003 IR Spectroscopy Flashcards
vibration occurring along the line of
the bond that changes the bond length
stretch
vibration that does not occur along the
line of the bond, but changes the bond
angle
bend
kinds of stretching
symmetric stretch
asymmetric stretch
kinds of bending
symmetric in-plane bend (scissor)
symmetric out-of-plane bend (twist)
asymmetric in-plane bend (rock)
asymmetric out-of-plane bend (wag)
instrument used to obtain an infrared spectrum
IR spectrometer
absorbed energy represented as downward
spike in the IR spectrum
Absorption bands
means of reporting absorption bands
Wavenumbers
has the right energy to correspond
to the stretching and bending vibrations of organic molecules
Infrared radiation (4000-600 cm-1)
The area on the left (4000-1400 cm-1) is where most of the functional groups show absorption bands. This is called the _____________
functional group region
The area on the right (1400-600 cm-1) is called the ___________________
fingerprint region
Just a note!
Because stretching a bond
requires more energy than
bending it:
✓ absorption bands for
stretching vibrations are
found in the functional
group region
✓ absorption bands for
bending vibrations are
typically found in the
fingerprint region.
NA
Just a note!
The intensity of an absorption band depends on the size of the change in dipole moment associated with the vibration: The greater the change in dipole moment, the more intense the absorption.
……….the polar is the bond, the greater IR absorption
NA
Just a note!
The intensity of an absorption band also depends on the number of bonds responsible for the absorption.
……………..the greater the number of bonds, the greater the IR absorption
NA
Just a note!
Lighter atoms show absorption
bands at larger wavenumber
Stronger bonds show absorption
bands at larger wavenumbers
NA
Just a note!
Increase in the bond order character will increase the wavenumber
NA
Just a note!
Electron withdrawal inductively, large wavenumber
Electron donating by resonance, lower wavenumber
…………..Resonance stabilized structure lowers the wavenumber
NA
Just a note!
Esters show C-O stretches at both ends of the range (between 1250 and 1050 cm-1).
If the C-O bond is in an alcohol or an ether, the stretch will occur toward the lower end of the range.
If the C-O bond is in a carboxylic acid, the stretch will occur at the
higher end of the range.
NA
Just a note!
Polar O-H bonds show intense absorption bands and the bands are quite broad
(dependent on the concentration of the solution).
▪ The O-H stretch of a concentrated (hydrogen-bonded) solution of an alcohol occurs at 3550 - 3200 cm-1, whereas the O-H stretch of a dilute solution (with little or no hydrogen bonding) occurs at 3650 - 3590 cm-1
▪ The absorption bands of non-hydrogen–bonded OH groups are sharper.
NA
~2100 cm-1 ……..
1650 cm-1……..
C≡C bond stretch
C=C bond stretch
~1700 cm-1………
1740 cm-1………..
1660 cm-1……
Strong ~1100 cm-1……..
C=O stretch
ester C=O sretch
amide C=O stretch
C-O stretch
3550–3200 cm-1…..
3300–2500 cm−1….
alcohol OH stretch
COOH OH stretch
Slightly right of 3000 cm-1….
Slightly left of 3000 cm-1…….
2820 cm-1 and ~2720 cm-1………..
sp3 C-H stretch
sp2 C-H stretch
aldehyde C-H stretch
~1600 cm-1 and one at 1500-1430 cm-1…………
3500-3300 cm-1………..
benzene ring
N-H stretch
slightly to the left of 1400 cm-1…………..
1000-600 cm-1……….
Broad and intense 1600 cm-1……….
left and right of 1400 cm-1……….
720 cm-1……….
890…….
990 and 910…..
sp3 CH bend
sp2 C-H bending
NH bending
methyl group
Acyclic compounds with more than four adjacent methylene (CH2) groups
terminal alkene (unsubstituted)
terminal alkene (monosubstituted)
