The Basics - How vibrational spectroscopy works

Properties of light

FTIR and Raman spectroscopy are related techniques forcharacterizing how light interacts with matter. Light is a type of electromagnetic radiation, which also includes radiowaves, microwave, infrared, ultra violet, X- and gamma rays.  Spectral analysis can utilize light at anyenergy level on the spectrum, but FTIR and Raman generally use infraredradiation.  We perceive IR radiation asheat; anything with a temperature over absolute zero is giving off IRradiation, and keeping in the nature of light, can also adsorb and transferenergy.

Light is a complicated phenomenon with both wave-like andparticle-light properties.  A particle oflight – electromagnetic radiation emitted from a source in discrete units – isa photon.  Photons have no mass orcharge, but do have energy proportional to their frequency.  They may be absorbed or emitted from amolecule in a transfer of energy.  Whilethe wave-like property is composed of two fields, electric and magnetic, IR andRaman spectroscopy are only concerned with the electric part as that is whatinteracts with molecules in a way that they can detect.  These oscillations, or waves, move at avelocity that is the product of the distance between peaks (wavelengths) and anumber of cycles per second (frequency). The reciprocal of the wavelength, the wavenumber (cm-1), isthe unit used to define spectral regions. Wave numbers are linearly proportional to energy, such that the higherthe wavelength, the greater the energy.

Most people can see light at roughly 400-700 μm (wavelength)or 25,000-14,286 cm-1 (wavenumber).  The infrared range covers 1.78-1000 μm (wavelength), or 10-12,800 cm-1(wavenumber) in the electromagnetic spectrum. Most FTIR and Raman analysis occurs in what is known as the mid-infraredregion, from 4000-400 cm-1, due to the nature of how it interactswith matter.

Interactions between light and molecules

Though not a perfectly accurate representation, for ourpurposes molecules can be thought of as a system with balls for atoms, bondedtogether by springs.  This system is inconstant motion with stretching and bending vibrations.  These vibrations move at a minimum vibrationenergy level known as the ground vibrational state, and the bond length at thispoint is the equilibrium bond distance (EBD). Much like a system with springs, the frequency of the vibrations isaffected not only by the nature of the bonds themselves (C-H vs C-O bonds) butalso by the whole molecule (C-H bonds in Chloroform vs Benzene) and itsenvironment (temperature, etc.).  Changesin the energy levels affect bond length – too much energy and they willbreak.  Unlike the spring system however,the energy levels in a molecule are quantized – i.e. they occur in discreteunits.  This means that rather thangradually increasing or decreasing, the energy amounts changes suddenly; theselevels are known as the vibrational energy difference (VED).

When hit by a proton, a molecule with absorb it, and thenraise to either a stable or unstable energy level.  To raise to a stable level, the energy mustbe the same as the VED.  Moving from theground vibration state to the next is known as a fundamental transition, whichfor most molecules requires 4,000 – 400 cm-1 of energy.   This is why the mid-IR spectrum is preferredfor most FTIR and Raman spectroscopy.  Totransition beyond this is an overtone transition, which tend to be weaker andare generally found in the NIR range (14,000 – 4,000 cm-1).  These transitions provide a different kind ofinformation and so are a popular frequency for other applications.

If the energy of the photon does not match the VED, itraises to an unstable or virtual energy state (VES).  In order to return to a stable level, themolecule releases a photon.  If theenergy of this photon is the same as the energy of the photon that hit it, itis called elastic or Rayleigh scattering. This is the most common kind of scattering, and is, for example, whatcauses the sky to be blue.  Very fewphotons will be released at different wavelength, causing inelastic or Ramanscattering, named after the man to who identified it. There are two differentforms of Raman scattering: stokes, in which the molecule is at a highervibrational state after the photon is emitted, and anti-stokes, in which themolecule starts at the higher vibrational state and ends up at the groundvibrational state after the photon is emitted. 

 

Returning to the ball and spring system, it can be modifiedby imaging an atom as having a heavy central nucleus and a cloud of lighterelectrons around it.  Atoms have aproperty called electronegativity, which is their ability to attractedelectrons.  A difference in the energybetween atoms in a single molecule is a dipole moment, and causes a slightasymmetry in the molecular charge.  Adipole moment may be permanent or induced. HCl is an example of a molecule with a permanent dipole moment – thechlorine atom is more electronegative than the hydrogen, so it attracts moreelectrons and consequently has a partial negative charge, leaving the hydrogenwith a partial positive charge.  CO2 isan example of a molecule with an induced dipole moment.  In this case, the oxygen atoms are pointed inopposite directions, so their electronegativity cancels each other out.  When light hits these bonds, the asymetircalpolarity in the electric component will attract electrons to one end, creatingan asymmetrical charge.  The ease withwhich a dipole moment is induced, or how easy it is to distort the electron cloudis called polarizability.

 

The factors that determine whether a bond can be detected byRaman or FTIR are called selection rules. The basic rules are as follows: for a vibration to be IR active, it must cause a change in the dipolemoment; for the vibration to be Raman active, it must cause a change in thepolarizability.  In each case, theintensity of the [band] is proportional to the intensity of the change.  For most molecules, bonds will either be IRor Raman active, known as the rule of mutual exclusion.

 

Because polarizability is a measure of how easily theelectron cloud is distorted, it can be conceptualized as how different theelectron cloud shapes are before and after equilibrium.  Using CO2 as an example, we see three modesof vibration:  symmetrical stretch,asymmetrical stretch and scissoring.  Forthe symmetrical stretch there is no change in the dipole moment (because thereisn’t one) and it is therefore IR inactive. The directions of the dipole moment are reversed however, in theasymmetrical stretch depending on whether the electron cloud is to the right orleft of the central carbon, and in the bend, depending on whether the carbon isabove or below the oxygen.  These areboth, therefore, IR active. Alternatively, for Raman the shape of the electron cloud is differentdepending on whether the molecule is stretched are not, and the symmetricstretching is therefore Raman active. For asymmetric stretching and scissoring, the electron cloud shapes areidentical before and after they pass through equilibrium, and are thereforeRaman inactive.  More complicatedmolecules do not necessarily hold to the rule of mutual exclusion as rigidly asa simple molecule like CO2, but it is a good portrayal of the selection rules.

Bibliography

Griffiths, P. R., & De Haseth, J. A. 2007. Fourier transform infrared spectrometry (Vol. 171). John Wiley & Sons.

Larkin, P. 2011. Infrared and Raman spectroscopy; principles and spectral interpretation. Elsevier.

Nakanishi, K., & Solomon, P. H. 1977. Infrared absorption spectroscopy.

Smith, Brian C. 1999. Infrared Spectral Interpretation: A Systematic Approach. CRC Press