Peter J Derrick and the Grand Scale ‘Magnificent Mass Machine’ mass spectrometer at Warwick

Article abstract of DOI:10.1177/1469066717737643

The value of the Grand Scale ‘Magnificent Mass Machine’ mass spectrometer in investigating the reactivity of ions in the gas phase is illustrated by a brief analysis of previously unpublished work on metastable ionised n-pentyl methyl ether, which loses predominantly methanol and an ethyl radical, with very minor contributions for elimination of ethane and water. Expulsion of an ethyl radical is interpreted in terms of isomerisation to ionised 3-pentyl methyl ether, via distonic ions and, possibly, an ion-neutral complex comprising ionised ethylcyclopropane and methanol. This explanation is consistent with the closely similar behaviour of the labelled analogues, C₃H₇CH₂CD₂OCH₃ ^+. and C₃H₇CD₂CH₂OCH₃ ^+., and is supported by the greater kinetic energy release associated with loss of ethane from ionised n-propyl methyl ether compared to that starting from directly generated ionised 3-pentyl methyl ether.

Full text of DOI:10.1177/1469066717737643

In Memoriam of Peter J Derrick
European Journal of Mass Spectrometry
2017, Vol. 23(6) 319–326
! The Author(s) 2017
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Peter J Derrick and the Grand Scale ‘Magnificent
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DOI: 10.1177/1469066717737643
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AW Colburn, Peter J Derrick and Richard D Bowen
Abstract
The value of the Grand Scale ‘Magnificent Mass Machine’ mass spectrometer in investigating the reactivity of ions in the gas
phase is illustrated by a brief analysis of previously unpublished work on metastable ionised n-pentyl methyl ether, which
loses predominantly methanol and an ethyl radical, with very minor contributions for elimination of ethane and water.
Expulsion of an ethyl radical is interpreted in terms of isomerisation to ionised 3-pentyl methyl ether, via distonic ions and,
possibly, an ion-neutral complex comprising ionised ethylcyclopropane and methanol. This explanation is consistent with
the closely similar behaviour of the labelled analogues, C3H7CH2CD2OCH3þ. and C3H7CD2CH2OCH3þ., and is supported by the
greater kinetic energy release associated with loss of ethane from ionised n-propyl methyl ether compared to that starting
from directly generated ionised 3-pentyl methyl ether.
Keywords
Peter Derrick, Grand Scale mass spectrometer, MMM, ionised ether, isomerisation, distonic ions, ion-neutral complex,
pentyl, methyl
Received 22 September 2017; accepted 24 September 2017
Introduction
sectors gave ample opportunity for fragmentation to
Professor Peter Derrick was appointed at the occur, thus resulting in an usually large contribution
University of Warwick in 1987 as the first Research from the spontaneous dissociation of metastable ions.
Professor in Mass Spectrometry in the UK.1 Shortly Consequently, the resultant MIKES were of an extraor-
afterwards, the Grand Scale Mass Spectrometer2 dinarily high quality, often showing unprecedented
(‘MMM’, given the sobriquet the ‘Magnificent Mass signal-to-noise ratio, thus permitting relatively minor
Machine’ by Steen Hammerum, during the early fragmentation pathways to be monitored effectively.
stages of a long-term collaboration), which he had
The power and versatility of the MMM instrument
designed and constructed in Australia, was installed facilitated the investigation of many themes in which
and brought into operation on the ground floor of mass spectrometry could be exploited, particularly
the Chemistry Department, where it was extensively those involving alternative ionisation methods or bio-
used for two decades in a wide range of scientific inves- logical applications,5–14 often as part of local, national
tigations, ranging from fundamental studies of the reac- or international collaborations. For example, 14 research
tions of ions to the application of mass spectrometry in papers by authors based at Warwick were published
biological and medicinal contexts.
during 1989–1993 in the field of ion chemistry
The MMM was constructed with ‘reverse geom- alone.15–28 The following brief summary of previously
etry’,3,4 in which ions were transmitted through the mag- unpublished results on ionised n-pentyl methyl ethers
netic sector before entering the electric sector. This obtained with MMM is presented as an illustration of
arrangement allowed for ions of interest to be selected, what could be achieved with this remarkable instrument.
then allowed or caused to dissociate before entering the
electric sector, thus permitting information on all the
resultant fragment ions to be acquired conveniently and
Department of Chemistry, University of Warwick, Coventry, UK
rapidly by Mass-Analysed Ion Kinetic Energy
Spectroscopy (‘MIKES’).3,4 Illustrative photographs of
Corresponding author:
Richard D Bowen, School of Chemistry and Biosciences, University of
the instrument are shown in Figures 1 and 2. The excep-
tionally long second field-free region between the two Email: R.D.Bowen@Bradford.ac.uk
Bradford, Richmond Road, Bradford, West Yorkshire, BD7 1DP, UK.

320
European Journal of Mass Spectrometry 23(6)
Figure 1. Part of the MMM instrument, showing the source and magnetic sector.
MMM: magnificent mass machine.
species that normally dissociate from their ground state
via the routes that require the lowest critical32 energy and
with excess energies in the transition states that are
comparable to, or smaller than, those found in classical
solution experiments.3,4,29–31,33 Many refinements in
understanding the chemistry of ions were made by study-
ing metastable ions and documenting their behaviour.
In particular, much progress was made in delineating
and explaining complex rearrangements by this method-
ology,3,4,29–31 often in conjunction with other techniques
that permitted the structure of the ionic34–37 and
neutral38–41 fragments to be established.
The chemistry of low-energy ionised ethers is more
complicated and interesting than might appear from a
cursory inspection. Although a-cleavage usually domin-
ates the 70 eV mass spectra of ethers,42,43 other processes
compete in slower reactions, such as those observed at 12
eV ionising energies or in the dissociation of metastable
ions.23,44–54 Thus, whereas ionised ethers of general struc-
þꢀ
ture R2CHOCH3 derived from secondary alcohols
eliminate Rꢀ in fast reactions, loss of RH (and, in many
cases, CH3OH) occurs for metastable ions. Similarly,
ionised ethers of general structure RCH2OCH3þꢀ derived
from primary alcohols tend to lose Rꢀ (and, sometimes,
CH3OH) in fast reactions, but elimination of a smaller
alkyl radical becomes more important at lower internal
energy. These trends are illustrated by the behaviour of
þꢀ
ꢀ
þꢀ
Figure 2. Peter turning off the MMM pumps for the last time at
the University of Warwick.
MMM: magnificent mass machine.
metastable n-C5H11OCH3 (which loses mainly C2H5
and CH3OH) and metastable (C2H5)2CHOCH3
(which loses almost exclusively C2H6).
It was well established by 1980 that the study of meta-
stable ions3,4,29–31 offers a powerful and highly advanta-
Experimental procedures
geous method for investigating the mechanism of ionic The methyl ethers required in this work were prepared
reactions. Metastable ions are long lived, low-energy by the condensation with a slight deficiency (0.8–0.9

Colburn et al.
321
equivalents) of methyl iodide (CH3I or CD3I, as appro- collision cell to reduce the intensity of the parent ion
priate) with the requisite alkoxide prepared from a sus- signal to 40% of its original value; data were accumu-
pension of mixture of a slight excess (1.2 equivalents) of lated over 10–20 scans.
sodium hydride and the parent pentanol in triglyme.
The crude product was distilled directly from the reac-
tion mixture, washed with water, dried and redistilled.
Results and discussion
The labelled analogues of n-pentanol were obtained by The reactions of metastable (C2H5)2CHOCH3þꢀ, 1þꢀ, n-
routine methods. The isotopic purity of the deuterated C5H11OCH3þꢀ, 2þꢀ, and three deuterium-labelled ana-
ethers was estimated to be at least 95%. Scheme 1 sum- logues of 2þꢀ are summarised in Table 1. Relevant CID
marises these synthetic procedures.
All mass spectra were obtained on the MMM instru- evident.
spectra are shown in Table 2. The following trends are
ment, which was equipped with a post-acceleration
First, the behaviour of 1þꢀ and 2þꢀ follows the general
detector. Typical values for the source pressure and trend shown by ionised ethers: 1þꢀ dissociates almost
accelerating voltage were 2 ꢁ 10ꢂ6 Torr (1 exclusively (>99.5%) by eliminating ethane, whereas 2þꢀ
Torr ¼ 133.3 Pa) and 8075 V, respectively. The dissoci- has a more varied chemistry, eliminating mainly methanol
ation of metastable ions in the second field-free region (ꢃ49%) or an ethyl radical (ꢃ47%), with only minor
was measured by the MIKES technique3,4, by repetitive contributions from loss of ethane (ꢃ3%) and water
scanning of the electric sector voltage over the range (ꢃ1%). According to the studies on other ionised second-
required to transmit the product ions. Data were accu- ary ethers, expulsion of ethane from 1þꢀ is logically inter-
mulated over 100–200 scans. Kinetic energy releases for preted as involving an ion-neutral complex (INC),56–60
each fragmentation were estimated from the width at leading eventually to ionised 1-methoxypropene,
half height of the associated metastable peaks3,4; the CH3OCH¼CHCH3þꢀ. This combination of products is
usual correction was applied for the width of the moreꢀ stable than those, [CH3OCHCH2CH3]þ and
main beam.55 The collision-induced dissociation C2H5 , accessible by simple a-cleavage.
(CID) mass spectra also were acquired by the MIKES
Second, the labelling data indicate that the elimin-
technique, after admitting sufficient helium to the ation of an ethyl radical from 2þꢀ involves not only the
terminal ethyl group but also the atoms attached to
the a- and b-carbon atoms. This observation may be
understood if 2þꢀ isomerises to 1þꢀ prior to dissociating.
If this rearrangement were rate limiting, the contrasting
behaviour of 1þꢀ and 2þꢀ may be explained. Starting
from 1þꢀ, energy is the dominant factor in governing
the competition between loss of an ethyl radical and
elimination of a molecule of ethane, thus explaining
the preference for the latter fragmentation, which has
a lower critical energy. In contrast, following the rate-
limiting isomerisation of 2þꢀ to 1þꢀ 2þꢀ, the rate at which
dissociation may occur becomes more important,61 so
ethyl radical elimination dominates because it requires
little or no further rearrangement but only simple cleav-
age. Moreover, the increased kinetic energy release
(i), (ii)
(i), (iii)
ROH
ROH
ROCH3
ROCD3
(iv), (v)
(i), (ii)
C4H9CO2CH3
C4H9CD2OH
C4H9CD2OCH3
(vii), (v)
(vi)
C3H7CH2CO2CH3
C3H7CD2CD2OH
C3H7CD2CO2CH3
(i), (ii)
C3H7CD2CD2OCH3
Scheme 1. Summary of synthetic routes used to prepare pentyl
methyl ethers.
Table 1. Reactions of metastable pentyl methyl ethers.
Neutral lost
ꢀ
ꢀ
H2O
HOD
C2H5
C2H6
RAa
C2H3D2
CH3OH
CH3OD
CD3OH
Structure
b
T1/2
RAa
RAa T1/2 RAa T1/2
RAa T1/2
T1/2 RAa T1/2
RAa T1/2 RAa T1/2
b
b
b
b
b
b
b
þꢀ
CH3CH2CH2CH2CH2OCH3þꢀ
CH3CH2CH2CH2CH2OCD3þꢀ
CH3CH2CH2CH2CD2OCH3þꢀ
CH3CH2CH2CD2CH2OCH3
0.7 1.89
1.3 1.87
46.8 1.17
38.5 1.15
3.2 2.28
2.6 2.36
49.3 1.65
57.5 1.52
0.8 1.90 0.4 1.82 29.3 1.16
0.9 1.94 0.2 1.82 29.0 1.13
1.0c 1.83 19.2 1.16 41.7d 1.51 7.4 1.40
1.0c 1.89 19.5 1.13 43.6d 1.52 5.7 1.4777
>99.5 1.44 <0.1
þꢀ
(CH3CH2)2CHOCH3
<0.1
<0.2
aRA: relative abundance, measured by ion counts (corresponding to metastable peak areas) and normalised to a total of 100 units.
bT1/2: Kinetic energy release, in KJ molꢂ1, estimated from the width at half height of the Gaussian metastable peak.
cThis signal may be partly attributable to loss of C2H4D.
dThis signal obscures the much weaker signal for loss of C2H4D2.

322
European Journal of Mass Spectrometry 23(6)
Table 2. Collision-induced dissociation spectra of C4H9Oþ ions.
m/z of fragment ion
Precursor and relative
intensity of signala
72 71
69
59
58
57
56
55b 53
45b
44 43b
42 41b
40
39
þꢀ
(CH3CH2)2CHOCH3
37 33
40 35
0.4
0.3
0.4
0.5
14 24
18 25
3.8 (12)
4.8 (16)
1.7 (375)
1.4 (440)
2
2
(68)
(77)
36 (347) 10
38 (375) 11
62
62
þꢀ
CH3CH2CH2CH2CH2OCH3
38 37
36
33
31b 30
29
28
27
26
25 16
15 14
13
12
þꢀ
(CH3CH2)2CHOCH3
12
11
5.8 ꢃ0.8
5.2 ꢃ1
3.2 (13)
3.2 (14)
8.3 100
8.4 100
32
31
65
68
18
18
ꢃ3
1.1 31
37
4.3
4.8
1.1
1.1
0.4
0.5
þꢀ
CH3CH2CH2CH2CH2OCH3
ꢃ3 ꢃ1
aRI, relative intensity, measured by peak heights and normalised to a total of 100 units for the most intense signal containing no significant contribution
from the dissociation of metastable ions; most values are quoted to two significant figures.
bThese signals contain a significant contribution from the dissociation of metastable ions, corresponding to loss of H2O, C2H4, CH2O, CH3OH and C3H6,
respectively, to form ions at m/z 55, 45, 43, 41 and 31, respectively; these RI values are enclosed in parentheses to emphasise that they contain a
contribution for the dissociation of metastableions.
H
1,4-H
.
.
+
HOCH3
+
.
OCH3
OCH3
+
2+.
3
H
4
.
+
+
+
OCH3
Faster reactions
OCH3
H
H OCH3
1,4-H
.
higher internal
energy of 1+.
.
C2H5
+
1+.
5
lower internal
energy of 1+.
Slower
reactions
.
+
+
H-transfer
OCH3
OCH3
.
H
+
C2H6
C2H5
+.
Scheme 2. Mechanisms of C2H5. and C2H6 loss from ‘‘C5H11OCH3
accompanying ethane elimination from 2þꢀ (ꢃ2.3 kJ elimination. There is limited, but significant, participa-
molꢂ1), compared to that starting from 1þ2þꢀ (1.44 kJ tion of the deuterium atoms on the a- and b-carbon
þꢀ
molꢂ1), is also explained if the rearrangement of 2þꢀ to atoms of CH3CH2CH2CH2CD2OCþHꢀ3þꢀ, 2-1,1-2H2þꢀ
,
1þꢀ is rate limiting because part of the excess internal and
CH3CH2CH2CD2CH2OCH3
,
2-2,2-2H2
,
energy is partitioned as translation when dissociation respectively, in methanol loss.
takes place.3,4,61
All these observations may be accommodated by the
Third, the close similarity of the CID34–37 spectra of general mechanism shown in Scheme 2. A 1,3-H shift
C4H9Oþ ions generated by simple cleavage of 1þꢀ and from the g-carbon atom to the oxygen atom of 2þꢀ gives
by ethyl radical elimination from 2þꢀ supports the inter- the distonic62 ion, 3, which may undergo cyclisation to
pretation that the same fragment ion, [CH3OCHCH2 an INC, 4, comprising ionised ethylcyclohexane and
CH3]þ, is formed from both ionised ethers.
methanol. Ring opening then forms a different distonic
Fourth, loss of methanol, which is the second major ion, 5, which may undergo a 1,3-hydrogen shift to
reaction of 1þꢀ, involves retention of the original meth- form 1þꢀ, which then dissociates via ethyl radical loss.
oxy group, together with one of the hydrogen atomsþoꢀf It is also possible to describe the crucial rearrangement
of 3 to 5 as a direct shift, without invoking the inter-
the pentylþcꢀhain. Thus, CH3CH2CH2CH2CH2OCD3
,
2-O-2CH3
,
loses only CD3OH in methanol mediacy of 4.

Colburn et al.
323
D
H
D
D
D
1,4-H
.
.
+
HOCH3
+
.
D
OCH3
OCH3
+
3-1,1-2H2
2-1,1-2H2
+.
H
D
4-2H2
H
1,4-H
.
.
+
HOCH3
+
.
OCH3
OCH3
+
D
D
D
D
H
+.
2-2,2-2H2
3-2,2-2H2
D
D
4-2H2'
+
.
+
+
+
OCH3
H OCH3
H OCH3
OCH3
D
1,4-H
H
.
.
D
D
+
D
.
+.
D
D
1-1,1-2H2
5-1,1-2H2
5-2,2-2H2
CH2CHD2
1,4-H
.
+
+
+
OCH3
OCH3
+
+
H
OCH3
OCH3
D
H
D
D
D
+
+
D
D
+.
.
.
.
1-2,2-2H2
C2H5
C2H5
CD2CH3
+.
+.
Scheme 3. Mechanisms of C2H5. and C2H3D2. loss from C3H7CH2CD2OCH3 and C3H7CD2CH2OCH3
This mechanism explains why 2-1,1-2H2
and direct rearrangement occurs, but the positions of the
þꢀ
2-2,2-2H2 behave similarly. Both loseꢀC2H5 (29.3% deuterium atoms in the cationic and neutral compo-
þꢀ
ꢀ
and 29.0%, respectively) and C2H3D2 , (19.2% and nents are affected.
19.5%, respectively) but essentially no C2H4Dꢀ
(<1%). These observations are explained by elaborat- tion, the preference for eliminating C2H5 , rather than
Irrespective of the precise details of the isomerisa-
ꢀ
ꢀ
ing Scheme 2 to produce Scheme 3.
C2H3D2 , may be attributable either to an isotope effect
Starting from 2-1,1-2H2 and 2-2,2-2H2þꢀ, the usual or to a tendency to expel the ethyl group that originates
þꢀ
1,4-H shift gives the distonic ions 3-1,1-2H2 and from the terminal part of the pentyl chain in
3-2,2-2H2, respectively. Ring closure of these species 2-1,1-2H2 and 2-2,2-2H2þꢀ, rather than that which is
þꢀ
2
2
0
leads to the INCs 4- H2 and 4- H2 , respectively; how- created by the rearrangement. The latter explanation,
ever, these two INCs are identical because the radical- which has been noted in other reactions that may be
cation component is the same in both cases. This INC explained by mechanisms involving INCs, is perhaps
may then isomerise to the distonic ions 5-1,1-2H2 and more likely.
5-2,2-2H2, which can rearrange by a 1,4-H shift to form
1-1,1-2H2 and 1-2,2-2H2þꢀ, which then can undergo preted on the basis of separation of the components
Finally, elimination of methanol may be inter-
þꢀ
ꢀ
a-cleavage to expel either C2H5 (by elimination of the of the INC, 4, almost certainly with rearrangement of
ethyl group tþhꢀat was originally in the terminal position the incipient radical-cation, to give a more stable
of 2-1,1-2H2 or 2-2,2-2H2þꢀ) or C2H3D2 (from the isomer of C5H10þꢀ. Limited exchange of the protium
ꢀ
dideuterated ethyl group created by the rearrange- (H) and deuterium (D) atoms between the compo-
ment). A parallel mechanism can be devised by nents of 4-2H2 (and other INCs easily accessible
direct rearrangement of 3-1,1-2H2 and 3-2,2-2H2 to from this structure) then accounts for the minor con-
þꢀ
1-1,1-2H2 or 1-2,2-2H2þꢀ, without postulating the tribution from CH3OD elimination in the dissociation
intermediacy of 4-2H2 and its degenerate isomer of 2-1,1-2H2 and 2-2,2-2þHꢀ 2þꢀ, via methanol expul-
þꢀ
2
4- H2 . The gross consequencþesꢀ for ethyl radical loss sion, whereas 2-O-2CH3
loses only CD3OH in
0
from 1-1,1-2H2 or 1-2,2-2H2 are not altered if this methanol elimination.
þꢀ

324
European Journal of Mass Spectrometry 23(6)
Figure 3. Relative Intensity, RI, values are normalised to 100 units for the total metastable ion current from m/z 104.
The superb quality of the MIKE spectra that were obtained on MMM testifies to the creativity and vision
acquired on the MMM instrument is illustrated in of Peter Derrick in designing and constructing such a
Figure 3, in which the closely simiþlaꢀr behaviour of novel and powerful mass spectrometer.
metastable 2-1,1-2H2 and 2-2,2-2H2 is evident. The
þꢀ
Dedication
signal-to-noise ratio is excellent, even for the very
minor fragmentation by loss of water, which accounts
Dedicated to Professor Peter J Derrick in particular com-
for less than 1% of the total metastable ion current for memoration of his contribution to science in designing and
constructing mass spectrometers and applying novel instru-
mentation in a variety of contexts.
dissociation of these isomeric ions.
Conclusion
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
The unsurpassed effectiveness of the MMM instrument
for experimental studies of the chemistry of isolated ions
is evident from this brief investigation of the reactions of
ionised pentyl methyl ethers. Ionised n-pentyl methyl
ether isomerises to ionised 3-pentyl methyl ether via a
series of simple steps involving distonic ions and, pos-
sibly, an INC, thus explaining the essentiallyþiꢀdentical
Funding
The author(s) disclosed receipt of the following financial sup-
port for the research, authorship, and/or publication of this
article: Financial support from the Science and Engineering
Research Council (SERC; an Advanced Fellowship to RDB)
behaviour of CH3CH2CH2CH2CD2OCH3
and
CH3CH2CH2CD2CH2OCH3þꢀ. The quality of the data

Colburn et al.
325
and the British Mass Spectrometry Society (a grant for pur-
chasing isotopically labelled starting materials) is gratefully
acknowledged.
NHþC4H9, CH3CH2CH¼NHþC4H9 and (CH3)2C¼
NHþC4H9. Org Mass Spectrom 1990; 25: 509–516.
17. Bowen RD and Derrick PJ. The mechanism of ethylene
loss from the oxonium ion CH3CH2þO¼CHCH2CH3.
J Chem Soc Chem Commun 1990; 1539–1541.
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