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IR Spectroscopy

Creates conformational changes in molecules with varying polarity

  • Is useful for identification of molecular structure, however is limited to identification of functional groups and “fingerprinting”
  • Is highly limited by its likelihood of being interfered with

Typical Spectrum

  • Measured as transmittance (peaks going down from 100%) instead of absorbance
  • Wavelength measured in \(\bar{\nu}=\frac{1}{\lambda (cm)}\)

Types of IR

  • Near-IR - Photometry/spectrophotometry similar to UV-VIS - quantification of organo-macromolecules
  • Mid-IR - Surface analysis, reflectance/diffuse reflectance/photoacoustic
  • Far-IR - Quantification of functional groups containing C, H, O rather than compound identification


  • measures an interferogram and uses FT to convert it to a spectra
  • IR radiation is primarily thermal energy and induces stronger molecular vibrations within bonds
    • Bonds act as springs, even with their own spring constant (see later)

Types of molecular interactions


  • Rotational transitions occur in the far-IR range (lower energy) at \(<100\:cm^{−1}\)
  • There are far more rotational transitions than vibrational transitions and only small amounts of energy are required to trigger them *This however means that they only are really observed in gasses


  • Vibrational modes are higher in energy and come in bond length and atom rotational forms.
  • Each mode is given a Greek letter to denote it’s behaviour


Symmetrical Stretching - \(\nu_s\) Asymmetrical Stretching - \(\nu_{as}\)


Scissoring - \(\sigma\) Rocking - \(\rho\)


Wagging - \(\omega\) Twisting - \(\tau\)

Peak height

  • Peaks can be (depending on their height):
    • Weak (w) - <30%
    • Medium (m) - 30−60%
    • Strong (s) - >60%
  • This is dependent on the polarity of the bond in question. The more polar the bond, the higher the absorbance at that peak.

Peak width

  • Peaks can also be:
    • Broad - wide and smooth * Typically seen in O-H bonds (water!)
    • Narrow - thin and pointy
  • This is caused by resolution limitations causing multiple absorbance peaks to blend into the one curve
    • This is dependent on the amount of excitation states for the particular bond


  • Only polar bonds will be active, as they need to be able to change their dipole when they rotate or vibrate
    • The movement of atoms causes a shift in the electron density and thus a shift in the bond’s dipole
  • As a result pure covalent bonds will not show up, such as \(\ce{O2, N2, H2}\) etc.
    • \(\ce{CO2}\) and \(\ce{H2O}\) are still envirnmental interferants however.

Symmetric bonds

  • If a bond has polarity but the molecule is symmetric, the resulting dipole will not move either, causing there to be no identifiable change
    • Strong * Strongly Polar bonds
    • Medium * Medium polarity bonds * Asymmetric bonds
    • Weak * Weakly polar bonds * Symmetric bonds

  • Symmetric molecules will also show a more simple spectra, as the bonds will be duplicated, since they’ll be in the same environment and will overlap in their absorbance.


  • If vibrations occur around a central atom, the effect of their shift may not cause a net change in the dipole
  • This is likely when:
    • The have a common atom in stretching modes
    • They have a common bond in bending modes
    • They have a common bond in bending and stretching modes
    • They have similar vibrational frequencies
  • This is unlikely when
    • The atoms are separated by two or more bonds
    • The symmetry is inappropriate

Electron density

  • When an EDG contributes more density to a bond, it gets stronger and as a result will take more energy to excite the group
  • The opposite is true for EWGs
  • E.g. \(\ce{C-O}\) bond in:
    • Methanol - \(1034\:cm^{−1}\)
    • Ethanol - \(1053\:cm^{−1}\)
    • Butanol - \(1105\:cm^{−1}\)

Anharmonic oscillation

  • The vibrational modes of bonds is dependent on the amount of energy put in. The process follows the Morse Potential pattern

  • Too much energy and the bond will break
  • The bond will get significantly longer rather than compressing significantly
  • This is due to a Coulombic effect as well harmonic oscillation

Diatomic formula

calculates where peaks appear


  • \(m_1\) and \(m_2=\) the masses of the atoms
  • \(k=\) the sprinc constant (Hooke’s law)
  • \(\mu=\text{reduced mass}=\frac{m_1m_2}{m_1+m_2}\)
\[ \bar{\nu}=\frac{1}{2\pi c}\big(\frac{k}{\mu}\big)^{\frac{1}{2}}=130.3\big(\frac{k}{\mu}\big)^{\frac{1}{2}} \]

\(k\) can be obtained from the following table

\(\ce{O\bond{-}H}\) \(\ce{C\bond{-}H (sp^3)}\) \(\ce{C\bond{-}H (sp^2)}\) \(\ce{C\bond{=}O}\) \(\ce{C\bond{#}C}\) \(\ce{C\bond{=}C}\) \(\ce{C\bond{-}C}\) \(\ce{C\bond{-}O}\)
780 480 510 1210 690 760 540 450

Peak count

  • To determine the amount of peaks we’d expect to see, we can use the following formula, which utilises the degrees of freedom for a molecule to determine what is possible
  • Where \(N=\) number of atoms:
\[ \text{Linear molecule - }3N−5 \]
\[ \text{Non-linear molecule - }3N−6 \]
  • Each vibration is called a normal mode and has a characteristic frequency
  • This will give the maximum number of fundamental peaks

Fewer peaks

Fewer peaks may be observed due to:

  • Symmetry of the molecule causing cancellations
  • Similar bonds sharing energy
  • Low absorption intensity
  • Energy of peak being out of the range of the spectrophotometer

More peaks

  • Harmonics of the fundamental frequency can cause overtones at multiples of the fundamental frequncy
    • uncommon but possible
    • occurs when an excitation happens from ground state to the second excited state (\(\ce{v=0 -> v=2}\))
  • When multiple vibrational modes are excited by a single photon, either the sum of the two modes or the difference can be seen as a result

Example - \(\ce{CO2}\)

  • Has \(3N−5=4\) vibrational modes
  • Actual peaks:
    • Symmetric stretching will result in no net change of the dipole
    • Asymmetric stretching will result in a net change and thus a peak
    • The two bending modes will have the same energy and thus will show up as a single peak
  • Thus only two peaks are observed, not the expected four