What is Fourier Transform? How to understand Fourier Transform Infrared Absorption Spectroscopy (FTIR)?

Introduction

We know that the light seen by the human eye is only a small part of the spectrum of electromagnetic radiation. The immediate high-energy side of the visible spectrum is the ultraviolet and the low-energy side is the infrared. The infrared region is most useful for analysing organic compounds, with a wavelength range of 2,500 to 16,000 nm and a corresponding frequency range of 1.9x1013 to 1.2x1014 Hz. Infrared photon energies (from 1 to 15 kcal/mol) are not sufficient to excite electrons but may cause vibrational energy level jumps in the covalent bonding of atoms and groups, which can change their dipole moments. Covalent bonds in molecules act like rigid springs that can be stretched and bent. This vibrational jump occurs when the frequency of the electromagnetic field in the infrared region is equal to the vibrational frequency. The figure below illustrates two types of molecular vibrations: stretching vibrations (symmetric and asymmetric stretching) and bending vibrations (tail-shearing, wobbling, swinging and twisting). Typically, the greater the change in dipole moment, the greater the absorption intensity. Thus, infrared spectroscopy can provide information on chemical structure and bonding.

Components of an FTIR instrument

The infrared beam is split by a beam splitter into two equal beams, one reflected by a fixed mirror and the other by a movable mirror. These two beams are reflected by the two mirrors and recombined to produce an interference wave. When the interfering beam passes through the sample, radiation of a specific frequency is absorbed by the sample due to excited vibrations of molecular bonds. The resulting beam is then recorded by a detector, digitised by an analogue-to-digital converter and stored in a computer. The computer takes the data and performs a Fourier transform calculation and presents the results as absorbance (A) or transmittance (T) versus wave number.

The figure below summarises the general regions of the infrared spectra of the various vibrational bands. Note that the blue portion above the dashed line indicates stretching vibrations and the green portion below the line indicates bending vibrations. Due to the complexity and uniqueness of the infrared spectra in the 1450 to 600 cm^-1 region, it is often referred to as the fingerprint region. The absorption bands in the 4000 to 1450 cm^-1 range are usually due to the stretching vibrations of the diatomic units and are referred to as the group frequency region.

Infrared absorption of common groups

The figure below provides the infrared absorption of the most common groups, with the blue shaded area listing the stretching absorption and the green shaded area listing the bending absorption. Since most organic compounds have C-H bonds, the absorption at 2850 to 3000 cm^-1 is attributed to sp3 stretching of the C-H bond. The absorption above 3000 cm^-1 is attributed to the sp2 stretching of the C-H bond, and the absorption at 3300 cm^-1 is attributed to the sp stretching of the C-H bond.