A solution and solid state study of niobium complexes University of ...

Chapter 3
The multiplicity of a multiplet is given by the number of equivalent protons in
neighbouring atoms plus one (n + 1). Equivalent nuclei do not interact with each
other and can only cause splitting of neighbouring protons. The signal splitting is a
consequence of indirect spin-spin coupling. The distances between the peaks of a
multiplet is called the coupling constant (J) and is measured in hertz (Hz). 3
3.3 Ultraviolet/visible Spectroscopy (UV/Vis)
The ultraviolet region falls in the range between 190 – 380 nm and the visible region
between 380 – 750 nm. Most commercial spectrophotometers cover a spectral range
of 185 – 900 nm. 4 Ultraviolet and visible radiation interacts with atoms which causes
electronic transitions. The following electronic transitions are possible:
π → π *
n → π *
σ → σ *
n → σ *
A molecule can absorb light and then transfer that energy by promoting electrons to
higher energy levels from the ground state (highest occupied molecular orbital –
HOMO) to the excited state (lowest unoccupied molecular orbital – LUMO). Three
types of molecular orbitals exist, bonding (σ or π), non-bonding (n) and anti-bonding
(σ * or π * ). The electrons are generally present in the bonding orbitals, or non-bonding
orbitals if they are lone pairs, and are excited to the anti-bonding orbitals. An
absorbance spectrum simply depicts what wavelength of light is absorbed by a
sample.
3.4 Infrared Spectroscopy (IR)
IR is used to gather information about the structure of a compound and as an
analytical tool to assess the purity of a compound. Infrared refers to that part of the
3 E. Breitmaier, G. Bauer, 13 C NMR Spectroscopy, Vol 3, Harwood Academic Publishers, London, 3, 1984.
4 F. Rouessac, A. Rouessac, Chemical Analysis: Modern Instrumentation Methods and Techniques, 2 nd Ed, Wiley
and Sons, New York, 167, 2007.
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