Vibrations Matti Hotokka
Identify the stuff I ve seen this spectrum before. I know what the stuff is
Identify the stuff Let s check the bands Film: Polymer Aromatic C-H Aliphatic C-H Group for monosubstituted benzene Ring modes CH 2 scissors Ring bend CH 2 rocking Aromatic CH rocking
Identify the vibrations IR 780 cm -1 (T 2 ) Fermi resonance 313 cm -1 (T 2 ) Raman 1 460 cm -1 (A 1 ) 3 214 cm -1 (E) 4 1 + 3 2 B. Schrader, Raman/Infrared Atlas of Organic Compounds, VCH, 1989.
Symmetry Every molecule has its point group A well defined set of symmetry types in each point group Every vibration follows one of the symmetry types Easy to calculate how many vibrations of each symmetry type there is Selection rules are based on symmetry types
Selection rules Theoretically from symmetry Hand-waving explanation IR: Strong band if the molecule s dipol moment oscillates with the vibration Raman: Strong band if the molecule s polarizability oscillates with the vibration
Selection rules Example: Carbon dioxide g + 1340 cm -1 IR inactive Raman active u + 2349 cm -1 IR active Raman inactive u 667 cm -1 IR active Raman inactive
CO 2 spectrum IR Raman 2 2
Group vibrations Basically all vibrations span over the whole molecule In practice Many vibrations stay to a large extent within a functional group but still follow the symmetry of that group Functional groups can be identified
Group vibrations Example: Methyl group Symmetric stretch 2870 cm -1 A 1 Asymmetric stretch 2960 cm -1 E All stretching vibrations are allowed both in IR and Raman in this point group.
Group vibrations Example: Methylene group Asymmetric stretch (as) 2925 cm -1 Symmetric stretch (as) 2850 cm -1 Bending ( ) Scissors 1465 cm -1 Rocking ( r ) 720 cm -1 Wagging ( w ) 1470 cm -1 Twisting ( w ) 1250 cm -1
What s in the spectrum Strong lines Allowed fundamental transitions Medium and weak lines Forbidden fundamentals Allowed overtones and combination bands Group vibrations region 4000 1500 cm -1 Fingerprint region below 1300 cm -1
What s in the spectrum Association band NH 2 s NH 2 as Ring C-H C=O Ring modes C-N NH 2 scissors NH 2 rock NH 2 wag Group vibrations Fingerprint region
Correlation tables CRC Handbook
Easily spotted bands Band position [cm-1] Assignment 3500-3200 O-H or N-H stretch 3200-2800 C-H stretch 2250-2000 C C or N C stretch 1800-1600 C=O stretch <1000 C=C stretch, phenyl ring
Alkanes, C-H modes Vibration CH 3 CH 2 Asym stretch 2970-2950 2935-2915 Sym stretch 2880-2860 2865-2845 CH 3 As bend 1470-1450 CH 3 Sym bend 1385-1365 CH 2 Scissors 1465-1445 CH 2 Rock 730-710
Branched alkanes Methine C-H stretch ~2900 cm -1 (w) Methine C-H bend 1350 (w) Split umbrella mode - Isopropyl and gem dimethyl 1385-1365 (2 bands, intensity ratio 1:1) - Isobutyl and t-butyl 1395-1365 (2 bands, intensity ratio 1:2)
Branched alkanes B. Smith, Inrared Spectral Interpretation, CRC 1998.
O-H and N-H Usually hydrogen bonded All related bands are very broad If (exceptionally) not hydrogen bonded the bands are at higher position and sharp Alcohols C-O stretch O-H stretch O-H bends All 3350 50 1350 50, 650 50 Primary 1075-1000 Secondary 1150-1075 Tertiary 1210-1100 Phenols 1260-1200 C-N stretch 1430-1390 NH2 stretch 3370-3170 NH2 scissors 1650-1620 NH2 wagging 750-600
Alcohol spectrum OH out-of-plane OH in-plane CCO s stretch OH CH 3 umbrella, split CCO as stretch CCC stretch
Ethers Ether type Asymmetric C-O-C Symmetric C-O-C Saturated, unbranched 1140-1070 (1 band) 890-820 Saturated, branched 1210-1070 (2 or more bands) 890-820 Alkyl/Aryl (mixed) 1300-1200 and 1050-1010 - Aryl 1300-1200 -
Ethers
Alkenes Group C-H stretch C=C stretch C-H Out-of-plane Vinyl 3090-3075 1660-1630 995-984, 915-905 Vinylidine 3090-3075 1660-1630 895-885 Cis 3050-3000 1660-1630 740-640 Trans 3050-3000 1680-1665 970-960 Trisubstituted 3050-3000 1680-1665 840-790 Tetrasubsituted 1680-1665
Phenyl rings Substitution C-H out-of-plane Ring bend 700-680 Mono 770-710 Yes Ortho 810-750 No Meta 770-735 Yes Para 860-790 No
Subsituted benzene Substitution pattern (w) C-H stretch Ring modes C-H outof-plane Ring bend
Substituted benzene ring Substitution pattern (w) Aromatic C-H Ring modes C-H outof-plane
Substituted benzene ring
Substituted benzene ring
Substitution pattern Substitution C-H Ring bend Pattern out-of-plane 690 10 cm -1 Mono 770-710 Yes Ortho 810-750 No Meta 770-735 Yes Para 860-790 No
Carboxylic group Vibration Saturated C=O stretch 1725-1705 Aromatic C=O stretch 1700-1640
Saturated ketone C=O C-C-C stretch
Aromatic ketone C=O C-C-C stretch
Aldehyde C=O C-H bend C-H
Carboxylic acid C=O O-H Overtones and combination bands In-plane O-H bend C-O stretch Out-of-plane O-H bend
Amides Amide I band: C=O stretch at 1690-1630 Amide II band: N-H bend (primary) 1650-1590 Amide II band: N-H bend (secondary) 1560-1500 O C C N (C in secondary)
Primary amide N-H Benzamide C=O Amide I NH 2 scissors Amide II C-N stretch NH 2 outof-plane Amide III
Secondary amide Nylon-6,6 C=O In-plane N-H bend N-H Overtone of NH bend C-N stretch Out-of-plane N-H bend
Protein Keratin N-H C=O N-H bend
Silicone Si-O-Si stretch Si-Me rock CH 3 umbrella mode
Silica Si-O stretch Si-O-Si As stretch Lone silanol SiO-H Silanolwater Si-O-Si S stretch Si-O-Si bend
Raman spectrum The vibrations are the same selection rules differ, intensities differ Use special Raman correlation tables Overtones and combination bands often weak Clean spectra
Selection rules Raman: polarisability determines Large scattering cross section for molecules with easily polarisable electrons Carbon tetrachloride, phenyl rings, etc Water is a poor scatterer: useful solvent
Same vibrations but...
Clean spectra Benzene
Benzene vib = 2A 1g + A 2g + A 2u + 2B 1u + 2B 2g + 2B 2u + E 1g + 3E 1u + 4E 2g + 2E 2u IR active: A 2u, E 1u Raman active: A 1g, E 1g, E 2g IR C-H out-of-plane 671 A 2 u CH stretch 3099 E 1u C C stretch 1485 E 1u C-H bend 1037 E 1u Raman C-H stretch 3062 A1g C-H stretch 3047 E2g C-C stretch 1585 E2g C-H bend 1178 E2g C-C stretch 992 A1g C-H out-of-plane 849 E1g C-C-C bend 605 E2g
Symmetric vibrations Symmetry determines selection rules Hand-waving Symmetric vibrations show in Raman but not in ir Seen previously for CO 2 Very clear-cut for diatomics
Symmetric vibrations
Advantages Water is a poor scatterer Good solvent; e.g., for bio applications