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Study Notes: Origin of UV-Vis Spectra

At room temperature a chemical species (C) exists in its lowest or ground state of energy. The species can absorb a photon of radiation of a specific or quantised energy value that exactly corresponds to the difference in energy between the ground state and a higher energy state. When this occurs the species is said to be excited (C*). The transfer of energy (hv) is illustrated by the following equation:

C + hv arrow right C*

Almost instantaneously the excited species gives up its absorbed energy and in so doing returns, or relaxes, to its ground state. It does this by transferring the energy, in the form of heat, to other atoms or molecules in the immediate environment. However, the thermal energy is so small as to be virtually undetectable.

C* arrow right C + heat

The process of absorption and subsequent relaxation occurs continuously while the species is radiated with energy.

A typical UV-Vis spectrum showing two broad peaks. The absorption of energy by a chemical species is characterised by its absorption spectrum - this is a plot commonly of intensity (as absorbance, transmission or molar absorptivity) against wavelength, frequency or wavenumber. Spectra will normally be presented in this unit as absorbance against wavelength (nm).

Ultraviolet (200-380 nm) and visible (380-780 nm) absorption spectra are most commonly obtained on a liquid sample or on a solution of the sample dissolved in a suitable solvent. Spectra can also be run on samples in solid and gaseous forms.

The important concept to understand about absorption of ultraviolet and visible radiation is that it involves transitions of electrons as the basis of the excited state. Electronic transitions of valence shell electrons are at the heart of this form of spectroscopy.

The spectrum of gaseous sodium atoms showing three distinct absorption lines at about 285, 330 and 590 nm. When gaseous atoms are radiated with polychromatic ultraviolet or visible light, the absorption spectrum comprises a number of discrete lines representing only a few wavelengths amongst the many radiated upon the sample. The figure depicts the spectrum for gaseous sodium atoms.

The three lines represent the transition of a single outer electron from its 3s (ground state) orbital to outer (vacant) 3p, 4p and 5p orbitals so creating different states of excitation. The energy of the absorbed photons (hv) associated with these transitions is exactly equal to the difference in energy between the ground and excited states for sodium.

Similarly, when a gaseous diatomic molecule is radiated with polychromatic ultraviolet or visible radiation, a number of discrete electronic transitions occur. These transitions come about as the result of the movement of electrons in ground state molecular orbitals to higher energy molecular orbitals. Again, the absorbed photons have exactly the same energy as the difference between the ground and electronically excited states. Skip flash movie

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For each electronic excited state there will be other (quantised) excited states that the molecule can simultaneously assume. These are mostly associated with the vibrational and rotational movements of the molecule and require a smaller input of energy to generate.

Vibrational transitions arise from bending and stretching movements of the bonds and atoms within an excited molecule.

Rotational transitions concern the rotation of an excited molecule about its centre of gravity.

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There are a multitude of vibrational and rotational excited states that a molecule can assume within each electronic excited state of which there are normally only a limited number.

In the UV-Vis spectrum of a molecule, these many-fold vibrational and rotational energy levels are superimposed on an electronic energy level. The resultant individual absorption lines can be seen in the spectrum of a gaseous sample of a molecular compound however there is little of this ‘fine’ structure when the compound is in solution. Due to interactions of the analyte with solvent molecules, these fine lines are blurred out to give a continuous spectrum of smooth, broad absorption bands.

A typical UV-Vis spectrum showing  two broad peaks. A typical UV-Vis spectrum consists of one or just a few absorption peaks.

The loss of fine structure is more evident with polar solvents such as water and alcohol than with non-polar solvents such as hexane. Some fine structure can be seen in the solutions of more complicated molecules.

Note that the majority of UV-Vis spectra are run on samples in solution.

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