Bonding molecular orbitals electronic transitions from

Here P°jj,)V is a constant (having energy units) characteristic of the bonding interaction between and %v its value is usually determined by forcing the molecular orbital energies obtained from such a qualitative orbital treatment to yield experimentally correct ionization potentials, bond dissociation energies, or electronic transition energies. 

This weak transition is due to the promotion of an electron from the non-bonding molecular orbital n to an anti-bonding tt orbital. This transition is usually observed in molecules that contain a heteroatom as part of an unsaturated system. The most common of these bands corresponds to the carbonyl band at around 270 to 295 nm, which can be easily observed. The molar absorption coefficient for this band is weak. The nature of the solvent influences the position of absorption bands because the polarity of the bond is modified during absorption. For example, ethanal Amax = 293 nm (e = 12 in ethanol as solvent). 

Absorption bands in the visible region arise from electronic transitions, from the ground state to excited states. The interpretation of these spectra can often lead to a detailed description of these states in terms of molecular orbital theory. This can reveal much about the structure of the chromophore and the nature of the chemical bonds therein. On a more empirical basis, we can often infer the identity of the axial ligands present in a haemoprotein from its spectrum (16). A proper understanding of the origins of the absorption bands of haemoproteins can be of great value in both structural and theoretical studies. 

Electronic transitions from bonding to antibonding molecular orbitals are often encountered. In this case the potential energy curve for the ground state will be quite different from that of the excited state because less bonding electron density is found in the excited state. An example is the transition from the ground state of NO+ ion to the first excited state, as shown in Fig 14.55. 

Indeed, as the pi/2 orbitals differ in behaviour from the ps/2 orbitals, it is obvious that the six electrons of a p orbital will not be equivalent when hybridization is involved to form bonding molecular orbitals including heavy atoms. As an illustration, we display in Fig. 7 the transition from LS to jj coupling down the Group 14 column of the Periodic Table as given by the Dirac-Fock method. 

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