Analytical techniques - IR spectroscopy

Question 1

what is IR?

Answer 1

concerns the interaction of Electromagnetic radiation with atoms and molecules to generate structural information
- the energy of photons in the IR region of the EMS is sufficient to excite vibrational transitions in molecules
Used to identify functional groups

Question 2

what is the wavenumber region of - C=O, N=O, ester, ketone, aldehyde, acid, amide, nitro group?

Answer 2

1550-1780 cm-1

Question 3

what is the wavenumber region of triple bonds, alkyne, nitrile?

Answer 3

2000-2400 cm-1

Question 4

what is the wavenumber region of OH, NH, NH2

Answer 4

3100-3600 cm-1

Question 5

what causes bonds to vibrate at a given intensity?

Answer 5

intensity of an IR signal is related to the change in dipole moment of the vibrating bond
Highly polar groups e.g C=O, N=O, OH absorb strongly as they enable a greater change in dipole when excited - hence why IR is good for functional group identification

Question 6

describe the steps that are required to prepare a KBr disc which is suitable for IR analysis

Answer 6

1. grind sample - ground with KBr (100 mg), to afford a well dispersed powder. Breaking the sample into small fragments reduces scattering of the radiation during analysis
2. compress- transfer a proportion of the mixture into a die and compress, the KBr liquifies and sets to afford a transparent disc containing a dispersion of the sample to be analysed
3. acquire spectrum - KBr disc containing sample placed onto sample holder and spectrum produced

Question 7

Describe the steps required to prepare a mull (nujol) which is suitable for IR analysis

Answer 7

1. solid sample is ground in pestle and mortar
2. drop of liquid paraffin (nujol) added and components mixed to produce an even dispersion
   3.drop of mixture is transferred onto a sodium chloride disc. and sandwiched between 2 NaCl discs to afford a thin film
   NOTE - there will be abs bands from the liquid paraffin so subtract

Question 8

How do we prepare thin films for IR analysis?

Answer 8

(a) solid sample is dissolved in a low BP solvent - and one drop is placed on KBr disc. After the solvent evaporates, the disc is placed in IR spec
(b) If the sample is a liquid - drop between 2 KBr discs