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The plot of absorption probability against wavelength is calledabsorption spectrum.

In the simpler case of single atoms (as opposed to multi-atom molecules), electronic

transitions lead to the occurrence of line spectra (see Section 12.7). Because of the

existence of more different kinds of energy levels, molecular spectra are usually

observed as band spectra (for example Fig. 12.7 below) which are molecule-specific

due to the unique vibration states.

A commonly used classification of absorption transitions uses thespin statesof

electrons. Quantum mechanically, the electronic states of atoms and molecules are

described byorbitalswhich define the different states of electrons by two parameters:

a geometrical function defining the space and a probability function. The combination

of both functions describes the localisation of an electron.

Electrons in binding orbitals are usually paired with antiparallelspin orientation

(Fig. 12.8). The total spinSis calculated from the individual electron spins. The multipli-

cityMis obtained byM¼ 2 Sþ1. For paired electrons in one orbital this yields:

S¼spinðelectron 1 Þþspinðelectron 2 Þ¼ðþ 1 = 2 Þþð 1 = 2 Þ¼ 0

The multiplicity is thusM¼ 2  0 þ 1 ¼1. Such a state is thus called asinglet state

and denotated as ‘S’. Usually, the ground state of a molecule is a singlet state,S 0.

In case the spins of both electrons are oriented in a parallel fashion, the resulting state

is characterised by a total spin ofS¼1, and a multiplicity ofM= 3. Such a state is called

atriplet stateand usually exists only as one of the excited states of a molecule, e.g.T 1.

According to quantum mechanical transition rules, the multiplicityMand the total

spinSmust not change during a transition. Thus, theS 0 !S 1 transition is allowed and

possesses a high transition probability. In contrast, theS 0 !T 1 is not allowed and has a

small transition probability. Note that the transition probability is proportional to the

intensity of the respective absorption bands.

Most biologically relevant molecules possess more than two atoms and, therefore,

the energy diagrams become more complex than the ones shown in Fig. 12.3.

Different orbitals combine to yieldmolecular orbitalsthat generally fall into one of

five different classes (Fig. 12.4):sorbitals combine to the bindingsand the anti-

bindings* orbitals. Someporbitals combine to the bindingpand the anti-bindingp*

Anti-binding

Non-binding

Binding

E

s*

p*

s

p

n

Fig. 12.4Energy scheme for molecular orbitals (not to scale). Arrows indicate possible electronic transitions.

The length of the arrows indicates the energy required to be put into the system in order to enable the

transition. Black arrows depict transitions possible with energies from the UV/Vis spectrum for some biological

molecules. The transitions shown by grey arrows require higher energies (e.g. X-rays).

481 12.1 Introduction