Water orbital model

Acetylene (C2H2) can be produced from the reaction of calcium carbide (CaC2) with water. Use both the localized electron and molecular orbital models to describe the bonding in the acetylide anion (Cj "). 

Murdachaew et used the SCP-NDDO semiempirical molecular orbital model to calculate an increase in the dipole moment from the equilibrium gas-phase value to the liquid-phase value from 2.16 D to 2.8 D, an increase of 30%, whereas with the older PM3 and PM6 NDDO-t3q)e method, which significantly underestimate the polarizability of water, they found that the increase was only 9% and 11%, respectively. 

To improve our model we note that s- and /7-orbitals are waves of electron density centered on the nucleus of an atom. We imagine that the four orbitals interfere with one another and produce new patterns where they intersect, like waves in water. Where the wavefunctions are all positive or all negative, the amplitudes are increased by this interference where the wavefunctions have opposite signs, the overall amplitude is reduced and might even be canceled completely. As a result, the interference between the atomic orbitals results in new patterns. These new patterns are called hybrid orbitals. Each of the four hybrid orbitals, designated bn, is formed from a linear combinations of the four atomic orbitals ... 

The square cell is convenient for a model of water because water is quadrivalent in a hydrogen-bonded network (Figure 3.2). Each face of a cell can model the presence of a lone-pair orbital on an oxygen atom or a hydrogen atom. Kier and Cheng have adopted this platform in studies of water and solution phenomena [5]. In most of those studies, the faces of a cell modeling water were undifferentiated, that is no distinction was made as to which face was a lone pair and which was a hydrogen atom. The reactivity of each water cell was modeled as a consequence of a uniform distribution of structural features around the cell. 

C09-0115. The H—O—H bond angle in a water molecule is 104.5°. The H—S—H bond angle in hydrogen sulfide is only 92.2°. Explain these variations in bond angles, using orbital sizes and electron-electron repulsion arguments. Draw space-filling models to illustrate your explanation. 

All but two of the known synthetic iron(IV)-oxo compounds are low-spin, 5=1 [202, 240]. The first example of an iron(IV)-oxo model compound with spin 5 = 2 was the quasioctahedral complex [(H20)5Fe =0] (5 = 0.38 mm s, A q = 0.33 mm s ) which was obtained by treating [Fe°(H20)6] with ozone in acidic aqueous solution [204]. The spin state of iron in this type of structure is determined by the energy gap between the d,2 y2 and the d y orbitals [241]. The weak water ligands induce a sufficiently small gap being less than the spin paring energy and stabilizing the HS state (Fig. 8.25, case a). 

In order to give a concrete expression for Feff(x), a simple model has been assumed (see Fig. 6). In this model, it is postulated that an electron in the HOMO (essentially the 2p orbital) of the surface oxygen atom (Osu g) of the polyanion partially transfers to the LUMO of water (the 4ai molecular orbital). Then Feff(x) can be expressed as... 

Equation (4-5) can be directly utilized in statistical mechanical Monte Carlo and molecular dynamics simulations by choosing an appropriate QM model, balancing computational efficiency and accuracy, and MM force fields for biomacromolecules and the solvent water. Our group has extensively explored various QM/MM methods using different quantum models, ranging from semiempirical methods to ab initio molecular orbital and valence bond theories to density functional theory, applied to a wide range of applications in chemistry and biology. Some of these studies have been discussed before and they are not emphasized in this article. We focus on developments that have not been often discussed. 

Figure 3.13 shows approximate representations of the 2px and 2p orbitals. To construct the two bonding orbitals b] and b2 we place the H atoms along the directions of the 2p and 2pv orbitals of the O atom to give maximum overlap of a Is orbital of H with the 2p, and 2py 0 orbitals (Figure 3.13). This bonding model implies a bond angle of 90° for the water molecule, which is not in very good agreement with the observed angle of 104.5°.

Callisto orbits Jupiter at a distance of 1.9 million kilometres its surface probably consists of silicate materials and water ice. There are only a few small craters (diameter less than a kilometre), but large so-called multi-ring basins are also present. In contrast to previous models, new determinations of the moon s magnetic field suggest the presence of an ocean under the moon s surface. It is unclear where the necessary energy comes from neither the sun s radiation nor tidal friction could explain this phenomenon. Ruiz (2001) suggests that the ice layers are much more closely packed and resistant to heat release than has previously been assumed. He considers it possible that the ice viscosities present can minimize heat radiation to outer space. This example shows the complex physical properties of water up to now, twelve different crystallographic structures and two non-crystalline amorphous forms are known Under the extreme conditions present in outer space, frozen water may well exist in modifications with as yet completely unknown properties. 

---