Spectroscopy in The Duncan Group
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Metal Ion Complexes
Metal cation complexes are models for metal-ligand bonding, metal
ion solvation and atmospheric meteor ablation chemistry. We study these
systems by mass selecting them and performing photodissociation spectroscopy in
a specially designed reflectron time-of-flight mass spectrometer (design
developed in our group). Over the last several years, these complexes
have been studied with electronic spectroscopy in wavelength regions near
atomic transitions on metal cations or with infrared spectroscopy probing the
ligand vibrations.
If
the electrostatic bonding is weak, the complexes will have electronic spectra
in the wavelength region near that of the free metal ion transitions.
Systems studied included Mg+-L and Ca+-L complexes, where
L = Ne, Ar, Kr, Xe, N2, CO2, and H2O.
The highlight of this work is the first determination for the structure
(bond distances and H-O-H bond angle) and spectroscopy of metal-water complexes
(Mg+-H2O and Ca+-H2O). These
studies determined the bond energies and the vibrational frequencies for the
slightly distorted water molecules. Other systems studied include
metal-benzene, -acetylene, and -ethylene complexes where p bonding
interactions are investigated.
New
experiments employ infrared optical parametric oscillator (OPO/OPA) systems to
investigate the infrared spectroscopy of these metal ion complexes through IR
photodissociation and IR-optical double resonance experiments. Infrared
experiments are harder technically than UV-Visible experiments because of the
available laser sources and because transitions are weaker in the IR than they
are in the UV-Visible. Additionally, the lower energy in IR photons make
it more difficult to break the bonds in the cluster, and dissociation is
required in our experiment in order to measure the spectrum. Better
interactions with theory are possible however, when measurements can be made in
the electronic ground state. These experiments excite ligand or solvent
vibrational modes, which then lead to dissociation of the cluster. Metal
ion complexes with strong IR active ligands (CO2, H2O,
etc.) are investigated to measure the perturbation caused by binding to metal
and especially the onset of solvation and multilayer clustering. These
new experiments have been successful so far for M+(CO2)n
complexes (M=Al, Mg, Si, Fe, Ni, V) with and without added argon in the
cluster, for M+(acetylene)n complexes (M=Ni, Co, Fe, Ag),
which are especially interesting because of the possibility of cyclization
chemistry that may occur to form cyclobutadiene, benzene or cyclo-octatetraene,
metal-benzene complexes, metal-carbonyl complexes and many metal ion-water
complexes (M=Al, Mg, V, Fe, Co, Mn, Cr, Sc, Cu, Ag, Au, Nb). New methods make it possible to obtain doubly
charged cation-water complexes, and their infrared spectroscopy can now be
compared to corresponding singly charged complexes. Argon or neon atoms are often added to the
cluster to promote efficient fragmentation without introducing significant
perturbations. The loss of the rare gas
"tag" provides the mass change needed to detect light absorption with
high sensitivity.
Figure
1. The infrared spectrum of various M+(C2H2)
complexes showing the two C-H stretching modes.
The positions and intensities of these bands confirm the predictions of
theory, which indicate the formation of p-complexes
for all the complexes except V+(C2H2), where a
three-membered ring "metalla-cycle" is formed.
Figure
2. The infrared spectrum of the Li+(H2O)
complex in the OH stretching region compared to the predictions of theory. The symmetric stretch band occurs as a single
feature at 3629 cm-1, while the asymmetric stretch has three main peaks
associated with its rotational structure.
Figure
3. The infrared spectrum of Ta(CO)7+,
which is an unusual seven-coordinate carbonyl made stable by its 18-electron
configuration.
This
research is sponsored by the National Science Foundation (main group metal and
protonated systems) and by the U.S. Department of Energy (transition metal
systems).
Selected Publications
Infrared
Spectroscopy:
B. Bandyopadhyay and M. A.
Duncan, "Infrared spectroscopy of V2+(H2O) complexes,"
Chem. Phys. Lett. 530, 10-15 (2012).
R. Garza-Galindo, M. Castro and M. A.
Duncan, "Theoretical study of nascent hydration in the Fe+(H2O)n
system," J. Phys. Chem. A 116,
1906-1913 (2012).
A. D. Brathwaite and M. A. Duncan, "Infrared
spectroscopy of Si(CO)n+ complexes: Evidence for
asymmetric coordination," J. Phys. Chem. A 116, 1375-1382 (2012).
A.
D. Brathwaite, Z. D. Reed and M. A. Duncan, "Infrared spectroscopy of
copper carbonyl cations," J. Phys.
Chem. A 115, 10461-10469 (2011).
A. M. Ricks, L. Gagliardi and
M. A. Duncan, "Oxides and superoxides of uranium detected by IR
spectroscopy in the gas phase," J.
Phys. Chem. Lett. 2, 1662
(2011).
A.
D. Brathwaite, Z. D. Reed and M. A. Duncan, "Infrared spectroscopy of
copper carbonyl cations," J. Phys.
Chem. A 115, 10461 (2011).
B. Bandyopadhyay, P. D.
Carnegie, and M. A. Duncan, "Infrared spectroscopy of Mn+(H2O)n
and Mn2+(H2O) complexes via argon complex
predissociation," J. Phys. Chem. A
115, 7602 (2011).
M. A. Duncan, "IR
spectroscopy of gas phase metal carbonyl cations," J. Mol. Spec. 266, 63
(2011) (invited feature article; cover art).
P.
D. Carnegie, B. Bandyopadhyay and M. A. Duncan, "Infrared spectroscopy of
Sc+(H2O) and Sc2+(H2O) via argon
complex predissociation: The charge
dependence of cation hydration," J.
Chem. Phys. 134, 014302 (2011).
A. M. Ricks, L. Gagliardi and
M. A. Duncan, "Infrared spectroscopy of extreme coordination: The
carbonyls of U+ and UO2+," J. Am. Chem. Soc. 132, 15905 (2010).
Z.
D. Reed and M. A. Duncan, "Infrared Spectroscopy and Structures of
Manganese Carbonyl Cations, Mn(CO)n+ (n=1-9)," J. Am. Soc. Mass Spectrom. 21, 739 (2010).
A. M. Ricks, Z. D. Reed and M. A. Duncan,
"Seven-coordinate
homoleptic metal carbonyls in the gas phase," J. Am. Chem. Soc. 131, 9176 (2009).
A. M. Ricks and M. A. Duncan, "IR
spectroscopy of Co+(CO)n complexes in the gas
phase," J. Phys. Chem. A 113, 4701 (2009).
P. D. Carnegie, A. B. McCoy and
M. A. Duncan, "Infrared spectroscopy and theory of Cu+(H2O)Ar2
and Cu+(D2O)Ar2: Fundamentals and combination
bands," J. Phys. Chem. A 113, 4849 (2009).
P.
D. Carnegie, B. Bandyopadhyay and M.A. Duncan, “Infrared spectroscopy of Cr+(H2O)
and Cr2+(H2O): The role of charge in cation hydration,” J. Phys. Chem. A 112, 6237 (2008).
J. Velasquez,
III and M. A. Duncan, "IR Photodissociation Spectroscopy of Pt+(CO)n
Complexes," Chem. Phys. Lett. 461, 28 (2008).
M.A. Duncan, “Structures, energetics and spectroscopy of gas phase
transition metal ion-benzene complexes,” Int.
J. Mass Spectrom. 272, 99 (2008)
(review).
J. Velasquez,
III, B. Njegic, M. S. Gordon and M. A. Duncan, "IR Photodissociation
Spectroscopy and Theory of Au+(CO)n Complexes:
Nonclassical Carbonyls in the Gas Phase," J. Phys. Chem. A 112,
1907 (2008).
Electronic Spectroscopy:
J. D. Mosley, T. C. Cheng,
S. Hasbrouck, A. M. Ricks and M. A. Duncan, "Electronic Spectroscopy of Co+-Ne
via Mass-selected Photodissociation," J.
Chem. Phys. 135, 104309 (2011).
M.
A. Duncan, "Frontiers in the Spectroscopy of Mass-Selected Molecular Ions,"
Intl. J. Mass Spectrom. 200, 545 (2000) (review).
J.
E. Reddic and M.A. Duncan, "Photodissociation Spectroscopy of Ca+-Ne,"
J. Chem. Phys. 112, 4974 (2000).
J.
E. Reddic and M.A. Duncan, "Photodissociation Spectroscopy of Mg+-Ne,"
J. Chem. Phys. 110, 9948 (1999).
M. A. Duncan, "Spectroscopy of Metal Ion Complexes: Gas Phase Models for Solvation, " Ann. Rev. Phys. Chem. 48, 69 (1997) (review).
Protonated Water Clusters, H+(H2O)n
Our group has recently obtained the first infrared
spectroscopy for protonated water clusters, H+(H2O)n,
in the size range of n=1-30. Smaller clusters (n=1-8) had been studied
before, but these are the first experiments to make measurements on the larger
clusters. In the late 1970's, John Fenn and coworkers noticed a strange
occurrence in the mass spectrum of such clusters. The n=21 mass peak was
much larger than others, indicating that it had special stability. Fenn
suggested that a near-spherical cage structure could explain this. A
hydrogen bonding network can be constructed with 20 water molecules forming a
"dodecahedron cage," leaving one left-over molecule. Fenn
suggested that this molecule would go inside the cage, explaining why n=21 was
apparently more stable than n=20. Because hydrogen bonding is so
important throughout chemistry and biology, many labs have tried to do
experiments and theory on these clusters. Until the recent work in
Our work uses a variation of the
laser plasma source that we use for metal complexes to produce the protonated
water clusters in a supersonic molecular beam. They are then
mass-selected in the same kind of time-of-flight mass spectrometer, and studied
with infrared photodissociation spectroscopy. The spectra obtained this
way do indeed find that the n=21 cluster is special. The spectra contain
strong vibrational bands in the free-OH region near 3700 cm-1.
Clusters smaller than n=21 have a multiplet here, which gradually evolves into
a single strong peak at n=21. This proves that this cluster has a high
symmetry structure like the proposed dodecahedron. Larger clusters have a
more complex multiplet of peaks in this region. Surprisingly, the IR
spectra reveal that the n=22 cluster also has a single peak in the free-OH
region at exactly the same frequency as the n=21 cluster. It must also
therefore have nearly the same structure. The only way this is possible
is that the n=22 cluster must have the same 20-molecule dodecahedron cage
structure, but with two interior molecules.
Ongoing studies are
collaborating with the experimental lab of Professor Mark Johnson at Yale
University and with the theory group of Professor Ken Jordan at the University
of Pittsburgh to refine the structures of these clusters and to determine where
the proton is located (inside or in the surface?) and whether it is localized
(i.e., in an H3O+ species; the so-called eigen structure)
or shared between two water molecules (the so-called Zundel structure, H2O
- H+- OH2). New
experiments have investigated the corresponding deuterated complexes.
Figure
4. The infrared spectrum of H+(H2O)5. The broad peaks in the hydrogen bonding
region and the sharp ones in the O-H stretching region agree well with the
predictions for a single isomeric structure, as shown.
Selected References
G.
E. Douberly, R. S. Walters, J. Cai, K. D. Jordan and M. A. Duncan,
"Infrared spectroscopy of small protonated water clusters H+(H2O)n
(n=2-5): Isomers, argon tagging and deuteration," J. Phys. Chem. A 114,
4570 (2010).
G. E. Douberly, A. M. Ricks and M. A.
Duncan, "Infrared spectroscopy of perdeuterated protonated water clusters
in the vicinity of the clathrate cage structure," J. Phys. Chem. A 113,
8449 (2009) (communication).
J. Headrick, E.G. Diken, R.S. Walters, N.I. Hammer,
R.A. Christie, J. Cui, E.M. Myshakin, M.A. Duncan, M.A. Johnson and K.D.
Jordan, "Spectral signatures of hydrated proton vibrations in water
clusters," Science 308, 1765 (2005).
J.-W. Shin, N.I. Hammer, E.G. Diken, M.A. Johnson,
R.S. Walters, T.D. Jaeger, M.A. Duncan, R.A. Christie and K.D. Jordan,
“Infrared signature of structural motifs associated with the H+(H2O)n,
n=6-27, clusters,” Science 304, 1137 (2004).
Other Protonated Molecular Clusters
We have synthesized a variety of other molecular clusters
containing protons or shared protons.
These species are studied with infrared photodissociation
spectroscopy. The free proton stretch
occurs at high frequency (near 3500-3800 cm-1, while shared proton
motions occur at very low frequency (1000-1200 cm-1). These data and their theoretical modeling are
relevant for proton transfer reactions that occur throughout Chemistry and
Biology.
This
research is sponsored by the National Science Foundation.
Selected Publications
B. Bandyopadhyay, T. C. Cheng and M. A.
Duncan, "Proton sharing in hydronium-nitrogen clusters probed with
infrared spectroscopy," Int. J. Mass
Spectrom. 297, 124 (2010).
T.
C. Cheng, B. Bandyopadhyay, Y. Wang, B. J. Braams, J. M. Bowman,
Michael A. Duncan, "The shared proton mode lights up the infrared spectrum
of fluxional cations H5+ and D5+,"
J. Phys. Chem. Lett. 1, 758 (2010).
A. M. Ricks, G. E. Douberly and M. A.
Duncan, "Infrared spectroscopy of proton-bridged nitrogen dimers: Complex
vibrational dynamics of the shared proton," J. Chem. Phys. 131,
104312 (2009).
G.E. Douberly,
A.M. Ricks, B.W. Ticknor and M.A. Duncan, “Infrared spectroscopy of protonated
acetone and its dimer,” Phys. Chem. Chem.
Phys. 10, 77 (2008).
G.E. Douberly,
A.D. Ricks, B.W. Ticknor and M.A. Duncan, “The structure of protonated carbon
dioxide clusters: Infrared photodissociation spectroscopy and ab initio
calculations,” J. Phys. Chem. A 112, 950 (2008).
G.E.
Douberly, A.M. Ricks, B.W. Ticknor, W.C. McKee, P.v.R. Schleyer and M.A. Duncan,
“Infrared photodissociation spectroscopy of protonated acetylene and its
clusters,” J. Phys. Chem. A 112, 1897 (2008).
Carbocations
A
number of small carbocations (formerly known as carbonium ions) have been
produced with pulsed discharge cluster sources and studied with infrared
photodissociation spectroscopy.
Unsaturated ions of this sort are expected to have several isomeric
structures and some have close energies.
Some of these species are also believed to be present in interstellar
gas clouds and may contribute to unassigned infrared or optical emission from
these environments.
Figure
5. The infrared spectrum of C3H3+
ions compared to the predictions of theory for the cyclopropenyl cation (red)
and propargyl cation (blue). As shown,
both are present in the experiment.
Selected Publications
A. M. Ricks, G. E. Douberly, P. v. R.
Schleyer and M. A. Duncan, "The infrared spectrum of gas phase C3H3+:
The cyclopropenyl and propargyl cations," J. Chem. Phys. 132, 051101 (2010) (communication).
A.
M. Ricks, G. E. Douberly, P. v. R. Schleyer and M. A. Duncan, "Infrared
spectroscopy of protonated ethylene: The nature of proton binding in the
non-classical structure," Chem.
Phys. Lett. 480, 17 (2009).
A.
M. Ricks, G. E. Douberly and M. A. Duncan, "The infrared spectroscopy of protonated
naphthalene and its relevance for the unidentified infrared bands," Astrophys. J. 702, 301 (2009).
G.E. Douberly,
A.M. Ricks, B.W. Ticknor, P.v.R. Schleyer and M.A. Duncan, “Infrared
spectroscopy of gas phase benzenium ions: Protonated benzene and protonated
toluene from 750 to 3400 cm-1,” J.
Phys. Chem. A 112, 4869 (2008)
(letter).
G.E. Douberly,
A.M. Ricks, P.v.R. Schleyer and M.A. Duncan, “Infrared spectroscopy of gas
phase C3H5+: The allyl and 2-propenyl
cations,” J. Chem. Phys. 128, 021102 (2008) (communication).
G.E.
Douberly, A.M. Ricks, B.W. Ticknor, W.C. McKee, P.v.R. Schleyer and M.A.
Duncan, “Infrared photodissociation spectroscopy of protonated acetylene and
its clusters,” J. Phys. Chem. A 112, 1897 (2008).
G.E. Douberly,
A.M. Ricks, B.W. Ticknor, P.v.R. Schleyer and M.A. Duncan, “The infrared
spectrum of the tert-butyl cation in the gas phase,” J. Am. Chem. Soc. 129,
13782 (2007) (communication).