Vu Thi Ngan

Carboxyl-Functionalized Task-Specific Ionic Liquids for Solubilizing Metal Oxides

Imidazolium, pyridinium, pyrrolidinium, piperidinium, morpholinium, and quaternary ammonium bis(trifluoromethylsulfonyl)imide salts were functionalized with a carboxyl group. These ionic liquids are useful for the selective dissolution of metal oxides and hydroxides. Although these hydrophobic ionic liquids are immiscible with water at room temperature, several of them form a single phase with water at elevated temperatures. Phase separation occurs upon cooling. This thermomorphic behavior has been investigated by (1)H NMR, and it was found that it can be attributed to the temperature-dependent hydration and hydrogen-bond formation of the ionic liquid components. The crystal structures of four ionic liquids and five metal complexes have been determined.

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation.

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.

Thermochemistry and electronic structure of small boron and boron oxide clusters and their anions.

Thermochemical properties of a set of small boron (B(n)) and boron oxide (B(n)O(m)) clusters, with n = 1-4 and m = 0-3, their anions, and the B(4)(2-) dianion, were calculated by using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, Q, 5) basis sets extrapolated to the complete basis set limit with additional corrections. Enthalpies of formation, bond dissociation energies, singlet-triplet or doublet-quartet separation gaps, adiabatic electron affinities (EA), and both vertical electron attachment and detachment energies were evaluated. The predicted heats of formation show agreement close to the error bars of the literature results for boron oxides with the largest error for OBO. Our calculated adiabatic EAs are in good agreement with recent experiments: B (calc, 0.26 eV; exptl, 0.28 eV), B(2) (1.95, 1.80), B(3) (2.88, 2.820 +/- 0.020), B(4) (1.68, 1.60 +/- 0.10), BO (2.50, 2.51), BO(2) (4.48, 4.51), BOB (0.07), B(2)O(2) (0.37), B(3)O (2.05), B(3)O(2) (2.94, 2.94), B(4)O (2.58), and B(4)O(2) (3.14, 3.160 +/- 0.015). The BO bond is strong, so this moiety is maintained in most of the clusters. Thermochemical parameters of clusters are not linearly additive with respect to the number of B atoms. The EA tends to be larger in the dioxides. The growth mechanism of small boron oxides should be determined by a number of factors: (i) formation of BO bonds, (ii) when possible, formation of a cyclic B(3) or B(4), and (iii) combination of a boron cycle and a BO bond. When these factors compete, the strength of the BO bonds tends to compensate the destabilization arising from a loss of binding in the cyclic boron clusters, in such a way that a linear boron oxide prevails. When the B(2) moiety is present in these linear clusters, the oxide derivatives prefer a high spin state.

Disparate effects of Cu and V on structures of exohedral transition metal-doped silicon clusters: a combined far-infrared spectroscopic and computational study.

The growth mechanisms of small cationic silicon clusters containing up to 11 Si atoms, exohedrally doped by V and Cu atoms, are described. We find that as dopants, V and Cu follow two different paths: while V prefers substitution of a silicon atom in a highly coordinated position of the cationic bare silicon clusters, Cu favors adsorption to the neutral or cationic bare clusters in a lower coordination site. The different behavior of the two transition metals becomes evident in the structures of Si(n)M(+) (n = 4-11 for M = V, and n = 6-11 for M = Cu), which are investigated by density functional theory and, for several sizes, confirmed by comparison with their experimental vibrational spectra. The spectra are measured on the corresponding Si(n)M(+)·Ar complexes, which can be formed for the exohedrally doped silicon clusters. The comparison between experimental and calculated spectra indicates that the BP86 functional is suitable to predict far-infrared spectra of these clusters. In most cases, the calculated infrared spectrum of the lowest-lying isomer fits well with the experiment, even when various isomers and different electronic states are close in energy. However, in a few cases, namely Si(9)Cu(+), Si(11)Cu(+), and Si(10)V(+), the experimentally verified isomers are not the lowest in energy according to the density functional theory calculations, but their structures still follow the described growth mechanism. The different growth patterns of the two series of doped Si clusters reflect the role of the transition metals 3d orbitals in the binding of the dopant atoms.

High Magnetic Moments in Manganese-Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese-doped silicon clusters cations, Si(n)Mn(+) with n=6-10, 12-14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si(n)Mn(+) (n=6-10) clusters are found to be substitutive derivatives of the bare Si(n+1)(+) cations, while the endohedral Si(n)Mn(+) (n=12-14 and 16) clusters adopt fullerene-like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si(n)Mn(+) (n=6-10) clusters have local magnetic moments of 4 μ(B) or 6 μ(B) and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition-metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.

Tuning the Geometric Structure by Doping Silicon Clusters

Ever since the discovery of C60, much effort has been expended in search of similar, finite-size stable clusters as building blocks for nanostructures. Apart from carbon, silicon has attracted much attention due to its vicinity to carbon in the periodic table as well as its importance in the semiconductor industry. In contrast to carbon, however, silicon favours sp hybridization and thus tetrahedral coordination, which leads to rather asymmetric and reactive structures for small, bare silicon clusters. 3] It has been argued that this deficiency can be solved by suitable doping of silicon clusters with transition metal ions. Following up on this idea, many theoretical studies have investigated SinM structures for various dopants and cluster sizes. [5, 6] Experimental information on doped silicon clusters has been obtained from mass spectrometry, photoelectron spectroscopy (PES), chemical probe methods, and photodissociation studies at fixed wavelengths. While there is no doubt that the structure of silicon clusters can be changed upon appropriate doping, detailed experimental studies on the growth mechanisms of doped silicon clusters are rather scarce, as it is difficult to investigate the structure of gas phase clusters experimentally. A deep knowledge about the influence of the dopant on the clusters’ structure, however, is necessary for the design and production of tailor-made silicon materials. It has recently been shown that infrared multiple photon dissociation (IR–MPD) of complexes of metal clusters with raregas atoms is a suitable experimental technique to obtain vibrational spectra for clusters in the gas phase. Comparison of experimental IR–MPD spectra of clusters with those obtained in calculations for different geometries, for example by using density functional theory (DFT), allows for the deduction of the cluster-size-specific structures. Herein we present the vibrational spectra of the small cationic copperand vanadium-doped silicon clusters SinCu + and SinV + (n=6–8). Copperand vanadium-doped silicon clusters show the same critical size for the transition from endohedral to exohedral structures, which has been rationalized by the similar atomic radii of the dopants. It is thus interesting to investigate whether doping with these two atoms will generate clusters with the same geometric structure. Figure 1 shows the vibrational spectra of Si8V + . The experimental spectrum (bottom panel) is obtained upon IR–MPD of its complex with one argon atom. In the case of resonant ab-

The Aromatic 8-Electron Cubic Silicon Clusters Be@Si-8, B@Si-8(+) and C@Si-8(2+)

The geometrical and electronic structures of the Si(8)(2-) dianion and isovalent silicon clusters doped by main second-row elements including Li@Si(8)(-), Be@Si(8), B@Si(8)(+), C@Si(8)(2+), N@Si(8)(3+), and O@Si(8)(4+), are investigated using quantum chemical methods. The analyses of phenomenological shell model (PSM) combined with partial electron localizability indicators (pELI-D) rationalize the existence of cubic silicon clusters. A cubic cluster can be formed, in the cases of Be@Si(8), B@Si(8)(+), and C@Si(8)(2+), when three conditions are satisfied, namely, a full occupancy of electronic shells (34 electrons), a presence of positive charge at the center, and a type of spherical aromaticity. A chemical bonding picture for the cubic cage of the doped silicon clusters is illustrated. Each Si atom has four lobes of sp(3) hybridization in which three lobes make three covalent sigma bonds with other Si atoms, and the fourth lobe makes a chemical bond with the dopant. The eight delocalized electrons distributed on the fourth lobes describing the bonding between dopant and Si cage follow the Hirsch rule. We demonstrate that a way of applying electron counting rule is to take into account delocalized electrons on the shell orbitals with N > 1 (2S and 2P shell orbitals).

The structures of neutral transition metal doped silicon clusters, SinX (n = 6−9; X = V, Mn)

We present a combined experimental and theoretical investigation of small neutral vanadium and manganese doped silicon clusters Si(n)X (n = 6-9, X = V, Mn). These species are studied by infrared multiple photon dissociation and mass spectrometry. Structural identification is achieved by comparison of the experimental data with computed infrared spectra of low-lying isomers using density functional theory at the B3P86∕6-311+G(d) level. The assigned structures of the neutral vanadium and manganese doped silicon clusters are compared with their cationic counterparts. In general, the neutral and cationic Si(n)V(0,+) and Si(n)Mn(0,+) clusters have similar structures, although the position of the capping atoms depends for certain sizes on the charge state. The influence of the charge state on the electronic properties of the clusters is also investigated by analysis of the density of states, the shapes of the molecular orbitals, and NBO charge analysis of the dopant atom.

Structure Assignment, Electronic Properties, and Magnetism Quenching of Endohedrally Doped Neutral Silicon Clusters, SinCo (n = 10–12)

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.

Energetics and Mechanism of the Decomposition of Trifluoromethanol

The thermal instability of alpha-fluoroalcohols is generally attributed to a unimolecular 1,2-elimination of HF, but the barrier to intramolecular HF elimination from CF3OH is predicted to be 45.1 +/- 2 kcal/mol. The thermochemical parameters of trifluoromethanol were calculated using coupled-cluster theory (CCSD(T)) extrapolated to the complete basis set limit. High barriers of 42.9, 43.1, and 45.0 kcal/mol were predicted for the unimolecular decompositions of CH2FOH, CHF2OH, and CF3OH, respectively. These barriers are lowered substantially if cyclic H-bonded dimers of CF3OH with complexation energies of approximately 5 kcal/mol are involved. A six-membered ring dimer has an energy barrier of 28.7 kcal/mol and an eight-membered dimer has an energy barrier of 32.9 kcal/mol. Complexes of CF3OH with HF lead to strong H-bonded dimers, trimers and tetramers with complexation energies of approximately 6, 11, and 16 kcal/mol, respectively. The dimer, CH3OH:HF, and the trimers, CF3OH:2HF and (CH3OH)2:HF, have decomposition energy barriers of 26.7, 20.3, and 22.8 kcal/mol, respectively. The tetramer (CH3OH:HF)2 gives rise to elimination of two HF molecules with a barrier of 32.5 kcal/mol. Either CF3OH or HF can act as catalysts for HF-elimination via an H-transfer relay. Because HF is one of the decomposition products, the decomposition reactions become autocatalytic. If the energies due to complexation for the CF3OH-HF adducts are not dissipated, the effective barriers to HF elimination are lowered from approximately 20 to approximately 9 kcal/mol, which reconciles the computational results with the experimentally observed stabilities.