The antibonding orbital is designated by an asterisk. Thus, the promotion of an electron from a π-bonding orbital to an antibonding (π *) orbital is indicated π → π * The concept of an antibonding orbital can be explained simply by considering the ultraviolet absorption of ethyl-ene. The ethylenic double bond, in the ground state, consists of a pair of bonding σ-electrons and a pair of bonding π -electrons. On absorption of ultraviolet radiation ear 165 nm, one of the bonding π -electrons is raised to the next higher energy orbital, an antibonding π *-orbital. The orbitals occupied by the π -electron in the ground state and in the excited state are diagrammed in Figure 3.
The shaded volumes indicate regions of maximum electron density. It can be seen that the antibonding π -electron no longer contributes appreciably to the overlap of the C-to-C bond,. In fact, it negates the bonding power of the remaining unexcited π -electron ; the olefinic bond has considerable single -bond character in the excited state.
The relationship between the energy absorbed in an electronic transition and the frequency (ν), wavelength (λ), and wave number (ν - ) of radiation producing the transition is
where h is Planck's constant and c is the velocity of light. ∆E is the energy absorbed in an electronic transition in a molecule from a low-energy state (ground state) to a high-energy state (excited state). The energy absorbed is dependent on the energy difference between the ground state and the excited state; the smaller the difference in energy, the longer the wavelength of absorption. The excess energy in the excited state may result in dissociation or ionization of the molecule, or it may be reemitted as heat or light. The release of energy as light results in fluorescence or phosphorescence
Since ultraviolet energy is quantized, the absorption spectrum arising from a single electronic transition should consist of a single, discrete line. A discrete line is not obtained since electronic absorption is superimposed( متداخل ) on rotational and vibrational sublevels.
The spectra of simple molecules in the gaseous state consist of narrow absorption peaks, each representing a transition from a particular combination of vibrational and rotational levels in the electronic ground state to a corresponding combination in the excited state. This is shown schematically in Figure 4,in which the vibrational levels are designated ν 0, ν 1, ν 2, so forth. At ordinary temperatures, most of the molecules in the electronic ground state will be in the zero vibrational level (G ν 0 ); consequently, there are many electronic transitions from that level.
In molecules containing more atoms, the multiplicity of vibrational sublevels and the closeness of their spacing cause the discrete bands to coalesce, and broad absorption bands or "band envelopes" are obtained.
The principal characteristics of an absorption band are its position and intensity. The position of absorption corresponds to the wavelength of radiation whose energy is equal to that required for an electronic transition. The intensity of absorption is largely dependent on two factors:
1- The probability of interaction between the radiation energy and the electronic system and 2-The difference between the ground and the excited state.
The probability of transition is proportional to the square of the transition moment. The transition moment, or dipole moment of transition, is proportional to the change in the electronic charge distribution occurring during excitation. Intense absorption occurs when a transition is accompanied by a large change in the transition moment. Absorption with ɛ max values > 10 4 is high-intensity absorption; low-intensity absorption corresponds to ɛ max value ˂10 3.Transitions of low probability are "forbidden" transitions. The intensity of absorption may be expressed as transmittance (T), defined by
where I 0 is the intensity of the radiant energy striking i sample, and I is the intensity of the radiation emerging from the sample. A more convenient expression of absorption intensity is that derived from the Lambert-Beer which establishes a relationship between the transmittance the sample thickness, and the concentration of the absorbing species. This relationship is expressed as