Ultraviolet spectroscopy is an important tool, which is helpful for the identification of organic compounds. However, it cannot be used to show the presence of individual functional groups (as is the case with infrared spectroscopy, discussed subsequently), but it gives information about the presence and extent of multiple bonds and conjugation in organic molecules.
This technique is routinely used to show relationship between functional groups, particularly conjugation between two or more carbon-carbon double (or triple bonds) between carbon-oxygen double bond and the aromatic moiety. It also indicates the number and location of the substituents attached to the conjugated system.
The region of primary interest to an organic chemist is between 200 to 380 nm. This region is known as the near UV region. Most of the unsaturated compounds absorb in this region. The region 380 to 80 urn is called visible region, in which all coloured compounds absorb. The UV portion of the electromagnetic spectrums is shown to fig below.
On subjecting a compound with multiple bands to electromagnetic radiation in the UV/visible region, a portion of the radiation is absorbed by the compound. The amount of absorption depends in the structure of the compound and the wave length of radiation. The ultraviolet spectrum is a plot of wavelength (or frequency) of absorption VS the absorption intensity (absorbance or transmittance). Thus, the uv spectrum records the wavelength of an absorption maximum which is represented by λ max in nanometers (1nm = 109m). The intensity of absorption is given by the Beer-Lambert Law.
log (lo/I) = ε c.l.
ε = A/c.I.
Where To is the intensity of the incident hight I is the intensity of the transmitted radiation. A is absorbance (optical density) ε is molar absorptivity (molar extinction coefficient) c is the concentration of the solution (g. mol/L) and I is the length of the cell containing the solution (in cm).
The plot of the curve of molar absorptivity (ε) or logarthm of molar absorptivity (log ε) versus wave length (λ) constituents the absorption band (Fig. below).
The molar absorptivity ε is constant for an organic compound at a given wave length, and is expressed as εmax,, i.e, molar absorptivity at as absorption maximum.
Organic substances give rise to several transitions involving both bonding as well as non banding electrons. Electronic transitions in organic molecules usually involve transitions of sigma (σ) electrons, n electrons, and π-electrons. σ-electrons are located in σ-bonds.
For example, a single valence bond is between two carbons as found in saturated hydrocarbons. The σ electrons are tightly held and energy of uv and visible region cannot sufficiently overcome this attraction. On the other hand, n electrons are less firmly held than σ electrons are non-bonding electrons and are found is atoms such as N, O, S and halogen. In case of π-electrons, the energy of UV and visible region is sufficient to cause the excitation process is take place. The n electrons may undergo two type of transistions.
n → π*
n → S*
An unsaturated bond contains 4 electrons (2π and 2 σ electrons). Of these π electrons are easiest to exite. Generally the transition of a π electron results is the absorption in the ultraviolet or visible region. Aromatic compounds have characteristic UV absorption spectra due to their π electrons.
For inorganic ions and molecules, the transitions may occur between the two electronic states of the individual atoms or there may be partial electron transfer from ligand to the central ion resulting is a charge transfer absorption spectrum. Absorption spectrum of the substances may be a result of contribution of the electronic transition and the electron transfer process.