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 same wavelength region as the free metal ion. 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. We obtained spectra for Ca+-acetylene, which
is the first ro-vibrationally resolved gas phase data for a metal p-complex.
New experiments
employ infrared optical parametric oscillator (OPO/OPA) systems to investigate
for the first time 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).
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.
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:
P. D. Carnegie, A.B. McCoy and M.A. Duncan,
“Infrared spectroscopy of Cu+(H2O)Ar2
and Cu+(D2O)Ar2: Fundamentals and combination
bands,” J. Phys. Chem. A,
submitted.
A.M. Ricks, J.M. Bakker, G.E. Douberly and M.A. Duncan,
“IR spectroscopy of Co+(CO)n
complexes in the gas phase,” J.
Phys. Chem. A, submitted.
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) (invited 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).
V. Kasalova, W.D. Allen, H.F.
Schaefer, E.D. Pillai and M.A. Duncan, "Model systems for probing metal
cation hydration: The V+(H2O)
and V+(H2O)Ar complexes," J. Phys. Chem. A 111, 7599 (2007).
E.D. Pillai, T.D. Jaeger and
M.A. Duncan, "Infrared Spectroscopy of
T.D. Vaden, J.M. Lisy, P.D. Carnegie,
E.D. Pillai and M.A. Duncan, "Infrared Spectroscopy of the Li+(H2O)Ar Complex: The Role of Internal
Energy and Its Dependence on Ion Preparation," Phys. Chem. Chem. Phys. 8,
3078 (2006).
J.
Velasquez, E.D. Pillai, P. Carnegie and M.A. Duncan, "IR Spectroscopy of M+(acetone) Complexes (M=Mg, Al, Ca):
Cation-Carbonyl Binding Interactions," J.
Phys. Chem. A 110,
2325 (2006).
N.R. Walker, R.S. Walters and
M.A. Duncan, "Frontiers in the Infrared Spectroscopy of Gas Phase Metal
Ion Complexes," New J. Chem. 29, 1495
(2005) (invited review).
R.S. Walters, E.D. Pillai,
P.v.R. Schleyer and M.A. Duncan, “Vibrational spectroscopy of Ni+(C2H2)n (n=1-4)
complexes,” J. Am. Chem. Soc. 127,
17030 (2005).
R.S. Walters and M.A. Duncan,
“Solvation Processes in Ni+(H2O)n
Complexes Revealed by Infrared Photodissociation Spectroscopy,” J. Am.
Chem. Soc. 127, 16599 (2005).
R.S. Walters, P.v.R. Schleyer,
C. Corminboeuf and M.A. Duncan, “Structural Trends in Transition Metal
Cation-Acetylene Complexes Revealed Through the C-H
Stretch Fundamentals,” J. Am. Chem. Soc. 127, 1100 (2005)
(communication).
J.B. Jaeger, E.D. Pillai, T.D.
Jaeger and M.A. Duncan, "Ultraviolet
and Infrared Photodissociation of Si+(C6H6)n
and Si+(C6H6)Ar Clusters," J. Phys. Chem. A 109, 2801 (2005).
T.D. Jaeger and M.A. Duncan,
"Infrared Photodissociation Spectroscopy of Ni+(benzene)x
Complexes," J. Phys. Chem. A 109, 3311 (2005).
E.D. Pillai, T.D. Jaeger and
M.A. Duncan, "Infrared spectroscopy and density functional theory of small
V+(N2)n
clusters," J. Phys. Chem. A 109, 3521 (2005).
N.R.
Walker, R.S. Walters, C.-S. Tasi, K.D. Jordan and M.A.
Duncan, “Infrared
Photodissociation Spectroscopy of Mg+(H2O)Arn
Complexes: Isomers in Progressive Microsolvation,” J. Phys.
Chem. A 109, 7057 (2005).
N.R. Walker, G.A. Grieves, R.S.
Walters and M.A. Duncan, “Growth Dynamics and Intracluster Reactions in Ni+(CO2)n
Complexes via Infrared Spectroscopy,” J. Chem. Phys. 121, 10498 (2004).
T.D. Jaeger and M.A. Duncan,
“Structure, Coordination and Solvation in V+(benzene)n
Complexes via Gas Phase Infrared Spectroscopy,” J. Phys. Chem. A 108, 6605 (2004).
R.S. Walters and M.A. Duncan,
“Infrared Spectroscopy of Solvation and Isomers in Fe+(H2O)1,2Arm
Complexes,” Austr. J. Chem. 57, 1145 (2004).
T.D. Jaeger, D. van Heijnsbergen, S. Klippenstein, G. von
Helden, G. Meijer and M.A. Duncan, “Infrared Spectroscopy and Density
Functional Theory of Transition Metal Ion-Benzene and
Dibenzene Complexes,” J. Am. Chem. Soc. 126, 10981 (2004).
N.R. Walker,
R.S. Walters and M.A. Duncan, “Infrared Photodissociation Spectroscopy of
V+(CO2)n and V+(CO2)nAr
Complexes,” J. Chem. Phys. 120, 10037 (2004).
M.A. Duncan, "Infrared Spectroscopy to Probe Structure and Dynamics
in Metal Ion-Molecular Complexes," Intl. Rev. Phys. Chem. 22,
407 (2003) (invited review).
Electronic Spectroscopy:
M.A. Duncan, "Frontiers in the Spectroscopy of Mass-Selected
Molecular Ions," Intl. J. Mass Spectrom. 200,
545 (2000) (invited 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.R. France, S.H. Pullins and M.A. Duncan, "Photodissociation
Spectroscopy of Ca+-C2H2 and Ca+-C2D2
p-Complexes," J. Chem. Phys. 109, 8842 (1998).
M.R. France, S.H. Pullins, and M.A. Duncan, "Spectroscopy of the Ca+-acetylene
π-complex," J. Chem. Phys. 108, 7049 (1998).
S.H. Pullins, J.E. Reddic, M.R. France, and M.A. Duncan,
"Photodissociation Spectroscopy of Ca+-N2
Complex," J. Chem. Phys. 108, 2725 (1998).
M.A. Duncan,
"Spectroscopy of Metal Ion Complexes: Gas Phase Models for Solvation,
" Ann. Rev. Phys. Chem. 48, 69 (1997) (invited 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.
Selected References
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).
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).
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
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.
Selected Publications
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).
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).
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).