IR spectroscopy of gas phase V(CO2)n+ clusters: solvation-induced electron transfer and activation of CO2.

Ion-molecule complexes of vanadium and CO2, i.e., V(CO2)n(+), produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (n < 7) are consistent with CO2 ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO2 molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (n ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V(CO2)n(+) complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a solvation-induced metal→ligand electron transfer reaction.

Coordination and spin states in vanadium carbonyl complexes (V(CO)n+, n = 1-7) revealed with IR spectroscopy.

The vibrational spectra of vanadium carbonyl cations of the form V(CO)(n)(+), where n = 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).

Mid- and Far-IR Spectra of H5(+) and D5(+) Compared to the Predictions of Anharmonic Theory.

H5(+) is the smallest proton-bound dimer. As such, its potential energy surface and spectroscopy are highly complex, with extreme anharmonicity and vibrational state mixing; this system provides an important benchmark for modern theoretical methods. Unfortunately, previous measurements covered only the higher-frequency region of the infrared spectrum. Here, spectra for H5(+) and D5(+) are extended to the mid- and far-IR, where the fundamental of the proton stretch and its combinations with other low-frequency vibrations are expected. Ions in a supersonic molecular beam are mass-selected and studied with multiple-photon dissociation spectroscopy using the FELIX free electron laser. A transition at 379 cm(-1) is assigned tentatively to the fundamental of the proton stretch of H5(+), and bands throughout the 300-2200 cm(-1) region are assigned to combinations of this mode with bending and torsional vibrations. Coupled vibrational calculations, using ab initio potential and dipole moment surfaces, account for the highly anharmonic nature of these complexes.

Covalent attachment of a rhenium bipyridyl CO2 reduction catalyst to Rutile TiO2.

We have characterized the covalent binding of the CO(2) reduction electrocatalyst ReC0A (Re(CO)(3)Cl(dcbpy) (dcbpy =4,4'-dicarboxy-2,2'-bipyridine)) to the TiO(2) rutile (001) surface. The analysis based on sum frequency generation (SFG) spectroscopy and density functional theory (DFT) calculations indicates that ReC0A binds to TiO(2) through the carboxylate groups in bidentate or tridentate linkage motifs. The adsorbed complex has the dcbpy moiety nearly perpendicular to the TiO(2) surface and the Re exposed to the solution in a configuration suitable for catalysis.

Infrared spectroscopy of extreme coordination: the carbonyls of U(+) and UO(2)(+).

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 UO(2)(CO)(5)(+), with D(4d) square antiprism and D(5h) pentagonal bipyramid structures. Back-bonding in U(CO)(8)(+) causes a red-shifted CO stretch, but back-donation is inefficient for UO(2)(CO)(5)(+), producing a blue-shifted CO stretch characteristic of nonclassical carbonyls.

Communications: Infrared spectroscopy of gas phase C3H3+ ions: the cyclopropenyl and propargyl cations.

C(3)H(3)(+) ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800-4000 cm(-1) region using the rare gas tagging method. Vibrational bands in the C-H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch (nu(5)) at 1293 cm(-1) and the asymmetric C-H stretch (nu(4)) at 3182 cm(-1), whereas propargyl has the CH(2) scissors (nu(4)) at 1445, the C-C triple bond stretch (nu(3)) at 2077 and three C-H stretches (nu(2), nu(9), and nu(1)) at 3004, 3093, and 3238 cm(-1). Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection.

Infrared spectroscopy of perdeuterated protonated water clusters in the vicinity of the clathrate cage.

We report infrared predissociation spectra of size-selected D(+)(D(2)O)(n) clusters in the size range n = 18-24 for comparison to previous studies of the corresponding H(+)(H(2)O)(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 D(2)O 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(+)(H(2)O)(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(+)(H(2)O)(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.

Seven-coordinate homoleptic metal carbonyls in the gas phase.

Gas-phase metal carbonyl cations of the vanadium-group metals (V(+), Nb(+), Ta(+)) were produced in a molecular beam by laser vaporization and then mass-analyzed and size-selected in a time-of-flight spectrometer and studied with IR laser photodissociation spectroscopy in the carbonyl-stretching region. The abundances in the mass spectra, the fragmentation patterns, and the IR spectra provided a combined approach that revealed the coordination numbers in these systems. Although seven-coordinate structures would have 18 electrons in each case, V(CO)(6)(+) was found to be formed rather than V(CO)(7)(+). Nb(+) formed both six- and seven-coordinate species, while Ta(+) formed only the Ta(CO)(7)(+) complex. Density functional theory computations were used to predict the IR spectra for these systems, which are dramatically different for the six- and seven-coordinate structures and in excellent agreement with the measurements. V(CO)(6)(+) and Nb(CO)(6)(+) have structures slightly distorted from octahedral, while Nb(CO)(7)(+) and Ta(CO)(7)(+) have C(3v) capped octahedral structures.

Infrared spectroscopy of gas phase C3H5+ : the allyl and 2-propenyl cations.

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800-3500 cm(-1) region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C(2nu) charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a C(s) symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group.

The structure of protonated acetone and its dimer: infrared photodissociation spectroscopy from 800 to 4,000 cm-1.

The infrared spectra of protonated acetone and the proton bound acetone dimer are obtained revealing vibrational resonances associated with the shared proton motions, which are in agreement with the predictions from ab initio, MP2, harmonic frequency calculations.