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.

 

Description: Description: Description: metal-acetylene

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.

Description: Description: Description: Li+(H2O)

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.

Description: Description: Description: Ta(CO)7+ IR & 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 of Metal Ion Complexes:

M. Castro, R. Flores Mena, and M. A. Duncan, "Theoretical Study of Nascent Solvation in Ni+(Benzene)m, m=3 and 4, Clusters," J. Phys. Chem. A, submitted.

A. D. Brathwaite and M. A. Duncan, "Infrared Spectroscopy of Vanadium Oxide Carbonyl Cations," J. Phys. Chem. A, in press.

B. Bandyopadhyay, K. N. Reishus and M. A. Duncan, " Infrared spectroscopy of solvation in small Zn+(H2O)n Complexes," J. Phys. Chem. A, in press.

A. D. Brathwaite and M. A. Duncan, " Infrared photodissociation spectroscopy of saturated group IV (Ti, Zr, Hf) metal carbonyl cations," J. Phys. Chem. A, in press. (Curt Wittig Festschrift) DOI: 10.1021/jp400793h.

A. M. Ricks, A. D. Brathwaite, M. A. Duncan, "Coordination and spin states of V+(CO)n clusters revealed by IR spectroscopy," J. Phys. Chem. A 117, 1001–1010 (2013). (Peter Armentrout Festschrift)  DOI: 10.1021/jp301679m.

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.M. Ricks, Z.E. Reed and M. A. Duncan, "IR spectroscopy of gas phase metal carbonyl cations," J. Mol. Spec. 266, 63-74 (2011) (invited feature article; cover art).  DOI:10.1016/j.jms.2011.03.006.

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).

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 Duncan's lab, however, there was no spectroscopy that might reveal the exact structures of these clusters.

        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.

Description: Description: Description: ToC

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

T. C. Cheng, B. Bandyopadhyay, M. A. Duncan, "Protonation in water-benzene nanoclusters: Hydronium, zundel and eigen at a hydrophobic interface," J. Am. Chem. Soc. 134, 13046 (2012).

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

J. W. Young, T. C. Cheng, B. Bandyopadhyay and M. A. Duncan, "Infrared photodissociation spectroscopy of H7+, H9+ and their deuterated analogs," J. Phys. Chem. A, in press. (Joel Bowman Festscrift) DOI: 10.1021/jp312630x.

T. C. Cheng, L. Jiang, K. R. Asmis, Y. Wang, J. M. Bowman, A. M. Ricks, M. A. Duncan, "Mid- and Far-IR spectra of H5+ and D5+ compared to the predictions of anharmonic theory," J. Phys. Chem. Lett. 3, 3160–3166 (2012).  DOI: 10.1021/jz301276f.

J. D. Moseley, A. M. Ricks, J. I. Wu, P. v. R. Schleyer, M. A. Duncan, "IR spectroscopy of α- and β-protonated pyrrole via argon complex photodissociation," J. Phys. Chem. A 116, 9689–9695 (2012).  DOI: 10.1021/jp307631n.

J. D. Moseley, T. C. Cheng, A. B. McCoy, M. A. Duncan, "Infrared spectroscopy of the mass 31 cation: Protonated formaldehyde vs methoxy," J. Phys. Chem. A 116, 9287 (2012).

T. C. Cheng, B. Bandyopadhyay, S. Wheeler, M. A. Duncan, "Vibrational spectroscopy and theory of the protonated benzene dimer and trimer," J. Phys. Chem. A 116, 2065 (2012).

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.

 

Description: Description: Description: C3H3+ Figure 2

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

M. A. Duncan, "Infrared laser spectroscopy of mass-selected carbocations," J. Phys. Chem. A 116, 11477–11491 (2012) (invited Feature Article).  DOI: 10.1021/jp309037d.

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).


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