Overlap p orbital

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible lo define three localized tt bonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six -tt electrons are completely delocalized around the ring. In resonance terms (Sections 2.4 and 2.5), benzene is a hybrid of two equivalent forms. Neither form... 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

We can therefore conclude that in 4n systems antiaromaticity will be at a maximum where a molecule is constrained to be planar (as in 59 or the dianion of 83) but, where possible, the molecule will distort itself from planarity and avoid equal bond distances in order to reduce antiaromaticity. In some cases, such as cyclooctatraene, the distortion and bond alternation are great enough for antiaromaticity to be completely avoided. In other cases (e.g., 96 or 103), it is apparently not possible for the molecules to avoid at least some p-orbital overlap. Such molecules show paramagnetic ring currents and other evidence of antiaromaticity, although the degree of is not as great as in molecules such as 59 or the dianion of 83. 

The n molecular orbitals described so far involve two atoms, so the orbital pictures look the same for the localized bonding model applied to ethylene and the MO approach applied to molecular oxygen. In the organic molecules described in the introduction to this chapter, however, orbitals spread over three or more atoms. Such delocalized n orbitals can form when more than two p orbitals overlap in the appropriate geometry. In this section, we develop a molecular orbital description for three-atom n systems. In the following sections, we apply the results to larger molecules. 

When two p orbitals overlap in a side-by-side configuration, they form a pi bond, shown in Figure 7.7. This bond is named after the Greek letter 7t. The electron clouds in pi bonds overlap less than those in sigma bonds, and they are correspondingly weaker. Pi bonds are often found in molecules with double or triple bonds. One example is ethene, commonly known as ethylene, a simple double-bonded molecule (Figure 7.8). The two vertical p orbitals form a pi bond. The two horizontal orbitals form a sigma bond. 

When p orbitals overlap in a side-by-side configuration, they form a pi bond. 

Here there is no formal electronic bar to interaction between the electrons, i.e. pairing to form the diamagnetic species (137) but this does not in fact happen, because the bulky chlorine atoms in the o-positions prevent the benzene rings from attaining a conformation close enough to coplanarity to allow of sufficient p orbital overlap for electron-pairing to occur. 

The parallel p orbitals overlap above and below the plane of the a framework. 

The p orbital of this new sp -hybridized carbon atom overlaps with the p orbital of the central carbon atom => in the allyl radical three p orbitals overlap to form a set of 7T MOs that encompass all three carbon atoms. 

In the C2H4 molecule, unhybridized p orbitals overlap in side by side and form a n bond. 

The ji-MO of butadiene are shown as below where the adjacent lobes of the same sign (phase) of p orbitals overlap while the adjacent lobes with opposite signs produce a node in the resulting MO. The 4 n electrons of butadiene are accommodated in the two bonding MO s i.e., /, and /2. 

In the benzene molecule, each carbon atom is sp hybridised and the three half-filled sp hybrid orbitals form a bonds with a hydrogen atom and two neighbouring carbon atoms. This leaves an electron occupying a p orbital on each carbon atom. Each of these p orbitals overlaps slde-on with p orbitals on neighbouring carbon atoms, and a tt molecular orbital Is formed, as shown in the diagram. 

A form of p orbital overlap in certain types of molecules in which two groups of moieties are not conjugated with each other yet both are conjugated with a third. 

Sigma bonds form when s or p orbitals overlap in a head-on manner. Single bonds cire usually sigma bonds. Pi bonds cire usually double or triple bonds. Figure 5-9 depicts these situations. 

Two parallel p orbitals overlap side-by-side to form a pi (tt) bond. Fig. 2-3(u), or a n bond. Fig. 2-3(6). The bond axis lies in a nodal plane (plane of zero electronic density) perpendicular to the cross-sectional plane of the tt bond. 

See Fig. 2-12. The C and N each have one cr bond and one unshared pair of electrons, and therefore each needs two sp hybrid HO s. On each atom one sp hybrid orbital forms a cr bond while the other has the unshared pair. Each atom has a p, AO and a p. AO. The two p, orbitals overlap to form a ir, bond in the xy -plane the two p, orbitals overlap to form a n, bond in the Arz-plane. Thus, two ir bonds at right angles to each other and a a bond exist between the C and N atoms. 

The H s on the C attached to the ring (the benzylic H s), although they are in this case 2°, are nevertheless more reactive toward Br- than are ordinary 3° H s. Like a C=C group in the allylic system, the Ph group can stabilize the free radical by electron donation through extended p orbital overlap. 

Covalent bonds are formed when atomic orbitals overlap. The overlap of atomic orbitals is called hybridization, and the resulting atomic orbitals are called hybrid orbitals. There are two types of orbital overlap, which form sigma (cr) and pi (tt) bonds. Pi bonds never occur alone without the bonded atoms also being joined by a ct bond. Therefore, a double bond consists of a O bond and a tt bond, whereas a triple bond consists of a ct bond and two tt bonds. A sigma overlap occurs when there is one bonding interaction that results from the overlap of two s orbitals or an s orbital overlaps a p orbital or two p orbitals overlap head to head. A tt overlap occurs only when two bonding interactions result from the sideways overlap of two parallel p... 

The six overlapping p orbitals overlap to form a set of six p molecular orbitals. Six tt electrons are completely delocalized around the ring, and... 

In SHMO, the core energies of heteroatoms, X, are specified in terms of a and / , and the interaction matrix elements for p orbitals overlapping in fashion on any pair of atoms, X and Y, are specified in terms of / . Thus,... 

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