2.12c: demonstrate an understanding that only molecules which change their polarity as they vibrate can absorb infrared radiation

Many molecules absorb infrared radiation

• the wavelengths they absorb depends on the natural frequencies of the stretching and bending vibrations of the individual bonds present
• INSERT DIAGRAM HERE
• by shining IR through a sample of a compound, we can determine which wavelengths are absorbed and therefore which bonds and functional groups are present
• only bonds that can change their polarity as they vibrate will interact with IR
• eg. single-bond vibrations: C-H, O-H, N-H = 4000 to 2500cm-1
• eg. triple-bond vibrations: C---C, C---N = 2500 to 1900cm-1
• eg. double-bond vibrations: C=C, C=O = 1900 to 1500cm-1
• carboxylic acids and alcohols have quite broad absorptions due to the added complication of hydrogen bonding