Basics of vibrational spectroscopy
CH stretchings
νCH > 3000 cm-1
Alkenes or aromatics?
Alkenes
In order to confirm the presence of an alkene group, its stretching mode must be correlated to two types of vibrations:
→ stretching C=C νC=C (cis) = 1660 – 1635 cm-1
νC=C (trans) = 1680 – 1665 cm-1
→ bending δ C-Hout of plan = 1000 – 650 cm-1
νC=C (trans) = 1680 – 1665 cm-1
→ bending δ C-Hout of plan = 1000 – 650 cm-1
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Aromatics
To confirm the presence of an aromatic, its stretching mode must be correlated to vibrations in three ranges of the spectrum:
→ 2000 à 1700 cm-1; combinations and overtones (broad and not intense)
→ 1620 à 1400 cm-1: cycle bands ( C=C stretchings)
→ 1200 à 700cm-1: bendings δin and out of plane C-H
Generally speaking, stretching and bending bands are very thin.
→ 1620 à 1400 cm-1: cycle bands ( C=C stretchings)
→ 1200 à 700cm-1: bendings δin and out of plane C-H
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Substitution identification:
| δC-H out of plan | δ C-C | |
|---|---|---|
| Mono | 770 – 710 | 690 ± 10 |
| Ortho | 810 – 750 | |
| Meta | 770 – 735 | 690 ± 10 |
| Para | 860 - 790 |