Electronic structures and chemical bonding in transition metal monosilicides MSi (M = 3d, 4d, 5d elements)

CrN, CuB, and AuB: A Tale of Two Dissociation Limits

Predissociation thresholds corresponding to dissociation at ground state separated atom limits (SALs) have been recorded in this group for more than 100 d- and f-block metal-containing molecules. The metal atom electronic degeneracies in these molecules generate a dense manifold of electronic states that allow high-lying vibronic levels to couple to pathways leading to dissociation. However, CrN, CuB, and AuB fail to dissociate at their ground SAL. Instead, the molecules remain bound at energies that far surpass their bond dissociation energies (BDEs), and their bonds break only when excited at or above an excited SAL. Sharp predissociation thresholds at excited SALs nevertheless allowed BDEs to be derived: D0(CrN): 3.941(22) eV; D0(CuB): 2.26(15) eV; D0(Au11B): 3.724(3) eV. A previous measurement of D0(AlCr) is re-evaluated as dissociating to a higher energy limit, giving a revised value of D0(AlCr) = 1.32(2) eV. A discussion of this physical behavior is presented.

Electron Detachments of NbSin-/0 (n = 1-3) Clusters from Density Matrix Renormalization Group-CASPT2 Calculations

The electronic states of NbSi_n^-/0/+ (n = 1-3) clusters have been explored using the state-of-the-art DMRG-CASPT2 method with relatively large active spaces. The leading configurations, bond distances, vibrational frequencies, and relative energies of the low-lying states were identified. Electron detachment energies of the anionic cluster and ionization energies of the neutral clusters were reported at the DMRG-CASPT2 level. The ground states of the NbSi_n^-/0/+ (n = 1-3) clusters were predicted as the ³Δ, ⁴Π, and ⁵Π states of the linear NbSi^-/0/+, the ³A₂, ⁴B₁, and ³B₁ states of cyclic NbSi₂^-/0/+, and the ¹A', ²A', and ³A″ states of tetrahedral NbSi₃^-/0/+ isomers. The first feature in the photoelectron spectrum of NbSi^- was attributed to the transitions from the anionic ³Δ ground state to the neutral ⁴Π, ⁴Δ, and ⁴Φ states, whereas the second feature was assigned to the transitions to the neutral ²Δ, ²Σ⁺, and ²Φ states. The first band in the photoelectron spectrum of NbSi₃^- was ascribed to the transition from the anionic ¹A' ground state to the neutral 1²A' and 1²A″ states; the second band was attributed to the transitions to 2²A', 2²A″, and 3²A' states; and the third band was assigned to the transition to 3²A' states.

Quantum chemistry calculations of the growth patterns, simulated photoelectron spectra, and electronic properties of LaASil (A = Sc, Y, La; l ≤ 10) compounds and their anions

The growth patterns, simulated photoelectron spectra, and electronic properties of LaASi_l (A = Sc, Y, and La; l ≤ 10) compounds and their anions were studied via quantum chemistry calculations using the Perdew-Burke-Ernzerhof (PBE) method and unprejudiced structural searching software ABCluster. The results revealed that the growth patterns of the most stable structures of neutral and anionic LaASi_l showed an adsorptive mode. The lowest-energy structures (LESs) of the LaASi_l (l ≤ 7) clusters were similar, except for those of anionic LaYSi₄^- and LaYSi₅^- and neutral LaScSi₇. Additionally, we investigated and calculated the photoelectron spectra, vertical detachment energies, adiabatic electron affinities, relative stability, charge transfer, magnetic moment, and chemical bond analysis of the LaASi_l ground-state structures. The La₂Si_l clusters exhibited higher stability than the LaYSi_l and LaScSi_l systems owing to their higher dissociation energies (DEs). The DEs of the LESs in the LaASi₃ molecule are higher than those of other clusters. Thus, the LaASi₃ cluster shows potential as a building framework for Si-based cluster materials with good stability. The natural population analysis data and chemical bond analysis results showed that the spd hybridization of the orbitals of the metal atoms in the LaASi_l system occurred. Except for the LaScSi₉ and LaScSi₁₀ clusters, the neutral LaASi_l compounds transform into the corresponding anions when an extra electron is accepted by the Si clusters.

Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies, Bond Lengths, and State Orderings

Transition metal silicides are promising materials for improved electronic devices, and this motivates achieving a better understanding of transition metal bonds to silicon. Here we model the ground and excited state bond dissociations of VSi, NbSi, and TaSi using a complete active space (CAS) wave function and a separated-pair (SP) wave function combined with two post-self-consistent field techniques: complete active space with perturbation theory at second order and multiconfiguration pair-density functional theory. The SP approximation is a multiconfiguration self-consistent field method with a selection of configurations based on generalized valence bond theory without the perfect pairing approximation. For both CAS and SP, the active-space composition corresponds to the nominal correlated-participating-orbital scheme. The ground state and low-lying excited states are explored to predict the state ordering for each molecule, and potential energy curves are calculated for the ground state to compare to experiment. The experimental bond dissociation energies of the three diatomic molecules are predicted with eight on-top pair-density functionals with a typical error of 0.2 eV for a CAS wave function and a typical error of 0.3 eV for the SP approximation. We also provide a survey of the accuracy achieved by the SP and extended separated-pair approximations for a broader set of 25 transition metal–ligand bond dissociation energies.

Structural and electronic properties of nanosize semiconductor HfSin0/-/2- (n = 6-16) material: a double-hybrid density functional theory investigation

Equilibrium geometries, thermodynamic stabilities, chemical reactivities, and electronic properties of neutral, mono-, and di-anionic Hf-doped silicon nanoclusters HfSi_n^0/-2- (n = 6-16) are calculated by employing an ABCluster global search technique combined with mPW2PLYP scheme. Based on the concordance between simulated and experimental PES, the true global minima are confirmed for n = 6, 9, and 12-16. Optimized geometries for neutral HfSi_n nanoclusters can be divided into three stages: first, Hf atom prefers locating on the surface site of the cluster for n = 6-9, which can be obtained by adding one, two, three, and four Si atoms to HfSi₅ tetragonal bipyramid, respectively (denoted as additive type); then, Hf atom is surrounded by Si atoms with half-cage configuration for n = 10-13; finally, Hf atom is encapsulated into Si cage pattern for n = 14-16. For mono-anions, it is from additive type (n = 6-11) to the cagelike configuration with Hf atom resided in silicon clusters (n = 12-16). For di-anions, it is additive type (n = 6-9) to the Hf-linked configuration (n = 10-11), and in the end to the Hf-encapsulated cagelike motif (n = 12-16). The thorough analysis of stability and chemical bonding revealed that the neutral HfSi₁₆ and di-anionic HfSi₁₅^2- are magic nanoclusters with good thermodynamic and chemical stability, which may make them as the most suitable building block for new functional materials. We suggest that the experimental PES of HfSi_n^- with n = 7, 8, 10, and 11 should be further examined due to the lack of comparably low electron binding energy peaks.

Bond dissociation energies of ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH

Predissociation thresholds of the ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH molecules have been measured using resonant two-photon ionization spectroscopy. It is argued that the dense manifold of electronic states present in these molecules causes prompt dissociation when the bond dissociation energy (BDE) is exceeded, allowing their respective predissociation thresholds to provide precise values of their bond energies. The BDEs were measured as 2.015(3) eV (ScSi), 2.450(2) eV (YSi), 2.891(5) eV (LaSi), 3.042(10) eV (ScC), 3.420(3) eV (YC), 4.718(4) eV (LaC), 3.899(13) eV (CoC), and 4.102(3) eV (Y-CH). Using thermochemical cycles, the enthalpies of formation, Δ_fH_0K°(g), were calculated as 627.4(9.0) kJ mol^-1 (ScSi), 633.1(9.0) kJ mol^-1 (YSi), 598.1(9.0) kJ mol^-1 (LaSi), 793.8(4.3) kJ mol^-1 (ScC), 805.0(4.2) kJ mol^-1 (YC), 687.3(4.2) kJ mol^-1 (LaC), 760.1(2.5) kJ mol^-1 (CoC), and 620.8(4.2) kJ mol^-1 (YCH). Using data for the BDEs of the corresponding cations allows ionization energies to be obtained through thermochemical cycles as 6.07(11) eV (ScSi), 6.15(13) eV (YSi), 5.60(10) eV (LaSi), 6.26(6) eV (ScC), 6.73(12) or 5.72(11) eV [YC, depending on the value of D₀(Y⁺-C) employed], and 5.88(35) eV (LaC). Additionally, a new value of D₀(Co⁺-C) = 4.045(13) eV was obtained based on the present work and the previously determined ionization energy of CoC. An ionization onset threshold allowed the measurement of the LaSi ionization energy as 5.607(10) eV, in excellent agreement with a prediction based on a thermochemical cycle. Chemical bonding trends are also discussed.

Study on Structural Evolution, Thermochemistry and Electron Affinity of Neutral, Mono- and Di-Anionic Zirconium-Doped Silicon Clusters ZrSin0/-/2- (n = 6-16)

We have carried out a global search of systematic isomers for the lowest energy of neutral and Zintl anionic Zr-doped Si clusters ZrSi_n^0/-/2- (n = 6–16) by employing the ABCluster global search method combined with the mPW2PLYP double-hybrid density functional. In terms of the evaluated energies, adiabatic electron affinities, vertical detachment energies, and agreement between simulated and experimental photoelectron spectroscopy, the true global minimal structures are confirmed. The results reveal that structural evolution patterns for neutral ZrSi_n clusters prefer the attaching type (n = 6–9) to the half-cage motif (n = 10–13), and finally to a Zr-encapsulated configuration with a Zr atom centered in a Si cage (n = 14–16). For Zintl mono- and di-anionic ZrSi_n^-/2-, their growth patterns adopt the attaching configuration (n = 6–11) to encapsulated shape (n = 12–16). The further analyses of stability and chemical bonding make it known that two extra electrons not only perfect the structure of ZrSi₁₅ but also improve its chemical and thermodynamic stability, making it the most suitable building block for novel multi-functional nanomaterials.

Structural growth pattern of neutral and negatively charged yttrium-doped silicon clusters YSi n 0/- (n=6-20): from linked to encapsulated structures

A global search for the low energy of neutral and anionic doped Si clusters YSi n 0/- (n = 6-20) was performed using the ABCluster global search technique coupled with a hybrid density functional method (mPW2PLYP). In light of the calculated energies and the measured photoelectron spectroscopy values, the true minima of the most stable structures were confirmed. It is shown that the structural growth pattern of YSi n - (n = 6-20) is from Y-linked two subcluster structure to a Y-encapsulated structure in Si cages, while that of YSi n (n = 6-20) is from substitutional to linked structures, and as the number of Si atoms increases, it evolves toward the encapsulated structure. Superatom YSi20 - with a high-symmetry endohedral I h structure has an ideal thermodynamic stability and chemical reactivity, making it the most suitable building block for novel optical, optoelectronic photosensitive or catalytic nanomaterials.

Bond dissociation energies of FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi

Resonant two-photon ionization spectroscopy has been used to investigate the spectra of the diatomic late transition metal silicides, MSi, M = Fe, Ru, Os, Co, Rh, Ir, Ni, and Pt, in the vicinity of the bond dissociation energy. In these molecules, the density of vibronic states is so large that the spectra appear quasicontinuous in this energy range. When the excitation energy exceeds the ground separated atom limit, however, a new decay process becomes available-molecular dissociation. This occurs so rapidly that the molecule falls apart before it can absorb another photon and be ionized. The result is a sharp drop to the baseline in the ion signal, which we identify as occurring at the thermochemical 0 K bond dissociation energy, D₀. On this basis, the measured predissociation thresholds provide D₀ = 2.402(3), 4.132(3), 4.516(3), 2.862(3), 4.169(3), 4.952(3), 3.324(3), and 5.325(9) eV for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Using thermochemical cycles, the enthalpies of formation of the gaseous MSi molecules are derived as 627(8), 700(10), 799(10), 595(8), 599(8), 636(10), 553(12), and 497(8) kJ/mol for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Likewise, combining these results with other data provides the ionization energies of CoSi and NiSi as 7.49(7) and 7.62(7) eV, respectively. Chemical bonding trends among the diatomic transition metal silicides are discussed.

Bond dissociation energies of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D₀(TiSi) = 2.201(3) eV, D₀(ZrSi) = 2.950(3) eV, D₀(HfSi) = 2.871(3) eV, D₀(VSi) = 2.234(3) eV, D₀(NbSi) = 3.080(3) eV, and D₀(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as Δ_f,0KH°(TiSi(g)) = 705(19) kJ mol^-1, Δ_f,0KH°(ZrSi(g)) = 770(12) kJ mol^-1, Δ_f,0KH°(HfSi(g)) = 787(10) kJ mol^-1, Δ_f,0KH°(VSi(g)) = 743(11) kJ mol^-1, Δ_f,0KH°(NbSi(g)) = 879(11) kJ mol^-1, and Δ_f,0KH°(TaSi(g)) = 938(8) kJ mol^-1. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi^- and NbSi^- anions, D₀(Zr-Si^-) ≤ 3.149(15) eV, D₀(Zr^--Si) ≤ 4.108(20) eV, D₀(Nb-Si^-) ≤ 3.525(31) eV, and D₀(Nb^--Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.

Probing the valence orbitals of transition metal-silicon diatomic anions: ZrSi, NbSi, MoSi, PdSi and WSi

Evolution of electronic properties and the nature of bonding of the 4d-transition metal silicides (ZrSi, NbSi, MoSi and PdSi) are discussed, revealing interesting trends in the transition metal-silicon interactions across the period. The electronic properties of select transition metal silicide diatomics have been determined by anion photoelectron imaging spectroscopy and theoretical methods. The electron binding energy spectra and photoelectron angular distributions obtained by 2.33 eV (532 nm) photons have revealed the distinct features of these diatomics. The theoretical calculations were performed at the density functional theory (DFT) level using the unrestricted B3LYP hybrid functional and at the ab initio unrestricted coupled cluster singles and doubles (triplets) (UCCSD(T)) methods to assign the ground electronic states of the neutral and anionic diatomics. The excited electronic states were calculated by the DFT (TD-DFT)/UB3LYP method. We have observed that the valence molecular orbital configuration of the ZrSi and NbSi anions are significantly different from that of the MoSi and PdSi anions. By combining our experimental and theoretical results, we report that the composition of the highest occupied molecular orbitals shift from a majority of transition metal s- and d-orbital contribution in ZrSi and NbSi, to mainly silicon p-orbital contribution for MoSi and PdSi. We expect these observed atomic scale transition metal-silicon interactions to be of increasing importance with the miniaturization of devices approaching the sub-nanometer size regime.

Toward accurate theoretical thermochemistry of first row transition metal complexes

The recently developed correlation consistent Composite Approach for transition metals (ccCA-TM) was utilized to compute the thermochemical properties for a collection of 225 inorganic molecules containing first row (3d) transition metals, ranging from the monohydrides to larger organometallics such as Sc(C(5)H(5))(3) and clusters such as (CrO(3))(3). Ostentatiously large deviations of ccCA-TM predictions stem mainly from aging and unreliable experimental data. For a subset of 70 molecules with reported experimental uncertainties less than or equal to 2.0 kcal mol(-1), regardless of the presence of moderate multireference character in some molecules, ccCA-TM achieves transition metal chemical accuracy of ±3.0 kcal mol(-1) as defined in our earlier work [J. Phys. Chem. A2007, 111, 11269-11277] by giving a mean absolute deviation of 2.90 kcal mol(-1) and a root-mean-square deviation of 3.91 kcal mol(-1). As subsets are constructed with decreasing upper limits of reported experimental uncertainties (5.0, 4.0, 3.0, 2.0, and 1.0 kcal mol(-1)), the ccCA-TM mean absolute deviations were observed to monotonically drop off from 4.35 to 2.37 kcal mol(-1). In contrast, such a trend is missing for DFT methods as exemplified by B3LYP and M06 with mean absolute deviations in the range 12.9-14.1 and 10.5-11.0 kcal mol(-1), respectively. Salient multireference character, as demonstrated by the T(1)/D(1) diagnostics and the weights (C(0)(2)) of leading electron configuration in the complete active self-consistent field wave function, was found in a significant amount of molecules, which can still be accurately described by the single reference ccCA-TM. The ccCA-TM algorithm has been demonstrated as an accurate, robust, and widely applicable model chemistry for 3d transition metal-containing species with versatile bonding features.

No quenching of magnetic moment for the GenCo (n=1-13) clusters: first-principles calculations

The authors predict that for the Ge(n)Co (n=1-13) clusters the magnetic moment does not quench, which is dark contrast to the previous results with transition-metal-doped Si(n) clusters. It may be due to the unpaired electrons of the Co atom in the clusters. For the ground state structures of the Ge(n)Co (n>or=9) clusters, the Co atom completely falls into the center of the Ge outer frame, forming metal-encapsulated Ge(n) cages. The doping of the Co atom enhances the stability of the host Ge(n) clusters. The Ge(10)Co cluster with the bicapped tetragonal antiprism structure is more stable than others, which agrees very well with the results of the experiment of the Co/Ge binary clusters by the laser vaporization.

Rotationally resolved spectra of jet-cooled RuSi

We report the first gas-phase spectroscopic investigation of diatomic ruthenium silicide (RuSi). The molecules were produced by laser ablation of a Ru disk into a flow of helium carrier gas containing 0.5% SiH(4), and were cooled in a supersonic expansion. The RuSi molecules were then studied using resonant two-photon ionization spectroscopy. Investigations conducted in the spectral range from 18,800 to 23,800 cm(-1) show a large number of excited vibronic levels that cannot readily be grouped into electronic band systems. The ground state is been demonstrated to be of (3)Delta(3) symmetry, deriving from the 2delta(3)14sigma(1) electronic configuration. Correcting for the effects of the spin-uncoupling operator, the ground state bond length (r(0)) is determined to be 2.0921+/-0.0004 A (1sigma error limit). Diatomic RuSi is shown to have strong dpi-ppi bonds, unlike the isovalent AlCo molecule.

Computational investigation of TiSin (n=2–15) clusters by the density-functional theory

The geometries, stabilities, and electronic properties of TiSin (n=2-15) clusters with different spin configurations have been systematically investigated by using density-functional theory approach at B3LYP/LanL2DZ level. According to the optimum TiSin clusters, the equilibrium site of Ti atom gradually moves from convex to surface, and to a concave site as the number of Si atom increases from 2 to 15. When n=12, the Ti atom in TiSi12 completely falls into the center of the Si outer frame, forming metal-encapsulated Si cages, which can be explained by using 16-electron rule. On the basis of the optimized geometries, various energetic properties are calculated for the most stable isomers of TiSin clusters, including the average binding energy, the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) gap, fragmentation energy, and the second-order difference of energy. It is found that at size n=6,8,12 the clusters are more stable than neighboring ones. According to the Mulliken charge population analysis, charges always transfer from Si atoms to Ti atom. Furthermore, the HOMO-LUMO gaps of the most stable TiSin clusters are usually smaller than those of Sin clusters.

Geometries, Stabilities, and Growth Patterns of the Bimetal Mo2-doped Sin (n = 9−16) Clusters: A Density Functional Investigation

The behaviors of the bimetal Mo-Mo doped cagelike silicon clusters Mo2Sin at the size of n=9-16 have been investigated systematically with the density functional approach. The growth-pattern behaviors, relative stabilities, and charge-transfer of these clusters are presented and discussed. The optimized geometries reveal that the dominant growth patterns of the bimetal Mo-Mo doped on opened cagelike silicon clusters (n=9-13) are based on pentagon prism MoSi10 and hexagonal prism MoSi12 clusters, while the Mo2 encapsulated Sin(n=14-16) frames are dominant growth behaviors for the large-sized clusters. The doped Mo2 dimer in the Sin frames is dissociated under the interactions of the Mo2 and Sin frames which are examined in term of the calculated Mo-Mo distance. The calculated fragmentation energies manifest that the remarkable local maximums of stable clusters are Mo2-doped Sin with n=10 and 12; the obtained relative stabilities exhibit that the Mo2-doped Si10 cluster is the most stable species in all different sized clusters. Natural population analysis shows that the charge-transfer phenomena appearing in the Mo2-doped Sin clusters are analogous to the single transition metal Re or W doped silicon clusters. In addition, the properties of frontier orbitals of Mo2-doped Sin (n=10 and 12) clusters show that the Mo2Si10 and Mo2Si12 isomers have enhanced chemical stabilities because of their larger HOMO-LUMO gaps. Interestingly, the geometry of the most stable Mo2Si9 cluster has the framework which is analogous to that of Ni2Ge9 cluster confirmed by recent experimental observation (Goicoechea, J. M.; Sevov, S. C. J. Am Chem. Soc. 2006, 128, 4155).