14 September 2001
Chemical Physics Letters 345 (2001) 277±281
www.elsevier.com/locate/cplett
Dissociation patterns of …H2O†‡
n cluster ions, for n ˆ 2±6
L. Angel, A.J. Stace *
School of Chemistry, Physics and Environmental Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, UK
Received 23 April 2001; in ®nal form 3 August 2001
Abstract
Metastable and collision-induced fragmentation patterns have been recorded for size-selected …H2 O†‡
n cluster ions,
where n ˆ 2±6. The results show evidence of a competitive loss of OH and H2 O from each of the ions; however, as the
size of the parent ion increases there is a corresponding increase in the relative loss of H2 O from each cluster. This
behaviour is interpreted as the predominant formation of an H3 O‡ ±OH central core with additional H2 O ligands
present in the ®rst solvation shell. The results are discussed with reference to the chemistry of the ionosphere, where
‰O2 …H2 O†n Š‡ intermediates are thought to contribute to the formation of proton hydrates, H‡ …H2 O†n . However,
recent laboratory measurements have shown that …H2 O†‡
n ions are the main fragmentation products from
‰O2 …H2 O†n Š‡ clusters [J. Phys. Chem. A 103 (1999) 2999]. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
‡
The fragmentation of …H2 O†n cluster ions,
where n ˆ 2±6, has been investigated by a method
where both the metastable (unimolecular) and
collision-induced dissociation (CID) of ions of a
single size can be monitored simultaneously. The
two dissociation channels of interest are
‡
‡
…H2 O†n ! …H2 O†n
…H2 O†‡
n
1
‡ H2 O
! H‡ …H2 O†n
1
‡ OH
…1†
…2†
The occurrence of either of these channels is considered to be related to the structural stability of the
‡
proposed isomers of …H2 O†n . Recent calculations
[1] and experiments [2] have suggested three possible isomeric forms for the ion, and these are: (i) a
central core of the disproportionated dimer,
*
Corresponding author. Fax: +44-01273-678-097.
E-mail address: [email protected] (A.J. Stace).
(ii)
a
hydrazine-like
core
…H3 O†‡ ±OH;
‡
…H2 O±OH2 † ; and (iii) a central H3 O‡ core with the
OH species contained in an outer solvation shell.
‡
The dissociation properties of …H2 O†n ions are
of interest because of their possible contribution to
the chemistry of the ionosphere. Mass spectrometric measurements of the lower reaches of the
ionosphere have detected two distinct layers. An
upper (E) region which consists primarily of NO‡
and O‡
2 ions, and a lower (D) region where the
dominant ions take the form of proton hydrates
(PHs) of the type H‡ …H2 O†n , with n in the range
1±20 [3]. Laboratory studies [4±7] have shown that
the successive hydration of NO‡ , leads to formation of the PHs H‡ …H2 O†3 and H‡ …H2 O†4 through
the following mechanism:
NO‡ …H2 O†n ‡ H2 O ! H‡ …H2 O†n ‡ HNO2
…3†
Detailed measurements of ion densities at
varying altitudes of the ionosphere [8] show that a
decrease in concentration of O‡
2 is accompanied
predominantly by an increase in concentration of
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 8 9 1 - 0
278
L. Angel, A.J. Stace / Chemical Physics Letters 345 (2001) 277±281
the two smallest PHs, H3 O‡ and H‡ …H2 O†2 . It has
been proposed that these species originate from
‡
‡
the ions ‰O2 …H2 O†2 Š and ‰O2 …H2 O†3 Š ; however, recent laboratory measurements on
O‡
2 …H2 O†n clusters have shown that the successive hydration of O‡
2 leads primarily to the formation of …H2 O†‡
ions
through the mechanism
n
[9,10]
‡
O‡
2 …H2 O†n ‡ H2 O ! …H2 O†n‡1 ‡ O2
…4†
High altitude mass spectrometer measurements
show no evidence of these unprotonated ions in
the ionosphere, and so an ecient mechanism is
‡
necessary to convert …H2 O†n ions into PHs. Alternatively, the proposed route from O‡
2 to PHs
may not be viable because of the enhanced
photodissociation cross-section recently measured
for `warm' O‡
2 H2 O ions [10].
Laboratory observations show the mass spectra
of water clusters to be dominated by PHs [11±13].
The large con®gurational di€erence between
‡
…H2 O†n and ionised …H2 O†n , results in the formation of a vibrationally excited ion, which then loses
OH and leads to the appearance of a PH. The
proton transfer reaction is exothermic by approximately 85 kJ mol 1 ; therefore, the production of
‡
unprotonated ions …H2 O†n requires this excess
vibrational energy to be dissipated in order to
prevent dissociation. One possible mechanism is
understood to be via the evaporation of argon
atoms where the initial ionisation of Arm …H2 O†n
clusters leads to the appearance of unprotonated
…H2 O†‡
n ions [14].
2. Experimental
The experiments were conducted on an apparatus that consists of a pulsed supersonic nozzle
coupled to a modi®ed high-resolution, doublefocusing VG ZAB-E mass spectrometer [15].
Argon, at a pressure of approximately 3.5 bar, was
directed through a reservoir containing de-ionised
water held at room temperature. The resulting
gas±vapour mixture was then expanded through a
200 lm conical nozzle into a vacuum chamber to
form …Ar†m …H2 O†n clusters. After passing through
a 1 mm diameter skimmer, the clusters entered the
ion source of the mass spectrometer where they
were ionised by electron impact with 70 eV electrons. To study their fragmentation patterns,
mass-analysed ion kinetic energy (MIKE) spectra
were recorded for mass-selected cluster ions [16].
…H2 O†‡
n ions for speci®c values of n were selected
using a magnet and then allowed to travel a further 1.5 m along a ¯ight-tube. Here the ions could
either dissociate in the ®eld-free region (metastable
decay) or within a cell containing a collision gas
(CID) that was located approximately half way
along the ¯ight-tube. The time spent travelling
through this region is ca. 5 10 5 s, which is
sucient for cluster ions to undergo a range of
unimolecular and internal bimolecular chemical
reactions. The CID of a cluster was promoted by
the introduction of collision gas (air) into the
collision cell, which was ¯oated at a potential of
)1000 V to distinguish the CID of ions within the
collision cell from metastable decay taking place
outside. Metastable decay was recorded with a
background pressure in the ¯ight tube of 10 8
mbar, and the CID data were recorded when the
pressure within the cell was approximately 10 5
mbar. The ion beam then entered an electrostatic
analyser where daughter ions were selected according to their laboratory-framed kinetic energies
and their relative intensities recorded using a
(Stanford Research Systems SR850 DSP) lock-in
ampli®er.
3. Results
Previous work in this laboratory on the metastable fragmentation of cluster ions [17±20], has
established a link between the pattern of decay and
the appearance of stable structures. For example,
metastable fragmentation was used to con®rm
‡
Ar‡
as stable ions [19]. In the
19 and …H2 O†21 H
results presented here, metastable and CID fragmentation patterns are used to comment on pos‡
sible structures adopted by …H2 O†n cluster ions.
‡
A MIKE spectrum of …H2 O†2 (Fig. 1a) shows
the following decay channel to be dominant in
both the metastable and CID processes
‡
…H2 O†2 ! H3 O‡ ‡ OH
…5†
L. Angel, A.J. Stace / Chemical Physics Letters 345 (2001) 277±281
279
The presence of this ion may be interpreted as either coming from the higher energy structure
‡
…H2 O±OH2 † , through simple bond ®ssion, or
‡
again from the …H3 O† ±OH isomer, but where the
ratio of OH to H2 O loss is taken as a measure of
the respective bond strengths B1 and B2 in the ion,
as illustrated in structure 1 shown in Fig. 2.
‡
A MIKE scan of …H2 O†3 (Fig. 1b) shows a
spectrum that is consistent with a disproportion‡
ated …H3 O† ±OH structure with an additional
water molecule hydrogen bond to the H3 O‡ central core. The OH moiety, which is considered to
be the most weakly bound component, is shown to
be the main loss through both metastable and CID
channels, resulting in the formation of the protonated H‡ …H2 O†2 ion
‡
…H2 O†‡
3 ! H …H2 O†2 ‡ OH
Fig. 1. Metastable and collision-induced fragmentation pat‡
terns recorded for: (a) …H2 O†‡
2 ; (b) …H2 O†3 .
This result is taken as a good evidence that a
majority of …H2 O†‡
2 ions take the form of the
‡
disproportionated ion …H3 O† ±OH; a result which
is supported by previous calculations on the
‡
structures of …H2 O†n ions [21]. Further support
‡
for the …H3 O† ±OH structure comes from the
shapes of the peaks shown in Fig. 1a. If either
collisional activation or metastable decay were
accompanied by a proton transfer reaction, then
observation of the H3 O‡ fragment would have
associated with it a large release of kinetic energy
(a classical equipartioning of internal energy
would give a kinetic release of between 7 and
14 kJ mol 1 ) [22]. Fig. 1a also shows a small
(15%) contribution in both the metastable and
CID channels from the alternative decay route
that leads to H2 O‡ daughter ions
‡
…H2 O†2 ! H2 O‡ ‡ H2 O
…6†
…7†
However, the fractional loss of H2 O has increased
with n (compare Figs. 1 and 3), and the ratio of
OH to H2 O loss is approximately the same in both
the metastable and the CID processes. This result
could again be understood as a measure of the
relative bond strengths B3 and B4 (structure 2, Fig.
2), or alternatively, as due to the presence of a
higher energy isomer. Studies of competitive
metastable decay in cluster ions have shown the
relative intensities of di€erent fragments to be very
sensitive to small di€erences in bond strength [18].
Both metastable and CID decay channels in
larger …H2 O†‡
n ions (Fig. 3) show signi®cant changes in the ratios of OH to H2 O loss, to those seen
for the smaller cluster ions. The data presented for
‡
…H2 O†6 represent the ®rst point of comparison with
the results presented by Jongman et al. [2], although the latter experiments made use of D2 O in
order to improve mass separation. The increased
loss of single water molecules observed here does
not match the proposal by Jongman et al., that
weakly bound OH is readily lost from the parent
ions through migration to the cluster surface [2].
From the data presented in Figs. 1 and 3 it is
clear that OH and H2 O loss are in competition,
and that the water-loss channel gains in intensity
as the clusters increase in size. However, the fact
that metastable peaks for both loss channels have
comparable intensities in the larger clusters, means
that the di€erence in critical energy between the
280
L. Angel, A.J. Stace / Chemical Physics Letters 345 (2001) 277±281
Fig. 2. Proposed structures for …H2 O†‡
n ions for n ˆ 2, 3, and 5.
two processes is not that signi®cant (<8 kJ mol 1 †.
Otherwise, a competitive shift would result in one
fragment being much more prominent than the
other [17±20]. It is also very likely that statistical
factors make a contribution to the gradual increase in intensity of the H2 O loss channel.
4. Discussion
The results presented here are consistent with
‡
…H2 O†n ions adopting the form …H2 O†n 2 ‡
…H3 O† ±OH, and with the OH moiety showing
evidence of becoming increasingly stabilised within
the clusters as n increases from 2 to 5. At n ˆ 5, the
®rst solvation shell around the …H3 O†‡ ±OH core
will be complete with three hydrogen bonded H2 O
molecules as shown in structure (3) of Fig. 2. This
gradual development of a solvation shell would be
expected to be accompanied by an increase in the
‡
relative stability of OH within the …H2 O†n ions,
which is exactly what is observed. There is no evidence of the OH moiety moving to the surface of
the larger cluster ions; a step that would be expected to facilitate its metastable loss with respect
to the competitive loss of H2 O.
With regard to the atmospheric signi®cance of
these results, ion identi®cation at lower altitudes in
the ionosphere (84±87 km) has suggested that both
‡
‡
‰O2 …H2 O†2 Š and ‰O2 …H2 O†3 Š are present with
signi®cant number densities [23]. Our earlier experiments [10] on the photodissociation, metastable and CID of these clusters show that they
‡
predominantly yield …H2 O†‡
2 and …H2 O†3 as the
primary daughter ions, although smaller quantities
of H3 O‡ and H‡ …H2 O†2 are also produced. Contrary to these laboratory observations, ions of the
‡
type …H2 O†n have not been identi®ed from mass
spectrometric studies of the ionosphere, therefore,
additional reactions have been postulated for
…H2 O†‡
n ions that could account for their rapid
conversion to PHs [9]. These reactions take the
form of collisions with additional water molecules
followed by loss of the OH moiety from the
‡
…H3 O† ±OH core.
…H3 O†‡ ±OH ‡ H2 O ! H‡ …H2 O†2 ‡ OH
…8†
and
‡
…H3 O† ±OH H2 O ‡ H2 O ! H‡ …H2 O†3 ‡ OH
…9†
L. Angel, A.J. Stace / Chemical Physics Letters 345 (2001) 277±281
281
‡
…H2 O†4 (Fig. 2a), yields the daughter ions
‡
H‡ …H2 O†3 and …H2 O†3 in equal ratio. However,
what may be an important distinction between the
‡
‡
two fragments, is that the …H2 O†3 ±…H2 O†4 interconversion is reversible, whereas the step leading
to H‡ …H2 O†3 is not.
Acknowledgements
The authors would like to thank EPSRC for the
award of a research studentship to L.A.
References
Fig. 3. Metastable and collision-induced fragmentation pat‡
‡
terns recorded for: (a) …H2 O†‡
4 ; (b) …H2 O†5 ; (c) …H2 O†6 .
If the …H2 O†‡
n (n ˆ 3 and 4) clusters are considered
as collision intermediates for reactions Eqs. (8)
and (9), then the primary products can be seen in
‡
Figs. 1 and 3. As Fig. 1b shows, …H2 O†3 predominantly decomposes to H‡ …H2 O†2 , whereas
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