CH435/535 Aromatic Fingerprint Vibrations |
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 |