close

Вход

Забыли?

вход по аккаунту

?

Coulomb Explosions and Stability of Multiply Charged Ions in the Gas Phase.

код для вставкиСкачать
Angewandte
Chemie
Molecular Beams
Coulomb Explosions and Stability of Multiply Charged
Ions in the Gas Phase**
Detlef Schr
der*
Keywords:
charge stripping · coulomb explosion · dianions ·
metal complexes · molecular beams
Since Pauling discussed the stability of
2+
[1]
the He2 ion in 1933, the existence of
multiply charged molecular ions in the
gas phase has attracted considerable
attention because it challenges the fundamentals of chemical bonding.[2]
Whereas an atom can get rid of all its
electrons if enough energy is provided,
molecules encounter an obvious problem in the absence of a stabilizing
solvent environment because the electrons removed are essential for binding.
Moreover, location of the charge on
more than a single nucleus leads to
strong electrostatic repulsion which may
eventually result in charge separation,
also referred to as Coulomb explosion.
The latter term describes the situation
quite adequately, because when the
charges separate the electrostatic repulsion leads to a rapid release of kinetic
energy, in the order of several electronvolts (eV). Consequently, almost all
molecules can be ionized to monocations in the gas phase, whereas only a
limited number of compounds form
molecular dications, and only few small
molecules exist as trications or in higher
charge states.[3, 4] In brief, the stability
range of a molecular dication is determined by the crossing between the
bonding potential-energy curve of
AB2+ and the entirely repulsive inter-
action of the monocations A+ and B+
(Figure 1 a).
Figure 1. Schematic potential-energy surface
of a) a dication [AB]2+ and b) a dianion [AB]2
with the associated charge-separation asymptotes A+ + B+ and [AB] + e , respectively. The
grey areas indicate the stability ranges of
[AB]2+ and [AB]2, and the horizontal arrows
indicate the occurrence of tunneling through
the potential-energy barrier.
[**] Financial support by the MCInet of the
European Commission is acknowledged.
The situation is even more extreme
in small, multiply charged anions because electrons can easily tunnel
through the potential-energy well
formed by the crossing of the potential
energy surfaces.As a result, the conditions required for the existence of longlived dianions are more challenging
(Figure 1 b). About a decade ago, Cederbaum et al. pioneered research on
dianions by means of extensive theoretical investigations, they suggested the
existence of several small dianions[5]
with [BeF4]2 as the smallest member.[6, 7]
Major progress in the research of
multiply charged ions came from the
invention of electrospray ionization
(ESI)[8] because this method allows the
generation of relatively small, multiply
Angew. Chem. Int. Ed. 2004, 43, 1329 –1331
DOI: 10.1002/anie.200301728
[*] Dr. D. Schr&der
Institut f(r Chemie
Technische Universit+t Berlin
Strasse des 17. Juni 135, 10623 Berlin
(Germany)
Fax: (+ 49) 30-314-21102
E-mail: df@www.chem.tu-berlin.de
charged ions directly from solution. ESI
is versatile in many respects and allows
the generation of a manifold of multiply
charged cations and anions. Other mass
spectrometric methods also provide important information about multiply
charged ions, particularly when it comes
to small species.[9] Herein, some recent
results about multiply charged cations
and anions are addressed.
As far as cations are concerned, a
number of interesting results have been
reported in 2003. Thus, Duncan et al.
have modified the conditions in a Smalley-type cluster-ion source to achieve
surprisingly good yields of molecular
dications, such as [Mg(CO2)n]2+ (n = 1–
3), [Si(Ar)n]2+ (n = 1–4), and [Co(H2O)]2+.[10] The [Co(H2O)]2+ ion is
particularly suited to illustrate the fine
balance that needs to be met in the
optimization of the ionization conditions. Thus, the ionization energy (IE) of
17.08 eV of the Co+ ion is more than
4 eV larger than IE(H2O) = 12.60 eV. A
strictly unimolecular collision of Co2+
ions formed upon laser ionization and
neutral H2O would therefore result in
immediate electron transfer rather than
association to the (metastable) bound
[Co(H2O)]2+ state. In the other extreme,
excessive cooling by an inert buffer gas
would instead result in equilibration and
thus lead to monocations only. The
highest laser power on one hand and
careful adjustment of gas pulses and gas
pressures on the other led to success.
While this new method has some limitations, it provides direct access to
several molecular dications which are
difficult to produce by other means.
A multiply charged ion which recently attracted much attention is the
[CO2]2+ dication[11] because it has been
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1329
Highlights
proposed to play a role in nonthermal
oxygen losses from the Martian atmosphere. In brief, the issue can be summarized as follows:[12] Unlike Earth,
Mars lacks the atmospheric layers which
protect from heavy radiation and thus
energetic photons can penetrate. In
higher layers of the Martian atmosphere, photons with energies above
37.4 eV can directly ionize the major
component CO2 to the corresponding
dication. Even for the ground state of
the [CO2]2+ dication, however, dissociation into the singly charged fragments
[CO]+ and O+ is highly exothermic (ca.
5.1 eV).[13] Most of this energy is released as kinetic energy (Coulomb explosion) leading to fast-moving O+ fragments which may eventually escape
from the gravitational field of Mars.
As far as long-lived small molecules
with even higher charge states are concerned, only a few trications are known
and just a single tetracation has been
detected.[4] In this respect, it is quite
remarkable that the diatomic species
[UF]3+ even corresponds to a thermochemically stable diatomic trication;[14]
here, the term “thermochemical stability” means that ground state of [UF]3+ is
lower in energy than all considerable
fragments including the charge-separation asymptote U2+ + F+. Recently,
Harvey and Kaczorowska proposed the
monocoordinate species [Zr(CH3CN)]4+
as a promising candidate for a small
tetracation with a lifetime sufficient for
detection, although the authors admit
that the ion might be difficult to generate.[15] The achievable charge state is,
of course, dependent on the size of the
system. For metal complexes with slightly more extended solvation, multiply
charged cations can easily be generated
by ESI, for example, [In(L)4]3+ and
[Y(L)5]3+ (with L = dimethylsulfoxide).[16, 17]
As mentioned above, there exist
more severe stability boundaries for
multiply charged anions because charge
reduction can proceed by electron tunneling. The oxalate dianion [C2O4]2 as
well as the oxocarbon dianion [C5O5]2,
for example, do not exist as free species
in the gas phase. They can only be
detected in the ms timeframe when
solvated by several water molecules,[18, 19] which is of interest for the
analysis of these species. Likewise, in the
1330
series of 1,w-diolatodiynes O-(CC)nO , the first species stable with respect
to electron detachment is the dodecahexayne derivative [C12O2]2 (n = 6).[20]
In addition to electron detachment from
multiply charged anions, Coulomb explosions can also occur without electron
loss through the formation of two singly
charged anions. Recent examples of
such reactions deal with the dianions of
iron–sulfur cubanes [Fe4S4X4]2 (X = Cl,
Br, SC2H5) which undergo symmetric
cleavage to afford two [Fe2S2X2]
units.[21, 22] Quite remarkably, the corresponding monoanions [Fe4S4X4] have
completely different dissociation patterns. Accordingly it was argued that
Coulomb repulsion forces a symmetric
cleavage of the cubane skeleton.[22]
A new methodological perspective
has recently been added by Nielsen
et al.[23] who generated reasonably small
dianions by charge stripping (CS) of
anions at a collision energy of 50 keV.
Charge stripping was introduced in 1973
by Beynon et al.[24] and refers to electron
removal from a mass-selected ion beam
in high-energy collisions with a quasistationary target gas; usually monocations
are converted into dications: [AB]+!
[AB]2+ + e . Nielsen et al. pursue the
opposite approach by adding an electron to a mass-selected anion beam, so
that the collision gas is also involved in
the CS process. Three requirements
were found to be essential for the
success of this approach: 1) complexes
of redox-active transition metals were
chosen as precursor ions, so that the
transition [AB] + e ![AB]2 can occur without major changes in the bonding pattern. 2) An unusually large collision energy of 50 keV was applied to
overcome the endothermicity of the CS
process and to minimize interaction
time between the fast-moving projectile
ion and the target gas. 3) Sodium vapor,
from which removal of an electron is
relatively facile (IE = 5.14 eV), was
chosen as a target. In marked contrast,
no dianions were observed at all when
dioxygen (IE = 12.07 eV) was used as a
collision gas.
In their exploratory studies, Nielsen
et al. demonstrate the occurrence of the
CS
processes
[Cr(SCN)4] + e !
2
[Cr(SCN)4] ,
[Fe(CN)4] + e !
2
[Fe(CN)4] , and [Pt(NO2)2] + e !
[Pt(NO2)2]2, where the electrons are
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
delivered by sodium in 50 keV collisions. In addition to the mere observation of CS occurring with anions, Nielsen et al. further investigated the dissociation behavior of the dianions formed
by CS by means of energy-resolved
experiments. The Coulomb explosion
of the dianion [Cr(SCN)4]2, for example, leads to the pair of the monoanions
[Cr(SCN)3] and [SCN] where both
monoanion signals show characteristic
peak broadenings owing to the large
amount of kinetic energy released upon
charge separation.
In conclusion, the chemistry and
physics of multiply charged ions remains
a formidable challenge to both experiment and theory, and recent progress in
experimental techniques helps to address existing questions and to pose new
ones. Upon first sight, small multiply
charged ions might appear as a mere
curiosity from mass spectrometric laboratories. More detailed consideration of
the above examples demonstrates, however, that multiply charged ions may
also play a role in practical applications
or even atmospheric processes, not to
speak about the enormous relevance of
multiply charged ions in the analysis of
biomolecules.
[1] L. Pauling, J. Chem. Phys. 1933, 1, 56.
[2] See for example the MCInet of the
European
Commission:
http://
www.chm.bris.ac.uk/pt/mcinet/welcome.htm.
[3] Large biomolecules can easily compensate many charges; for a recent example
of a pentadecakisanion, see: J. Wu, S. A.
McLuckey, Int. J. Mass Spectrom. 2003,
228, 577.
[4] D. SchrKder, H. Schwarz, J. Phys. Chem.
A 1999, 103, 7385.
[5] A. Dreuw, L. Cederbaum, Chem. Rev.
2002, 102, 181.
[6] H. G. Weickert, L. S. Cederbaum, J.
Chem. Phys. 1993, 99, 8877.
[7] For experimental evidence, see: R. Middleton, J. Klein, Phys. Rev. A 1999, 60,
3515.
[8] J. B. Fenn, Angew. Chem. 2003, 115,
3999; Angew. Chem. Int. Ed. 2003, 42,
3871.
[9] For an example, see: D. SchrKder, H.
Schwarz, J. Wu, C. Wesdemiotis, Chem.
Phys. Lett. 2001, 343, 258, and references
therein.
[10] N. R. Walker, G. A. Grieves, J. B. Jaeger,
R. S. Walters, M. A. Duncan, Int. J. Mass
Spectrom. 2003, 228, 285.
Angew. Chem. Int. Ed. 2004, 43, 1329 –1331
Angewandte
Chemie
[11] P. Franceschi, R. Thissen, J. Zabka, J.
Roithova, Z. Herman, O. Dutuit, Int. J.
Mass Spectrom. 2003, 228, 507, and
references therein.
[12] O. Witasse, O. Dutuit, J. Lilenstein, R.
Thissen, J. Zabka, C. Alcaraz, P.-L.
Blelly, S. W. Bougher, S. Engel, L. H.
Andersen, K. Seiersen, Geophys. Res.
Lett. 2002, 29, 104/1.
[13] H. Hogreve, J. Phys. B 1995, 28, L263.
[14] D. SchrKder, M. Diefenbach, T. M. KlapKtke, H. Schwarz, Angew. Chem. 1999,
Angew. Chem. Int. Ed. 2004, 43, 1329 –1331
[15]
[16]
[17]
[18]
[19]
111, 206; Angew. Chem. Int. Ed. 1999,
38, 137.
J. N. Harvey, M. Kaczorowska, Int. J.
Mass Spectrom. 2003, 228, 517.
A. A. Shvartsburg, J. Am. Chem. Soc.
2002, 124, 12 343.
A. T. Blades, P. Jayaweera, M. G. Ikonomou, P. Kebarle, Int. J. Mass Spectrom. Ion Processes 1990, 101, 325.
X.-B. Wang, X. Yang, J. B. Nicholas, L.S. Wang, J. Chem. Phys. 2003, 119, 3631.
T. B. Arthur, M. Peschke, P. Kebarle, Int.
J. Mass Spectrom. 2003, 228, 1017.
www.angewandte.org
[20] P. Schwerdtfeger, A. Hammerl, R. Wesendrup, Int. J. Mass Spectrom. 2003,
228, 341.
[21] X. Yang, X.-B. Wang, S. Q. Niu, C. J.
Pickett, T. Ichiye, L.-S. Wang, Phys. Rev.
Lett. 2002, 89, 163 401.
[22] X. Yang, X.-B. Wang, L.-S. Wang, Int. J.
Mass Spectrom. 2003, 228, 797.
[23] A. B. Nielsen, P. Hvelplund, B. Liu, S. B.
Nielsen, S. Tomita, J. Am. Chem. Soc.
2003, 125, 9592.
[24] R. G. Cooks, T. Ast, J. H. Beynon, Int. J.
Mass Spectrom. Ion Phys. 1973, 11, 490.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1331
Документ
Категория
Без категории
Просмотров
0
Размер файла
600 Кб
Теги
explosion, coulomb, gas, ions, multiple, phase, stability, charge
1/--страниц
Пожаловаться на содержимое документа