Bond Ab Research Articles

A Fluorooxosilicophosphate with an Unprecedented SiO2 F4 Species.

The large number and structural beauty of silicates are largely due to the variety of connection mode of SiO4 tetrahedra. SiO6 and SiF6 octahedra are also known and give rise for structural versatility of inorganic silicates. However, to date, the crystal structure of inorganic fluorooxosilicates with oxofluoride SiOx F4-x or SiOx F6-x species are unknown. Now, fluorine was successfully introduced into the silicophosphates, and the first fluorooxosilicophosphate K4 Si3 P2 O7 F12 with an unprecedented SiO2 F4 species was synthesized. The existence of Si-F bonds was verified by comprehensively experimental and theoretical work. Using ab initio and bond valence calculations, the oxofluoride SiOx F6-x species is shown to be stable when oxygen atoms connect to other atoms with strong covalent interactions. This work will contribute to the structural diversity of silicate chemistry by the exploration of the new fluorooxosilicates.

A Fluorooxosilicophosphate with an Unprecedented SiO2F4 Species

AbstractThe large number and structural beauty of silicates are largely due to the variety of connection mode of SiO4 tetrahedra. SiO6 and SiF6 octahedra are also known and give rise for structural versatility of inorganic silicates. However, to date, the crystal structure of inorganic fluorooxosilicates with oxofluoride SiOxF4−x or SiOxF6−x species are unknown. Now, fluorine was successfully introduced into the silicophosphates, and the first fluorooxosilicophosphate K4Si3P2O7F12 with an unprecedented SiO2F4 species was synthesized. The existence of Si−F bonds was verified by comprehensively experimental and theoretical work. Using ab initio and bond valence calculations, the oxofluoride SiOxF6−x species is shown to be stable when oxygen atoms connect to other atoms with strong covalent interactions. This work will contribute to the structural diversity of silicate chemistry by the exploration of the new fluorooxosilicates.

Generalization of Wertheim's theory for the assembly of various types of rings.

We generalize Wertheim's first order perturbation theory to account for the effect in the thermodynamics of the self-assembly of rings characterized by two energy scales. The theory is applied to a lattice model of patchy particles and tested against Monte Carlo simulations on a fcc lattice. These particles have 2 patches of type A and 10 patches of type B, which may form bonds AA or AB that decrease the energy by εAA and by εAB ≡ rεAA, respectively. The angle θ between the 2 A-patches on each particle is fixed at 60°, 90° or 120°. For values of r below 1/2 and above a threshold rth(θ) the models exhibit a phase diagram with two critical points. Both theory and simulation predict that rth increases when θ decreases. We show that the mechanism that prevents phase separation for models with decreasing values of θ is related to the formation of loops containing AB bonds. Moreover, we show that by including the free energy of B-rings (loops containing one AB bond), the theory describes the trends observed in the simulation results, but that for the lowest values of θ, the theoretical description deteriorates due to the increasing number of loops containing more than one AB bond.

Quantum characteristics of the hydrogen bond

Most analyses of hydrogen bond strengths and dynamics (e.g. vibrational spectroscopy and transport) have been considered in terms of (model) potential energy functions defined at the Born–Oppenheimer level of approximation. This paper reports an analysis of the quantum dynamics of hydrogen in the bond that focuses on the quantum ground state of the binding hydrogen in the fields of the other atoms. In view of its mass and size, hydrogen is delocalized in its bond much as is the electron in a covalent bond. It is shown, under certain circumstances, that the compression of the A–B distance, with hydrogen in the A–H–B bond space, yields a flatter effective A–B bond potential due to the reduction of hydrogen density along the direct AB bond line. The quantum behaviour of bonding hydrogen also influences dynamical processes such as vibrational spectroscopy and hydrogen or proton transport. †Also Adjunct Professor, Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5 Canada.

Elastic and thermodynamic properties of potentially superhard carbon boride materials

Boron icosahedral structures are the basic building structures of many important hard borides and especially B4C. The structural and thermodynamic properties of B4C have been examined applying molecular dynamics simulation with the use of both ab initio and bond order Tersoff potentials. Various physical quantities of B4C including the elastic constants, thermal expansion coefficients, and specific heat have been examined.

How patchy can one get and still condense? The role of dissimilar patches in the interactions of colloidal particles

We investigate the influence of strong directional, or bonding, interactions on the phase diagram of complex fluids, and in particular on the liquid–vapour critical point. To this end we revisit a simple model and theory for associating fluids which consist of spherical particles having a hard-core repulsion, complemented by three short-ranged attractive sites on the surface (sticky spots). Two of the spots are of type A and one is of type B; the interactions between each pair of spots have strengths , and . The theory is applied over the whole range of bonding strengths and results are interpreted in terms of the equilibrium cluster structures of the coexisting phases. In systems where unlike sites do not interact (i.e. where ), the critical point exists all the way to . By contrast, when , there is no critical point below a certain finite value of . These somewhat surprising results are rationalised in terms of the different network structures of the two systems: two long AA chains are linked by one BB bond (X-junction) in the former case, and by one AB bond (Y-junction) in the latter. The vapour–liquid transition may then be viewed as the condensation of these junctions and we find that X-junctions condense for any attractive (i.e. for any fraction of BB bonds), whereas condensation of the Y-junctions requires that be above a finite threshold (i.e. there must be a finite fraction of AB bonds).

Predicting the activation energy of catalytic dissociation of the heteroatomic AB bond

Two analytical formalisms, adiabatic and diabatic ones, were developed for the description of catalytic dissociation of heteroatomic bond AB interacting with a metal surface in the adsorption processes. In the adiabatic formalism, the transition state was localized on a four-dimensional potential energy surface in classical approximation. This approach generalizes the previous three-dimensional model for dissociative adsorption of homonuclear molecules X 2 on metals surfaces, and it was used for studying the effect of non-parallel orientation to a surface of O 2 molecules in the adsorption precursor state. The second formalism takes into account a possible quantum character of vibrations along the chemical bond AB. The calculation of the activation energy in this approach is performed by the density matrix method. This approach is applied for studying catalytic dissociation of CO molecule on a Ni(1 1 1) surface. The calculated apparent activation energy for this reaction is compared with published data for this system.

On AB Bond Percolation on the Square Lattice and AB Site Percolation on Its Line Graph

We prove that AB site percolation occurs on the line graph of the square lattice when \(p \in (1 - \sqrt {1 - p_c } ,\sqrt {1 - p_c } )\), where p c is the critical probability for site percolation in \(\mathbb{Z}^2\). Also, we prove that AB bond percolation does not occur on \(\mathbb{Z}^2\) for p = \(\frac{1}{2}\).

Exact geometries from quantum chemical calculations

For seventeen molecules, complete basis set (CBS) geometries are obtained for Møller–Plesset perturbation methods at second (MP2), fourth (MP4), and sixth order (MP6) as well as for the Coupled Cluster methods CCD, CCSD, and CCSD(T). The correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ were systematically applied and calculated geometries extrapolated to the limit of an infinitely large basis set. MP6 equilibrium geometries are more accurate than MP2 or MP4 geometries at the CBS limit and provide AH bond lengths with an accuracy of 0.001Å. However, AB bonds are always predicted too long because of the lack of sufficient coupling effects between p-electron correlation at MP6. CCSD(T) provides reasonable AB bond lengths although these are in general too short by 0.003Å. Due to error cancellation very accurate geometries are obtained at the CCSD(T)/cc-pVTZ and CCSD(T)/cc-pVQZ level of theory. With the help of the accurate equilibrium geometries obtained in this work, several experimentally based geometries could be corrected. The effects of HF-optimized basis sets, diffuse functions or the frozen core approximation on geometry optimizations are discussed. It is emphasized that the use of the cc-pVDZ or any other VDZ+P basis set should be avoided in correlation corrected ab initio calculations.

Chemistry of vibronic coupling. Part 2. How to maximize the dynamic diagonal vibronic coupling constant for T1 states in AB systems (A, B=H, Li, Na, K, Rb, Cs, F, Cl, Br or I)?

The dynamic diagonal vibronic coupling constant (VCC) in several series of AB and AA molecules (A, B = H, Li, Na, K, Rb, Cs, F, Cl, Br or I) has been investigated. The electronic states considered are the singlet ground state (“ionic ” for heteronuclear AB species) and first excited singlet or triplet states (“covalent”). The VCC is thus studied for a charge transfer lowest lying triplet state. Qualitative trends in the VCC within the families of systems studied have been sought, with the aim of finding “ a chemistry of vibronic coupling”. Two interesting correlations emerge: the VCC for the charge transfer states in an AB system grows as the sum of the electronegativities of the A and B elements increases, as well as with decreasing AB bond length. A parameter f was defined as the sum of the electronegativities of the A and B elements divided by the AB bond length. This leads to a nearly monotonic correlation between computed values of VCC and f for 55 molecules originating from three distinct classes with a formal single bond: intermetallic species M1M2 (M = alkali metal), interhalogens X1X2 (X = halogen) and salt-like compounds MX. It emerges that contracted p-type orbitals making up the σ* MO (occupied by one electron in the excited state) seem to provide higher values of VCC than diffused s orbitals. The energy of the singlet–triplet gap is also correlated with the sum of the electronegativities of the A and B elements within two families of diatomics. Quantitative explanations of these two trends are still sought.

Chemistry of Vibronic Coupling. 3. How One Might Maximize Off-Diagonal Dynamic Vibronic Coupling Constants for Intervalence Charge-Transfer (IVCT) States in an ABA• System (A, B = Alkali Metal, H, Halogen)?

We investigate computationally the vibronic coupling constant in a large group of simple uncharged ABA• systems (A, B = H, Li, Na, K, Rb, Cs, F, Cl, Br, I), which may be formally mixed-valence or intermediate-valence ones. These open-shell species belong to three chemically quite distinct groups: intermetallic, interhalogen, and salt-like triatomics. Constraining these systems to a linear symmetric geometry, we optimize the A−B bond length at a certain level of theory and determine the electronic and vibrational structure of such species. At this common geometry we calculate a vibronic stability parameter for the antisymmetric mode (G), defined as the ratio of force constants for the antisymmetric to the symmetric stretching mode. G is a sensitive indicator of the magnitude of vibronic coupling. Values of G smaller than 1 (asymmetric mode softening) indicate large values of the off-diagonal vibronic coupling constant. Negative G indicate very large values of that constant, leading to vibronic instability. The smallest values of G are obtained for interhalogen species (most of them are vibronically unstable). Salt-like ABA and BAB molecules, as well as intermetallic species are vibronically stable. Two further interesting correlations evolve: the calculated force constants for the symmetric stretching mode correlate well with f, a parameter defined as the sum of the electronegativities of the A and B elements divided by the AB bond length. Large values of f also indicate strong vibronic coupling.

Is CO a Special Ligand in Organometallic Chemistry? Theoretical Investigation of AB, Fe(CO)4AB, and Fe(AB)5 (AB = N2, CO, BF, SiO)

Carbon monoxide, CO, is a ubiquitous ligand in organometallic and coordination chemistry. In the present paper we investigate the neutral isoelectronic molecules AB = N2, CO, BF, and SiO and their coordination in the model complexes Fe(CO)4AB and Fe(AB)5, using nonlocal density functional theory and a large, polarized STO basis set of triple-ζ quality (NL-SCF/TZ(2P)). Our aim is to get more insight into the ligating properties of SiO and BF in comparison to CO and N2. The computed 298 K Fe(CO)4−AB bond dissociation enthalpies of C3v-symmetric Fe(CO)4AB are 18.1, 42.3, 67.9, and 35.6 kcal/mol for N2, CO, BF, and SiO, respectively; the corresponding values for C2v-symmetric Fe(CO)4AB are comparable: 19.0, 42.3, 66.7, and 39.7 kcal/mol. Good, balanced σ donation (through 5σ) and π acceptance (through 2π) are what makes CO a good donor, of course. The gap between these frontier orbitals (5σ and 2π) becomes even smaller in SiO and BF. The analysis of the bonding mechanism of the Fe−AB bond shows that SiO is a better σ donor but a worse π acceptor ligand than CO and that BF should be superior to CO in terms of both σ donor and π acceptor properties. However, these polar ligands are therefore also more reactive; and more sensitive, e.g. to nucleophilic attack, because of a low-energy 2π LUMO. Our results suggest that BF and SiO should, in principle, be excellent ligands. We also find interesting side-on and O-bound local minima, not very unstable, for SiO bound to an Fe(CO)4 fragment.

Properties of Transition Species in the Reactions of Hydroxyl with Ammonia and with Itself

Quantum transition state theory was used to model two hydrogen atom abstraction reactions of hydroxyl radicals. Moments of inertia for the transition species were calculated from geometries obtained by ab initio or bond energy bond order methods. The transition state equation was fit by nonlinear least squares to experimental rate constants to determine parameters of the transition species. For the reaction of hydroxyl with ammonia, the geometric mean, ωb, of four transitional vibrational frequencies was found to be 660 ± 20 cm-1, the activation barrier height, Vb, including zero point energy, to be 8.5 ± 0.5 kJ mol-1, and the barrier thickness, at half its height, to be 65 ± 30 pm. For the disproportionation reaction of hydroxyl radicals, the corresponding values of the first two parameters, based on the most recent experimental data, were 890 ± 20 cm-1 and 2.7 ± 0.8 kJ mol-1. Ab initio calculations on the latter reaction gave ωb = 746 cm-1 at the HF/6-31G(d,p) level and Vb = 13.6 kJ mol-1 at the MP4/6-3...

Pearson′s chemical hardness, heterolytic dissociative version of Pauling′s bond-energy equation and a novel approach towards understanding Pearson′s hard–soft acid–base principle

The empirical chemical hardness parameters developed recently by Pearson have been examined critically. It was found that these parameters normally characterise a group but not the corresponding ion as postulated. Equation (i) has been found quite successful when the heterolytic dissociation of D(A+B–)–½[D(A+A–)+D(B+B–)]= 2|Δγ|2(i) AB into A++ B– follows the general notion of electronegativity (i.e. the more electronegative part of AB dissociates as the anion), where D(A+B–) is the energy required to dissociate the AB bond into A++ B– and D(A+A–) and D(B+B–) are the heterolytic bond-dissociation energies of the molecules AA and BB respectively. Equation (i) has been used to define a new parameter γ, where |Δγ|=|γA+–γB–|. The γ parameters characterise the ions, and have been used to rank various monovalent anions and cations in a unified way to explain the hard–soft acid–base principle of Pearson. It was found that a reaction of type (II) proceeds from left to right in the gas phase if the AB + CD → AC + BD (ii)|ΔΔγ| value of the right-hand side is greater than that of the left. Out of some 282 gas-phase reactions considered, there are few exceptions. Emphasis has been placed on explaining the course of inorganic reactions. It is felt that the γ parameter can be identified with the chemical hardness of an ion.

Indice de liaison en méthode CNDO. Relation entre indice de liaison et distance de liaison

The use of p AB = Σ μ A Σ ν B ϱ μν S μν as bond index in the CNDO approximation is theoretically justified as it is an elaborated form of Coulson's bond order and an approximation of Roby's shared electron density. This bond index correlates with diatomic energy E AB, a measure of binding energy, and with interatomic distance R AB, for several bonds involving hydrogen and atoms of the n = 2 period. As an example of utilization of p AB bond lengths in butadiene and a series of ethylenic compounds are determined.

Electronegativity and Electron Affinity

Various consequences of the supposition that the value of the resonance integral βAB depends on the nature of the bond are evaluated within the framework of the simple Hückel method. The resulting equation for βAB in function of the polarity 1 of the AB bond is: βAB =(1-I2)-½ [C+(½) ∫ (αA-αB)dI] where αX is the Coulomb integral. A solution is proposed corresponding with βAB = βAA ·βBB)½. Further elaboration then suggests that the simple Hückel method in this form is in favour of the ionic approximation to chemical bonding. The Coulomb integral turns out to be the electron affinity, which must now be considered as the electronegativity of elements when forming chemical bonds.

Complexes of alkaline earths and cyanate in alkali halide crystals

Single crystals of KCl and KBr produced from the molten salt simultaneously doped with Ca +2 or Ba +2 and NCO − exhibit numerous new infrared absorption lines indicating a perturbed cyanate spectrum relative to the well known lines due to cyanate alone in the alkali halides. In most cases, ν 1 and ν 3 were shifted to higher frequencies. A detailed comparison of force constants for the pairs NCO − and HNCO, N − 3 and NH 3, NCS − and HNCS shows a similar shift in the bonding in which the AB bond is softened, the BC bond stiffened in HABC relative to ABC −, although the frequencies of ν 1 and ν 3 shift in different directions in these examples. The new lines reported here closely parallel the HNCO frequencies, and are attributed to complexes occurring in the crystals.