Download T IR

C-H Stretch
• 2962 and 2872 cm-1 C-H in CH3
• 2926 and 2853 cm-1 C-H in CH2
• 2890 cm-1 tertiary C-H
strong
strong
weak
• All ± 10 cm-1
• 2720 cm-1 C-H stretch in aldehydes
C-H Bend
• ~ 1450 cm-1 Asymmetric methyl bend
Methylene scissoring
• ~ 1380 cm-1 Symmetric methyl bend
•Alkene and alkyne C-H bonds
display sharp stretching
absorptions in the region 31003000 cm-1.
•The bands are of medium
intensity and are often obscured
by other absorbances in the
region (i.e., OH).
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.
Vibration position (as well as
strength) depends on substitution
•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.
Terminal acetylenic bond
•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.
•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.
•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.
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
• 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.
• O-H gets broader and moves to lower
frequency as the amount of H-bonding
increases
• Carboxylic acid O-H is particularly broa
• 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
ESTERS
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
1550, 1380 cm-1