Reactions of Laser-Ablated Y and La Atoms, Cations and Electrons with O2. Infrared Spectra and Density Functional Calculations of the MO, MO+, MO2, MO2+, and MO2- Species in Solid Argon

Threshold Photoionization and Density Functional Theory Investigations of Small Lanthanum Monoxide Clusters, LanO (n = 2-10)

Small lanthanum monoxide clusters, LanO (n = 2-10), have been studied by laser threshold photoionization (PI) spectroscopy and density functional theory (DFT). Ionization energies (IEs) gradually decrease with cluster size with oscillatory behavior at certain sizes at n = 4, 7, and 9, and the IEs are slightly lower than the corresponding bare Lan clusters. Mulliken charge analysis of the neutral and cation clusters indicates that the threshold PI occurs from a delocalized metal-based 5d orbital. The stable geometric structures and IEs for the LanO (n = 2-8) clusters have been determined from DFT. Zero electron kinetic energy and PI spectra have been calculated based on DFT calculations and compared with the experimental PI spectra. On the basis of favorable agreement between the simulated and experimental PI spectra, reliable ground-state geometric structures have been assigned. The O atom is negatively charged, equivalent to one electronic charge mainly transferred from La atoms bonded to it. Chemical bonding between the La and O atoms is ionic, whereas La atoms are mostly covalently bonded. No direct correlation is found between the electronic properties like the IE, electron affinity, and HOMO-LUMO gap and the stability of these clusters.

Anion Photoelectron Spectroscopy and Quantum Chemical Calculations of Bimetallic Oxide Clusters YCu2On-/0 (n = 2-5)

The structural and electronic properties of bimetallic oxide clusters, YCu₂O_n^- and YCu₂O_n (n = 2-5), are investigated using anion photoelectron spectroscopy and density functional theory calculations. The experimental vertical detachment energies of YCu₂O₂^-, YCu₂O₃^-, YCu₂O₄^-, and YCu₂O₅^- were measured to be 1.59, 1.76, 3.85, and 3.78 eV, respectively. Vibrationally resolved photoelectron spectra have been obtained for YCu₂O₂^-, with a spacing of 726 ± 80 cm^-1 assigned to the Y-O stretching vibrational mode. It is found that YCu₂O₂^- and YCu₂O₂ have C_2v symmetric planar five-membered ring structures. YCu₂O₃^- and YCu₂O₃ have C_2v symmetric planar six-membered ring structures. The most stable structure of YCu₂O₄^- is a quasi-planar structure which can be viewed as one O atom interacting with the Y atom of the YCu₂O₃ six-membered ring, while the most stable structure of YCu₂O₄ is a planar seven-membered ring. YCu₂O₅^- and YCu₂O₅ have nonplanar structures, which can be viewed as an O₂ unit interacting with the Y atom of the YCu₂O₃ six-membered ring. In YCu₂O_3,4,5^-/0, the Y-O and Cu-O bonds are dominant, while the Y-Cu and Cu-Cu interactions are weak.

The decisive role of 4f-covalency in the structural direction and oxidation state of XPrO compounds (X: group 13 to 17 elements)

Lanthanide oxo compounds are of vital importance in lanthanide chemistry, as well as in environmental and materials sciences. Praseodymium, as an exceptional element in lanthanides which can form a +V formal oxidation state (OSf) besides the dominant +III among the 4f-block element, displays the significant participation of the Pr 4f orbitals in bonding interactions which is commonly crucial in stabilizing the high oxidation state of Pr in PrO2+ and NPrO species. Here, we report a systematic theoretical study on the structures and stabilities of a series of XPrO (X: B, Al, C, Si, N, P, As, O, S, F, Cl) compounds along with [XPrO]+ cation (X: O, S) and [X3PrO] complexes (X: F and Cl). This work reveals that Pr is able to achieve the lowest and highest OSf and the OSf exhibits periodic variation from +I in BOPr and AlOPr to +II in SiOPr to +III in CPrO, FPrO, ClPrO and AsPrO to +IV in OPrO and SPrO and even to +V in NPrO, [OPrO]+, [SPrO]+, F3PrO and Cl3PrO. We found that the molecular structures are correlated to the Pr oxidation state due to the highly important 4f orbital in the chemical bonding of the high oxidation state compounds. Thus, not only the electronegativity of the ligand but also the quasi-degenerate Pr valence 4f orbitals, namely energetic covalency, control the oxidation state and play a fundamental role in affecting the electronic structural stability of Pr(v) compounds as well. This work demonstrates the structurally directing role of the f-orbital in the formation of the linear structure and is constructive for achieving the higher oxidation state of a given element by tuning the ligand.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Gadolinium cation (Gd+) reaction with O2: Potential energy surface mapped experimentally and with theory

Guided ion beam tandem mass spectrometry is used to measure the kinetic energy dependent cross sections for reactions of the lanthanide metal gadolinium cation (Gd⁺) and GdO⁺ with O₂ and for collision-induced dissociation (CID) of GdO₂⁺ with Xe. Gd⁺ reacts with O₂ in an exothermic and barrierless reaction to form GdO⁺ and O. GdO₂⁺ is also formed in this reaction, but this product ion is formed in a sequential reaction, as verified by pressure dependent measurements and comparison with the results for the reaction of GdO⁺ with O₂. The CID experiments of GdO₂⁺ indicate the presence of two GdO₂⁺ precursor ion populations, assigned to a weakly bound oxygen molecule adduct (Gd⁺-O₂) and an inserted cyclic Gd⁺ dioxide species (O-Gd⁺-O). Analysis of the resulting product ion cross sections yields bond dissociation energies (BDEs, D₀) for Gd⁺-O₂ and OGd⁺-O, where the latter BDE is also independently measured in an exchange reaction between GdO⁺ and O₂. The CID experiments also provide the energy of the barrier for the rearrangement of the Gd⁺-O₂ adduct to the inserted O-Gd⁺-O structure (as identified by loss of a single oxygen atom). The thermochemistry measured here yields D₀(OGd⁺-O) = 2.86 ± 0.08 eV, D₀(Gd⁺-O₂) = 0.75 ± 0.11 eV, and a barrier height relative to Gd⁺-O₂ of 0.31 ± 0.07 eV. These data are sufficient to characterize in some detail the potential energy surface of the Gd⁺ reaction with O₂ entirely from experiment. Theoretical calculations are performed for comparison with the experimental energetics and for further insight into the reaction mechanisms.

DFT calculations on polarizabilities and hyperpolarizabilities for the neutral and anionic yttrium oxide clusters

The electronic properties, polarizabilities, first and second hyperpolarizabilities of YOn clusters of [Formula: see text]–12 were studied using the quantum chemical method. The vertical ionization potential (VIP) values for the anionic clusters increase monotonically with the cluster size. Among the neutral clusters YO3 and YO8 have the least chemical hardness values, where in anionic clusters with size [Formula: see text] possesses the least chemical hardness. Anionic clusters have more electrons attracting tendency than the neutral clusters. The computed static mean polarizability of neutral yttrium oxides has positive values but is close to zero. The incorporation of oxygen atom quenches the polarizability of yttrium. The computed polarizability anisotropy of neutral clusters shows an oscillatory effect both at static and at dynamic conditions. The first hyperpolarizability for many YOn clusters are close to zero. The existence of high symmetry in these clusters reduces the first hyperpolarizability values which was supported by the small dipole moments. The computed [Formula: see text] values for the static neutral and anionic clusters show only a small variation. The decrease in the polarizability and second hyperpolarizability with size can be interpreted in terms of the electronic delocalization and chemical bonding in the clusters.

Structural evolution, sequential oxidation and chemical bonding in tri-yttrium oxide clusters: Y3Ox− and Y3Ox (x = 0–6)

The evolution of regularities for Y₃O_x^−/0 (x = 0–6) and all-metal aromaticity of the Y₃⁻ cluster have been discovered.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.

Infrared Spectroscopic and Density Functional Theory Study on the Reactions of Lanthanum Atoms with Carbon Dioxide in Rare-Gas Matrices

Reactions of laser-ablated La atoms with CO2 molecules in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. On the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments, absorptions at 1839.9 and 753.6 cm-1 in argon and 1855.9 and 771.3 cm-1 in neon are assigned to the C-O and La-O stretching vibrations of the OLaCO molecule, respectively. Ultraviolet-visible photoinduced isomerization of OLaCO to La-(eta2-OC)O and OLa-(eta2-CO) have been observed under different wavelength photolyses in the solid matrix. The neon matrix experiments give the C-O and La-O stretching vibrations of the OLaCO- anion at 1769.5 and 779.3 cm-1, respectively. Density functional theory calculations have been performed on these products, which support the experimental assignments of the infrared spectra. The present study reveals that the C-O stretching vibrational frequencies of OMCO decrease from Sc to La, which indicates an increase in metal d orbital --> CO pi* back-donation in this series.

Infrared Spectra of the CH3−MX and CH2−MHX Complexes Formed by Reactions of Laser-Ablated Group 3 Metal Atoms with Methyl Halides

Reactions of laser-ablated group 3 metal atoms with methyl halides have been carried out in excess of Ar during condensation and the matrix infrared spectra studied. The metals are as effective as other early transition metals in providing insertion products (CH3-MX) and higher oxidation state methylidene complexes (CH2-MHX) (X = F, Cl, Br) following alpha-hydrogen migration. Unlike the cases of the group 4-6 metals, the calculated methylidene complex structures show little evidence for agostic distortion, consistent with the previously studied group 3 metal methylidene hydrides, and the C-M bond lengths of the insertion and methylidene complexes are comparable to each other. However, the C-Sc bond lengths are 0.013, 0.025, and 0.029 A shorter for the CH2-ScHX complexes, respectively, and the spin densities are consistent with weak C(2p)-Sc(3d) pi bonding. The present results reconfirm that the number of valence electrons on the metal is important for agostic interaction in simple methylidene complexes.

Reactions of Group 3 Transition Metal Atoms with CS2 and OCS: Matrix Isolation Infrared Spectra and Density-Functional Calculations of SMCS, SM-(η2-CS), SMCO, and SM-(η2-CO) in Solid Argon

Laser-ablated scandium, yttrium, and lanthanum atoms were reacted with CS2 and OCS molecules in an argon matrix. Products of the type SMCX and S-M(eta2-CX) (X = S or O) were formed on sample deposition. Photolysis favored the S-M(eta(2)-CX) complex, while annealing increased the more stable SMCX isomer. Product absorptions are identified by density-functional frequency calculations and isotopic substitutions. This work reports the first vibrational spectroscopic characterization of Sc, Y, and La reaction products with CS2 and OCS and the subsequent interconversion between SMCX and S-M(eta2-CX) structural isomers.

Infrared Spectra and Density Functional Calculations for M(OH)2,3 and HOMO Molecules and M(OH)2+ Cations (M = Y, La)

Reactions of laser-ablated Y and La atoms with H2O2 gives the M(OH)2 and M(OH)3 molecules and the HOMO dehydration product, and the cation M(OH)2+ in solid argon. Density functional calculations show that the dihydroxide molecules and cations are bent at the metal center, and the symmetric and antisymmetric O-H stretching modes are both observed in the infrared spectra. The trihydroxide molecules have calculated C(3h) structures characterized by strong antisymmetric O-H and M-O stretching modes. Mulliken charges increase for all product molecules going down the Group 3 family and increase as one, two, and three OH ligands are bonded to the metal center. Evidence is also presented for the Y(OH)4- anion.

Coordination of ScO+ and YO+ by Multiple Ar, Kr, and Xe Atoms in Noble Gas Matrixes: A Matrix Isolation Infrared Spectroscopic and Theoretical Study

The combination of matrix isolation infrared spectroscopic and quantum chemical calculation results provide strong evidence that scandium and yttrium monoxide cations, ScO+ and YO+, coordinate multiple noble gas atoms in forming noble gas complexes. The results showed that ScO+ coordinates five Ar, Kr, or Xe atoms, and YO+ coordinates six Ar or Kr and five Xe atoms in solid noble gas matrixes. Hence, the ScO+ and YO+ cations trapped in solid noble gas matrixes should be regarded as the [ScO(Ng)5]+ (Ng = Ar, Kr, or Xe), [YO(Ng)6]+ (Ng = Ar or Kr) or [YO(Xe)5]+ complexes. Experiments with dilute krypton or xenon in argon or krypton in xenon produced new IR bands, which are due to the stepwise formation of the [ScO(Ar)(5-n)(Kr)n]+, [ScO(Kr)(5-n)(Xe)n]+ (n = 1-5), [YO(Ar)(6-n)(Kr)n]+ (n = 1-6), and [YO(Ar)(6-n)(Xe)n]+ (n = 1-4) complexes.

Persistent Photo-Reversible Transition-Metal Methylidene System Generated from Reaction of Methyl Fluoride with Laser-Ablated Zirconium Atoms and Isolated in a Solid Argon Matrix

A photoreversible transition-metal methylidene system has been formed for the first time by reaction of methyl fluoride and laser-ablated Zr atoms, isolated in solid argon, and investigated by means of infrared spectroscopy. Four different groups of absorptions are characterized on the basis of behaviors upon broad-band irradiation and sample annealing. Growth of Group I is accompanied by demise of Group II on irradiation with visible light (lambda > 530 nm) and vice versa with UV light (240 < lambda < 380 nm). The methylidene complex CH(2)=ZrHF is responsible for Groups I and II either in different singlet-triplet spin states or argon matrix packing configurations. The ground singlet state is stabilized by an agostic interaction. On the other hand, Group III, which arises from the Grignard type compound CH(3)-ZrF, disappears upon irradiation of UV light (lambda > 380 nm), increasing the concentration of CH(2)=ZrHF by alpha-H elimination. Fragments of methyl fluoride such as the CH(2)F radical comprise Group IV. Theoretical calculations are carried out for the alkylidene complex and other plausible products, and the results are compared with the experimental frequencies.

Homoleptic tetrahydrometalate anions MH(4)(-) (M = Sc, Y, La). Matrix infrared spectra and DFT calculations

Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH(2) and (H(2))MH(2). The homoleptic tetrahydrometalate anions ScH(4)(-), YH(4)(-), and LaH(4)(-) are formed by electron capture and identified by isotopic substitution (D(2), HD, and H(2) + D(2) mixtures). Doping with CCl(4) to serve as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The observed vibrational frequencies are in agreement with the results of density functional theory calculations, which predict electron affinities in the 2.8-2.4 eV range for the (H(2))ScH(2), (H(2))YH(2), and (H(2))LaH(2) complexes, and indicate high stability for the MH(4)(-) (M = Sc, La, Y) anions and suggest the promise of synthesis on a larger scale for use as reducing agents.