003 IR Spectroscopy

Question 1

vibration occurring along the line of
the bond that changes the bond length

Answer 1

stretch

Question 2

vibration that does not occur along the
line of the bond, but changes the bond
angle

Answer 2

bend

Question 3

kinds of stretching

Answer 3

symmetric stretch
asymmetric stretch

Question 4

kinds of bending

Answer 4

symmetric in-plane bend (scissor)
symmetric out-of-plane bend (twist)
asymmetric in-plane bend (rock)
asymmetric out-of-plane bend (wag)

Question 5

instrument used to obtain an infrared spectrum

Answer 5

IR spectrometer

Question 6

absorbed energy represented as downward
spike in the IR spectrum

Answer 6

Absorption bands

Question 7

means of reporting absorption bands

Answer 7

Wavenumbers

Question 8

has the right energy to correspond
to the stretching and bending vibrations of organic molecules

Answer 8

Infrared radiation (4000-600 cm-1)

Question 9

The area on the left (4000-1400 cm-1) is where most of the functional groups show absorption bands. This is called the _____________

Answer 9

functional group region

Question 10

The area on the right (1400-600 cm-1) is called the ___________________

Answer 10

fingerprint region

Question 11

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.

Answer 11

NA

Question 12

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

Answer 12

NA

Question 13

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

Answer 13

NA

Question 14

Just a note!

Lighter atoms show absorption
bands at larger wavenumber

Stronger bonds show absorption
bands at larger wavenumbers

Answer 14

NA

Question 15

Just a note!

Increase in the bond order character will increase the wavenumber

Answer 15

NA

Question 16

Just a note!

Electron withdrawal inductively, large wavenumber

Electron donating by resonance, lower wavenumber

…………..Resonance stabilized structure lowers the wavenumber

Answer 16

NA

Question 17

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.

Answer 17

NA

Question 18

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.

Answer 18

NA

Question 19

~2100 cm-1 ……..
1650 cm-1……..

Answer 19

C≡C bond stretch
C=C bond stretch

Question 20

~1700 cm-1………
1740 cm-1………..
1660 cm-1……
Strong ~1100 cm-1……..

Answer 20

C=O stretch
ester C=O sretch
amide C=O stretch
C-O stretch

Question 21

3550–3200 cm-1…..
3300–2500 cm−1….

Answer 21

alcohol OH stretch
COOH OH stretch

Question 22

Slightly right of 3000 cm-1….
Slightly left of 3000 cm-1…….
2820 cm-1 and ~2720 cm-1………..

Answer 22

sp3 C-H stretch
sp2 C-H stretch
aldehyde C-H stretch

Question 23

~1600 cm-1 and one at 1500-1430 cm-1…………

3500-3300 cm-1………..

Answer 23

benzene ring
N-H stretch

Question 24

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…..

Answer 24

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)