Vibration Assignments

1. Vibration Assignments

2. C-H Stretch • 2962 and 2872 cm-1 C-H in CH3 strong • 2926 and 2853 cm-1 C-H in CH2 strong • 2890 cm-1 tertiary C-H weak • All ± 10 cm-1 • 2720 cm-1 C-H stretch in aldehydes

3. C-H Bend • ~ 1450cm-1 Asymmetric methyl bend Methylene scissoring • ~ 1380 cm-1 Symmetric methyl bend

7. Alkene and alkyne C-H bonds display sharp stretching absorptions in the region 3100-3000 cm-1. • The bands are of medium intensity and are often obscured by other absorbances in the region (i.e., OH).

8. Carbon-carbon double bond stretching occurs in the region around 1650-1600 cm-1. Generally sharp, medium intensity. Trans substituted bonds absorb much less than cis Aromatic compounds will typically display a series of sharp bands in this region.

9. Vibration position (as well as strength) depends on substitution

11. Triple bond stretching absorptions occur in the region 2400-2200 cm-1. • Absorptions from nitriles are • generally of medium intensity and are clearly defined. • Alkynes absorb weakly in this region unless they are highly asymmetric • Symmetrical alkynes do not show absorption bands.

12. Terminal acetylenic bond

13. Alcohols and amines display strong broad O-H and N-H stretching bands in the region 3400-3100 cm-1. • The bands are broadened due to hydrogen bonding and a sharp 'non-bonded' peak can often be seen at around 3400 cm-1.

14. Carbon-oxygen single bonds display stretching bands in the region 1200-1100 cm-1. • The bands are generally strong and broad. • You should note that many other functional groups have bands in this region which appear similar.

16. Carbonyl stretching bands occur in the region 1800-1700 cm-1. • The bands are generally strong and broad. • Carbonyl compounds which are more reactive in nucleophilic addition reactions (acyl halides, esters) are generally at higher cm-1 than simple ketones and aldehydes. • Amides are the lowest, absorbing in the region 1700-1650 cm-1.

17. Band Shifts • Influenced by molecular environment • Stronger bonds absorb at higher frequency • In general the carbonyl of an aldehyde is at higher frequency than carbonyl of ketone • Shift depends on • Physical state of sample • Substitution effects • Conjugation • H-bonding

18. Frequency increases in polar solvent because of reduced intermolecular interactions – shifts of up to ~25 cm-1 • Normal ketone carbonyl is at 1715 cm-1 • If the α-substituent is electron withdrawing, the П bond will tighten, so there will be an increased force constant – so band will be at higher frequency.

19. O-H gets broader and moves to lower frequency as the amount of H-bonding increases • Carboxylic acid O-H is particularly broa

23. A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation. Note the spectrum of aniline, below.) • The N–H bending vibration of primary amines is observed in the region 1650-1580 cm-1. Usually, secondary amines do not show a band in this region and tertiary amines never show a band in this region. (This band can be very sharp and close enough to the carbonyl region to cause students to interpret it as a carbonyl band.) • Another band attributed to amines is observed in the region 910-665 cm-1. This strong, broad band is due to N–H wag and observed only for primary and secondary amines

33. ESTERS

35. Amines • Primary amine – asymmetric and symmetric stretch – see two bands • Secondary amine – only one band • N-H in- plane bend – strong ~ 1600 cm-1 • N-H out- of -plane bend 750 – 850 cm-1 • Very broad for primary amines

36. 1550, 1380 cm-1