Water O—H bonds

Optical components are manufactured from transparent materials of certain refractive indices. We have already discussed the refractive properties of glass and quartz in Sect.6.1.1. Here we will consider the transmission properties of optical materials. Optical glass is transparent from about 350 nm to 2.6 /im. This region of transmission can be extended by using quartz. In Fig.6.48 transmission curves for different qualities of quartz are shown. As can be seen the best quartz has a transmission down to 170 nm. However, strong absorption bands can occur in the near IR region, particularly at 2.7 /xm. These are due to the presence of water (O-H bonds) in the quartz. Water-free quartz can be used up to about 3.5 /xm. 

Not only functions but also displacement vectors can be analyzed for symmetry properties. Displacements can be associated with movement of atoms (vibration, rotation, and translation of molecules), and these may transform as symmetry representations. Consider a vector that corresponds to simultaneously stretching both water O-H bonds by like amounts. A picture of this displacement is that of two vectors of the same magnitude on the two hydrogen centers. 

One can start building up a list of MM3 parameters by use of the TINKER analyze command. Don t expect to build up the entire set, which occupies about 100 pages in the MM3 user s manual, but do obtain a few representative examples to get an idea of how a parameter set is constr ucted. From previous exercises and projects, you should have input and output geometries for an alkene, an alkane, and water. From these, the object is to determine the stretching and bending parameters for the C—C, C=C, C—H, and O—H bonds. The C—H bond parameters are not the same... 

Properties. SUica gel (see Eig. 8) is a coherent, rigid, continuous three-dimensional network of spherical particles of coUoidal sUica. Both sUoxane, —Si—O—Si—, and sUanol, —Si—O—H, bonds are present in the gel stmcture. The pores are intercoimected and fUled with water and/or alcohol from the hydrolysis and condensation reactions (40). A hydrogel is a gel in which the pores are filled with water. A xerogel is a gel from which the hquid medium... 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

Alcohols and phenols have nearly the same geometry around the oxygen atom as water. The R-O—H bond angle has an approximately tetrahedral value (109° in methanol, for example), and the oxygen atom is sp3-hybridized. 

The ionization energy of the hydrogen atom, 313.6 kcal/mole, is quite close to that of fluorine, so a covalent bond between these two atoms in HF is expected. Actually the properties of HF show that the molecule has a significant electric dipole, indicating ionic character in the bond. The same is true in the O—H bonds of water and, to a lesser extent, in the N—H bonds of ammonia. The ionic character of bonds to hydro-... 

O-H bond length was 1.08A, a value similar to that previously reported by Szy-tula et al. in a neutron diffraction study of Ni(OH)2 [23]. The O-H bond is both well crystallized and as precipitated materials is parallel to the c-axis. The difference between well-crystallized and as precipitated material is important since the well-crystallized material is not electrochemi-cally active. The differences between the materials are attributed to a defective structure that accrues from the large concentration of surface OH ion groups in the high-surface-area material [22]. These are associated with absorbed water. This is a consistent with an absorption band in the infrared at 1630cm 1. This is not seen in the well-crystallized material. 

The protons come from the water molecules that hydrate these metal cations in solution (Fig. 10.19). The water molecules act as Lewis bases and share electrons with the metal cations. This partial loss of electrons weakens the O -H bonds and allows one or more hydrogen ions to be lost from the water molecules. Small, highly charged cations exert the greatest pull on the electrons and so form the most acidic solutions. 

There are two kinds of bond energy. The energy necessary to cleave a bond to give the constituent radicals is called the dissociation energy D. For example, D for H2O—>HO -f H is 118 kcal mol (494kJmol ). However, this is not taken as the energy of the O—H bond in water, since D for H—O H -f O is 100 kcal... 

Regardiess of the conditions, the reaction of methane with molecular oxygen to form water and carbon dioxide invoives breakage of two ODO bonds and four C—H bonds and subsequent formation of four O— H bonds and two C O bonds for every molecule of methane that reacts. 

Imagine that this reaction occurs along a two-step path. In the first step, the bonds In the reactants break, yielding four hydrogen atoms and two oxygen atoms. In the second step, the hydrogen and oxygen atoms form two water molecules that contain four O—H bonds. 

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. 

The structure of ice. (a) Each oxygen atom is at the center of a distorted tetrahedron of hydrogen atoms. The tetrahedron is composed of two short covalent O—H bonds and two long H—O hydrogen bonds, (b) Water molecules are held in a network of these tetrahedra. 

Hydrogen bonding in solid ice creates a three-dimensional network that puts each oxygen atom at the center of a distorted tetrahedron. Figure 11-16 shows that two arms of the tetrahedron are regular covalent O—H bonds, whereas the other two arms of the tetrahedron are hydrogen bonds to two different water molecules. 

Remember from Chapter H that the hydrogen atoms of water molecules form hydrogen bonds with electronegative O and N atoms, whereas the oxygen atom of a water molecule forms hydrogen bonds with hydrogen atoms in highly polar N—H and O—H bonds. 

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