Molecular vibration is achieved when atoms that makes up a molecule constant cyclic motion but the whole molecule has repetitive translational and rotational shift. Vibrations in a molecule are occurs since bonds are not technically rigid but have some range of flexibility. Vibration frequency is the regularity of occurrence of a periodic motion. Fourier transform infrared spectroscopy is a technique used to obtain an infrared spectrum. The spectrum may take different characteristics such as emission photoconductivity, absorption or even scattering in the three forms of matter i.e. gas, liquid and solid.
Generally, the two major molecular vibrations are bending and stretching depending with bond length and deflection of angel between the atoms in the molecule. From these main categories, six subgroups arise. Stretching can be, either symmetrical or asymmetrical while bending can be scissoring, rocking, wagging or twisting. Practically, molecular vibrations that results to stretching have higher frequencies compared to bending frequencies irrespective of a molecule. Another factor that directly affects stretching frequencies is number of bonds where the frequency increases with bond. Stretching is observed when a molecule has a significant change in bond size between atoms. Bending mode is noted when the angle between three atom (one of them acts as a pivot) such as the one made between water Hydrogen atoms and the core oxygen atom. In the rocking mode, there is an eminent change in angle between clusters of atoms in relation to the whole molecule. E.g. in methylene structure. Wagging on the other hand, means vibration mode where there is a notable change in an atomic plane and overall molecular plate. Twisting varies from wagging in that the changes affect two groups of atomic planes in a molecule or compound. To help understand this clearly consider the following examples, water molecule has three atoms, the molecule exhibits three distinct vibration modes namely symmetrical stretch, bend, and asymmetrical
The symmetrical stretching results in equal bond length increment there by not affecting the identity of the two anatomical planes as seen in the asymmetrical stretching where one arm increases and the others decreases.
Mathematically it can be represented as Q= Left + Right (symmetrical stretch) and Q=Left - Right (asymmetrical stretch)
In another aspect, the number of vibration of water can be expressed as 3X3 - 6=3(non-linear) and 3X3 - 5(linear molecule). Water has both linear and non-linear. A non-linear molecule of Y atoms has 3Y-6 while that of a linear molecule has 3n-5. Hydrogen cyanide and carbonyl sulfide give a perfect display of simple rotational spectrum for linear molecular vibration while hydrogen isocyanides (HN=C) demonstrates a non-linear molecular vibration flawlessly. Molecule such as methane can have as many as 9 modes of vibration. As delineated by the formulae 3X5 – 6 =9 but due to the symmetrical nature of this compound, only five modes are degenerated.
Due to the dipole moment that is possessed by nitrous oxide, it does not display pure rotational spectrum the compound displays three fundamental programs. The molecule has linear and asymmetrical molecule structure. It worth noting that vibration modes of twisting, wagging and rocking do affect the angle but have no effect the coordinate bonds of the molecule. Both stretching and bending molecular vibrations can be mathematically predicted for analysis.