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Bond length

Figure 2.10 Morse curve. Inter-atomic attractive and repulsive forces result in the formation of a bond length with a minimum energy level.

Bond length

Figure 2.10 Morse curve. Inter-atomic attractive and repulsive forces result in the formation of a bond length with a minimum energy level.

the relationship between these forces and illustrates the fact that at a particular bond length the energy associated with the bond is minimal and the molecule is said to be in its ground state. However, this only describes the most stable and hence the most frequent form of the molecule. More energetic forms will also exist and these are indicated by the higher energy levels in the Morse curve diagram.

A new Morse curve is required to describe the energy levels associated with an excited molecule and this is displaced to the right and above the original curve. The excitation of a molecule with a particular bond length results in an excited molecule with the same bond length but with a higher internal energy as illustrated by the new Morse curve (Figure 2.11). An excited molecule initially will revert to the minimum energy state for the excited molecule

Potential + energy

Figure 2.11 Morse curve for an excited molecule. The energy required for excitation (A) is lost as the molecule returns to the ground state but only the energy lost between states (C) may be emitted as radiation. Energy losses due to internal rearrangements (B and D) are non-radiative.

before reverting to the ground state, with the result that the energy originally absorbed is lost in at least three stages (B, C + D) as the molecule returns to its ground state. Some energy losses are due to internal rearrangements of each of the two molecular forms and as such involve a non-radiant loss of energy: it is only the energy lost during the interchange between the two molecular forms that may possibly be emitted as radiation. Any energy lost by radiation (C) must be less than the total energy absorbed (A) and hence the wavelength emitted will be greater than that which was absorbed in the first place.

A molecule in the ground state has a pair of electrons with opposite spin in each molecular orbital. On excitation, one electron is elevated to an anti-bonding orbital and because it is not restricted by the presence of another electron it can exist with either its original spin (a singlet transition) or reversed spin (a triplet transition). If the return to the ground state involves a fall from the singlet transition and radiation is emitted, the compound is said to fluoresce. However, in a return to the ground state from a triplet transition, fluorescence will not occur and the energy loss will probably be non-radiative. The intensity of fluorescence depends upon the proportion of the total molecules which undergo singlet transitions and it is mainly for this reason that different compounds show different

> Luminescence - see Section 8.3.4.

>• Scintillation counter ■ see Section 5.2.2.

degrees of fluorescence. Some compounds do show radiative emission during triplet transitions and these compounds are said to phosphoresce. The most demonstrable difference between these two types of emission is the fact that fluorescence occurs within about 1 X 10~8 seconds and persists for only about 1 to 1 X 103 nanoseconds and is much faster than phosphorescence, which persists for up to 1 X 10~3 seconds due to the time taken for the spin change to occur.

Chemiluminescence is another form of molecular emission in which the initial electronic transition is caused by an exergonic reaction rather than the absorption of radiant energy. Most chemiluminescence reactions are of the oxidative type and those involving hydrogen peroxide are particularly useful biochemically. Luminol (5-amino-2,3-dihydrophthalazine-l,4-dione), for instance, will emit light when reacting with hydrogen peroxide and may be used in monitoring many oxidative enzymes such as glucose oxidase, amino acid oxidase, etc. Bioluminescence is a special type of chemiluminescence in which the process of light emission is catalysed by an enzyme. The enzyme luciferase, extracted from the firefly, uses ATP to oxidize the substrate luciferin with the emission of radiation and can be used to measure ATP concentrations. The enzyme extracted from bacterial sources can oxidize long chain aliphatic aldehydes in the presence of oxygen, FMN and NADH with the emission of radiation.

Luminescence methods are very sensitive, quantities as little as 1 fem-tomole of ATP being detectable and while measurements may be made using scintillation counters, much simpler equipment that requires neither a radiation source nor a monochromating system is satisfactory.

Self test questions

Section 2.1

1 What describes radiation with a wavelength of 340 nm?

(a) Ultraviolet radiation.

(b) Visible radiation.

(c) Near infrared radiation.

(d) Far infrared radiation.

2 Ultraviolet radiation will frequently induce which of the following molecular transitions?

(a) Inner shell electronic transition.

(b) Valence electronic transition.

(c) Molecular vibrational transition.

(d) Molecular rotational transition.

3 An effect of an increased level of conjugation in a molecule is to shift the absorption maximum to a longer wavelength

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