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

Chapter 3
electromagnetic spectrum between the visible and microwave regions and this area
is divided into three regions: near (14 000 – 4 000 cm -1 ), mid (4 000 – 400 cm -1 ) and
far (400 – 10 cm -1 ) IR. The technique is based on the vibrations of atoms of a
molecule. Important parameters are the frequency (v), wavelength (λ, length of 1
wave) and wavenumber (, number of waves per unit length) and they are related to
one another by the following equation 5 . Here, c is the speed of light and n the
refractive index of the medium it is passing through:
=
⁄
= 1
46
λ (3.4)
A spectrum is obtained by passing infrared radiation through a sample and
determining the fraction of incident radiation that is absorbed at a specific energy.
Radiation is considered as two perpendicular electric and magnetic fields, oscillating
in a single plane. This radiation can be regarded as a stream of particles for which
the energy (E) can be calculated as follows. Where h is the Planck constant (h =
6.626 x 10 -34 J.s).
E = hν (3.5)
Two important characteristics to the process are the radiation frequency and the
molecular dipole moment (µ). The interaction of radiation with molecules involves a
resonance condition where the specific oscillating radiation frequency matches the
natural frequency of a particular normal mode of vibration. The molecular vibration
must cause a change in the dipole moment of the molecule in order for the energy to
be transferred from the IR photon to the molecule, via absorption 6 .
IR spectroscopy depends on the specific frequencies at which chemical bonds
vibrate or rotate. Chemical bonds can be excited by IR radiation to cause bond
“stretching” (high energy) or bond “bending” (low energy) vibrations. This stretching
or bending of bonds can be classified into various vibrational modes. 7 In the case of
stretching, the modes can either be symmetrical or asymmetrical. The modes of
bending include rocking, scissoring, wagging and twisting. As a rule, the stretching
5
B. H. Stuart, Infrared Spectroscopy: Fundamentals and Applications, Wiley and Sons, New York, 3, 2004.
6
D. N. Sathyanarayana, Vibrational Spectroscopy: Theory and Applications, New Age International, New Delhi,
44, 2004.
7 th
L. D. Field, S. Sternhell, J. R. Kalman, Organic Structures from Spectra, 4 Ed, Wiley and Sons, New York, 15,
2007.