Laser Synthesis and Spectroscopy of Molybdenum Oxide Nanorods

Molybdenum oxide nanorods have been synthesized using laser vaporization, oxidation reactions, and ligand coating in a gas flow reactor, followed by capture in a cryogenic trap and transfer to solution. This method produces small ligand-coated (acetonitrile) nanorods, 100 × 20 nm in size, with excellent size uniformity that are soluble in several common solvents. The electronic and optical properties are consistent with those of other known α-MoO3 materials. Raman spectroscopy indicates these rods are semiamorphous as produced but can be annealed to form crystalline MoO3. The as-produced material is highly reactive as a catalyst (degradation of methylene blue) but becomes less active following annealing. The laser synthesis method produces smaller, more reactive molybdenum oxide nanorods than other methods

Infrared Spectroscopy of Extreme Coordination: The Carbonyls of U⁺ and UO₂⁺

Uranium and uranium dioxide carbonyl cations produced by laser vaporization are studied with mass-selected ion infrared spectroscopy in the C−O stretching region. Dissociation patterns, spectra, and quantum chemical calculations establish that the fully coordinated ions are U(CO)8+ and UO2(CO)5+, with D4d square antiprism and D5h pentagonal bipyramid structures. Back-bonding in U(CO)8+ causes a red-shifted CO stretch, but back-donation is inefficient for UO2(CO)5+, producing a blue-shifted CO stretch characteristic of nonclassical carbonyls

Theoretical Study of Nascent Solvation in Ni⁺(Benzene)_<i>m</i>, <i>m</i> = 3 and 4, Clusters

The ligand versus solvent behavior of Ni⁺(C₆H₆)_3,4 complexes was studied using density functional theory all-electron calculations. Dispersion corrections were included with the BPW91-D2 method using the 6-311++G­(2d,2p) basis set. The ground state (GS) for Ni⁺(C₆H₆)₃ has three benzene rings 3d−π bonded to the metal. A two-layer isomer with two moieties coordinated η³–η² with Ni⁺, and the other one adsorbed by van der Waals interactions to the Ni⁺(C₆H₆)₂ subcluster, i.e., a 2 + 1 structure, is within about 8.4 kJ/mol of the GS. Structures with 3 + 1 and 2 + 2 ligand coordination were found for Ni⁺(C₆H₆)₄. The binding energies (<i>D</i>₀) of 28.9 and 26.0 kJ/mol for the external moieties of Ni⁺(C₆H₆)_3,4 are much smaller than that for Ni⁺(C₆H₆)₂, 193.0 kJ/mol, obtained also with BPW91-D2. This last <i>D</i>₀ overestimates somehow the experimental value, of 146.7 ± 11.6 kJ/mol, for Ni⁺(C₆H₆)₂. The abrupt fall for <i>D</i>₀(Ni⁺(C₆H₆)_3,4) shows that such molecules are bound externally as solvent species. These results agree with the <i>D</i>₀(Ni⁺(C₆H₆)₃) < 37.1 kJ/mol limit found experimentally for this kind of two-layer clusters. The ionization energies also decrease for <i>m</i> = 2, 3, and 4 (580.8, 573.1, and 558.6 kJ/mol). For Ni⁺(C₆H₆)_3,4, each solvent moiety bridges the benzenes of Ni⁺(C₆H₆)₂; their position and that of one internal ring mimics the tilted T-shape geometry of the benzene dimer (Bz₂). The distances from the center of the external to the center of the internal rings for <i>m</i> = 3 (4.686 Å) and <i>m</i> = 4 (4.523 Å) are shorter than that for Bz₂ (4.850 Å). This and charge transfer effects promote the (C^δ−–H^δ+)_int dipole−π_ext interactions in Ni⁺(C₆H₆)_3,4; π–π interactions also occur. The predicted IR spectra, having multiplet structure in the C–H region, provide insight into the experimental spectra of these ions

Uranium Oxo and Superoxo Cations Revealed Using Infrared Spectroscopy in the Gas Phase

The UO₄⁺ and UO₆⁺ cations are produced in a supersonic molecular beam by laser vaporization and studied with infrared laser photodissociation spectroscopy using rare gas atom predissociation. The argon complexes UO₄⁺Ar₂ and UO₆⁺Ar₂ are mass-selected in a reflectron time-of-flight spectrometer and excited with an IR-OPO laser system in the range of the O–U–O and O–O stretching vibrations. These same systems are studied with computational quantum chemistry. UO₄⁺ is found to have a central UO₂ core, with an additional η² coordinated oxygen molecule. Charge transfer/oxidation gives the system the character of a UO₂²⁺, O₂^– ion pair. UO₆⁺ has this same core structure, with an additional weakly bound oxygen molecule in an η¹ coordination configuration. The O–U–O stretch is sensitive to the local environment and approximates the vibration of the isolated uranyl cation in these systems

Solvation Dynamics in Ni⁺(H₂O)<i>_n</i> Clusters Probed with Infrared Spectroscopy

Infrared photodissociation spectroscopy is reported for mass-selected Ni+(H2O)n complexes in the O−H stretching region up to cluster sizes of n = 25. These clusters fragment by the loss of one or more intact water molecules, and their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water. The first evidence for hydrogen bonding, indicated by a broad band strongly red-shifted from the free OH region, appears at the cluster size of n = 4. At larger cluster sizes, additional red-shifted structure evolves over a broader wavelength range in the hydrogen-bonding region. In the free OH region, the symmetric stretch gradually diminishes in intensity, while the asymmetric stretch develops into a closely spaced doublet near 3700 cm-1. The data indicate that essentially all of the water molecules are in a hydrogen-bonded network by the size of n = 10. However, there is no evidence for the formation of clathrate structures seen recently via IR spectroscopy of protonated water clusters

Infrared Spectroscopy of Perdeuterated Protonated Water Clusters in the Vicinity of the Clathrate Cage

We report infrared predissociation spectra of size-selected D+(D2O)n clusters in the size range n = 18−24 for comparison to previous studies of the corresponding H+(H2O)n species (Shin, J.-W.; Hammer, N. I.; Diken, E. G.; Johnson, M. A.; Walters, R. S.; Jaeger, T. D.; Duncan, M. A.; Christie, R. A.; Jordon, K. D. Science 2004, 304, 1137). For n = 18−20, two “free” OD stretch bands are observed and assigned to D2O molecules in acceptor−acceptor−donor (AAD) and acceptor−donor (AD) hydrogen bonding arrangements. Only the AAD band is observed for the n = 21 perdeuterated species. This behavior is identical to that observed previously for the corresponding H+(H2O)n clusters. Similar to the all-H protonated species, the AD “free” OD stretch band is also absent for the perdeuterated n = 22 cluster but returns for clusters larger than n = 22. Like the H+(H2O)n systems, the perdeuterated clusters have no spectral band in the lower frequency range where the signature of the hydronium cation is predicted. These observations shed new light on the intriguing spectroscopy and dynamics of large protonated water clusters

Charge-Transfer Spectroscopy of Ag⁺(Benzene) and Ag⁺(Toluene)

Gas-phase ion–molecule complexes of silver cation with benzene or toluene are produced via laser vaporization in a pulsed supersonic expansion. These ions are mass-selected and photodissociated with tunable UV–visible lasers. In both cases, photodissociation produces the organic cation as the only fragment via a metal-to-ligand charge-transfer process. The wavelength dependence of the photodissociation produces electronic spectra of the charge-transfer process. Broad structureless spectra result from excitation to the repulsive wall of the charge-transfer excited states. Additional transitions are detected correlating to the forbidden 1S → 1D silver cation-based atomic resonance and to the HOMO–LUMO excitation on the benzene or toluene ligand. Transitions to these states produce the same molecular cation photofragments produced in the charge-transfer transitions, indicating an unanticipated excited-state curve-crossing mechanism. Spectra measured for these ions are compared to those for ions tagged with argon atoms. The presence of argon causes a significant shift on the energetic positions of these electronic transitions for both Ag+(benzene) and Ag+(toluene)

Theoretical Study of Nascent Hydration in the Fe⁺(H₂O)_<i>n</i> System

The interactions of the iron monocation with water molecules and argon atoms in the gas phase were studied computationally to elucidate recent infrared vibrational spectroscopy on this system. These calculations employ first-principles all-electron methods performed with B3LYP/DZVP density functional theory. The ground state of Fe⁺(H₂O) is found to be a quartet (<i>M</i> = 2<i>S</i> + 1 = 4, <i>S</i> is the total spin). Different binding sites for the addition of one or two argon atoms produce several low-lying states of different geometry and multiplicity in a relatively small energy range for Fe⁺(H₂O)–Ar₂ and Fe⁺(H₂O)₂–Ar. In both species, quartet states are lowest in energy, and sextets and doublets lie at higher energies from the respective ground states. These results are consistent with the conclusion that the experimentally determined infrared photodissociation spectra (IRPD) of Fe⁺(H₂O)–Ar₂ and Fe⁺(H₂O)₂–Ar are complicated because of the presence of multiple isomeric structures. The estimated IR bands for the symmetric and asymmetric O–H stretches from different isomers provide new insight into the observed IRPD spectra

Infrared Spectroscopy of Solvation in Small Zn⁺(H₂O)_<i>n</i> Complexes

Singly charged zinc-water cations are produced in a pulsed supersonic expansion source using laser vaporization. Zn⁺(H₂O)_<i>n</i> (<i>n</i> = 1–4) complexes are mass selected and studied with infrared laser photodissociation spectroscopy, employing the method of argon tagging. Density functional theory (DFT) computations are used to obtain the structures and vibrational frequencies of these complexes and their isomers. Spectra in the O–H stretching region show sharp bands corresponding to the symmetric and asymmetric stretches, whose frequencies are lower than those in the isolated water molecule. Zn⁺(H₂O)_<i>n</i>Ar complexes with <i>n</i> = 1–3 have O–H stretches only in the higher frequency region, indicating direct coordination to the metal. The Zn⁺(H₂O)_2–4Ar complexes have multiple bands here, indicating the presence of multiple low energy isomers differing in the attachment position of argon. The Zn⁺(H₂O)₄Ar cluster uniquely exhibits a broad band in the hydrogen bonded stretch region, indicating the presence of a second sphere water molecule. The coordination of the Zn⁺(H₂O)_<i>n</i> complexes is therefore completed with three water molecules

Coordination and Spin States in Vanadium Carbonyl Complexes (V(CO)_<i>n</i>⁺, <i>n</i> = 1–7) Revealed with IR Spectroscopy

The vibrational spectra of vanadium carbonyl cations of the form V­(CO)_<i>n</i>⁺, where <i>n</i> = 1–7, were obtained via mass-selected infrared laser photodissociation spectroscopy in the carbonyl stretching region. The cations and their argon and neon “tagged” analogues were produced in a molecular beam via laser vaporization in a pulsed nozzle source. The relative intensities and frequency positions of the infrared bands observed provide distinctive patterns from which information on the coordination and spin states of these complexes can be obtained. Density functional theory is carried out in support of the experimental spectra. Infrared spectra obtained by experiment and predicted by theory provide evidence for a reduction in spin state as the ligand coordination number increases. The octahedral V­(CO)₆⁺ complex is the fully coordinated experimental species. A single band at 2097 cm^–1 was observed for this complex red-shifted from the free CO vibration at 2143 cm^–1