Formation and photodepletion of cluster ion-messenger atom complexes in a cold ion trap: Infrared spectroscopy of VO+, VO2+, and VO3

A novel experimental technique is described in which radiation from a free electron laser is used to measure infrared spectra of gas-phase cluster ions via vibrational predissociation of the corresponding ion-messenger atom complexes. The weakly bound complexes are formed in a temperature-controllable, radio frequency ion trap. This technique is applied to the study of the vibrational spectroscopy of the monovanadium oxide cluster cations VO+, VO2+, and VO3+. (C) 2003 American Institute of Physics

Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4+(NH3)(1-4)

The gas phase vibrational spectroscopy of the protonated ammonia dimer N2H7+, a prototypical system for strong hydrogen bonding, is studied in the spectral region from 330 to 1650 cm(-1) by combining infrared multiple photon dissociation and multidimensional quantum mechanical simulations. The fundamental transition of the antisymmetric proton stretching vibration is observed at 374 cm(-1) and assigned on the basis of a six-dimensional model Hamiltonian, which predicts this transition at 471 cm(-1). Photodissociation spectra of the larger protonated ammonia clusters NH4+(NH3)(n) with n=2-4 are also reported for the range from 1050 to 1575 cm(-1). The main absorption features can be assigned within the harmonic approximation, supporting earlier evidence that hydrogen bonding in these clusters is considerably weaker than for n=1

Gas-phase infrared spectrum of the protonated water dimer

The protonated water dimer is a prototypical system for the study of proton transfer in aqueous solution. We report infrared photodissociation spectra of cooled H+(H2O)(2) [and D+(D2O2] ions, measured between 620 and 1900 wave numbers (cm(-1)). The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600 cm(-1) and one at 1740 cm(-1) (for H5O2+). From a comparison to multidimensional quantum calculations, the three lower energy bands were assigned to stretching and bending fundamentals involving the O...H+...O moiety, and the highest energy band was assigned to a terminal water bend. These results highlight the importance of intermode coupling in shared proton systems

Kinetic study of the reaction of vanadium and vanadium-titanium oxide cluster anions with SO2

The reactivity of mass-selected V4O10- cluster anions towards sulphur dioxide is investigated in an ion trap under multi-collision conditions. Gas phase reaction kinetics are studied as a function of temperature (T-R = 150-275 K). The binding energy of SO2 to V4O10- is obtained by analyzing the experimental low pressure rate constants, employing the Lindemann energy transfer model for association reactions in conjunction with statistical RRKM theory. In addition, infrared multiple photon dissociation spectroscopy is used in conjunction with density functional theory for the structural assignment of the [V4O10-, SO2] complex, revealing a square pyramidal structure with the SO2 molecule incorporated in the vanadium oxide framework. Energy profiles are calculated for the reaction between V4O10- and V6O15- with SO2. Whereas the transition structures along the reaction pathway of V4O10- with SO2 have energies below those of the separated partners, the reaction of V6O15- with SO2 proceeds via a transition structure with energy higher than the educts. The role of cluster size and composition is investigated by studying the reaction kinetics of larger (V6O15- and V8O20-) and titanium doped (V3TiO10- and V2Ti2O10-) vanadium oxide clusters with SO2. The observed cluster size and composition dependencies are discussed

Isomorphous substitution in bimetallic oxide clusters

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V2O5)(n-1)(VTiO5)(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V2O5)(n) clusters. Upon systematic exchange of V by Ti atoms in V4-nTinO10- (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti4O10-

Vibrational spectroscopy of hydrated electron clusters (H2O)(15-50)(-) via infrared multiple photon dissociation

Infrared multiple photon dissociation spectra for size-selected water cluster anions (H2O)(n)(-), n=15-50, are presented covering the frequency range of 560-1820 cm(-1). The cluster ions are trapped and cooled by collisions with ambient He gas at 20 K, with the goal of defining the cluster temperature better than in previous investigations of these species. Signal is seen in two frequency regions centered around 700 and 1500-1650 cm(-1), corresponding to water librational and bending motions, respectively. The bending feature associated with a double-acceptor water molecule binding to the excess electron is clearly seen up to n=35, but above n=25; this feature begins to blueshift and broadens, suggesting a more delocalized electron binding motif for the larger clusters in which the excess electron interacts with multiple water molecules. (C) 2007 American Institute of Physics

Gas phase vibrational spectroscopy of mass-selected vanadium oxide anions

The vibrational spectra of vanadium oxide anions ranging from V(2)O(6)(-) to V(8)O(20)(-) are studied in the region from 555 to 1670 cm(-1) by infrared multiple photon photodissociation (IRMPD) spectroscopy. The cluster structures are assigned and structural trends identified by comparison of the experimental IRMPD spectra with simulated linear IR absorption spectra derived from density functional calculations, aided by energy calculations at higher levels of theory. Overall, the IR absorption of the V(m)O(n)(-) clusters can be grouped in three spectral regions. The transitions of (i) superoxo, (ii) vanadyl and (iii) V-O-V and V-O(center dot) single bond modes are found at similar to 1100 cm(-1), 1020 to 870 cm(-1), and 950 to 580 cm(-1), respectively. A structural transition from open structures, including at least one vanadium atom forming two vanadyl bonds, to caged structures, with only one vanadyl bond per vanadium atom, is observed in-between tri- and tetravanadium oxide anions. Both the closed shell (V(2)O(5))(2,3)VO(3)(-) and open shell (V(2)O(5))(2-4)(-) anions prefer cage-like structures. The (V(2)O(5))(3,4)(-) anions have symmetry-broken minimum energy structures (C(s)) connected by low-energy transition structures of C(2v) symmetry. These double well potentials for V-O-V modes lead to IR transitions substantially red-shifted from their harmonic values. For the oxygen rich clusters, the IRMPD spectra prove the presence of a superoxo group in V(2)O(7)(-), but the absence of the expected peroxo group in V(4)O(11)(-). For V(4)O(11)(-), use of a genetic algorithm was necessary for finding a non-intuitive energy minimum structure with sufficient agreement between experiment and theory

Identification of Conical Structures in Small Aluminum Oxide Clusters: Infrared Spectroscopy of (Al2O3)(1-4)(AlO)(+)

The vibrational spectroscopy of the electronically closed-shell (Al2O3)(n)(AlO)(+) cations with n = 1-4 is studied in the 530-1200 cm(-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C-3v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions