Interpreting and explaining the infrared spectrum of ethene (ethylene)

 See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and ethene

Introductory note on the infrared spectrum of ethene

Students and teachers please note my explanation of the infrared spectrum of ethene is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for ethene, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if ethene has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of ethene is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing ethene or following its change of concentration in a reaction.

Spectra obtained from a liquid film of ethene. The right-hand part of the of the infrared spectrum of ethene, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of ethene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of ethene.

The molecular structure and naming of alkenes

Interpretation of the infrared spectrum of ethene

The most prominent infrared absorption lines of ethene

The absorption C-H stretching vibrations of the CH₂ group peak at wavenumber ~3026

There are absorption bands at ~1400 cm^-1 due to scissoring H-C-H vibrations and ~3100  cm^-1 from H-C-H asymmetric stretching vibrations - the latter overlaps with the C-H stretching vibrations.

Strong absorption band peaking at 900 cm^-1 due to a H-C-H 'wagging' vibration.

Not sure what the absorption band at ~1900 cm^-1 is due to?

Note: Unlike most other alkenes, it is a totally symmetrical molecule and cannot absorb infrared radiation at the characteristic C=C stretching vibration frequency of  ~1690 to 1620 cm^-1 - the there is no change in the dipolar character of this molecule (this is university level theory).

Lots of peaks appear due to rotation-vibration interactions - but this is again, university level theory and is observed much more clearly in the infrared spectra of gaseous molecules.

The absence of other specific functional group bands will show that a particular functional group is absent from the ethene molecular structure.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of ethane and ethene NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethane and ethene image sizes.
INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking differences are (i) the band at ~1900 cm-1 for ethene, absent in the ethane spectrum, (ii) the bands at 800 cm-1 for ethane (CH3 vibrations), absent or much weaker in ethene, and (iii) the strong absorptions at ~1000 cm-1 for ethene, completely absent in the ethane spectrum. Because of its complete symmetry, the ethene infrared spectrum does not show the characteristic C=C vibration absorption at wavenumber ~1680 to 1620 cm-1 of alkenes.
MASS SPECTRA: Both ethane and ethene show some similarities in their mass spectra e.g. m/z ions 25 to 28 for [C2Hx]+ (x = 1 to 4) ions and in both cases the base ion peak has an m/z of 28. However, the molecular ion peaks will be different because of their different relative molecular masses i.e. ethane m/z 30 and ethene m/z 28. Ethane also has a prominent m/z ion peak of 29, which is tiny in the ethene mass spectrum (and only due to 1% 13C atoms in the parent molecular ion).
1H NMR SPECTRA: The 1H NMR spectra of ethane and ethene are similar in that that both give one single singlet resonance line in their proton NMR spectra. All the protons in each molecule are equivalent to each other and occupy the same chemical environment due to the symmetry of the molecule, so no resonance splitting. However the two 1H chemical shifts are significantly different due the different shielding effects of the -CH3 and =CH2 groupings respectively.
13C NMR SPECTRA: The 1C NMR spectra of ethane and ethene are similar in that that both give one single resonance line in their carbon-13 NMR spectra. In both molecules the two carbon atoms occupy the same chemical environment due to the symmetry of the molecule.  However the two 13C chemical shifts are significantly different due the different shielding effects of the -CH3 and =CH2 groupings respectively.