isomer (CH3)2N+CD=CD2 (CH3)2ND+CD=CD2or cyclic isomer
18 (24) 27 (24) 29 (39) 45 (100) 41 (46) 43 (33) 59 (56) 64 (95) 60 (43) 77 (95) 78 (24)
CD; C2H~ CEH; CD 3N+=CH
C3H~
C3H~ CD 3N +~CCH 3 (CD3)2N+-CH2 CD3N+=CHCH30r cyclic isomer (CD3)2N+CH=CH2 (CD3)2NH+CH=CH2or cyclic isomer
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
directly from the cyclic structures 5 or 5'. Instead, it is proposed that the diradical ion 9' (Scheme 2) is the initial MSF product ion from 2, with an analogous structure 9 initially formed from 1. This proposal clearly carries implications for the mechanism of the MSF reaction itself. It is not apparent whether or not such diradical structures represent local minima on the potential energy surface, but they must be species of higher energy than the closed-shell structures 5 - 8 (or 5'-8'). Accordingly, the diradical ions, e.g., 9', may be expected to rearrange to more stable closed-shell structures such as 5', 6' and 10' as illustrated in Scheme 3 (note that 10' is a special case of 8'). In turn these proposed intermediates contain structural elements which correspond to structures of several secondgeneration fragment ions listed in Table 2, although the sequence of events leading to these ions is not obvious in all cases (Schemes 3 and 4). Alternatively, diradical ion structures such as 9' can fragment directly to produce the remaining fragment ions listed in Table 2, as illustrated in Scheme 4. Of course, as for most mass spectrometric fragmentation mechanisms, the proposals summarized in Schemes 2 - 4 are
D3~. _CD3 -/-.~ at, •
235
largely speculative. However, they do have the merit of accounting for the experimental observations (Table 2), based on a minimal number of assumptions which do not violate chemical intuition derived from more substantive experimental evidence from physical organic studies of reactions in solution. The less intense first-generation fragment ions at m/z 123 (from 1) and 124 (from 2) arise from CSR expulsion of C10X21, where X is D or H for precursors 1 and 2 respectively. The secondgeneration fragment spectra (Fig. 5(C), Fig. 6(D), and Table 3) are broadly similar to those of their even-electron CSR counterparts (Fig. 5(A) and Fig. 6(C)). However, these are probably distonic ions, e.g. (CH3)3N+(CaD8) ', and their odd-electron character might be expected to facilitate rearrangement reactions. Indeed, Table 3 is appreciably more complicated than either Table 1 or Table 2, and the proposed interpretation is correspondingly less secure. It is now more difficult to match up relative intensities of corresponding fragments because of multiple possibilities of interpretation. For example, the relative intensity of the fragment at m/z 64 from 1 (Table 3) is a good match for that of the
D3C_ i D3
HH
-.
H
H
+N"
H"[
X"~ t/t~ m/z 120
m/z 120
at
.j ÷
D3C--N----C--CH 3 m/z 59
/\ m/z 78
>
D3Q -. N+
/\
m/z 60
Scheme 4. Proposed mechanisms for formation of some second-generation fragment ions of 2 via the MSF ion at m/z 120 (Table 2).
236
K. Whalen et aL/lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
Table 3 Interpretation of fragment ion spectra obtained by MS/MS/MS experiments on ! and 2, with m/z 123 and m/z 124 respectively as the firstgeneration fragments presumed to have been formed in CSR expulsions of Ct0X21 radicals, where X is D or H for precursors 1 and 2 respectively. Data, both m/z values and relative intensities (R.I.), are from Fig. 5(C) and Fig. 6(D) 1 ~ [(CH3)3N+(C4D8), m/z 123] ~ X
2 ~ [(CD3)3N+(C4Hs) , m/z 124] ~ Y
2nd generation X, m/z (R.I.)
Interpretation of X
2nd generation Y, m/z (R.I.)
Interpretation of Y
58 (13)
(CH3)2N+=CH2
66 (100)
(CD3)2N+=CD2 and (CD3)2N+H(CH3) and (CD3)zN÷CH2D
59 60 61 62
(CH3)3N + (CH3)zN÷-CD2 (CH3)3N+D and (CH3)2N* CD2H C4D~ and DN+=-C-CzD5 and (CH3)zN+-CD3 (CH3)zNH+CD3 C4D~ and (CH3)2N+D(CD3) and CH3NH+C2D5 (CH3)3N+(C2D4) (CH3)2-N+=CDC3D7
68 (30) 64 (26) 69 (26) 55 (20) 65 (76) 67 (35) 56 (30) 63 (7) 96 (24) 106 (22)
(CD3)3N÷
(39) (48) (87) (100)
63 (22) 64 (6) 91 (11) 108 ( < 5)
ion at m/z 63 from 2, suggesting that the possible interpretations of the m/z 64 ion other than CH3NH+C2D5 do not make significant contributions. If correct, this conclusion implies that the main contribution to m/z 56 from 2 is that of HN+=CCzHs, for which the corresponding ion from 1 is at m/z 62, the base peak in Fig. 5(C). Also, the proposal that (CH3)zN+D(CD3) does not contribute significantly to the relative intensity of m/z 64 from 1 (Table 3) implies that its counterpart (CD3)zN+H(CH3) does not contribute much to m/z 66 from 2, which must therefore correspond mostly to (CD3)2N+=CD2 and (CD3)zN+CH2 D. In turn, the corresponding ion (CH3)2N+CDzH from 1 must make a significant contribution to the intensity at m/z 61, and so on. In view of these complicated relationships it is not surprising that the matching of relative intensities for corresponding fragments is not as satisfactory for Table 3 as for Tables 1 and 2. Nonetheless, the proposed interpretations of Table 3 do have a measure of self-consistency, and do indeed indicate an increased extent of (presumably radical-site initiated) rearrangements. The very low-intensity peaks at m/z 122, apparent in Fig. 3(A) and 3(B), appear to
(CD3)2N+=CH2 (CD3) 3N+H C4H~ (CD3)2N+'CH3 (CD3) 2ND+CH3 C4H~" and HN+=CC2Hs CD3ND+C2H 5 (CD3) 3N+(C2H4) (CD3)2-N+=CHC3H 7
correspond to isotopic variants of the CSR fragments at m/z 121 and 123, respectively. There appear to be two possibilities for isotopic substitution to occur. The first possibility involves incomplete deuteration of the precursor. In the case of l, the sequence summarized in Eq. (1) could account for formation of a CSR fragment at m/z 122 containing one 13C atom (CH3)3 N÷ C4DsC10D20 H [13C~2C16, m/z 285] --~ (CH3)3 N÷ C4D7 [13C~2C6, m/z 122] + (C10, H, D21)
(1)
In the case of 2, the incomplete deuteration would be retained in the CSR ion at m/z 122, which now contains no 13C atom, as described in Eq. (2) (CD3)z(CD2H)N + C14H29 [13C~2C16,
m/z
265]
(CD3)z(CDzH)N + C4H7 [13C~2C7, m/z 122] + (C10, H22) [13C~2C9] (2) The alternative possibility invokes H/D scrambling in the precursor ion prior to CSR fragmentation with no need to consider 13C
K. Whalen et al./International Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
C4D~,arising
variants, as shown in Eq. (3) for 1 (CH3)3 N+ C14D29 [1, m/z 285] ---* (CH3)2(CH2D)N + C4DsC10D20 H [m/z 285] ---* (CH3)z(CHzD)N + C4D 7 [m/z 122] + (C10, H, D21 )
(3)
and correspondingly for 2, as in Eq. (4) (CD3)3N + C14H29 [2,
m[z
265]
(CD3)2(CD2H)N + C4HsC10H20 D [m/z 265] ---* (CD3)z(CD2H)N + C4H 7 [m/z 122] + (C10 , D, H21 )
237
(4)
Another possibility is that the first-generation fragment at m/z 122, formed from 1 in FFR1, represents an artifact of the B/E technique [19] used to define both the precursor ion and the FFR1 fragment ion. As discussed above, the effects of kinetic energy release will result in a sufficiently poor resolving power for the precursor such that the 13C1 version of the 1 ----, 3 CSR reaction (i.e. m/z 286 ~ m/z 122) will be observed to some extent. Any second-generation fragments at m/z 122 produced thus from 1 as B/E artifacts [19] will be indistinguishable from those produced via the sequence described by Eq. (1). However, such considerations are unlikely to apply to the ion at m/z 122 formed from 2, since the CSR ion is now at m/z 123 (i.e. already at higher m/z) and the intense MSF ion is at m/z 120 (Fig. 3(B)), too far away to make a significant contribution via a B/E artifact from the second 13C isotope peak of 2 as FFR1 precursor. Comparison of the fragment ion spectrum of m/z 122 from 1 (Fig. 5(B)) with that of its unmodified CSR counterpart 3 (m/z 121, Fig. 5(A)) shows that the only qualitative difference is the appearance in Fig. 5(B) of a fragment ion at m/z 63 which is not observable in Fig. 5(A). If the interpretations summarized in Table 1 are valid, the base peak at m/z 62 is C4D~. The secondgeneration fragment at m/z 63 in Fig. 5(B) is then readily interpreted as the ~3C1 version of
either via the sequence described in Eq. (1) or as a B/E artifact as described above. The alternative sequence of Eq. (3) cannot give rise to this species. However, the intensity of m/z 63 is only 12% of that of m/z 62 in Fig. 5(B), whereas random distribution of the 13C atom in m/z 122 ions of this type predicts that the intensity ratio of these two ions should be 4:3. Therefore, while sequence Eq. (1) and/or the B/E artifact route do occur, they appear to be appreciably less important for 1 than sequence Eq. (3). In the case of 2, comparison of Fig. 6(B) and Fig. 6(C) shows that the C4H~ions at rn/z 55 are unaccompanied by any observable intensity at m/z 56, as predicted both by Eqs. (2) and (4) and by the impossibility of a B/E artifact of the required type, so that no discrimination amongst these mechanisms is possible on this basis. The most striking difference between Fig. 6(B) and Fig. 6(C) is the shift of the second most intense peak at m/z 69 in Fig. 6(C) to m/z 68 in Fig. 6(B). This observation is consistent with a change from (CD3)3N÷H (m/z 69, Table 1) to (CD3)2(CD2H)N+H (m/z 68) as predicted by both Eqs. (2) and (4). The corresponding predictions for 1, from both Eqs. (1) and (3), are that the (CH3)3N+D ion (m/z 61, Fig. 5(A) and Table 1) should shift to m/z 62 in Fig. 5(B), but the latter is the base peak in the spectrum and the predicted shift is observable only via changes in relative intensities. The overall conclusion drawn from these observations on the ions at m/z 122 is that the effects of incomplete deuteration and/or B/E artifacts are indeed observable, but are not as important as H/D scrambling in the precursors prior to CSR fragmentation. However, it should be emphasized that these effects make very minor contributions, as indicated by the relative intensities at m/z 122 in Fig. 3(A) and Fig. 3(B).
3.2. Other CSR fragment ions The discussion thus far has been restricted to the intense sets of peaks in the range m/z
238
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
correspond to CSR reactions, though in this case the mechanism presumably involves either 1,2elimination of CX4 with X being D or H, or successive expulsions of CX3 plus X [9], rather than the 1,4-elimination reaction [5,6] shown in Scheme 1. In contrast, loss of CH3D from 1 and of CD3H from 2 are most likely chargesite initiated reactions forming structures similar to 6 and 6', but with the entire C14 backbone intact. Fragment ion spectra of the first-generation fragment ions at m/z 249 and 246 from 2 are shown in Fig. 7. The spectrum of m/z 249 (Fig. 7(A)) is very similar to that of 2 itself (Fig. 2) with both CSR and MSF fragments, exactly as predicted for the proposed structure
120-125, as it was hoped that the structural interpretation of these smaller ions would be relatively straightforward. The fragment ion spectra of the larger CSR product ions (not shown) contained the CSR and MSF fragment ions predicted for precursor ions with a terminal double bond (Scheme 1). These spectra contained no surprises. However, Figs. 1 and 2 show that both 1 and 2 can also lose methane neutrals, by which losses of CD4 and of C H 3 D from 1 occur with roughly equal probabilities to form m/z 265 and 268, and correspondingly losses of CH4 and of C D 3 H from 2 to form m/z 249 and 246. Expulsion of C D 4 from 1 and of CH4 from 2 might be expected to x2o
lOO % -
x200
÷
,~ 2 4 9
69 (A)
80 'k
120
60
55 123
40
20 ,,,
J.,,l
L-i i j ..,.
, ''. . . . ,.
,
. . ;
+
x20
100 %
•
(B)
! ki,,.p
.J+,L]. •
'
'
--/JL"
~
I
,d
246
x200
90
80 64 60 77 40
104
20
0
, ,.,,,.
.,It, .J. . 'lI 20
40
+!l,+J,,L, 60
80
I
"'1, -
118
-,..:-,: .: ..,..+ ~. :-. , . . . . - . . - + . . - , 120
140
160
180
'i. 200
..~. ++,-,-.. +...+-, 220
m/z Fig. 7. (A) Fragment ion spectrum of (2-CH4), m/z 249; (B) fragment ion spectrum of (2-CD3H), m/z 246.
240
, ,
K. Whalen et al./lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
(CD3)3N+(CH2)12CH=CH2 . Similarly, the structure (CD3)2N+=CHCa3H27 proposed for m/z 246 from 2 is expected to form CSR fragments but no MSF ions, owing to the inflexibility introduced by the double bond, and indeed this is observed in Fig. 7(B). The low-intensity ions (Figs 1 and 2) at m/z 269 and 245, corresponding to losses of C H 4 f r o m 1 and of C D 4 f r o m 2 , correspond to e.g. a structure CD3N+(=CDz)C14H29 in the case o f ( 2 - CD4).
3.3. Other MSF product ions Examination of Figs. 1 and 2 shows that the MSF ions, e.g. the fragment ions of formula (CH3)2N+(CD2)m from 1, are prominent only for m -- 4, 5 and 6 (m/z 108, 124 and 140 for 1 in Fig. 1, and m/z 106, 120 and 134 for 2 in Fig. 2). The MSF ions for other values of m are appreciably less intense than the products of the competing CSR reaction channels, and this trend presumably reflects the probabilities of formation and stabilities of different ring sizes in cyclic transition states. Note, however, that the MSF ions with m -- 9 (m/z 188 and 176 in Figs 1 and 2, respectively) are more competitive relative to their CSR counterparts than for any value of m other than 4, 5 and 6. The nature of these MSF reactions will require further study.
4. Conclusions The present study of the competition between CSR and MSF reactions of long-chain tetraalkylammonium ions upon collisional activation has confirmed that the product ions of these two types are very different from one another. The MS/MS/MS experiments used to probe the structures of the CSR product ions produced no major surprises. The MSF reaction channels are competitive with their CSR counterparts when the assumed cyclic transition states involve rings incorporating five, six or seven atoms (including the nitrogen). This observation fits in well with
239
established principles of organic chemistry. The fragment ion spectra of these MSF intermediate ions, however, are difficult to rationalize as arising directly from cyclic structures, and at least the activated forms of these ions are best considered to arise from a diradical ion intermediate. On this basis it was possible to postulate rearrangement reactions to forms which include structural elements corresponding to all of the observed second-generation fragment ions. The notion of a diradical intermediate is consistent with the earlier proposals of Wysocki and Ross [9] for CSR reactions. Although these mechanistic proposals (Schemes 2-4) are necessarily speculative, they do account for the MS/MS/ MS spectra in a reasonable way. Clarification of these MSF reactions will require additional experiments.
Acknowledgements This paper benefited greatly from the perceptive comments of an anonymous referee.
References [1] F.W. McLafferty, Mass Spectral Correlations, Advances in Chemistry Series 40, American Chemical Society, Washington, DC, 1963. [2] F.W. McLafferty (Ed.), in Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p. 309. [3] F.W. McLafferty, Interpretation of Mass Spectra, W.A. Benjamin, New York, 1966. [4] F.W. McLafferty and F. Ture6ek, Interpretation of Mass Spectra, 4th edn., University Science Books, Mill Valley, CA, 1993. [5] K.B. Tomer, F.W. Crow and M.L. Gross, J. Am. Chem. Soc., 105 (1983) 5487. [6] N.J. Jensen, K.B. Tomer and M.L. Gross, J. Am. Chem. Soc., 107 (1985) 1863. [7] J. Adams, Mass Spectrom. Rev., 9 (1990) 141. [8] M.L. Gross, Int. J. Mass Spectrom. Ion Processes, 118/119 (1992) 137. [9] V.H. Wysocki and M.M. Ross, Int. J. Mass Spectrom. Ion Processes, 104 (1991) 179. [10] M. Claeys, L. Nizigiyimana, H. Van den Heuvel and P.J. Derrick, Rapid Commun. Mass Spectrom., 10 (1996) 770. [11] M.M. Cordero and C. Wesdemiotis, Anal. Chem., 66 (1994) 861.
240
K. Whalen et aL/lnternational Journal of Mass Spectrometry and Ion Processes 160 (1997) 223-240
[12] T.L. Selby, C. Wesdemiotis and R.P. Lattimer, J. Am. Soc. Mass Spectrom., 5 (1994) 1081. [13] A.A. Tuinman, K.D. Cook and L.J. Magid, J. Am. Soc. Mass Spectrom., 1 (1989) 85. [14] A.A. Tuinman and K.D. Cook, Proc. 38th ASMS Conf. Mass Spectrom. Allied Topics, Tucson, AZ, 1990, p. 938. [15] K. Whalen, J.S. Grossert and R.K. Boyd, Rapid Commun. Mass Spectrom., 9 (1995) 1366. [16] M. Bambagiotti-Alberti, S.A. Coran, F. Benvenuti, P.
[17] [18] [19] [20]
LoNostro, S. Catinella, D. Favretto and P. Traldi, J. Mass Spectrom., 30 (1995) 1742. M. Bambagiotti-Alberti, S.A. Coran, V. Giannellini, D. Favretto and P. Traldi, Rapid Commun. Mass Spectrom., 8 (1994) 439. R.H. Bateman, M.R. Green, G. Scott and E. Clayton, Rapid Commun. Mass Spectrom., 9 (1995) 1227. R.K. Boyd, Mass Spectrom. Rev., 13 (1995) 359. A.M. Falick, K.F. Medzihradszky and F.C. Walls, Rapid Commun. Mass Spectrom., 4 (1990) 318.