On the zirconium oxide neutral cluster distribution in the gas phase: Detection through 118 nm single photon, and 193 and 355 nm multiphoton, ionization

Ethylene C-H Bond Activation by Neutral Mn2O5 Clusters under Visible Light Irradiation

A photo excitation fast flow reactor coupled with a single-photon ionization (118 nm, 10.5 eV) time-of-flight mass spectrometry (TOFMS) instrument is used to investigate reactions of neutral MnmOn clusters with C2H4 under visible (532 nm) light irradiation. Association products Mn2O5(C2H4) and Mn3O6,7(C2H4) are observed without irradiation. Under light irradiation, the Mn2O5(C2H4) TOFMS feature decreases, and a new species, Mn2O5H2, is observed. This light-activated reaction suggests that the visible radiation can induce the chemistry, Mn2O5 + C2H4 + hv(532 nm) → Mn2O5*(C2H4) → Mn2O5H2 + C2H2. High barriers (0.67 and 0.59 eV) are obtained on the ground-state potential energy surface (PES); the reaction is barrierless and thermodynamically favorable on the first excited-state PES, as performed by time-dependent density functional theory calculations. The calculational and experimental results suggest that Mn2O5-like structures on manganese oxide surfaces are the appropriate active catalytic sites for visible light photocatalysis of ethylene dehydrogenation.

Experimental and theoretical studies of H2O oxidation by neutral Ti2O4,5 clusters under visible light irradiation

The Ti₂O₅ cluster has a high activity for H₂O oxidation under visible light irradiation in the gas phase.

Gas-Phase Neutral Binary Oxide Clusters: Distribution, Structure, and Reactivity toward CO

Neutral binary (vanadium-cobalt) oxide clusters are generated and detected in the gas phase for the first time. Their reactivities toward carbon monoxide (CO) are studied both experimentally and theoretically. Experimental results suggest that neutral VCoO4 can react with CO to generate VCoO3 and CO2. Density functional theory studies show parallel results as well as provide detailed reaction mechanisms.

Oxidation reactions on neutral cobalt oxide clusters: experimental and theoretical studies

Reactions of neutral cobalt oxide clusters (Co(m)O(n), m = 3-9, n = 3-13) with CO, NO, C(2)H(2), and C(2)H(4) in a fast flow reactor are investigated by time of flight mass spectrometry employing 118 nm (10.5 eV) single photon ionization. Strong cluster size dependent behavior is observed for all the oxidation reactions; the Co(3)O(4) cluster has the highest reactivity for reactions with CO and NO. Cluster reactivity is also highly correlated with either one or more following factors: cluster size, Co(iii) concentration, the number of the cobalt atoms with high oxidation states, and the presence of an oxygen molecular moiety (an O-O bond) in the Co(m)O(n) clusters. The experimental cluster observations are in good agreement with condensed phase Co(3)O(4) behavior. Density functional theory calculations at the BPW91/TZVP level are carried out to explore the geometric and electronic structures of the Co(3)O(4) cluster, reaction intermediates, transition states, as well as reaction mechanisms. CO, NO, C(2)H(2), and C(2)H(4) are predicted to be adsorbed on the Co(ii) site, and react with one of the parallel bridge oxygen atoms between two Co(iii) atoms in the Co(3)O(4) cluster. Oxidation reactions with CO, NO, and C(2)H(2) on the Co(3)O(4) cluster are estimated as thermodynamically favorable and overall barrierless processes at room temperature. The oxidation reaction with C(2)H(4) is predicted to have a very small overall barrier (<0.23 eV). The oxygen bridge between two Co(iii) sites in the Co(3)O(4) cluster is responsible for the oxidation reactions with CO, NO, C(2)H(2), and C(2)H(4). Based on the gas phase experimental and theoretical cluster studies, a catalytic cycle for these oxidation reactions on a condensed phase cobalt oxide catalyst is proposed.

Formation and Characterization of the Oxygen-Rich Hafnium Dioxygen Complexes: OHf(η2-O2)(η2-O3), Hf(η2-O2)3, and Hf(η2-O2)4

Hafnium atom oxidation by dioxygen molecules has been investigated using matrix isolation infrared absorption spectroscopy. The ground-state hafnium atom inserts into dioxygen to form primarily the previously characterized HfO(2) molecule in solid argon. Annealing allows the dioxygen molecules to diffuse and react with HfO(2) to form OHf(eta(2)-O(2))(eta(2)-O(3)), which is characterized as a side-on bonded oxo-superoxo hafnium ozonide complex. Under visible light (532 nm) irradiation, the OHf(eta(2)-O(2))(eta(2)-O(3)) complex either photochemically rearranges to a more stable Hf(eta(2)-O(2))(3) isomer, a side-on bonded di-superoxo hafnium peroxide complex, or reacts with dioxygen to form an unprecedented homoleptic tetra-superoxo hafnium complex: Hf(eta(2)-O(2))(4). The Hf(eta(2)-O(2))(4) complex is determined to possess a D(2d) geometry with a tetrahedral arrangement of four side-on bonded O(2) ligands around the hafnium atom, which thus presents an 8-fold coordination. These oxygen-rich complexes are photoreversible; that is, formation of Hf(eta(2)-O(2))(3) and Hf(eta(2)-O(2))(4) is accompanied by demise of OHf(eta(2)-O(2))(eta(2)-O(3)) under visible (532 nm) light irradiation and vice versa with UV (266 nm) light irradiation.

Photodissociation of Chromium Oxide Cluster Cations

Chromium oxide cluster cations, Cr(n)O(m)+, are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum exhibits a limited number of stoichiometries for each value of n, where m > n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic output of a Nd:YAG laser. At either wavelength, multiphoton absorption is required to dissociate these clusters, which is consistent with their expected strong bonding. Cluster dissociation occurs via elimination of molecular oxygen, or by fission processes producing stable cation species and/or eliminating stable neutrals such as CrO3, Cr(2)O(5), or Cr(4)O(10). Specific cation clusters identified to be stable because they are produced repeatedly in the decomposition of larger clusters include Cr(2)O(4)+, Cr(3)O(6)+, Cr(3)O(7)+, Cr(4)O(9)+, and Cr(4)O(10)+.

Formation, detection, and stability studies of neutral vanadium sulfide clusters

Neutral vanadium sulfide clusters are generated by the reaction of seeded hydrogen sulfide in a helium carrier gas with laser ablated vanadium metal within a supersonic nozzle. The exiting clusters are expanded into a vacuum in a molecular beam and are ionized by both ultraviolet (UV) and vacuum UV (VUV) laser radiation. The generated ions are detected by a time of flight mass spectrometer. With single photon ionization (SPI) employing VUV (118 nm) radiation, sulfur rich clusters (V(m)S(n), n>m+1) and hydrogen containing clusters (V(m)S(n)H(x), x>0) are observed. With multiphoton ionization (MPI) through nanosecond UV (193 nm) radiation, these sulfur rich and hydrogen containing clusters cannot be observed, indicating severe fragmentation generated by MPI and the importance of SPI in determining the neutral vanadium sulfide cluster distribution. With MPI through femtosecond UV (226 nm) radiation, a few sulfur rich and hydrogen containing clusters are detected, but most clusters observed by SPI are still undetected even by femtosecond MPI. Density functional theory calculations are applied to optimize energies and structures of the clusters with m=1-3 and n=0-7. The experimental results are well interpreted based on the calculations. The calculated and experimental results for vanadium sulfides are compared with those of vanadium oxides in literature.

Matrix Isolation Infrared Spectroscopic and Theoretical Study of Group IV Metal Oxide Clusters: M2O2 and M2O4

Dinuclear titanium, zirconium, and hafnium oxide clusters, M2O2 and M2O4 (M = Ti, Zr, Hf) have been prepared and characterized by matrix isolation infrared spectroscopy and quantum chemical calculations. The M2O2 clusters were formed through the reactions of metal dimers and O2 in solid argon upon sample annealing. Theoretical calculations indicate that the Ti2O2 cluster has a singlet ground state with a nonplanar cyclic C(2v) structure with a strong Ti-Ti bond, while the Zr2O2 and Hf2O2 clusters have planar cyclic structures. The M2O4 clusters were characterized to have a closed-shell singlet ground state with a nonplanar C2h symmetry, which were formed from the dimerization of the metal dioxide molecules.

Formation and distribution of neutral vanadium, niobium, and tantalum oxide clusters: Single photon ionization at 26.5eV

Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV/photon soft x-ray laser. During the ionization process the metal oxide clusters are almost free of fragmentation. The most stable neutral clusters of vanadium, niobium, and tantalum oxides are of the general form (MO2)0,1(M2O5)y. M2O5 is identified as a basic building unit for these three neutral metal oxide species. Each cluster family (Mm, m=1,...,9) displays at least one oxygen deficient and/or oxygen rich cluster stoichiometry in addition to the above most stable species. For tantalum and niobium families with even m, oxygen deficient clusters have the general formula (MO2)2(M2O5)y. For vanadium oxide clusters, oxygen deficient clusters are detected for all cluster families Vm (m=1,[ellipsis (horizontal)],9), with stable structures (VO2)x(V2O5)y. Oxygen rich metal oxide clusters with high ionization energies (IE>10.5 eV, 118 nm photon) are detected with general formulas expressed as (MO2)2 (M2O5)y O1,2,3. Oxygen rich clusters, in general, have up to three attached hydrogen atoms, such as VO3H1,2, V2O5H1,2, Nb2O5H1,2, etc.

Photodissociation of vanadium, niobium, and tantalum oxide cluster cations

Transition-metal oxide clusters of the form M(n)O(m) (+)(M=V,Nb,Ta) are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. Consistent with earlier work, cluster oxides for each value of n produce only a limited number of stoichiometries, where m>n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG (yttrium aluminum garnet) laser. All of these clusters require multiphoton conditions for dissociation, consistent with their expected strong bonding. Dissociation occurs by either elimination of oxygen or by fission, repeatedly producing clusters having the same specific stoichiometries. In oxygen elimination, vanadium species tend to lose units of O(2), whereas niobium and tantalum lose O atoms. For each metal increment n, oxygen elimination proceeds until a terminal stoichiometry is reached. Clusters having this stoichiometry do not eliminate more oxygen, but rather undergo fission, producing smaller M(n)O(m) (+) species. The smaller clusters produced as fission products represent the corresponding terminal stoichiometries for those smaller n values. The terminal stoichiometries identified are the same for V, Nb, and Ta oxide cluster cations. This behavior suggests that these clusters have stable bonding networks at their core, but additional excess oxygen at their periphery. These combined results determine that M(2)O(4) (+), M(3)O(7) (+), M(4)O(9) (+), M(5)O(12) (+), M(6)O(14) (+), and M(7)O(17) (+) have the greatest stability for V, Nb, and Ta oxide clusters.

Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.

Identification, Structure, and Spectroscopy of Neutral Vanadium Oxide Clusters

Neutral vanadium oxide clusters are studied by photoionization time-of-flight (TOF) mass spectroscopy, electronic spectroscopy, and density functional theory (DFT) calculations. Mass spectra of vanadium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Mechanisms of 118 nm single photon ionization and 193 and 355 nm multiphoton ionization/fragmentation of vanadium oxide clusters are discussed on the basis of observed mass spectral patterns and line widths of the mass spectral features. Only the 118 nm laser light can ionize vanadium oxide neutral species by single photon ionization without fragmentation. The stable vanadium oxide neutral clusters under saturated oxygen growth conditions are found to be of the form (VO2)x(V2O5)y. Structures of the first few members of this series of clusters are determined through high level DFT calculations. Fragmentation of this series of clusters through 355 and 193 nm multiphoton ionization processes is discussed in light of these calculated structures. The B(2)B2 <-- X(2)A1 transition is observed for the VO2 neutral species, and nu1 and nu2 vibrations are assigned for both electronic states. From this spectrum, the VO2 rotational and vibrational temperatures are found to be approximately 50 and approximately 700 K, respectively.

On the Titanium Oxide Neutral Cluster Distribution in the Gas Phase: Detection through 118 nm Single-Photon and 193 nm Multiphoton Ionization

Titanium oxide clusters are generated in a supersonic expansion by laser ablation of the metal and reaction with oxygen (0.1-6%) in He expansion gas. Mass spectra of the titanium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Only the 118 nm (10.5 eV) light can ionize Ti(m)O(n) neutral clusters without fragmentation. Both the 193 nm (6.4 eV) and 355 nm (3.5 eV) multiphoton ionization cause fragmentation of the neutral clusters during the ionization process and, thus, can complicate the determination of the stable neutral Ti(m)O(n) gas-phase species. Employing 118 nm single-photon ionization and line-width data, the Ti(m)O(2m) and Ti(m)O(2m+1) series are found to be the most stable neutral cluster species for high oxygen content in the expansion gas. Fragmentation during the multiphoton ionization process for 193 nm light yields the cluster ions Ti(m)O(2m-1,-2)+. These ions are formed by the loss of one or two oxygen atoms from Ti(m)O(2m,2m+1) neutral species. The dominant cluster growth process is suggested to be through the addition of TiO2 species. For low oxygen content (<2%) in the expansion gas, oxygen-deficient clusters of the form Ti(m)O(2m-1,-2) are also observed. These latter series are not fragmented by the 193 nm ionization process.