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Ion Rearrangement at the Beginning of Cluster Formation
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001)
Ion rearrangement at the beginning of cluster
formation: methyl substitution effects on
the internal SN2 reaction in the proton-bound
dimers of acetonitrile and alcohols
Richard A. Ochran and Paul M. Mayer*
Chemistry Department, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5. E-mail: pmayer@science.uottawa.ca
Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular decompositions of proton-bound
dimers consisting of acetonitrile with n- and i-propanol. Common to both systems is a competition between dissociation of the hydro-
gen bond in the proton-bound dimer and isomerization to (CH3CNR)(H2O)+[R = CH2CH2CH3and CH(CH3)2]. The minimum-energy-
reaction pathways for the isomerization in these two systems, as well as those for the dimers containing methanol and ethanol, are
presented and compared. The dominant isomerization pathway for these ions is an internal SN2 reaction that proceeds via a stable
intermediate CH3CN···ROH2
+ion [R = CH3,CH
2CH3,CH
2CH2CH3and CH(CH3)2]. The mass spectra for the four butanol-containing
dimers (n-, s-, i- and t-butanol) follow similar behavior.
Keywords: tandem mass spectrometry, dimer ions, rearrangement, unimolecular reaction mechanism, ab initio calculations,
thermochemistry
Introduction
A central feature of the chemistry of gaseous ions is
their tendency to rearrange prior to reaction. Over the years,
a variety of thermodynamically stable structures have been
discovered including distonic ions (for example, CH2OH2
+),1
ion–neutral complexes2,3 and bridged ions (such as proton-
bound dimers).4These ion structures are ubiquitous to ion
dissociation mechanisms, and indeed, the isomerization of
gas-phase organic ions is a common occurrence.5–9 This ten-
dency for isomerization is not limited to covalently bound
ions, however, as any ion that has preferred, thermodynami-
cally stable and kinetically accessible isomers can undergo
rearrangement. Even electrostatically bound ions such as
proton-bound dimers undergo extensive rearrangement
reactions, as evidenced by the proton-bound dimers of alco-
hols (ROH)(R´OH)H+which almost universally exhibit a
tendency to lose H2O via an internal rearrangement pro-
cess.10–15
A family of proton-bound dimers that share this propen-
sity for rearrangement are those consisting of CH3CN and
the alcohols methanol, ethanol, n- and i-propanol and the
four butanol isomers (n-, s-, i- and t-butanol). All of these
ions exhibit a dehydration reaction that competes on the
microsecond timescale with the dissociation of the hydrogen
bond in the dimer. The competition between the two chan-
nels means that there is a fine balancing of the thresholds for
dissociation with the barriers to isomerization. Recent work
in our laboratory16,17 showed that the metastable proton-
bound dimers of acetonitrile and methanol,
(CH3CN)(CH3OH)H+, and ethanol, (CH3CN)(CH3CH2
OH)H+, undergo two types of unimolecular reactions on the
microsecond timescale, simple hydrogen bond cleavage
reactions and the loss of water to form CH3CNR+(R=CH
3,
CH2CH3). The water-loss channel is preceded by rearrange-
ment of the proton-bound dimer to a second isomer,
(CH3CNR)(H2O)+(R=CH
3,CH
2CH3). The isomerization
proceeds via an internal SN2-type reaction that involves the
backside attack of CH3CN to form an intermediate complex
(CH3CN···ROH2)+, followed by the formation of the thermo-
dynamically stable isomer (CH3CNR)(H2O)+.16–18 The present
study concerns itself with the other members of this family
of proton-bound dimers, those between acetonitrile and the
propanols, (CH3CN)(CH3CH2CH2OH)H+and (CH3CN)
((CH3)2CHOH) H+and between acetonitrile and the
butanols, (n-C4H9OH, s-C4H9OH, i-C4H9OH and t-C4H9OH).
Mass spectrometry is used to identify the key features of the
potential energy surfaces and theoretical calculations on the
© IM Publications 2001, ISSN 1356-1049
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 267
four dimer systems (CH3CN)(CH3OH)H+, (CH3CN)
(CH3CH2OH)H+, (CH3CN)(CH3CH2CH2OH)H+and
(CH3CN)((CH3)2CHOH) H+allow us to compare the key
transition states governing the rearrangement process in this
family of dimers.
Experimental procedures
The experiments were performed on a modified VG
ZAB-2HF mass spectrometer19 incorporating a magnetic
sector followed by two electrostatic sectors (BEE geome-
try). Protonated cluster ions were generated in the chemical
ionization ion source of the instrument. The pressures in the
ion source chamber, read with an ionization gauge located
above the ion source diffusion pump, were typically between
10–5 and 10–4 Torr (the pressure in the ion source itself being
approximately two orders of magnitude higher). Cluster ions
were not observed when the pressure was below 10–5 Torr,
and there was no evidence of higher order clusters at any of
the pressures used in these experiments. The accelerating
voltage was held constant at 8 kV. Metastable ion (MI) and
collision-induced dissociation (CID) mass spectra were
recorded in the usual manner in both the second and third
field-free regions (2FFR and 3FFR, respectively) of the
instrument.20 Helium collision gas was used in all CID exper-
iments and was introduced into the collision cells to achieve
a 10% reduction in the ion flux (i.e. single collision condi-
tions). All chemicals were commercially obtained and used
without further purification. Isotopically labeled compounds
(C/D/N Isotopes Ltd) were of 99% purity.
Computational procedures
Standard ab initio molecular orbital calculations21 were
performed using the Gaussian 9822 suite of programs. Geom-
etries were optimized and harmonic vibrational frequencies
were calculated, at both the HF/6-31G(d) and MP2/6-
31+G(d) levels of theory. Two recent assessments of theo-
retical procedures for calculating the properties of proton-
bound dimers involving HCN and CH3CN with a variety of
first-row hydrides have shown that geometries optimized at
the MP2/6-31+G(d) level of theory provide an adequate
foundation for high-level single-point energy calcula
-
tions.23,24 Relative energies have also been found to be rea-
sonably estimated at this level.23–25 Extending the basis set at
the MP2 level changes the binding energies of (HCN)2H+,
(HCN)(NH3)H+, (HCN)(H2O)H+and (HCN)(HF)H+by at
most 3 kJ mol–1.25 For species related to
(CH3CN)(CH3OH)H+,G2
26 level calculations were per-
formed on the MP2/6-31+G(d) geometries to check the
validity of the MP2/6-31+G(d) results. The scaling factors
for the HF/6-31G(d) zero-point energies (ZPE) used in the
G2 calculations was 0.8929.26 For species related to the etha-
nol complex (CH3CN)(CH3CH2OH)H+, G2(MP2,SVP)27 sin-
gle-point calculations were performed on the MP2/6-
31+G(d) optimized geometries. Again, scaled (by 0.8929)
zero-point energy corrections were used in these calcula-
tions. In the case of the two propanol-containing systems,
relative energies were obtained at the MP2/6-31+G(d) level
of theory employing scaled (by 0.943) MP2/6-31+G(d)
zero-point energies.
Results and discussion
In two previous publications,16,17 mass spectrometry was
used to explore the unimolecular decomposition of proton-
bound dimers consisting of acetonitrile and the primary
alcohols methanol and ethanol. Both proton-bound dimers
(CH3CN)(CH3OH)H+(1) and (CH3CN)(CH3CH2OH)H+(2)
exhibit a competition between simple hydrogen bond cleav-
age and dehydration, the latter channel involving the initial
isomerization of (CH3CN)(ROH)H+(R=CH
3,CH
3CH2)to
an isomer of the form (CH3CNR)(H2O)+. The isomerization
was assumed to proceed via an internal SN2 rearrangement in
which the CH3CN moiety migrates to the back of the alcohol
and displaces the H2O group. At the time, detailed calcula-
tions of the transition states for this process were not avail-
able and so they have been included in the present study for
comparison with the results for the dimers incorporating n-
and i-propanol (see below). Recent calculations on the
(CH3CN)(CH3OH)H+system by Fridgen et al. support this
mechanism.18
(CH3CN)(CH3CH2CH2OH)H+and (CH3CN)((CH3)2CHOH)H+
The 2FFR MI mass spectrum [Figure 1(a)] of the pro-
ton-bound dimer (CH3CN)(CH3CH2CH2OH)H+(3), m/z 102,
exhibits four peaks, m/z 84 (– 18 amu), m/z 61 (– 41 amu),
m/z 60 (– 42 amu) and m/z 42 (– 60 amu) having relative
intensities of 1.0, 0.93, 0.06 and 0.03, respectively. The loss
of 18 amu to form m/z 84 can only be attributed to the loss of
water, whereas the other three reactions can be attributed to
the loss of acetonitrile (to form CH3CH2CH2OH2
+), propene
(to form m/z 60) and propanol (to form CH3CNH+), respec-
tively. The identities of the metastably-generated m/z 42 and
m/z 61 ions were confirmed by transmitting them into the
3FFR and obtaining their CID mass spectra. These spectra
were found to be identical to those of CH3CNH+and
CH3CH2CH2OH2
+generated in the ion source by self-
protonation. The 3FFR CID of the metastably-generated m/z
60 contained a single peak at m/z 42. Loss of 18 suggests that
this ion is (CH3CN)(H2O)H+. More evidence for this ion
structure will be presented later. Due to its low abundance in
the ion source, it was not possible to examine source-gener-
ated m/z 60 ions.
In the MI mass spectrum, the m/z 61 peak is 30 times
more intense than the peak with m/z 42, which is consistent
with the relative proton affinities (PA) of propanol and
acetonitrile. The most recent critically evaluated compen-
268 Ion Rearrangement at the Beginning of Cluster Formation
dium lists the PA of CH3CH2CH2OH as 786.5 kJ mol–1 and
that of CH3CN as 779.2 kJ mol–1.28 Introduction of a trace
amount of collision gas into a collision cell in the 2FFR
results in a dramatic increase in the intensities of m/z 61 and
m/z 42 relative to m/z 84 and m/z 60, the latter two signals
being largely unaffected. This suggests that the fragmenta-
tion channels leading to m/z 84 and m/z 60 involve rear-
rangement of the initially generated proton-bound dimer. In
this respect the results are analogous from those of the meth-
anol-acetonitrile and ethanol–acetonitrile proton-bound
dimer ions.16,17 The kinetic energy release (KER) values for
the competing MI processes [reported as the full-width at
half-height (T0.5) of the three peaks] are 8.3 meV (m/z 42),
12.8 meV (m/z 61) and 34 meV (m/z 84). The first two val-
ues are typical values for simple bond dissociation reactions,
whereas the latter is consistent with the dissociation of a
weakly bound species.
The 2FFR MI mass spectrum [Figure 1(b)] of the pro-
ton-bound dimer (CH3CN)((CH3)2CHOH)H+(4), m/z 102,
exhibits three peaks at m/z 84 (– H2O), m/z 61
((CH3)2CHOH2
+) and m/z 60 having relative intensities of
1.0, 0.07 and 0.03, respectively. The products were identi-
fied as described above. The absence of CH3CNH+is consis-
tent with the PA of acetonitrile being 13.8 kJ mol–1 lower
than that of i-propanol.28 The kinetic energy release (KER)
values for the competing processes (T0.5) are 27 meV (m/z
84) and 18.8 meV (m/z 61) in the MI mass spectrum (m/z 60
being too small to obtain a reliable value). The CID mass
spectrum of m/z 102 also contains ions with m/z 42
(CH3CNH+), m/z 43 (C3H7
+) and m/z 45. Propene loss to form
m/z 60 also increases in abundance. Both the MI and CID
results show that ions 3and 4are distinct species that do not
interconvert on the microsecond time scale of these
experiments.
Isotopically-labeled cluster ions were studied to deter-
mine the extent of interchange of the hydrogen atoms in the
two proton-bound dimers. The cluster ion
(CD3CN)(CH3CH2CH2OH)H+(m/z 105, formed by the reac-
tion of CD3CN with CH3CH2CH2OH in the ion source)
exhibits four fragment ion peaks in the MI mass spectrum,
m/z 87 (– 18 amu), m/z 63, m/z 61 (CH3CH2CH2OH2
+) and
m/z 45 (CD3CNH+) [Figure 2(a)]. These observations indi-
cate that there is no mixing of the methyl hydrogens on
acetonitrile with the bridging hydrogens (the H+bridge and
hydroxyl hydrogen) or those in the propyl group. The shift of
three mass units of m/z 60 to m/z 63, suggests that this ion has
an intact acetonitrile group. This supports the assignment of
the (CH3CN)(H2O)H+structure to m/z 60. Propanol-d7was
introduced into the ion source and reacted with CH3CN to
form m/z 109 (CH3CN)(CD3CD2CD2OH)H+. The MI mass
spectrum contained five peaks, m/z 42 (CH3CNH+), m/z 61
[nominally (CH3CN)(H2O)D+], m/z 68 CD3CD2CD2OH2
+and
two peaks having m/z 90 and 91 [Figure 2(b)]. These latter
two peaks represent the loss of HOD and H2O, respectively,
in the ratio 0.05 : 1.0. These results show that a small
amount of interchange occurs between the propyl hydrogens
and the bridge hydrogens, but that this mixing occurs only
after isomerization of the originally formed proton-bound
dimer. The mixing is a minor process, with the rearrange-
ment reaction preferentially resulting in dissociation to H2O.
The results from the isotopically-labeled
(CD3CN)((CH3)2CHOH)H+ion (m/z 105, formed by the reac-
tion of CD3CN with (CH3)2CHOH in the ion source) are simi-
lar to the case involving n-propanol in that the acetonitrile
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 269
Figure 1. (a) MI mass spectrum of (CH3CN)(CH3CH2CH2OH)H+
obtained in the second field-free region of the VG ZAB-2HF, (b)
MI mass spectrum of (CH3CN)((CH3)2CHOH)H+obtained in the
second field-free region of the VG ZAB-2HF.
Figure 2. MI mass spectra of isotopically-labeled
(CH3CN)(CH3CH2CH2OH)H+. (a) (CD3CN)(CH3CH2CH2OH)H+, (b)
(CH3CN)(CD3CD2CD2OH)H+.
methyl hydrogens do not lose their positional identity [Fig-
ure 3(a)]. Isopropanol-d8was introduced into the ion source
and reacted with CH3CN which resulted in the formation of
three cluster ions, m/z 109 (CH3CN) ((CD3)2CDOH)H+,m/z
110 (CH3CN)((CD3)2CDOD)H+and m/z 111
(CH3CN)((CD3)2CDOD)D+. Although the location of the
label among the bridging hydrogens cannot be determined,
m/z 109 was selected to ensure that only H was in the bridge
and to avoid contamination of the ion beam with isotopic
contribution from a lower mass ion. The MI mass spectrum
contained seven peaks, weak signals at m/z 42 (CH3CNH+),
m/z 49 [nominally (CD3)2CH+] and m/z 61 [nominally
(CH3CN)(H2O)D+], and stronger signals at m/z 68 [nomi-
nally (CD3)CDOH2
+], m/z 89, 90 and 91 [Figure 3(b)]. The
latter three peaks represent the loss of D2O, HOD and H2O,
respectively, in the ratio 0.19 : 0.36 : 1.0. In contrast to the
n-propanol-containing dimer ion, there is extensive mixing
of the bridging hydrogens with those on the propyl group in
the present system. This further distinguishes the proton-
bound dimer ions.
Identity of m/z 84 in the (CH3CN)(CH3CH2CH2OH)H+and
(CH3CN)((CH3)2CHOH)H+systems
Both proton-bound dimers 3and 4exhibit water loss to
form an ion having m/z 84. The CID mass spectra of the
source- and metastably-generated m/z 84 ions starting from
3are the same, as are the source- and metastably-generated
m/z 84 ions originating from 4. Metastable source-generated
m/z 84 ions from both precursors exhibit a single peak, m/z
42, in their MI mass spectra; the measured T0.5 values were
34 meV (for the ions originating from 3) and 27 meV (for
the ions originating from 4). Collision-induced dissociation
270 Ion Rearrangement at the Beginning of Cluster Formation
Figure 3. MI mass spectra of isotopically-labeled
(CH3CN)((CH3)2CHOH)H+. (a) (CD3CN)((CH3)2CHOH)H+, (b)
(CH3CN)((CD3)2CDOH)H+.
Precursor m/z
69 68 56 55 54 43 42 41 40 39 38 28 27 26
(CH3CN)(CH3CH2CH2OH)H+a 0.01 0.01 0.02 0.01 < 0.01 0.14 1.0 0.07 0.02 0.04 0.01 0.01 0.03 0.01
(CH3CN)(CH3CH2CH2OH)H+b 0.02 0.02 0.02 0.04 0.02 0.16 1.0 0.13 0.03 0.05 0.02 0.03 0.05 0.02
(CH3CN)((CH2)2CHOH)H+a 0.01 0.02 — — < 0.01 0.12 1.0 0.07 0.03 0.05 0.01 0.01 0.03 0.02
(CH3CN)((CH2)2CHOH)H+b 0.02 0.02 — < 0.01 < 0.01 0.15 1.0 0.10 0.03 0.04 0.01 0.02 0.04 0.02
(CH3CN)(CH2=CHCH3)H+c — — — — — 0.07 1.0 — — — – — — —
aSource-generated m/z 84
bMetastably-generated m/z 84
cGenerated by ion/molecule reaction in the ion source. There was a background signal at m/z 84 and only the fragment ions m/z 43 and 42 were observed to be affected by changes in
acetonitrile and propene pressure. However, due to low signal levels, the presence of other fragment ions cannot be discounted
Table 1. Relative peak intensities in the He CID mass spectra of m/z 84 ions
ionization (CIDI)29 experiments on the respective source-
generated ions result in mass spectra having consecutive
peaks from m/z 26 through m/z 28 and m/z 39 through m/z 43,
indicating that the neutral lost is C3H7in both cases..From
these results, it is possible that a common m/z 84 ion is
formed upon dissociation of 3and 4. However, the He CID
mass spectra of the ions formed from 3and 4are distinguish-
able, indicating that a distinct m/z 84 ion is formed from each
precursor (Table 1). The differences lie in the intensities of
two small peaks, m/z 54 and m/z 55 (nominally losses of C2H6
and C2H5, respectively). The He CID mass spectrum of m/z
84 ions from 3exhibits an m/z 55 : 54 : 42 ratio of
0.04 : 0.02 : 1.0 whereas the ratio in the spectrum of ions
generated from 4is < 0.01 : < 0.01 : 1.0. Thus, the m/z 84
ions generated from 3and 4are distinct species that do not
interconvert on the time scale of the experiment. Based on
this data, and from analogy to the methanol–acetonitrile and
ethanol-acetonitrile dimer ion reaction surfaces, we propose
three possible isomers for m/z 84: CH3CNCH2CH2CH3
+(5),
CH3CNCH(CH3)2
+(6) and (CH3CN)(C3H6)H+(7). The CID
results can be rationalized if ethane and ethyl loss are
assumed to be more facile from an ion containing an n-
propyl group, 5. The He CID mass spectrum of ion 7(formed
by ion/molecule reaction between CH3CN and CH3CH=CH2
in the ion source) is also distinguishable from the
metastably-generated m/z 84 ions from 3and 4in that it
shows a smaller m/z 43 : 42 ratio (0.07, Table 1). The results
for ion 7are not conclusive as the signal was weak and a
background source of m/z 84 contaminated the spectrum.
This is further indicated by the m/z 43 : 42 ratio which is
large considering that the PA values for CH3CN and propene
differ by almost 30 kJ mol–1. Based on these results, tentative
assignments of the m/z 84 ions, resulting from the dissocia-
tion of 3and 4, can be made to ions 5and 6, respectively.
This will be discussed in more detail in a later section.
(CH3CN)(ROH)H+(R = n-butyl, s-butyl, i-butyl and t-butyl)
The 2FFR MI mass spectrum [Figure 4(a)] of the pro-
ton-bound dimer (CH3CN)(CH3CH2CH2CH2OH)H+(8), m/z
116, exhibits three peaks, a water-loss signal at m/z 98 and
ions at m/z 75 (loss of CH3CN) and m/z 60 (loss of butene)
having relative intensities of 0.54, 1.0 and 0.06, respectively.
The lack of a signal at m/z 42 is consistent with the relative
PAsofn-butanol and acetonitrile, 789.2 kJ mol–1 and
779.2 kJ mol–1, respectively.28 However, the four protonated
butanol isomers have closely similar CID mass spectra
(Table 2), so that it was not possible to unequivocally iden-
tify the protonated butanol being generated in these systems.
That the ion is likely generated by simple-bond cleavage
reaction in the proton-bound dimer is supported by the fact
that the introduction of a trace amount of collision gas into a
collision cell results in a significant increase in the intensity
of m/z 75 relative to m/z 98 and m/z 60, which remain largely
unaffected. In this respect, the results are analogous to those
from the previous systems. The He CID mass spectrum of
the source-generated m/z 60 ions exhibited a series of peaks
with m/z 42, 41, 40, 39 and 28 having relative intensities of
1.0, 0.14, 0.05, 0.02 and 0.02, respectively. The CID of
source-generated m/z 63 from the (CD3CN)
(CH3CH2CH2CH2OH)H+dimer ion exhibits a base peak at
m/z 45. This shift of three mass units indicates that m/z 60 is
likely to be CH3CN(H2O)H+as it loses water to form m/z 42
(CH3CNH+).
The 2FFR MI mass spectrum [Figure 4(b)] of the pro-
ton-bound dimer (CH3CN)(s-C4H9OH)H+(9) exhibits the
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 271
Figure 4. MI mass spectra of (a) (CH3CN)(n-C4H9OH)H+, (b) (CH3CN)(s-C4H9OH)H+, (c) (CH3CN)(i-C4H9OH)H+and (d) (CH3CN) (t-
C4H9OH)H+.
same three peaks as 8, but in very different ratios (an m/z
98 : 75 : 60 ratio of 0.56 : 1.0 : 0.17). Again, the absence of
CH3CNH+is consistent with the PA of acetonitrile being
35.8 kJ mol–1 lower than that of s-butanol.28 Introduction of a
trace amount of collision gas into a collision cell results in an
increase in the intensity of m/z 75 and the appearance of m/z
42 which is consistent if the two channels result from simple
bond cleavage in the dimer.
The MI mass spectrum [Figure 4(c)] of the proton-
bound dimer (CH3CN)(i-C4H9OH)H+(10) exhibits the same
three peaks as the above two dimers (m/z 98, 75 and 60) and a
fourth signal at m/z 57 (an m/z 98 : 75 : 60 : 57 ratio of
1.0 : 0.29 : 0.16 : 0.12). The KER values are 6.5 meV (m/z
57), 6.9 meV (m/z 75), 27.2 meV (m/z 60) and 30.1 meV
(m/z 98). The CID mass spectrum of m/z 116 shows an
increase in the intensities of both m/z 75 and m/z 57 and the
appearance of m/z 42, whereas both m/z 60 and m/z 98
remain unchanged. Both of these results support the assign-
ment of m/z 57 (which is likely due to the consecutive loss of
H2O from m/z 75) and m/z 75 to simple cleavage reactions
and m/z 60 and m/z 98 to rearrangement processes. The small
kinetic energy release value for the m/z 57 ion in the MI mass
spectrum precludes its formation from m/z 98, even though
the 3FFR MI mass spectrum of the metastably-generated m/z
98 exhibits a single fragment-ion peak at m/z 57 (–CH3CN),
indicating that the m/z 98 fragment ion is itself metastable.
The proton-bound dimer (CH3CN)(t-C4H9OH)H+(11)
has a unique MI mass spectrum [Figure 4(d)] that contains
three peaks at m/z 60, 98 and 101 (– 15 amu). The lack of
simple dissociation products indicates that the barriers lead-
ing to the water and butene loss channels are significantly
lower than the dissociation thresholds for the proton-bound
dimer.
Isotopically-labeled cluster ions were studied to deter-
mine the extent of interchange of the hydrogen atoms in the
proton-bound dimers 8–11. The cluster ions formed by the
reaction of CD3CN with n-C4H9OH, s-C4H9OH, i-C4H9OH
and t-C4H9OH in the ion source exhibit no mixing of the
methyl hydrogens on acetonitrile with those of the bridging
hydrogens or those on the butyl group. The butanols were
not labeled but, from the results of the methanol-, ethano-
and propanol-containing dimer ions, there may be secondary
reactions after isomerization of the proton-bound dimer ions
resulting in some exchange of hydrogens.
Identity of m/z 98 in the (CH3CN)(ROH)H+(R = n-butyl, s-butyl,
i-butyl and t-butyl) systems
The He CID mass spectra of the metastably-generated
m/z 98 ions are summarized in Table 3, along with the corre-
sponding source-generated ions. Unlike the propyl-contain-
ing systems, the mass spectra for source- and metastably-
generated ions from corresponding precursors are rarely the
same; the only case in which an argument can be made for
them being the same is for the m/z 98 ions generated from
(CH3CN)(CH3CH2CH(CH3)OH)H+(Table 3). Each of the
metastably-generated m/z 98 ions has, however, a unique
272 Ion Rearrangement at the Beginning of Cluster Formation
Precursor m/z
57 56 52 45 43 42 41 39 38 31 29 27
CH3CH2CH2CH2OH2
+a 1.0 0.04 — — < 0.01 — 0.02 < 0.01 — < 0.01 < 0.01 0.01
CH3CH2CH(CH3)OH2
+a 1.0 0.03 — 0.02 < 0.01 — 0.05 0.03 — < 0.01 0.03 0.02
(CH3)2CHCH2OH2
+a 1.0 0.01 — — < 0.01 < 0.01 0.02 0.01 — < 0.01 0.01 < 0.01
(CH3)3COH2
+a 1.0 0.03 ————0.06 0.05 — < 0.01 0.03 0.02
(CH3CN)(CH3CH2CH2CH2OH)H+b 1.0 0.03 — — < 0.01 0.02 0.08 0.02 — 0.02 0.02 0.02
(CH3CN)(CH3CH2CH(CH3)OH)H+b 1.0 0.06 0.11 0.06 0.06 — 0.07 — 0.08 — 0.06 0.02
(CH3CN)((CH3)2CHCH2OH)H+b 1.0———————————
am/z 75 generated when only the butyl alcohol was present in the ion source
bMetastably-generated m/z 75
Table 2. Relative peak intensities in the He CID mass spectra of m/z 75 ions.
CID mass spectrum, indicating that a unique structure is
formed from each precursor. The m/z 98 ion generated from
(CH3CN)(CH3CH2CH2CH2OH)H+is characterized by an
intense peak at m/z 70 which is due to loss of 28. This can
only be due to loss of ethene, which may be more likely from
a precursor containing the n-butyl moiety, perhaps
CH3CNCH2CH2CH2CH3
+. Also, simple bond dissociation in
such an ion would lead to the unfavorable primary cation,
CH3CH2CH2CH2
+, explaining the lack of a prominent m/z 75
in the CID mass spectrum (Table 3). The ion generated from
(CH3CN)(CH3CH2CH(CH3)OH)H+has a weaker, but signifi-
cant, m/z 70 peak in its CID mass spectrum and is dominated
by m/z 56. A possible structure of CH3CNCH(CH3)CH2CH3
+
includes the ethyl group that can lead to ethene loss. The CID
mass spectra of the m/z 98 ions generated from
(CH3CN)((CH3)2CHCH2OH)H+and (CH3CN)((CH3)3
COH)H+are dominated by m/z 57, presumably the t-butyl
cation. However, they differ in that the ions generated from
(CH3CN)((CH3)3COH)H+also include significant peaks at
m/z 42, 41 and 39.
The He CID mass spectra of the (CH3CN)(butene)H+
ions, generated by the reaction of acetonitrile and the butene
isomers, 1-butene, trans-2-butene and isobutene, are also
presented in Table 3 for comparison. The spectra of these
three ions are dominated by m/z 57 but are distinguishable
based on the intensities of m/z 83, 70 and 42 (Table 3). Based
on the results in Table 3, it is possible that the m/z 98 ion gen-
erated from metastable (CH3CN)((CH3)2CHCH2OH)H+pre-
cursor ions has the (CH3CN)((CH3)2C=CH2)H+structure.
Ab initio calculations
The mass spectrometric results indicate that there is
likely to be a thermodynamically-stable isomer or isomers in
the reactions of 3and 4that are responsible for water loss. Ab
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 273
Precursor m/z
83 70 69 68 58 57 56 43 42 41 39 29 28 27
(CH3CN)(CH3CH2CH2CH2OH)H+a 0.01 0.02 0.04 0.02 —0.45 0.03 —1.0 0.03 0.01 0.01 < 0.01 0.02
(CH3CN)(CH3CH2CH2CH2OH)H+b 0.02 0.86 0.05 0.06 —0.18 1.0 — 0.36 0.06 — — ——
(CH3CN)(CH3CH2CH(CH3)OH)H+a 0.13 0.27 0.05 0.02 0.16 0.20 1.0 0.03 0.08 < 0.01 0.04 0.02 0.02 0.02
(CH3CN)(CH3CH2CH(CH3)OH)H+b 0.10 0.13 0.06 0.10 0.04 0.10 1.0 0.04 0.25 0.16 0.12 0.05 0.03 0.05
(CH3CN)((CH3)2CHCH2OH)H+a < 0.01 ——0.04 —1.0 0.09 —0.88 0.09 0.05 0.05 — 0.04
(CH3CN)((CH3)2CHCH2OH)H+b —————1.0 —— 0.09 0.03 0.01 0.01 — —
(CH3CN)((CH3)3COH)H+a 1.0 0.18 0.11 0.04 0.88 0.29 0.61 0.07 0.08 0.35 —0.06 0.02 0.05
(CH3CN)((CH3)3COH)H+b ———— — 1.0 — —0.13 0.15 0.12 — ——
(CH3CN)(CH3CH2CH=CH2)H+a 0.03 0.03 0.02 — 0.04 1.0 0.07 0.04 0.14 0.05 0.03 0.03 0.04 0.02
(CH3CN)(t-CH3CH=CHCH3)H+a 0.12 0.12 0.03 < 0.01 0.05 1.0 0.61 < 0.01 0.07 0.07 0.03 0.02 0.01 0.02
(CH3CN)((CH3)2CH=CH2)H+a < 0.01 < 0.01 ——— 1.0 0.05 —0.04 0.04 0.02 0.01 — 0.01
aSource-generated m/z 98
bMetastably-generated m/z 98
Table 3. Relative peak intensities in the He CID mass spectra of m/z 98 ions.
Figure 5. Equilibrium structures for products (m/z 84, isomers
5–7) resulting from water loss from the proton-bound dimers
(CH3CN)(CH3CH2CH2OH)H+and (CH3CN)((CH3)2CHOH)H+and the
transition structures for their interconversion. All geometries
are optimized at the MP2/6-31+G(d) level of theory. Bond
lengths are in Angstroms, bond angles in degrees.
initio calculations have been used to identify possible struc-
tures and model the reaction surface for the propanol- con-
taining dimers. Structures optimized at the MP2/6-31+G(d)
level of theory can be found in Figures 5 and 6 and relative
energies are listed in Tables 4 and 5.
274 Ion Rearrangement at the Beginning of Cluster Formation
Erel
a,b
30
TS(3–13)77
13 76
TS(13–8) 106
8– 16
10 77
5+ H2O27
5´ + H2O28
TS (5–7) + H2O 191
7+ H2O 122
a(CH3CN)(H2O)H++ C3H681
CH3CH2CH2OH2
++ CH3CN 132
CH3CNH++ CH3CH2CH2OH 144
CH3CNH++ C3H6+ H2O 186
H3O++ CH3CN + C3H6292
aMP2/6-31+G(d) values incorporating a scaled (by 0.943)30 ZPE
correction based on MP2/631+G(d) vibrational frequencies
bValues are in kJ mol–1 at 0 K
Table 4. Calculated relative energies for (CH3CN)(CH3CH2CH2OH)H+.
Figure 6. Equilibrium and transition structures for the rear-
rangement of the proton-bound dimers (a) (CH3CN)(CH3OH)H+,
(b) (CH3CN)(CH3CH2OH)H+, (c) (CH3CN)(CH3CH2CH2OH)H+and
(d) (CH3CN)((CH3)2CHOH)H+to (CH3CNR)(H2O)+(R=CH
315,
CH3CH216,CH
3CH2CH28and (CH3)2CH 9). All geometries are
optimized at the MP2/6-31+G(d) level of theory. Bond lengths
are in Angstroms, bond angles in degrees.
Figure 7. Calculated minimum-energy-reaction pathways for the unimolecular processes of (CH3CN)(CH3CH2CH2OH)H+. All energies
were obtained at the MP2/6-31+G(d) level of theory and incorporate a scaled (by 0.943) MP2/6-31+G(d) zero-point energy
correction.
Isomeric forms of m/z 84
The three isomers of m/z 84 discussed previously,
CH3CNCH2CH2CH3
+(5), CH3CNCH(CH3)2
+(6) and
(CH3CN)(C3H6)H+(7) were found to be equilibrium struc-
tures on the MP2/6-31+G(d) surface. Ion 5has two conform-
ers resulting from a cis or trans arrangement of the propyl
group relative to the nitrile functionality. The cis form lies
only 1 kJ mol–1 lower in energy than the trans configuration
(labeled 5´ in Figure 5). The most stable isomer is 6, which
lies 19 kJ mol–1 lower in energy than 5which, in turn, is
94 kJ mol–1 lower in energy than 7. Isomerization of the n-
propyl-containing ion 5to the proton-bound
acetonitrile–propene dimer 7occurs over a high barrier that
is predicted to be 69 kJ mol–1 above ion 7and is actually
5kJmol
–1 above the dissociation products CH3CNH++C
3H6
(Table 4 and Figure 7). The transition structure for the
interconversion of 6and 7lies 12 kJ mol–1 above 7and
53 kJ mol–1 below the dissociation products
CH3CNH++C
3H6(Table 5 and Figure 8). These results con-
firm that isomers 5and 6do not interconvert prior to loss of
propene and are consistent with the experimental observa-
tions when the m/z 84 ions from 3have structure 5and those
from 4have structure 6.
Isomeric forms of m/z 102
A number of isomeric forms of the proton-bound dimers
3and 4were investigated to model the reaction surface.
Since m/z 84 is formed by loss of water, it is highly likely
that there are isomeric forms of the proton-bound dimers
consisting of the m/z 84 isomers (5,6and 7) with an electro-
statically-bound water molecule (Figure 6). Isomers 8,9
(water bound to isomers 5and 6, respectively) and 10 (water
bound to isomer 7, not shown) represent ion–water com-
plexes of the three m/z 84 products. At the MP2/6-31+G(d)
level of theory, isomers 8and 9are both thermodynamically
more stable than their respective proton-bound dimers 3and
4(Tables 4 and 5). The relative energies of the three ions
indicate that 9is the most stable, followed by 8(20 kJ mol–1
higher than 9) and 10 (113 kJ mol–1 higher than 9). Isomer 10
has a very small threshold to dissociation to m/z 60
(CH3CN)(H2O)H+(5 kJ mol–1) and so any isomerization pro-
cess that leads to 10 will likely proceed directly to m/z 60.
Hence, the presence of m/z 60 in the mass spectrum is an
indication of the extent to which such a process competes
with the alternative path, which is the isomerization of 3to 8
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 275
Erel
a,b
40
TS(4–14)70
14 69
TS(14–9)97
9– 10
10 103
6+ H2O35
TS(6–7) + H2O 160
7+ H2O 148
CH3CN(H2O)H++ C3H6108
(CH3)2CHOH2
++ CH3CN 127
CH3CNH++ (CH3)2CHOH 150
CH3CNH++ C3H6+ H2O 213
H3O++ CH3CN + C3H6292
aMP2/6-31+G(d) values incorporating a scaled (by 0.943)30 ZPE
correction based on MP2/631+G(d) vibrational frequencies
bValues are in kJ mol–1 at 0 K
Table 5. Calculated relative energies for (CH3CN)((CH3)2CHOH)H+.
Figure 8. Calculated minimum-energy-reaction pathways for the unimolecular processes of (CH3CN)((CH3)2CHOH)H+. All energies
were obtained at the MP2/6-31+G(d) level of theory and incorporate a scaled (by 0.943) MP2/6-31+G(d) zero-point energy
correction.
and 4to 9. For the two propanol-containing dimers studied
here, formation of m/z 60 is a minor process [Figures 1(a)
and (b)].
Isomerization mechanism of nitrile–alcohol proton-bound
dimers
The isomerization from the proton-bound dimers incor-
porating CH3CN and the alcohols methanol,18 ethanol, n-
propanol and i-propanol involves an SN2 type mechanism.
The first step in the process consists of the backside attack of
CH3CN on the carbon adjacent to the OH2group in the
protonated alcohol to form an intermediate complex
(CH3CN···ROH2)+[R=CH
3(11), CH2(CH3)(12),
CH2(CH2CH3)(13) and CH(CH3)2(14)]. The second step
involves the stretching of the C–O bond in the protonated
alcohol moiety and the shortening of the N–C bond,
[CH3CN–R–OH2]+‡, followed by the formation of the ther-
modynamically stable isomer(s) (CH3CNR)(H2O)+. Equilib-
rium structures corresponding to intermediate ion–dipole
complexes 11–14 have been located at the MP2/6-31+G(d)
level of theory (Figure 6). Transition structures are shown in
Figure 6 for the interconversion of the proton-bound dimers
1–4and the complexes 11–14, as well as the transition states
for the second step of the isomerization from 11–14 to
(CH3CNCH3)(H2O)+(15), (CH3CNCH2CH3)(H2O)+(16), 8
and 9. The relative energies for the four isomerization pro-
cesses are listed in Table 6. The minimum-energy-reaction
pathways for ions 3and 4are presented in Figures 7 and 8.
In each of the four dimer systems presented in Table 6,
the key transition state governing the isomerization to
(CH3CNR)(H2O)+is that which connects this isomer to the
intermediate complex (CH3CN···ROH2)+. This transition
state involves the shortening of the N–C bond and lengthen-
ing of the C–O bond in the complex. In effect, it also sets the
bond dissociation threshold for the intermediate complex.
Examination of the relative energies in Table 6 (comparing
values at the MP2 level of theory for the four systems) shows
that the dissociation energy of (CH3CN···ROH2)+changes
from 24 kJ mol–1 for the methanol system to 35 kJ mol–1 for
the ethanol system and then drops back to 30 and 28 kJ mol–1
for the two propanol-containing ions. So, the C–O bond
strength changes significantly, but not dramatically, upon
alkyl substitution of the central methyl group in the interme-
diate complex. The overall relative barrier height [i.e. the
relative energy of TS (complex–isomer)] is fairly consistent
at the MP2 level of theory: 107, 110, 106 and 97 kJ mol–1
going across Table 6. The most notable value is that for the i-
propyl-containing system. Dimethyl substitution of the cen-
tral methyl group in the complex results in the lowest transi-
tion-state energy (by approximately 10 kJ mol–1) of the four
species studied here. This also accounts for the predomi-
nance of m/z 84 in the MI mass spectrum of
(CH3CN)((CH2)2CHOH)H+.
Conclusion
The proton-bound dimers of acetonitrile with the alco-
hols methanol, ethanol, n- and i-propanol all exhibit a com-
petition between isomerization and dissociation in their MI
mass spectra. Mass spectrometry and ab initio theory have
been used to elucidate the unimolecular reaction pathways
276 Ion Rearrangement at the Beginning of Cluster Formation
R=CH
3
bCH3CH2
cCH3CH2CH2(CH3)2CH
G2 MP2/6-31+G(d) G2(MP2,SVP) MP2/6-31+G(d) MP2/6-31+G(d) MP2/6-31+G(d)
(CH3CN)(ROH)H+00000 0
TS(dimer-complex) 89 87 98 77 77 70
(CH3CN···ROH2)+84 83 96 75 76 69
TS(complex–isomer) 103 107 124 110 106 97
(CH3CNR)(H2O)+– 21 – 22 2 – 14 – 16 – 10
Dimer →productsd121 129 152 139 132 127
Isomer →productse19 25 42 31 27 35
aValues in kJ mol-1 at 0 K
bA portion of this data has been previously published in Reference 16. The results are similar to MP2/6-311G(d,p) values obtained by Fridgen
et al.18
cA portion of this data has been previously published in Reference 17
dRefers to the lowest-energy simple dissociation products of the respective proton-bound dimers
eRefers to the products resulting from water loss from (CH3CNR)(H2O)+ions
Table 6. Comparison of the relative energies of the isomerization process for (CH3CN)(ROH)H+[R=CH
3,CH
3CH2,CH
3CH2CH2and
(CH3)2CH].a
of these systems. The main isomerization channel is an inter-
nal SN2 reaction that involves the formation of an intermedi-
ate complex (CH3CN···ROH2)+prior to the loss of water to
form (CH3CNR)+. Methyl substitution of the carbon atom
adjacent to the OH2moiety lowers the energy of the interme-
diate complex and, hence, the barrier to isomerization.
Acknowledgments
PMM thanks the Natural Sciences and Engineering
Research Council of Canada for financial support and the
Computing Centres at the University of Montreal and
Queen’s University for generous donations of computing
time through the HPNET and HPCVL programs, respec-
tively.
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Received: 26 June 2001
Revised: 13 August 2001
Accepted: 29 August 2001
Web Publication: 5 November 2001
R.A. Ochran and P.M. Mayer, Eur. J. Mass Spectrom.7, 267–277 (2001) 277
