Electronic transitions

Colours as perceived by the sense of vision are simply a human observation of the inverse of a visible absorption spectrum. The underlying phenomenon is that of an electron being raised from a low-energy molecular orbital (MO) to one of higher energy, where the energy difference is given as ΔE = hν. For a collection of molecules that are in a particular MO or electronic state, there will be a distribution among the accessible vibrational and rotational states. Any electronic transition will then be accompanied by simultaneous changes in vibrational and rotational energy states. This will result in an absorption spectrum which, when recorded under high-resolution conditions, will exhibit considerable fine structure of many closely spaced lines. Under low-resolution conditions, however, the spectrum will show the absorption of a broad band of frequencies. When the energy change is sufficiently large that the associated absorption frequency lies above 7.5 × 10¹⁴ hertz the material will be transparent to visible light and will absorb in the ultraviolet region.

The concept of MOs can be extended successfully to molecules. For electronic transitions in the visible and ultraviolet regions only the outer (valence shell) MOs are involved. The ordering of MO energy levels as formed from the atomic orbitals (AOs) of the constituent atoms is shown in Figure 8. In compliance with the Pauli exclusion principle each MO can be occupied by a pair of electrons having opposite electron spins. The energy of each electron in a molecule will be influenced by the motion of all the other electrons. So that a reasonable treatment of electron energies may be developed, each electron is considered to move in an average field created by all the other electrons. Thus the energy of an electron in a particular MO is assigned. As a first approximation, the total electronic energy of the molecule is the sum of the energies of the individual electrons in the various MOs. The electronic configuration that has the lowest total energy (i.e., the ground state) will be the one with the electrons (shown as short arrows in Figure 8) placed doubly in the combination of orbitals having the lowest total energy. Any configuration in which an electron has been promoted to a higher energy MO is referred to as an excited state. Lying above the electron-containing MOs will be a series of MOs of increasing energy that are unoccupied. Electronic absorption transitions occur when an electron is promoted from a filled MO to one of the higher unfilled ones.

Although the previous description of electron behaviour in molecules provides the basis for a qualitative understanding of molecular electronic spectra, it is not always quantitatively accurate. The energy calculated based on an average electric field is not equivalent to that which would be determined from instantaneous electron interactions. This difference, the electron correlation energy, can be a substantial fraction of the total energy.

Factors determining absorption regions

The factors that determine the spectral region in which an electronic transition lies (i.e., the colour of the material) will be the energy separation between the MOs and the allowed quantum mechanical selection rules. There are certain types of molecular structures that characteristically exhibit absorptions in the visible region and others that are ultraviolet absorbers. A large class of organic compounds, to which the majority of the dyes and inks belong, are those that contain substituted aromatic rings and conjugate multiple bonds. For example, the broad 254-nanometre transition in benzene (C₆H₆) can be shifted by the substitution of various organic groups for one or more of the hydrogen atoms attached to the carbon ring. The substitution of a nitroso group (NO) to give nitrosobenzene, C₆H₅NO, modifies the energy level spacings and shifts the absorption from the ultraviolet into the violet-blue region, yielding a compound that is pale yellow to the eye. Such shifts in spectral absorptions with substitution can be used to aid in characterizing the electron distributions in the bonds of a molecule.

A second class of highly coloured compounds that have distinctive visible absorption are coordination compounds of the transition elements. The MOs involved in the spectral transitions for these compounds are essentially unmodified (except in energy) d-level atomic orbitals on the transition-metal atoms. An example of such a compound is the titanium (III) hydrated ion, Ti(H₂O)₆³⁺, which absorbs at about 530 nanometres and appears purple to the eye.

A large number of compounds are white solids or colourless liquids and have electronic absorption spectra only in the ultraviolet region. Inorganic salts of this type are those that contain nontransition metals and do not have any atomic d-electrons available. Covalently bonded molecules consisting of nonmetal atoms and carbon compounds with no aromatic rings or conjugated chains have all their inner orbitals fully occupied with electrons, and for the majority of them the first unoccupied MOs tend to lie at considerably higher energies than in visibly coloured compounds. Examples are sodium chloride (NaCl), calcium carbonate (CaCO₃), sulfur dioxide (SO₂), ethanol C₂H₅OH, and hydrocarbons (C_nH_m, where n and m are integers).

Low-resolution electronic spectra are useful as an aid in the qualitative and quantitative identification of compounds. They can serve as a fingerprint for a particular species in much the same manner as infrared spectra. Particular functional groups or molecular configurations (known as chromophores) tend to have strong absorptions that occur in certain regions of the visible-ultraviolet region. The precise frequency at which a particular chromophore absorbs depends significantly on the other constituents of the molecule, in general the frequency range over which its absorption is found will not be as narrow as the range of the infrared vibrational frequency associated with a specific structural entity. A strong electronic absorption band, especially in the visible region, can be used to make quantitative measurements of the concentration of the absorbing species.

Both rotational and vibrational energies superimpose on an electronic state. This results in a very dense spectrum. The analysis of spectra of this type can provide rotational constants and vibrational frequencies for molecules not only in the ground state but also in excited states. Although the resolution is not as high as for pure rotational and vibrational spectra, it is possible to examine electronic and vibrational states whose populations are too low to be observed by these methods. Improvements in resolution of electronic spectra can be achieved by the use of laser sources (see below Laser spectroscopy).