2.7 INFRARED SPECTROSCOPY

Question 1

What is a ground state

Answer 1

molecular vibration which is in the lowest possible energy state

Question 2

What is the wavenumber

Answer 2

The reciprocal of the wavelength and it is measured in cm-1

Question 3

What is the label of the axis on an IR Spectrum

Answer 3

y-axis: Transmission/%
x-axis: Wavenumber/cm-1

Wavenumber is in the range of 4000-600cm-1 from the left

Question 4

State and explain the effect of infrared radiation on molecules

Answer 4

Molecules will absorb IR if the frequency of the radiation matches the frequency at which the molecule will vibrate.

Bonds in molecules will vibrate more vigorously when the molecule absorbs IR

Question 5

What are the type of vibrations in a water molecule

Answer 5

Asymmetric stretching
Symmetric stretching
Bending

Question 6

Why do covalent bonds absorb IR at different frequencies/wavenumbers?

Answer 6

Different atoms attached
Atoms may have different masses
Bonds may have different lengths
Bonds may have different strengths
bonds may be in a different environment (near different atoms/functional groups)

Question 7

Analysis of IR Spectrum

Answer 7

Presence or absence of a peak (trough) tells us what functional groups are present/absent and provides evidence for identification

Fingerprint region: 1500-500cm-1
It is very difficult to attribute troughs to individual bonds
Pattern obtained in this region is characteristic of compound - Fingerprint region of IR spectrum is compared to spectra of known compounds in a computer database for identification

Question 8

Aldehydes

Answer 8

end in -al

R-(C=O)-H