• All?covalent bonds?act rather like springs, as opposed to rigid bars
• Like springs, the bonds can vibrate in a number of different ways
• The frequency of vibration occurs in the?infra-red region?of the?electromagnetic spectrum
• If an organic molecule is irradiated with?infra-red?energy?that matches the?natural vibration frequency?of its bonds, it absorbs some of that energy and the amplitude of vibration increases
• This is known as?resonance

Different modes of vibration in molecules. Each mode has a characteristic frequency of vibration

Infrared (IR) spectroscopy

• Infrared (IR) spectroscopy?is a technique used to identify compounds based on changes in?vibrations?of atoms when they?absorb?IR of certain?frequencies
• A?spectrophotometer?irradiates the sample with IR radiation and then detects the?intensity?of?IR radiation?absorbed by the molecule
• IR?energy?is absorbed only if a molecule has a?permanent dipole?that changes as it vibrates
  • Symmetrical molecules such as O₂?or H₂, are therefore?IR inactive
• The?resonance frequency?is the specific frequency at which the bonds will vibrate
• Rather than displaying frequency, an?IR spectrum?shows a unit called?wavenumber
  • Wavenumber?is the reciprocal of the?wavelength?and has units of?cm^-1
• Characteristic absorptions can be matched to specific bonds in molecules
  • This enables chemists to determine the?functional groups?present

Absorption Range of Bonds

• Due to some absorption bands overlapping each other, other analytical techniques such as?mass spectroscopy?should be used alongside?IR spectroscopy?to identify an unknown compound

• The best way to understand how to interpret an?IR spectrum?is by looking at examples and becoming familiar with the characteristic features of an?IR spectrum

Answer:

• • IR spectrum?A?is?propanone?and spectrum?B?is?propan-2-ol.
  • In IR spectrum?A?the presence of a strong, sharp absorption around 1710 cm^-1?corresponds to the characteristic C=O, carbonyl, group in a ketone.
  • In spectrum?B?the presence of a strong, broad absorption around 3200-3500 cm^-1?suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2-ol.

Fingerprint Region

• The region below about 1500 cm^-1?is called the?fingerprint region?and is unique to every molecule
• It has many peaks that can be difficult to assign
• These peaks represent the complex vibrational interactions that occur between different bonds within a molecule
• The value of the?fingerprint region?is in being able to compare the?IR spectrum?to a known compund from a database and coming up with an exact match
• This is particularly useful, for example, in identifying a specific member of a?homologous series
  • All members of the series will show the same type of bonds present, but no two molecules will have the same fingerprint region