Potential covalent character

A discrepancy which is certainly related to the covalent character of bonding is the sign of the Cauchy pressures Ci2-C66 and C]3-C44. The experimental values of these pressures are -6.4 and -34.2 MPa, respectively, while the potentials give 31 and 30 MPa, respectively. It is a general feature of the Flnnls-Sinclair type potentials and the EAM that negative Cauchy pressures cannot be reproduced. Such Cauchy pressures... 

Particularly desirable among film deposition processes are solution-based techniques, because of the relative simplicity and potential economy of these approaches. However, the covalent character of the metal chalcogenides, which provides the benefit of the desired electronic properties (e.g., high electrical mobility), represents an important barrier for solution processing. Several methods have been developed to overcome the solubility problem, including spray deposition, bath-based techniques, and electrochemical routes, each of which will be discussed in later chapters. In this chapter, a very simple dimensional reduction approach will be considered as a means of achieving a convenient solution-based route to film deposition. 

The alkali halogenide gas molecules MX present a still more extreme case, the bonds being essentially ionic with only a small amount of covalent character. For cesium chloride, involving the most electropositive of the metals and one of the most electronegative of the nonmetals, the electron affinity of the nonmetal (86 kcal/mole) is about as great as the ionization potential of the metal (89 kcal/mole), so that at large intemuclear distances the ionic structure Cs+Cl is about as... 

Authors do not really agree on the charge redistribution which takes place at the MgO(lOO) surface, because the ionicity of the oxide appears to be nearly total in HF approaches [61,63], while in DFT and semi-empirical methods, the Mg-0 bonds present a small but non-negligible covalent character [60,66]. The surface projected Density of States (DOS) displays surface states just at the top of the VB and bottom of the conduction band (CB), originating from a reduction of the Madelung potential on the surface atoms. These states also exist on NiO(lOO) [74] and CoO(100)[75] and were shown to play a role in the formation of STM images (Fig. 2). A small reduction of the HOMO-LUMO gap results in MgO(lOO) as well as CaO(lOO) [60,64,67,68], and NiO(lOO) [74], which is qualitatively consistent with experimental observation [76-79]. 

Aside from the assumptions not open to direct test [e.g., equations (1) and (2)], there is a serious we2ikness in the assumption of constant A—H distance. Experiment shows that a H bond of over-all length 2.5 A would have an O—H bond length near 1.07 A (see Fig. 9-1). Because equation (2) is extremely sensitive to a change in r(A—H), this raises the estimate of the amount of covalent character to 22 percent, double the value obtained by Coulson and Danielsson. Since this is in fair agreement with a second estimate (based on a Morse potential curve) by Coulson and Danielsson, it would seem that the appropriate conclusion of this paper should not be that the H bond is essentially ionic but rather that the covalent contribution is quite important for... 

Another family of potential models takes into account the covalent character of chemical bonds in zeolite lattices more explicitly and uses for the PES representation a set of potential functions like those employed in molecular mechanics (MM) force fields for organic molecules. Such a force field describes the potential energy of the system in two terms ... 

Since G(r) > 0 and V(r) < 0, the lowering of the potential energy dominates the total energy in those regions of space where electronic charge is concentrated, where V p(r) < 0, while the kinetic energy is dominant in regions where V p(r) > 0. We can define the amount of covalent character H rt) where... 

In the study of silica and silicates, MD was primarily used to simulate molten and vitreous states using ionic and pairwise interatomic potentials [1, 2, 3, 4], where the interatomic potential was obtained empirically by trial and error. Although the fourfold oxygen coordination of silicon atoms and non-linear Si-O-Si angle widely observed in silica and silicates seem to suggest covalent character of the Si-0 bonds, it has been shown that these structural properties are roughly reproduced by pairwise interatomic potentials. 

A number of functional forms for the short-range potentials have been used. In the earliest work[7, 8], only pair potentials were employed, but it was quickly realised that the covalent character of silica was too strong to be neglected. Central force models could not reproduce the tetrahedral geometry closely enough, so three-body terms were introduced to provide a measure of the direetional nature of the covalent bonding[9]. These have taken a number of different forms as well. 

The explanation of this effect can be conceived as follows. Phenol has a fairly high dipole moment and has no low-energy acceptor orbitals, whereas iodine has no dipole moment hence interactions with iodine may be expected to have more covalent character than the analogous reactions with phenol. Accordingly, iodine will react more readily with the better polarizable reaction partners possessing lower ionization potentials. Similar considerations may be employed to interpret, for example, the sequence of basic strengths of primary, secondary and tertiary amines [Dr 63], and the sequence of acid strengths of iodine monochloride, elemental bromine, elemental iodine, phenol and sulphur dioxide [Dr 62]. 

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