Truong Ba Tai

Thermochemistry and Electronic Structure of Small Boron Clusters (Bn, n = 5−13) and Their Anions

Thermochemical parameters of a set of small-sized neutral (B(n)) and anionic (B(n)(-)) boron clusters, with n = 5-13, were determined using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, and Q) basis sets extrapolated to the complete basis set limit (CBS) plus addition corrections and/or G3B3 calculations. Enthalpies of formation, adiabatic electron affinities (EA), vertical (VDE), and adiabatic (ADE) detachment energies were evaluated. Our calculated EAs are in good agreement with recent experiments (values in eV): B(5) (CBS, 2.29; G3B3, 2.48; exptl., 2.33 +/- 0.02), B(6) (CBS, 2.59; G3B3, 3.23; exptl., 3.01 +/- 0.04), B(7) (CBS, 2.62; G3B3, 2.67; exptl., 2.55 +/- 0.05), B(8) (CBS, 3.02; G3B3, 3.11; exptl., 3.02 +/- 0.02), B(9) (G3B3, 3.03; exptl., 3.39 +/- 0.06), B(10) (G3B3, 2.85; exptl., 2.88 +/- 0.09), B(11) (G3B4, 3.48;, exptl., 3.43 +/- 0.01), B(12) (G3B3, 2.33; exptl., 2.21 +/- 0.04), and B(13) (G3B3, 3.62; exptl., 3.78 +/- 0.02). The difference between the calculated adiabatic electron affinity and the adiabatic detachment energy for B(6) is due to the fact that the geometry of the anion is not that of the ground-state neutral. The calculated adiabatic detachment energies to the (3)A(u), C(2h) and (1)A(g), D(2h) excited states of B(6), which have geometries similar to the (1)A(g), D(2h) state of B(6)(-), are 2.93 and 3.06 eV, in excellent agreement with experiment. The VDEs were also well reproduced by the calculations. Partitioning of the electron localization functions into pi and sigma components allows probing of the partial and local delocalization in global nonaromatic systems. The larger clusters appear to exhibit multiple aromaticity. The binding energies per atom vary in a parallel manner for both neutral and anionic series and approach the experimental value for the heat of atomization of B. The resonance energies and the normalized resonance energies are convenient indices to quantify the stabilization of a cluster of elements.

A Stochastic Search for the Structures of Small Germanium Clusters and Their Anions: Enhanced Stability by Spherical Aromaticity of the Ge10 and Ge12(2-) Systems.

Investigations on germanium clusters in the neutral, anionic, and dianion states Gen(x) (n = 2-12 and x = 0, -1, -2) are performed using quantum chemical calculations with the B3LYP functional and the coupled-cluster singles and doubles [CCSD(T)] methods, in conjunction with the 6-311+G(d) basis set. An improved stochastic method is implemented for searching the low-lying isomers of clusters. Comparison of our results with previous reports on germanium clusters shows the efficiency of the search method. The Ge8 system is presented in detail. The anionic clusters Gen(-/2-) are studied theoretically and systematically for the first time, and their energetics are in good agreement with available experiments. The clusters Ge10, Ge10(2-), and Ge12(2-) are, in their ground state, characterized by large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, high vertical and adiabatic detachment energies, and substantial average binding energies. The enhanced stability of these magic clusters can consistently be rationalized using the jellium electron shell model and the spherical aromatic character.

Disk Aromaticity of the Planar and Fluxional Anionic Boron Clusters B20−/2−

The presence of excess electrons modifies the structural landscape and tends to extend the planarity of boron clusters. While the neutral B(20) is tubular, both the anion and dianion B(20)(-/2-) become planar. Geometrical features of the stable anions suggest the existence of a new type of cluster that is planar and doubly cyclic with one atom located at the center (see figure), as well as being fluxional.

Thermochemical properties and electronic structure of Boron oxides BnOm (n=5-10, m=1-2) and their anions

The molecular and electronic structures of a series of small boron monoxide and dioxide clusters B(n)O(m) (n = 5-10, m = 1, 2) plus their anions were predicted. The enthalpies of formation (DeltaH(f)s), electron affinities (EAs), vertical detachment energies, and energies of different fragmentation processes are predicted using the G3B3 method. The G3B3 results were benchmarked with respect to more accurate CCSD(T)/CBS values for n = 1-4 with average deviations of +/-1.5 kcal/mol for DeltaH(f)s and +/-0.03 eV for EAs. The results extend previous observations on the growth mechanism for boron oxide clusters: (i) The low spin electronic state is consistently favored. (ii) The most stable structure of a neutral boron monoxide B(n)O is obtained either by condensing O on a BB edge of a B(n) cycle, or by binding one BO group to a B(n-1) ring. The balance between both factors is dependent on the inherent stability of the boron cycles. (iii) A boron dioxide is formed by incorporating the second O atom into the corresponding monoxide to form BO bonds. (iv) A B(n)O(m)(-) anion is constructed with BO groups bound to the B(n-1) or B(n-2) rings (yielding the B(n-2)(BO)(2)(-) species). This becomes the preferred geometry for the larger boron dioxides, even in the neutral state. The boronyl group mainly behaves as an electron-withdrawing substituent reducing the binding energy and resonance energy of the oxides. (v) The boron oxides conserve some of the properties of the parent boron clusters such as the planarity and multiple aromaticity.

Electronic Structure and Thermochemical Properties of Small Neutral and Cationic Lithium Clusters and Boron-Doped Lithium Clusters: Lin0/+ and LinB0/+ (n = 1―8)

The stability, electronic structure, and thermochemical properties of the pure Li(n) and boron-doped Li(n)B (n = 1-8) clusters in both neutral and cationic states are studied using electronic structure methods. The global equilibrium structures are established, and their heats of formation are evaluated using the G3B3 and CCSD(T)/CBS methods based on the density functional theory geometries. Theoretical adiabatic ionization energies (IE(a)) for the Li(n) clusters are in good agreement with experiment: Li(2) (G3B3, 5.21 eV; CCSD(T), 5.14 eV; expt, 5.1127 ± 0.0003 eV), Li(3) (4.16, 4.11, 4.08 ± 0.10), Li(4) (4.76, 4.68, 4.70 ± 0.05), Li(5) (4.11, 4.06, 4.02 ± 0.10), Li(6) (4.46, 4.32, 4.20 ± 0.10), Li(7) (4.07, 3.99, 3.94 ± 0.10), and Li(8) (4.49, 4.31, 4.16 ± 0.10). The Li(4) experimental IE(a) has been revised on the basis of the Franck-Condon simulations. Species Li(5)B, Li(6)B(+), Li(7)B, and Li(8)B(+) exhibit high stability as compared to their neighbors, which can be understood by considering the magic numbers of the phenomenological shell model (PSM).

Electronic Structures and Thermochemical Properties of the Small Silicon-Doped Boron Clusters BnSi (n=1–7) and Their Anions

We perform a systematic investigation on small silicon-doped boron clusters B(n)Si (n=1-7) in both neutral and anionic states using density functional (DFT) and coupled-cluster (CCSD(T)) theories. The global minima of these B(n)Si(0/-) clusters are characterized together with their growth mechanisms. The planar structures are dominant for small B(n)Si clusters with n≤5. The B(6)Si molecule represents a geometrical transition with a quasi-planar geometry, and the first 3D global minimum is found for the B(7)Si cluster. The small neutral B(n)Si clusters can be formed by substituting the single boron atom of B(n+1) by silicon. The Si atom prefers the external position of the skeleton and tends to form bonds with its two neighboring B atoms. The larger B(7)Si cluster is constructed by doping Si-atoms on the symmetry axis of the B(n) host, which leads to the bonding of the silicon to the ring boron atoms through a number of hyper-coordination. Calculations of the thermochemical properties of B(n)Si(0/-) clusters, such as binding energies (BE), heats of formation at 0 K (ΔH(f)(0)) and 298 K (ΔH(f)([298])), adiabatic (ADE) and vertical (VDE) detachment energies, and dissociation energies (D(e)), are performed using the high accuracy G4 and complete basis-set extrapolation (CCSD(T)/CBS) approaches. The differences of heats of formation (at 0 K) between the G4 and CBS approaches for the B(n)Si clusters vary in the range of 0.0-4.6 kcal mol(-1). The largest difference between two approaches for ADE values is 0.15 eV. Our theoretical predictions also indicate that the species B(2)Si, B(4)Si, B(3)Si(-) and B(7)Si(-) are systems with enhanced stability, exhibiting each a double (σ and π) aromaticity. B(5)Si(-) and B(6)Si are doubly antiaromatic (σ and π) with lower stability.

Structure, Thermochemical Properties, and Growth Sequence of Aluminum-Doped Silicon Clusters SinAlm (n = 1–11, m = 1–2) and Their Anions

A systematic examination of the aluminum doped silicon clusters, Si(n)Al(m) with n = 1-11 and m = 1-2, in both neutral and anionic states, is carried out using quantum chemical calculations. Lowest-energy equilibrium structures of the clusters considered are identified on the basis of G4 energies. High accuracy total atomization energies and thermochemical properties are determined for the first time using the G4 and CCSD(T)/CBS (coupled-cluster theory with complete basis set up to n = 3) methods. In each size, substitution of Si atoms at different positions of a corresponding pure silicon clusters by Al dopants invariably leads to a spectrum of distinct binary structures but having similar shape and comparable energy content. Such an energetic degeneracy persists in the larger cluster sizes, in particular for the anions. The equilibrium growth sequences for Al-doped Si clusters emerge as follows: (i) neutral singly doped Si(n)Al clusters favor Al atom substitution into a Si position in the structure of the corresponding cation Si(n+1)(+), whereas the anionic Si(n)Al(-) has one Si atom of the isoelectronic neutral Si(n+1) being substituted by the Al impurity; and (ii) for doubly doped Si(n)Al2(0/-) clusters, the neutrals have the shape of Si(n+1) counterparts in which one Al atom substitutes a Si atom and the other Al adds on an edge or a face of it, whereas the anions have both Al atoms substitute two Si atoms in the Si(n+2)(+) frameworks. The Al dopant also tends to avoid high coordination position.

Theoretical study of AunV-CO, n = 1–14: The dopant vanadium enhances CO adsorption on gold clusters

The CO adsorption on vanadium-doped gold clusters Au(n)V with n = 1-14 is studied by density functional theory computations, using the BB95 and B3LYP functionals along with the cc-pVDZ-PP basis for metals and cc-pVTZ for non-metals. When both Au and V sites are exposed, CO adsorption on V is thermodynamically favorable because with partially filling d orbitals vanadium is more willing to interact with CO empty or filled orbitals. When vanadium is confined inside a gold cage, the low-coordinated Au atoms become the preferred sites for CO attachment. The presence of V tends to reinforce CO adsorption as compared with the bare gold clusters. The diatomic AuV is predicted to have the largest CO adsorption affinity as it has a typical π-back donation bond. Au(n)V-CO complexes typically have the larger CO binding energies and larger CO frequency shift than the isoatomic gold-carbonyl Au(n+1)-CO counterparts.

The π-conjugated P-flowers C16(PH)8 and C16(PF)8 are potential materials for organic n-type semiconductors

Following the theme of this special issue, two new compounds, the P-flowers C(16)(PH)(8) and C(16)(PF)(8), are designed by us and subsequently characterized by quantum chemical computations. Their geometries and infrared signatures are analyzed and compared to those of the well-known sulflower C(16)S(8). Their electronic structure and aromaticity are examined using the electron localization function (ELF) and also by the total and partial densities of state (DOS). Both C(16)(PF)(6) and C(16)(PH)(8) molecules exhibit small energy barrier of electron injection (Φ(e) = 0.33 eV for the gold electrode for the former, and Φ(e) = 0.1 eV for the calcium electrode for the latter), remarkably low reorganization energy and high rate of electron hopping. Thus, both theoretically designed P-flower molecules are predicted to be excellent candidates for organic n-type semiconductors.

Electronic structure and thermochemical properties of silicon‐doped lithium clusters LinSi0/+, n = 1–8: New insights on their stability

A theoretical investigation on small silicon‐doped lithium clusters LinSi with n = 1–8, in both neutral and cationic states is performed using the high accuracy CCSD(T)/complete basis set (CBS) method. Location of the global minima is carried out using a stochastic search method and the growth pattern of the clusters emerges as follows: (i) the species LinSi with n ≤ 6 are formed by directly binding one Li to a Si of the smaller cluster Lin−1Si, (ii) the structures tend to have an as high as possible symmetry and to maximize the coordination number of silicon. The first three‐dimensional global minimum is found for Li4Si, and (iii) for Li7Si and Li8Si, the global minima are formed by capping Li atoms on triangular faces of Li6Si (Oh). A maximum coordination number of silicon is found to be 6 for the global minima, and structures with higher coordination of silicon exist but are less stable. Heats of formation at 0 K (ΔfH0) and 298 K (ΔfH298), average binding energies (Eb), adiabatic (AIE) and vertical (VIE) ionization energies, dissociation energies (De), and second‐order difference in total energy (Δ2E) of the clusters in both neutral and cationic states are calculated from the CCSD(T)/CBS energies and used to evaluate the relative stability of clusters. The species Li4Si, Li6Si, and Li5Si+ are the more stable systems with large HOMO–LUMO gaps, Eb, and Δ2E. Their enhanced stability can be rationalized using a modified phenomenological shell model, which includes the effects of additional factors such as geometrical symmetry and coordination number of the dopant. The new model is subsequently applied with consistency to other impure clusters LinX with X = B, Al, C, Si, Ge, and Sn.