Infrared Spectroscopy

Question 1

Infrared Spectroscopy (IR):

Answer 1

Measures vibrations of bonds to identify functional groups

Question 2

Ultraviolet Spectroscopy (UV):

Answer 2

Measures electronic transitions to provide information on the electronic bonding in the sample

Question 3

Mass Spectrometry (MS):

Answer 3

Breaks the molecule into fragments (destructive) and measures the MW of fragments to give information to the structure and functional groups present

Question 4

Nuclear Magnetic Resonance (NMR):

Answer 4

Uses electromagnetic fields to measure spinning of nuclei to identify chemical environments leading to identification of functional groups and structure of the sample

Question 5

What is he spectrum called?

Answer 5

Electromagnetic Spectrum

Question 6

Electromagnetic Radiations

Answer 6

Travel at the speed of light (c) but at different frequencies and wavelengths

Question 7

Frequency (v)

Answer 7

= number of complete wave cycles that pass a fixed point per second, Hz (cycles per second)

Question 8

Wavelength (λ) =

Answer 8

distance between any two peaks

Question 9

Relationship:

Answer 9

λ = c/v, or c = λv, or v = c/ λ

Question 9

Relationship:

Answer 9

λ = c/v, or c = λv, or v = c/ λ

Question 10

Photons =

Answer 10

massless packs of energy E = hv (h = Planck’s Constant)

Question 11

Molecule struck by a photon:

Answer 11

may absorb the photon’s energy and will result in the molecule’s energy increasing by an amount equal to the photon’s energy E = hv

Question 12

Infrared Region

Answer 12

Molecular vibrations can be detected in the infrared region.
The covalent bonds between atoms act like springs. If the bond is stretched / compressed and then released, the atoms vibrate

Question 13

Heavier atoms vibrate more ______.
Frequency decreases with __________ atomic mass/

Answer 13

slowly
increasing

Question 14

Stronger bonds are more ______ so they vibrate faster.
More ______ needed to stretch / compress (higher frequency)

Answer 14

rigid

force

Question 15

What are the three vibrational modes?

Answer 15

1 symmetric stretching
2) antisymmetric stretching
3) Bonding (scissoring)

Question 16

Non-linear:

Answer 16

Non-linear molecules with n atoms will have 3n – 6 vibrational modes (i.e. 6 atoms = 12 modes)

Question 17

What are the combinations and overtunes of fundamental vibrations called?

Answer 17

complex vibrations

Question 18

IR-Active and IR-Inactive vibrations

Answer 18

The bond must have a dipole moment.
The vibration must change the dipole moment of the molecule

Question 19

IR-Inactive:
charicteristic

Answer 19

Symmetrical / has no dipole moment

Question 20

Infrared Spectroscopy of Hydrocarbons

Answer 20

stronger bonds absorb at higher frequencies because of their greater stiffness

Question 21

c-c bond stretching

Answer 21

• isolated
• conjugated
• aromatic

Question 22

Different O-H stretching frequencies: important

Answer 22

Alcohol (e.g. CH3OH); 3300 cm-1, Broad
Acid (e.g. CH3COOH)l 3300 cm-1, Broad

Question 22

Different O-Hstretching frequesncies:

Answer 22

Alcohol (e.g. CH3OH); 3300 cm-1, Broad
Acid (e.g. CH3COOH)l 3300 cm-1, Broad

Question 23

Different C-O frequencies

Answer 23

Alcohols and ethers: 1000 – 1200 cm-1

Question 24

N-H Stretching Frequencies

Answer 24

Amine: 3300 cm-1 , Broad
1 Spike = N-H of secondary amine (R2NH)
2 Spikes = N-H of primary amine (RNH2)

Question 25

C=O stretching frequencies:

Answer 25

Intense absorption at 1700 cm-1
Small overtone peaks at 3400 cm-1
Aldehyde: 1725 cm-1
Ketone: 1710 cm-1

Question 26

C-H stretching frequencies:

Answer 26

Aldehyde has two: 2700 and 2800 cm-1

Question 27

Infrared Spectroscopy of Ketones & Aldehydes

Answer 27

ketone - 1710cm-1

aldehyde - 1724 / 2700, 2800

Question 28

Strengths and Limitations of IR Spectroscopy

Answer 28

1. Ability to indicate functional groups in the compound
2. Highlights the absence of other functional groups that would give strong absorptions if they were present
3. Can confirm identity of a compound by comparison with a known sample
4. It is non-destructive (sample can be retrieved)

Question 29

what is an overtone in a graph?

Answer 29

any resonant frequency above the fundamental frequency
(does not mean anything)

Question 30

revice sprectrum

Answer 30

lecture slides - identify absorption location/ intensity of each functional group

Question 31

What is IR:

Answer 31

a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves