SPECTROSCOPY IN THE DUNCAN LAB 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 cation-pi 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. The lower traces show spectra predicted by theory for the capped octahedron versus pentagonal bipyramid structures. This research is sponsored by the National Science Foundation (main group metal and protonated systems) and by the U.S. Department of Energy and Air Force Office of Scientific Research (transition metal systems). Selected Publications Infrared Spectroscopy of Metal Ion Complexes: A. D. Brathwaite, J. H. Marks, I. J. Webster, A. G. Batchelor, T. B. Ward, M. A. Duncan, "Coordination and Spin States in Fe+(C2H2)n Complexes Studied with Infrared Spectroscopy," J. Phys. Chem. A 126, 9680‒9690 (2022). DOI: 10.1021/acs.jpca.2c07556. J. H. Marks, B. M. Rittgers, M. J. van Stipdonk, M. A. Duncan, "Photodissociation and Infrared Spectroscopy of Uranium-Nitrogen Cation Complexes," J. Phys. Chem. A 125, 7278−7288 (2021). (Dan Neumark Festschrift). DOI: 10.1021/acs.jpca.1c05823. J. H. Marks, E. Miliordos, M. A. Duncan, "Infrared Spectroscopy of RG-Co+(H2O) Complexes (RG = Ar, Ne, He): The Role of Rare Gas "Tag" Atoms," J. Chem. Phys. 154, 064306 (2021). DOI: 10.1063/5.0041069. A. D. Brathwaite, T. B. Ward, J. H. Marks, M. A. Duncan, "Coordination and Solvation in Gas Phase Ag+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy," J. Phys. Chem. A 124, 8562-8573 (2020). DOI: 10.1021/acs.jpca.0c08081. J. H. Marks, T. B. Ward, A. D. Brathwaite, M. A. Duncan, "Infrared Spectroscopy of Zn(Acetylene)n+ Complexes: Ligand Activation and Nascent Polymerization," J. Phys. Chem. A 124, 4764−4776 (2020). DOI: 10.1021/acs.jpca.0c03358. P. D. Carnegie, J. H. Marks, A. D. Brathwaite, T. B. Ward, M. A. Duncan, "Microsolvation in V+(H2O)n Clusters Studied with Selected-Ion Infrared Spectroscopy," J. Phys. Chem. A 124, 1093−1103 (2020). DOI: 10.1021/acs.jpca.9b11275. J. H. Marks, T. B. Ward, A. D. Brathwaite, S. Ferguson, M. A. Duncan, "Cyclotrimerization of Acetylene in Gas Phase V+(C2H2)n Complexes: Detection of Intermediates and Products with Infrared Spectroscopy," J. Phys. Chem. A 123, 6733-6743 (2019). DOI: 10.1021/acs.jpca.9b04962. J. H. Marks, T. B. Ward, M. A. Duncan, "Infrared Spectroscopy of the Coordination and Solvation in Cu+(ethylene)n (n = 1-9) Complexes," Int. J. Mass Spectrom. 435, 107-113 (2019). DOI: 10.1016/j.ijms.2018.10.008. T. B. Ward, A. D. Brathwaite, M. A. Duncan, "Infrared Spectroscopy of Au(Acetylene)n+ Complexes in the Gas Phase," Top. Catal. 61, 49–61 (2018). DOI: 10.1007/s11244-017-0859-0. T. B. Ward, E. Miliordos, P. D. Carnegie, S. S. Xantheas, M. A. Duncan, "Ortho-Para Interconversion in Cation-Water Complexes: The Case of V+(H2O) and Nb+(H2O) Clusters," J. Chem. Phys.146, 224305 (2017). DOI: 10.1063/1.4984826. A. D. Brathwaite, H. L. Abbott-Lyon, M. A. Duncan, "Distinctive Coordination of CO vs N2 to Rhodium Cations: An Infrared and Computational Study," J. Phys. Chem. A 120, 7659-7670 (2016). DOI: 10.1021/acs.jpca.6b07749. T. B. Ward, P. D. Carnegie, M. A. Duncan, "Infrared Spectroscopy of the Ti(H2O)Ar+ Ion-Molecule Complex: Electronic State Switching Induced by Argon," Chem. Phys. Lett. 654, 1–5 (2016). DOI: 10.1016/j.cplett.2016.04.065. J. A. Maner, D. T. Mauney, M. A. Duncan, "Imaging Charge Transfer in a Cation-π System: Velocity-Map Imaging of Ag+(Benzene) Photodissociation," J. Phys. Chem. Lett. 6, 4493−4498 (2015). DOI: 10.1021/acs.jpclett.5b02240. A. D. Brathwaite, T. B. Ward, R. S. Walters, M. A. Duncan, "Cation-π and CH-π Interactions in the Coordination and Solvation of Cu+(Acetylene)n Complexes," J. Phys. Chem. A 119, 5658–5667 (2015). DOI: 10.1021/acs.jpca.5b03360. K. N. Reishus, A. D. Brathwaite, J. D. Mosley, M. A. Duncan, "Infrared Spectroscopy of Coordination versus Solvation in Al+(Benzene)1-4 Complexes," J. Phys. Chem. A 118, 7516−7525 (2014) (Ken Jordan festschrift). DOI: 10.1021/jp500778w. A. D. Brathwaite, J. A. Maner, M. A. Duncan, "Testing the Limits of the 18-Electron Rule: The Gas Phase Carbonyls of Sc+ and Y+," Inorg. Chem. 53, 1166–1169 (2014). DOI: 10.1021/ic402729g. A. D. Brathwaite, A. M. Ricks, M. A. Duncan, "Infrared Spectroscopy of Vanadium Oxide Carbonyl Cations," J. Phys. Chem. A 117, 13435−13442 (2013). (Terry Miller festschrift) DOI: 10.1021/jp4068697. B. Bandyopadhyay, K. N. Reishus, M. A. Duncan, " Infrared Spectroscopy of Solvation in Small Zn+(H2O)n Complexes," J. Phys. Chem. A 117, 7794−7803 (2013). DOI: 10.1021/jp4046676. A. M. Ricks, A. D. Brathwaite, M. A. Duncan, "IR Spectroscopy of V+(CO2)n Clusters: Solvation-Induced Electron Transfer and Activation of CO2," J. Phys. Chem. A 117, 11490–11498 (2013). DOI: 10.1021/jp4089035. Electronic Spectroscopy: Electronic spectroscopy probes the excited states of metal ion complexes and the threshold energies required for photodissociation, which can in turn provide metal-ligand bond energies. J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, "Photodissociation Spectroscopy and Photofragment Imaging to Probe the Dissociation Energy of the Fe+(Acetylene) Complex," J. Phys. Chem. A 127, 1244‒1251 (2023). DOI: 10.1021/acs.jpca.2c08456. J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, "Photodissociation Spectroscopy and Photofragment Imaging to Probe the Dissociation Energies of the Fe+(Benzene)1,2 Complexes," J. Phys. Chem. A 127, 2795‒2804 (2023). DOI: 10.1021/acs.jpca.3c00735. J. E. Colley, D. S. Orr, M. A. Duncan, "Charge-Transfer Spectroscopy of Ag+(Benzene) and Ag+(Toluene)," J. Phys. Chem. A 127, 4822–4831 (2023). DOI: 10.1021/acs.jpca.3c01790. 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). M. A. Duncan, "Spectroscopy of Metal Ion Complexes: Gas Phase Models for Solvation, " Ann. Rev. Phys. Chem. 48, 69 (1997) (review). Photofragment Imaging: Photofragment imaging experiments measure the excess energy in a photodissociation process that produces kinetic energy release (KER) in the fragments produced. These experiments investigate a number of metal ion-ligand complexes, especially those with benzene as the ligand. J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, "Photodissociation Spectroscopy and Photofragment Imaging to Probe the Dissociation Energy of the Fe+(Acetylene) Complex," J. Phys. Chem. A 127, 1244‒1251 (2023). DOI: 10.1021/acs.jpca.2c08456. J. E. Colley, N. J. Dynak, J. R. C. Blais, M. A. Duncan, "Photodissociation Spectroscopy and Photofragment Imaging to Probe the Dissociation Energies of the Fe+(Benzene)1,2 Complexes," J. Phys. Chem. A 127, 2795‒2804 (2023). DOI: 10.1021/acs.jpca.3c00735. B. M. Rittgers, J. H. Marks, D. J. Kellar, M. A. Duncan, "Photoinduced Charge Transfer in the Zn-Methanol Cation Studied with Selected-Ion Photofragment Imaging," J. Chem. Phys. 157, 114302 (2022). DOI: 10.1063/5.0108467. B. M. Rittgers, D. Leicht, M. A. Duncan, "Cation-π Complexes of Silver Studied with Photodissociation and Velocity-Map Imaging," J. Phys. Chem. A 124, 9166-9176 (2020). DOI: 10.1021/acs.jpca.0c08498. 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. 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 D. C. McDonald II, J. P. Wagner, A. B. McCoy, M. A. Duncan, "Near-Infrared Spectroscopy of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape," J. Phys. Chem. Lett. 9, 5664–5671 (2018). DOI: 10.1021/acs.jpclett.8b02499. J. P. Wagner, D. C. McDonald II, M. A. Duncan, "Near-Infrared Spectroscopy and Anharmonic Theory of the H2O+Ar1,2 Cation Complexes," J. Chem. Phys. 147, 104302 (2017). DOI: 10.1063/1.4998419. 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. P. Wagner, D. C. McDonald, P. R. Franke, M. A. Duncan, "Infrared Spectroscopy of the Protonated HCl Dimer and Trimer," J. Chem. Phys. 155, 134302 (2021). DOI: 10.1063/5.0065477. D. Leicht, G. E. Douberly, J. P. Wagner, D. C. McDonald, D. T. Mauney, M. Tsuge, Y.-P. Lee, M. A. Duncan, "Infrared Spectroscopy of the H+(CO)2 Ion in the Gas Phase and in para-Hydrogen Matrices," J. Chem. Phys. 153, 084305 (2020). DOI: 10.1063/5.0019731. D. C. McDonald II, B. M. Rittgers, R. A. Theis, R. C. Fortenberry,J. H. Marks, D. Leicht, M. A. Duncan, "Infrared Spectroscopy and Anharmonic Theory of H3+Ar2,3 Complexes: The Role of Symmetry in Solvation," J. Chem. Phys. 153, 134305 (2020). DOI: 10.1063/5.0023205. P. R. Franke, M. A. Duncan, G. E. Douberly, "Infrared Photodissociation Spectroscopy and Anharmonic Vibrational Study of the HO4+ Molecular Ion," J. Chem. Phys. 152, 174309 (2020). DOI: 10.1063/5.0005975. M. A. Boyer, C. S. Chiu, D. C. McDonald II, J. P. Wagner, J. E. Colley, D. S. Orr, M. A. Duncan, A. B. McCoy, "The Role of Tunneling in the Spectra of H5+ and D5+up to 7300 cm-1," J. Phys. Chem. A 124, 4427–4439 (2020). DOI: 10.1021/acs.jpca.0c02299. D. C. McDonald II, J. P. Wagner, M. A. Duncan, "Infrared Photodissociation Spectroscopy of the H6+ Cation in the Gas Phase," J. Chem. Phys. 149, 031105 (2018). (communication; selected as Feature Article) DOI: 10.1063/1.5043425. J. P. Wagner, D. C. McDonald II, M. A. Duncan, "Spectroscopy of Proton Coordination with Ethylenediamine," J. Phys. Chem. A 122, 5168−5176 (2018). DOI:10.1021/acs.jpca.8b03592. J. P. Wagner, D. C. McDonald II, M. A. Duncan, "Infrared Spectroscopy of the Astrophysically Relevant Protonated Formaldehyde Dimer," J. Phys. Chem. A 122, 192−198 (2018). DOI: 10.1021/acs.jpca.7b10573. J. P. Wagner, D. C. McDonald II, M. A. Duncan, "An Argon-Oxygen Covalent Bond in the ArOH+ Molecular Ion," Angew. Chem., Int. Ed. 57, 5081-5085 (2018). (communication) DOI: 10.1002/anie.201802093. D. C. McDonald, J. P. Wagner, M. A. Duncan, "Mid-/Near-IR Spectroscopy and Anharmonic Theory of the H2Cl+Ar Cation Complex," Chem. Phys. Lett. 691, 51-55 (2018). DOI: 10.1016/j.cplett.2017.10.056. D. T. Mauney, J. A. Maner, M. A. Duncan, "IR Spectroscopy of Protonated Acetylacetone and its Water Clusters: Enol-Keto Tautomers and Ion→Solvent Proton Transfer," J. Phys. Chem. A 121, 7059-7069 (2017). DOI: 10.1021/acs.jpca.7b07180. D. C. McDonald, D. T. Mauney, D. Leicht, J. H. Marks, J. A. Tan, J.-L. Kuo, M. A. Duncan, "Communication: Trapping a Proton in Argon: Spectroscopy and Theory of the Proton-Bound Argon Dimer and its Solvation," J. Chem. Phys. 145, 231101 (2016). DOI: 10.1063/1.4972581. 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 J. P. Wagner, S. M. Giles, M. A. Duncan, "Gas Phase Infrared Spectroscopy of the Methaniminium Cation," Chem. Phys. Lett. 726, 53-56 (2019). DOI: 10.1016/j.cplett.2019.04.032. J. P. Wagner, D. C. McDonald II, M. A. Duncan, "Mid-Infrared Spectroscopy of C7H7+ Isomers in the Gas Phase: Benzylium and Tropylium," J. Phys. Chem. Lett. 9, 4591–4595 (2018). DOI: 10.1021/acs.jpclett.8b02121. J. P. Wagner, M. A. Bartlett, W. D. Allen, M. A. Duncan, "Tunneling Isomerizations on the Potential Energy Surfaces of Formaldehyde and Methanol Radical Cations," ACS Earth Space Chem. 1, 361-367 (2017). DOI: 10.1021/acsearthspacechem.7b00068. D. T. Mauney, J. D. Mosley, L. R. Madison, A. B. McCoy, M. A. Duncan, "Infrared Spectroscopy and Theory of Formaldehyde Cation and its Hydroxymethylene Isomer," J. Chem. Phys. 145, 174303 (2016). DOI: 10.1063/1.4966214. D. Leicht, T. C. Cheng, M. A. Duncan, "Infrared Spectroscopy of the Glyoxal Radical Cation: The Charge Dependence of Internal Rotation," Chem. Phys. Lett. 643, 89–92 (2016) . DOI: 10.1016/j.cplett.2015.11.018. J. D. Mosley, J. W. Young, M. Huang, A. B. McCoy, M. A. Duncan, "Infrared Spectroscopy of the Methanol Cation and its Methylene-Oxonium Isomer," J. Chem. Phys. 142, 114301 (2015). DOI: 10.1063/1.4914146. J. D. Mosley, J. W. Young, M. A. Duncan, "Infrared Spectroscopy of the Acetyl Cation and Its Protonated Ketene Isomer," J. Chem. Phys. 141, 024306 (2014). DOI: 10.1063/1.4887074. J. D. Mosley, J. W. Young, J. Agarwal, H. F. Schaefer, III, P. v. R. Schleyer, M. A. Duncan, "Structural Isomerization of the Gas Phase 2-Norbornyl Cation Revealed with Infrared Spectroscopy and Computational Chemistry," Angew. Chem. Int. Ed. 53, 5888–5891 (2014). DOI: 10.1002/ange.201311326. M. A. Duncan, "Infrared laser spectroscopy of mass-selected carbocations," J. Phys. Chem. A 116, 11477–11491 (2012) (invited Feature Article). 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). Carbon Clusters Carbon clusters produced by laser vaporization are studied with infrared and UV-visible spectroscopy as well as with photofragment imaging. N. J. Dynak, B. M. Rittgers, J. E. Colley, D. J. Kellar, M. A. Duncan, "Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture," J. Phys. Chem. Lett. 13, 4786−4793 (2022). DOI: 10.1021/acs.jpclett.2c00950. J. E. Colley, D. S. Orr, M. A. Duncan, "Electronic Transition of l-C6+ at 417 nm," J. Chem. Phys. 157, 121102 (2022). DOI: 10.1063/5.0106183. CLUSTERS IN THE DUNCAN LAB Metal-Carbide and Oxide Cages and Nanocrystals Metal carbon clusters are produced using the same techniques originally used to produce C60 ("Buckminsterfullerene"). These species are now recognized to form metal-carbon cages (so-called "met-cars" clusters - M8C12) or metal carbon "nanocrystals" (M13C14), depending on the metals employed and the growth conditions. We produce these clusters and study their chemistry via gas phase adsorption reactions and we study their decomposition via mass-selected photodissociation. Our laboratory was the first to document the competition between met-cars cage and nanocrystal production, the first to show that certain nanocrystals could dissociatively reconstruct to form met-cars cages, and the first to show that laser excitation of large nanocrystals leads to photo-induced crystal cleavage to produce smaller nanocrystals. Exciting work has been done in collaboration with the research group of Prof. Gerard Meijer at the University of Nijmegen, The Netherlands. In their lab, we have used the Free Electron Laser for Infrared eXperiments ("FELIX") together with a metal cluster experiment. (visit the FELIX lab) Using FELIX, we have obtained the first-ever infrared spectra of gas phase metal clusters for the met-cars and nanocrystal species. The experiments use resonance-enhanced multiphoton ionization with infrared radiation (IR-REMPI) in the 400-1700 cm-1 region from the pulsed free electron laser at the F.O.M. Institute for Plasma Physics (Utrecht). Infrared spectra are obtained for Ti8C12, Ti14C13 and for many larger metal carbide nanocrystals. The nanocrystals have a strong resonance at 500 cm-1(20 microns), and these spectra are essentially unchanged with the size of the cluster. In a collaboration with astronomers, these TiC nanocrystal spectra have been identified as having the identical signature of the so-called "21 micron line" (which actually occurs at 20.1 microns) seen in the IR spectra of carbon rich stars as they burn out in the last stage of their growth. This identifies metal carbide nanocrystals in space for the first time and provides significant new insight into interstellar dust formation. Closely related to the work on metal carbide clusters are studies of other metal compound systems (metal oxides, nitrides, etc.) which also form strongly bound clusters with novel geometries, and which also have potential astrophysical significance. XMCD experiments at the BESSY light source in Berlin have investigated the magnetism of small metal clusters and how this varies with ligand coatings. This research is sponsored by the Air Force Office of Scientific Research and by the U.S. Department of Energy. Selected Publications: J. H. Marks, P. Kahn, M. Vasiliu, D. A. Dixon, M. A. Duncan, "Photodissociation and Theory to Investigate Uranium Oxide Cluster Cations," J. Phys. Chem. A 124, 1940−1953 (2020). DOI: 10.1021/acs.jpca.0c00453 J. H. Marks, T. B. Ward, M. A. Duncan, "Photodissociation of Manganese Oxide Cluster Cations," J. Phys. Chem. A 122, 3383−3390 (2018). DOI: 10.1021/acs.jpca.8b01441. S. Dillinger, M. P. Klein, A. Steiner, D. C. McDonald II, M. A. Duncan, M. M. Kappes, G. Niedner-Schatteburg, "Cryo IR Spectroscopy of N2 and H2 on Ru8+: The Effect of N2 on the H-Migration," J. Phys. Chem. Lett. 9, 914-918 (2018). DOI: 10.1021/acs.jpclett.8b00093. S. T. Akin, V. Zamudio-Bayer, K. Duanmu, G. Leistner, K. Hirsch, C. Bülow, A. Ławicki, A. Terasaki, B. von Issendorff, D. G. Truhlar, J. T. Lau, M. A. Duncan, "Size-Dependent Ligand Quenching of Ferromagnetism in Co3(Benzene)n+ Clusters studied with XMCD Spectroscopy," J. Phys. Chem. Lett. 7, 4568-4575 (2016). DOI: 10.1021/acs.jpclett.6b01839. S. T. Akin, S. G. Ard, B. E. Dye, H. F. Schaefer, M. A. Duncan, "Photodissociation of Cerium Oxide Nanocluster Cations," J. Phys. Chem. A 120, 2313-2319 (2016). DOI: 10.1021/acs.jpca.6b02052. Synthesis of Macroscopic Amounts of Metal Clusters and Metal Nanoparticles A new laser vaporization flow reactor (LVFR) has been constructed consisting of a laser ablation cluster source combined with a fast flowtube reactor for the production and isolation of ligand-coated metal clusters. The source includes high repetition rate laser vaporization with a 100 Hz KrF (248 nm) excimer laser, while cluster growth and passivation with ligands takes place in a flowtube with ligand addition via a nebulizer spray. Samples are isolated in a low temperature trap and solutions containing the clusters are analyzed with laser desorption time-of-flight mass spectrometry. Initial experiments with this apparatus have trapped Tix(ethylenediamine)y complexes which apparently have linear metal units with octahedral ligand coordination. Other experiments have produced and isolated clusters of the form TixOy(THF)z that apparently have linear metal oxide cores and larger (TiO2)x(THF)y nanoparticle species, where x=10-14 and y=5-8. The isolation of these new cluster species suggest that the LVFR instrument has considerable potential for the production of new nanocluster materials. This research is sponsored by the Air Force Office of Scientific Research. Selected Publications: M. P. Woodard, M. A. Duncan, "Laser Synthesis and Spectroscopy of Molybdenum Oxide Nanorods," J. Phys. Chem. C 123, 9560-9566 (2019). DOI: 10.1021/acs.jpcc.9b00627. M. P. Woodard, S. T. Akin, C. J. Dibble, M. A. Duncan, "Synthesis and Spectroscopy of Ligand-Coated Chromium Oxide Nanoclusters," J. Phys. Chem. A 122, 3606–3620 (2018). DOI: 10.1021/acs.jpca.8b01219. S. T. Akin, X. Liu, M. A. Duncan, "Laser Synthesis and Spectroscopy of Acetonitrile/Silver Nanoparticles," Chem. Phys. Lett. 640, 161−164 (2015). DOI:10.1016/j.cplett.2015.10.022. S. Ard, C. Dibble, S. T. Akin, M. A. Duncan, "Ligand-coated vanadium oxide nanoclusters: Capturing gas phase magic numbers in solution," J. Phys. Chem. C 115, 6438 (2011). Laser Desorption Mass Spectrometry We have built a special version of a time-of-flight mass spectrometer for analysis of involatile materials. Recent studies show that fast pulsed laser excitation can lead to volatilization and ionization of molecules with virtually no intrinsic vapor pressure. We have used this technique to analyze novel polymer films of C60 and metal colloidal "quantum dot" materials. Additional studies in our lab use Matrix Assisted Laser Desorption Ionization ("MALDI") to produce mass spectra of proteins, enzymes and polymers. We are collaborating to analyze particle sizes and extent of polymerization in materials produced in other laboratories, and we are investigating the mechanism of laser desorption/ionization in its various forms. This research is sponsored by the University of Georgia Research Foundation and the Air Force Office of Scientific Research. This is the laser desorption mass spectrometer system. A "MiniLite" YAG laser is used to desorb samples. Selected Publications: I. J. Webster, J. L. Beckham, N. D. Johnson, M. A. Duncan, "Photochemical Synthesis and Spectroscopy of Covalent PAH Dimers," J. Phys. Chem. A 126, 1144-1157 (2022). DOI: 10.1021/acs.jpca.1c10606. T. M. Ayers, S. T. Akin, C. J. Dibble, M. A. Duncan, "Laser Desorption Time-of-Flight Mass Spectrometry of Inorganic Nanoclusters: An Experiment for Physical Chemistry or Advanced Instrumentation Laboratories," J. Chem. Educ. 91, 291–296 (2014). DOI: 10.1021/ed4003942. T. C. Cheng, S. T. Akin, C. J. Dibble, S. Ard and M. A. Duncan, "Tunable infrared laser desorption ionization of fullerene films," Int. J. Mass. Spectrom. 354–355, 159 (2013). A. M. Rao, P. Zhou, K. A. Wang, G. T. Hager, J. M. Holden, Y. Wang, W. T. Lee, X. X. Bi, P. C. Eklund, D. S. Cornett, M. A. Duncan, I. J. Amster, "Photo-induced polymerization of solid C60 films," Science 259, 955 (1993).
