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CH435/535
Structure Determination by Spectrosciopic Methods

Aromatic Fingerprint Vibrations

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Aromatic compounds display several absorbances in the fingerprint region that can be useful as structural identifiers, particularly with regard to substitution patterns. These generally arise from out-of-plane C-H bending motions, and the frequencies indicate the number of adjacent hydrogens:

Number of adjacent hydrogens

Frequency, cm-1

1 855
2 813
3 780
4 760
5 750

Obviously, in something like a meta-disubstituted ring, there will be a combination of bands, while in a para-disubstituted ring, there will be only one. There are also ring deformations that crop up in to 600-900 cm-1, so one normally looks for characteristic patters.

Monosubstituted 736-769 plus 690 Toluene
Ortho (1,2)-disubstituted 756-769 ortho-Xylene
Meta (1,3)-disubstituted 855-901 plus 775-794 plus 690 meta-Xylene
Para (1,4)-disubstituted 806-826 para-Xylene
1,2,3-trisubstituted 763-781 plus 700 1,2,3-trimethylbenzene
1,3,5-trisubstituted 813-862 plus 700 1,3,5-trimethylbenzene
1,2,4-trisubstituted 806-820 plus 870-884 1,2,4-trimethylbenzene
1,2,3,4-tetrasubstituted 800-813 1,2,3,4-tetramethylbenzene
1,2,3,5-tetrasubstituted 840-847 1,2,3,5-tetramethylbenzene
1,2,4,5-tetrasubstituted 855-869 1,2,4,5-tetramethylbenzene
Pentasubstituted 855 Pentamethylbenzene