Formation of Met-Cars and Face-Centered Cubic Structures: Thermodynamically or Kinetically Controlled?

Investigation of Structures, Stabilities, and Electronic and Magnetic Properties of Niobium Carbon Clusters Nb7Cn (n = 1-7)

The geometrical structures, relative stabilities, and electronic and magnetic properties of niobium carbon clusters, Nb7Cn (n = 1-7), are investigated in this study. Density functional theory (DFT) calculations, coupled with the Saunders Kick global search, are conducted to explore the structural properties of Nb7Cn (n = 1-7). The results regarding the average binding energy, second-order difference energy, dissociation energy, HOMO-LUMO gap, and chemical hardness highlight the robust stability of Nb7C3. Analysis of the density of states suggests that the molecular orbitals of Nb7Cn primarily consist of orbitals from the transition metal Nb, with minimal involvement of C atoms. Spin density and natural population analysis reveal that the total magnetic moment of Nb7Cn predominantly resides on the Nb atoms. The contribution of Nb atoms to the total magnetic moment stems mainly from the 4d orbital, followed by the 5p, 5s, and 6s orbitals.

Velocity correlated emission of secondary clusters by a single surface impact of a polyatomic ion: A new mechanism of clusters emission and subpicosecond probing of extreme spike conditions

Emission of secondary clusters off clean solid surfaces following impact of a projectile ion at kiloelectronvolt (keV) kinetic energies is important from both the practical and fundamental points of view....

Role of metcar on the adsorption and activation of carbon dioxide: a DFT study

Metallocarbohedrenes or metcars belong to one of the classes of stable nanoclusters having a specific stoichiometry. In spite of the available theoretical and experimental studies, the structure of pristine Ti8C12 metcar is still uncertain. We study the geometric structure of a titanium metcar, Ti8C12, together with its electronic properties and chemical activity towards adsorption and activation of CO2 molecule by means of density functional theory. Our results suggest that the CO2 molecule is strongly adsorbed and undergoes a significantly high degree of activation onto the Ti8C12 metcar. The migration of charge from titanium metcar to CO2 molecule attributes the high degree of activation of this molecule. In the infrared vibrational spectra for CO2 molecule adsorbed onto Ti8C12, we find a new signal which is absent in the corresponding spectra for gaseous CO2. In addition to adsorption energy, we also estimate the energy barrier for the dissociation of CO2 molecule to CO and O fragments on a Ti8C12 cluster. As a whole, this work reveals the ground state geometry of Ti8C12 metcar and highlights the role of this metcar in CO2 adsorption and activation, which are the key steps in designing potential catalysts for CO2 capture and its conversion to industrially valuable chemicals.

The reactivity of Nbn+ clusters with acetylene and ethylene to produce a cubic aromatic metal carbide Nb4C4+

The reactions of niobium cationic clusters with acetylene and ethylene under sufficient gas collision conditions give rise to dominant dehydrogenation and produce a main metal carbide Nb4C4+ which is associated with cubic aromaticity.

Anion photoelectron spectroscopy and density functional investigation of diniobium-carbon clusters

Experimental photoelectron and computational results show diniobium-carbon (Nb(2)C(n)) clusters to coexist in multiple structural isomers: three-dimensional geometries, planar rings, and linear chains. Three-dimensional clusters having up to five carbons are formed preferentially with Nb-Nb bonding, whereas only Nb-C bonding is observed experimentally at six carbons. Clusters consisting of an odd number of atoms are also observed with linear geometries. The larger binary clusters (n > or = 7) display properties similar to those of pure carbon clusters. We provide evidence for niobium substitution of carbon atoms.

A density-functional study of the structural, electronic, magnetic, and vibrational properties of Ti8C12 metallocarbohedrynes

Calculations are presented for the structural, electronic, and vibrational properties of the different Ti8C12 metallocarbohedrynes. (Please note that we adopt the name "metallocarbohedrynes" instead of "metallocarbohedrenes" to denote the acetylenic nature of C2 units in this class of clusters demonstrated by several contributions in literature.) The density-functional theory (DFT) calculations are performed with the all-electron projector augmented-wave method and generalized gradient approximation for the exchange-correlation functional. We study the seven low-energy isomers of the Ti8C12 metallocarbohedrynes using spin-polarized DFT, where we find a correlation between the number of rotated carbon dimers and the cohesive energy of the structure. The electronic density of states (eDOS) show that C3nu, D*3d, and D3d isomers are spin polarized. The partial eDOS shows that, depending on the dimer orientation, carbon atoms and a subgroup of the metal atoms form a covalent framework while other metal atoms are bonded to this framework more ionically. This picture is further supported by the charge density of the different structures, where we see that the Ti atoms with higher charge density show less contribution to the covalent bonding of the Ti-C framework. The vibrational spectra of the different structures are calculated using the frozen-vibration method. Also, we calculate the vibrational spectra of the C3nu and C2nu structures using molecular-dynamics simulations at two different temperatures. The results of the simulations demonstrate the local stability of the structures beyond the harmonic limit explored by the frozen-vibration method.

Photoabsorption spectra of Ti8C12 metallocarbohedrynes: Theoretical spectroscopy within time-dependent density functional theory

The photoabsorption spectra of several of the most stable isomers of the Ti8C12 metallocarbohedryne are calculated using time-dependent density functional theory. Several ground-state magnitudes have been also calculated, such as cohesive energies, electronic gaps between the highest occupied and lowest unoccupied molecular orbitals, and static polarizabilities. Since significant differences are found among the photoabsorption spectra of the different isomers in the low energy region (0-5 eV), we propose the comparison of experimental and the calculated absorption spectra as a tool to elucidate the isomers that appear to form in the experiments. Between 10 and 13 eV all the spectra show a region of high absorption that we identify as due to collective electronic excitations. The existence of this prominent feature explains the occurrence of delayed ionization and delayed ion emission phenomena observed in previous experiments.

Ionization Potentials of Tantalum−Carbide Clusters: An Experimental and Density Functional Theory Study

We have used photoionization efficiency spectroscopy to determine the ionization potentials (IP) of the tantalum-carbide clusters, Ta3Cn (n = 1-3) and Ta4Cn (n = 1-4). The ionization potentials follow an overall reduction as the number of carbon atoms increases; however, the trend is not steady as expected from a simple electrostatic argument. Instead, an oscillatory behavior is observed such that clusters with an odd number of carbon atoms have higher IPs and clusters with an even number of carbon atoms have lower IPs, with the Ta4C4 cluster exhibiting the lowest IP. Excellent agreement is found with relative IPs calculated using density functional theory for the lowest energy structures, which are consistent with the development of a 2 x 2 x 2 face-centered nanocrystal. This work shows that IPs may be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory. The variation in IP can also be interpreted qualitatively with application of a simple model based upon isolobal frontier orbitals.

Theoretical study of the electronic states of Nb4, Nb5 clusters and their anions (Nb4−,Nb5−)

Geometries and energy separations of the various low-lying electronic states of Nb(n) and Nb(n) (-) (n=4,5) clusters with various structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field method followed by multireference singles and doubles configuration interaction (MRSDCI) calculations that included up to 52x10(6) configuration spin functions have been used to compute several electronic states of these clusters. The ground states of both Nb(4) ((1)A('), pyramidal) and Nb(4) (-) ((2)B(3g), rhombus) are low-spin states at the MRSDCI level. The ground state of Nb(5) cluster is a doublet with a distorted trigonal bipyramid (DTB) structure. The anionic cluster of Nb(5) has two competitive ground states with singlet and triplet multiplicities (DTB). The low-lying electronic states of these clusters have been found to be distorted due to Jahn-Teller effect. On the basis of the energy separations of our computed electronic states of Nb(4) and Nb(5), we have assigned the observed photoelectron spectrum of Nb(n) (-) (n=4,5) clusters. We have also compared our MRSDCI results with density functional calculations. The electron affinity, ionization potential, dissociation and atomization energies of Nb(4) and Nb(5) have been calculated and the results have been found to be in excellent agreement with the experiment.

Structure determination of gas-phase niobium and tantalum carbide nanocrystals via infrared spectroscopy

Niobium and tantalum carbide clusters have been isolated in the gas phase and irradiated with intense tunable infrared (IR) light. Stable neutral clusters are selectively ionized and subsequently detected in a mass spectrometer. By tuning the IR frequency, infrared multiphoton absorption spectra are obtained for a whole range of clusters. These mass-selective IR spectra lead to insights into the structures of small niobium and tantalum carbide clusters and nanocrystals.