Oxygen nuclei equivalent

In the plane of the nuelei, p again exhibits maxima only at the positions of the nuclei and ridges linking the oxygen nucleus to the protons, indicating the presence of two bond paths. The negative of the Laplaeian distribution, on the other hand, in addition to the maxima at the positions of the nuclei, exhibits two equivalent bonded maxima in the valence-shell eharge eoncen-... 

Fig. 8.30. A shock therapy. The localized molecular orbitals of water molecule, (a) An orbital, which is strougly localized on the oxygen nucleus and nearly spherical (practically Is of the oxygen atom), (b) and (c) are the OH bond orbitals, one for each OH bond, (d) The first of the two lone parr orbitals is a hybrid with its axis within the molecular plane, (e) The second of the two lone pair orbitals (orthogonal to the plane of the molecule) is a pure 2p orbital of oxygen. It is clear that a sum of the last two orbitals leads to a hybrid, which has to go up off the plane. On the other hand, a subtraction of these orbitals brings a twin hytaid but down from the plane. In this way, we recover (in a fully correct way) the equivalent lone pairs of water molecules seen in textbooks.

Reduced nicotinamide-adenine dinucleotide (NADH) plays a vital role in the reduction of oxygen in the respiratory chain [139]. The biological activity of NADH and oxidized nicotinamideadenine dinucleotide (NAD ) is based on the ability of the nicotinamide group to undergo reversible oxidation-reduction reactions, where a hydride equivalent transfers between a pyridine nucleus in the coenzymes and a substrate (Scheme 29a). The prototype of the reaction is formulated by a simple process where a hydride equivalent transfers from an allylic position to an unsaturated bond (Scheme 29b). No bonds form between the n bonds where electrons delocalize or where the frontier orbitals localize. The simplified formula can be compared with the ene reaction of propene (Scheme 29c), where a bond forms between the n bonds. 

Figure 2.11 Cartoons of contours of i/r for the equivalent representations of oxygen lone pairs in an ether as in two sp orbitals or one p and one sp orbital and an electron density contour. Note that hybrid sp orbitals do not have a node at the nucleus, as a pure p orbital does, but rather one behind the nucleus, between it and the small lobe.

Because the 2p orbitals have a node at the atomic nucleus, the electron distribution of the VB wavefunction formed by their side-on overlap does not have cylindrical symmetry, but instead changes sign when rotated by 180° about the bond axis. Bonds that have this property are called pi (tt) bonds. Thus, the double bond in the oxygen molecule consists of a sigma bond and a pi bond, which are not equivalent. The existence of two different types of bonds in double bond formation is something that is not predicted by simple Lewis theory. A triple bond such as that in N2 is described within the VB approach as a sigma bond formed from the head-on overlap of 2p orbitals and two pi bonds formed by the overlap of the 2py and 2p orbitals on one N atom with their counterparts on the other atom (Figure 3.7). 

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