Transition metal-doped magnesium hydride solids are leading candidates as hydrogen storage materials. Here, a double-hybrid density functional theory method is used for the first time to explore the ground state geometries and electronic properties of small MgScHn and MgTiHn (n = 1–18) clusters. It is determined that hydrogen atoms aggregate to the metal core of the cluster up to a saturation limit of MgScH13 and MgTiH14 for each transition metal. Additional hydrogen atoms exist as weakly interacting dissociated H2 molecules. These saturated clusters containing scandium and titanium contain a large hydrogen mass percent of 15.9% and 16.4%, respectively. A detailed discussion of cluster growth mechanisms, hydrogen dissociation pathways, the effect of each different transition metal, and the cluster stabilities is presented as determined at the DSDPBEP86/6-311++G(3df,3pd) level of theory.

The structures of silicon clusters doped with two palladium atoms have been explored using an unbiased global optimization technique, and the candidate structures further analyzed using density functional theory. The optimized geometries, stabilities, and electronic structures of Si1Pd2 – Si17Pd2 are reported, and several new lowest energy isomers are located for the first time impacting the relative cluster stabilities. Based on the relative energies, Si5Pd2, Si10Pd2, and Si16Pd2 are predicted to be the most stable clusters for the sizes considered here. A growth pattern in this size range is noted that primarily consists of trigonal and pentagonal structural motifs, the latter dominating for larger clusters once Pd atoms become encompassed in the silicon cluster framework. Palladium atoms bind exohedrally to the base cluster for smaller sized clusters, but begin to bind endohedrally once the cluster size reaches Si11Pd2 (for the first Pd atom) and Si16Pd2 (for the second Pd atom). Detailed analysis of the cluster's electronic structure indicate that palladium atoms have only a small partial charge, and that the frontier molecular orbitals consist primarily of palladium d and silicon p atomic orbitals.

Magnesium hydride solids doped with transition metals have received attention recently as prospective hydrogen storage materials for a green energy source and a hydrogen economy. In this study, MgTiHn (n = 1–20) clusters were investigated for the first time by employing the B3PW91 hybrid density functional theory computational chemistry technique with all electron basis sets to determine precise cluster structures and the maximum hydrogen capacity for this model system. We find that hydrogen atoms bind to the metal cluster core until a MgTiH14 saturation limit is reached, with hydrogen dissociation from this system occurring for MgTiH15 and larger cluster sizes. This MgTiH14 cluster contains a large 16.4% hydrogen by mass. This saturation size limit and hydrogen mass percent is larger than the analogous MgScHn system previously reported. The clusters relative stabilities and electronic properties are discussed along with a possible novel hydrogen dissociation pathway. MgTiH10 and MgTiH13 clusters are predicted to be especially stable species in this size range.

The geometrical structures of the lowest energy states of anionic niobium-doped hydrogen clusters are determined based on Crystal structure AnaLYsis by the Particle Swarm Optimization (CALYPSO) isomer search method. The geometries and relative stabilities are determined by the B3LYP hybrid density functional theory (DFT) method. The calculated results show that a niobium atom can absorb up to twelve hydrogen atoms in the anionic clusters. In exploring the stability, electronic properties, and photoelectron spectra of NbH n − clusters, the NbH 12 − cluster is found to possesses a relatively high binding energy, high second-order differential energy, and a large HOMO–LUMO gap. The hydrogen storage density of the NbH 12 − cluster is calculated as 11.5 wt%. The present findings enrich the database of transition metal doped hydrogen clusters and the insights are important in the pursuit of efficient hydrogen energy storage materials.

Strongly bound atomic clusters are often used as models for the active sites in heterogeneous catalysis and materials chemistry. Building up from a single atom to bulk material progresses through the cluster size region, where individual cluster structures and electronic properties can drastically change with the addition of a single atom. In this chapter, we describe our strategy for addressing novel questions in this research field at a primarily undergraduate institution (PUI). Starting with our experimental endeavors, we depict collaborative gas-phase infrared multiple photon dissociation spectroscopic measurement on silicon, transition metal, and metal-doped silicon clusters. We also detail our recent advances with matrix isolation infrared investigations to explore reactivity locally. Computational chemistry techniques, including global optimization schemes, employed in our laboratory to predict the geometry, internal bonding, and reactivity of atomic clusters are explained. After these methods are covered, we describe in the subsequent sections results related to cluster structure determination first, then reactivity, and lastly commenting on cluster growth patterns and electronic properties. Safety considerations that are associated with these studies are provided and a section discusses student recruitment, training, and success. Lastly, we describe throughout the chapter unique challenges and opportunities that exist at primarily undergraduate institutions for performing research in cluster science and building upon student’s formal education during these endeavors at a PUI.

Niobium hydrides are attractive superconductors. Exploring the formation process of niobium hydrides is essential to elucidate the mechanism of superconductivity. One of the key issues is to clarify the atomic stacking patterns of Nb and H atoms, i.e., the structural evolution of Nb-H clusters. Here, the low-energy structural isomers of NbHn (n = 2-15) clusters are determined using the CALYPSO method combined with density functional theory calculations. Geometries were fully optimized at the B3LYP/LANL2DZ/6-311++G(d) level of theory to determine global minimum structures for each size. The results indicate that NbH13 is the most stable cluster in this size range. The 4d atomic orbital of Nb and the hydrogen 1s atomic orbital participate largely to the internal binding of the NbH13 cluster. They hydrogen storage density and adsorption energy of this cluster are calculated to be 12.4 wt% and 2.58 eV, respectively. The high hydrogen storage density, suitable hydrogen adsorption energy, and high stability of NbH13 shows promise as a hydrogen storage material. These results provide fundamental information for further design of metal hydrogen storage materials. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Transition metal doped magnesium hydride solids are a leading candidate for hydrogen storage materials. In this investigation, MgScHn clusters (n = 1-20) are theoretically studied using density functional theory and Moller-Plesset perturbation theory. It is determined that hydrogen binds successively to the MgSc diatomic metal center up to MgScH13 when the cluster becomes saturated at 15.9% hydrogen by mass. In contrast to earlier predictions, we shown that for MgScH14 and larger clusters molecular hydrogen dissociates from the core cluster structure. A local minimum is observed on the potential energy surface for larger clusters where dissociated hydrogen interacts with a negatively charged hydride of the core cluster in a dipole-induced dipole intermolecular force, providing insight into the dissociation pathway in bulk magnesium hydride materials doped with transition metals. Analysis of the frontier orbitals and natural bonding analysis of these clusters support this logical dissociation pathway. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

We predict the structures of neutral gas-phase gold clusters ($Au_n$, $n$ = 5$-$13) at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral $Au_5$$-$$Au_{13}$ clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and free-energy calculations. The critical gold cluster size where the most stable isomer changes from planar to nonplanar is $Au_{11}$ (capped-trigonal prism, $D_{3h}$) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for $Au_8$, $Au_9$, and $Au_{10}$ clusters. Density-functional theory exchange-correlation functionals within the generalized gradient or hybrid approximation must be corrected for long-range van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our work gives insight into the stable structures of gas-phase gold clusters by highlighting the impact of temperature, and therefore the importance of free-energy over total energy studies, and long-range van der Waals interactions on gold cluster stability.

The geometric structures and properties of mixed silicon/gold clusters (Si n Au2; n = 1–10) are theoretically studied using density functional theory and coupled cluster calculations. Several new low lying isomers are found for most cluster sizes. New lowest energy isomers are found for Si7Au2, Si9Au2, and Si10Au2. Si1Au2 and Si3Au2 are predicted to be planar structures, whereas other sizes are three dimensional. Energetically, Si5Au2 and Si8Au2 are predicted to be more stable clusters compared to other sizes investigated here. For all lowest energy structures, gold atoms are bound exohedrally on the cluster and the frontier orbitals often show interactions between silicon p and gold d atomic orbitals. Only for the largest sized cluster, Si10Au2, does the cluster take the same structure of the pure Si10 cluster with Au atoms capping it. For other cluster sizes, at least one Au atom makes up part of the central structural motif. Natural bond order analysis is performed to further explore the bonding in each cluster.

The structure of cobalt-doped silicon clusters, SinCo+ (n = 5-8) and SinCo2+ (n = 8-12), is investigated in a combined infrared multiple photon dissociation spectroscopy and density functional theory study. The singly doped clusters have exohedral structures in which the Co atom substitutes an atom of bare Si-n+1(+) dusters. In the doubly doped SinCo2+ clusters, the second Co atom is adsorbed to the singly doped counterparts and, for n >= 9, one of the Co atoms is encapsulated by a silicon cage. Computational analysis of the electronic and magnetic properties of the identified isomers indicates a distance dependent magnetic coupling between the Co atoms in the SinCo2+ dusters.

The geometric structures of Si-n Au+ (n = 2-11, 14, and 15) clusters are investigated using density functional theory computations in combination with infrared multiple-photon dissociation spectra measured on the corresponding duster.argon and duster.xenon complexes. The SinAu+ clusters adopt planar structures for the smallest sizes (n = 2-4) and have three-dimensional geometries for larger sizes (n >= 5). All of the investigated SinAu+ clusters have exohedral structures in which the Au dopant atom is adsorbed on a surface site of the bare Si-n(+) cluster at a low-coordinated position. The growth mechanism of SinAu+ clusters is discussed and compared with those of SinCu(+) and SinAg+. The present results indicate that the filled d shell and the atomic radii of the dopant atoms May play important roles in the cage formation of the transition-metal-doped Si clusters. Moreover, it is found that the localization of charge on the Au dopant atoms in Si Au determines the extent of complex formation with argon and xenon.

Far-IR vibrational spectra of small ruthenium cluster cations (Ru-7(+), Ru-8(+), and Ru-9(+)) are measured via infrared multiple photon dissociation (IR-MPD) spectroscopy using Ar atoms as messenger for the absorption. These spectra are compared to results from density functional theory calculations, leading to structural assignments. The pure functional PBE leads to better agreement with the experimental data compared to the hybrid functional PBEO. For all three cluster sizes the structures assigned are based on a cubic motif and are 4 distorted cube missing one corner atom (Run a slightly distorted cube (Ru-8(+)), and a face-capped cube (Ru-9(+)).

Laser-ablated vanadium, niobium, and tantalum atoms were reacted with CH2X2, CHX3, and CX4 (X = F and Cl) molecules in condensing argon, and the products were investigated by matrix isolation infrared spectroscopy. The major reaction products are new CH2-MX2, CHX-MX2, HC-MX3, and XC-MX3 complexes. These reactive species were identified by comparing their matrix infrared spectra with frequencies, intensities, and isotopic shifts from density functional theory calculations. Product structures and energies from these calculations are also presented. Results from previously studied Group 4 and 6 metal reaction products are compared. Little change is found in the calculated metal-carbon bond lengths in the early first row CH2═MF2 methylidene σ(2)π(2) series; however, the methylidyne complexes HC{}MF3 show considerable increase in bond strength for the nominally σ(2)π(1)π(1)(Ti), σ(2)π(2)π(1)(V), and σ(2)π(2)π(2)(Cr) carbon{}metal bonds left to right. The Group 5 HC{}MF3 complexes have only a plane of symmetry whereas the Group 4 and 6 analogues have 3-fold symmetry.

Cationic silver-doped silicon clusters, Si(n)Ag(+) (n=6-15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin-hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low-energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition-metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si(n)Ag(+) (n=7-9) has a pentagonal bipyramidal building block, whereas the larger Si(n)Ag(+) (n=10-12, 14, 15) clusters have trigonal prism-based structures. On comparing the structures of Si(n)Ag(+) with those of Si(n)Cu(+) (for n=6-11) it is found that both Cu and Ag adsorb on a surface site of bare Si(n)(+) clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si(n)(+) framework than the Cu dopant atom.

The structures of neutral cobalt-doped silicon clusters have been assigned by a combined experimental and theoretical study. Size-selective infrared spectra of neutral Si(n)Co (n = 10-12) clusters are measured using a tunable IR-UV two-color ionization scheme. The experimental infrared spectra are compared with calculated spectra of low-energy structures predicted at the B3P86 level of theory. It is shown that the Si(n)Co (n = 10-12) clusters have endohedral caged structures, where the silicon frameworks prefer double-layered structures encapsulating the Co atom. Electronic structure analysis indicates that the clusters are stabilized by an ionic interaction between the Co dopant atom and the silicon cage due to the charge transfer from the silicon valence sp orbitals to the cobalt 3d orbitals. Strong hybridization between the Co dopant atom and the silicon host quenches the local magnetic moment on the encapsulated Co atom.

Vibrational spectra of small neutral gold clusters containing up to 8 Au atoms are measured in the far-infrared (46-222 cm(-1)) via photodissociation of their complexes with krypton atoms. Comparisons with calculated IR spectra for bare Au-n clusters using density functional theory allow for structural assignment. For these small sizes, all clusters are found to be planar and of comparably high symmetry. For Au-6 no data is available, as this cluster size is not detected in the photoionization mass spectra due to its high ionization energy. The structures assigned are for n = 4: rhombus (D-2h); 5: trapezoid (C-2v); 7: edge-capped triangle (C-s), 8: 4-fold edge-capped square (D-4h).

The activation of nitrogen molecules when forming complexes with neutral Ru clusters in the gas phase has been investigated by probing their N–N stretching frequencies using infrared multiple photon dissociation spectroscopy. The measured frequencies for RunN2m (n = 5–16; m = 1, 2) fall in the range between 2110 and 2201 cm–1 and can be attributed to chemisorbed σ-bonded N2, which corresponds to the γ-state on metal surfaces. The band positions are not dependent on the N2 coverage, but significant variations are found depending on cluster size.