Orbital non-bonding

The representation of non-bonding orbitals on an atom again uses the concept of. T-systems, though they may have any kind of hybridization (p, sp etc.), In Figure 2-56 the three possibilities arc shown lone pairs, radicals, and orbitals without electrons can be accommodated by this eoneept. 

For example, in formaldehyde, H2CO, one forms sp hybrids on the C atom on the O atom, either sp hybrids (with one p orbital "reserved" for use in forming the n and 7i orbitals and another p orbital to be used as a non-bonding orbital lying in the plane of the molecule) or sp hybrids (with the remaining p orbital reserved for the n and 7i orbitals) can be used. The H atoms use their 1 s orbitals since hybridization is not feasible for them. The C atom clearly uses its sp2 hybrids to form two CH and one CO a bondingantibonding orbital pairs. 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

In a second example, the three CH bonds, three CH antibonds, CO bond and antibond, and three 0-atom non-bonding orbitals of the methoxy radical H3C-O also cluster into ai and e orbitals as shown below. In these cases, point group symmetry allows one to identify degeneracies that may not have been apparent from the structure of the orbital interactions alone. 

Figure 7 The splitting of the adjacent non-bonding orbitals in pyridazine...

The UV absorption of pyrimidine occurs in two bands centred at 243 and 298 nm in cyclohexane. The second band is ascribed to the electronic transition from a nitrogen lone pair non-bonding orbital to an empty ring tt-orbital, in short an n transition, on... 

Combine POs with the same symmetry label(s) to form approximate MOs. From each couple, an in-phase and an out-of-phase combination must again be obtained. POs whose symmetry is such that they cannot mix, will remain as non-bonding orbitals. The in-phase combination will have a lower energy than the out-of-phase one. 

The most evident of these is the marked stability of radical cations formed in an aprotic medium by the oxidation of compounds where the first ionization potential (in the sense of photoelectron spectroscopy) is for the removal of an electron from a non-bonding orbital, e. g. thianthrene... 

In Fig. 2 are depicted anion and neutral potential curves that are qualitatively illustrative of (lb,4d) the X II NH case mentioned earlier. In this anion, the HOMO is a non-bonding 2pn orbital localized almost entirely on the N atom. As such, its LCAO-MO coefficients are not strongly affected by motion of the N-H bond (because it is a non-bonding orbital). Moreover, the anion and neutral surfaces have nearly identical Re and a>e values, and similar De values, as a result of which these two sur ces are nearly parallel to one... 

Bonds interact with one another in molecules. The bond interactions are accompanied by the delocahzation of electrons from bond to bond and the polarization of bonds. In this section, bond orbitals (bonding and antibonding orbitals of bonds) including non-bonding orbitals for lone pairs are shown to interact in a cychc manner even in non-cychc conjugation. Conditions are derived for effective cychc orbital interactions or for a continuous orbital phase. 

Electron-donating orbitals are those occupied by electrons, i.e., bonding orbitals of bonds, non-bonding orbitals of lone pairs, HOMOs of molecnles, gronps and others. Electron-accepting orbitals are vacant orbitals, i.e., antibonding orbitals of bonds, vacant atomic orbitals on cationic centers, LUMOs of molecnles, gronps, etc. 

Scheme 7 illnstrates the mechanism of the polarization of the n bond. In one path, an electron in the non-bonding orbital a of the anionic center A is transferred to the antibonding orbital r of the double bond TI. The A TI delocalization is... 

The unsymmetrization of the nitrogen non-bonding orbital (n ) was due to out-of-phase interaction of the electron-rich a orbital at the (3 positions (115), which leads to syn addition. Furthermore, in-phase interaction of the nitrogen non-bonding orbital (n ) with the low-lying vacant a orbitals (116, due to the electron-withdrawing CF3 groups) can contribute to the syn preference. 

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