Basics of vibrational spectroscopy

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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
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Different types of alkenes

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.
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Information pictogram

Substitution identification:
δC-H out of planδ C-C
Mono770 – 710690 ± 10
Ortho810 – 750
Meta770 – 735690 ± 10
Para860 - 790
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