Question

On an IR spectrum, why is it that some readings reach further down on the graph than others?

Answer 1

When the electronegativity difference between two IR active bonded atoms are under IR radiation they will stretch more rapidly consequently their stretching frequency is higher. Groups with a small difference in electronegativity e.g. C-H, C-C, C=C, etc., will usually show weak or medium sized peaks in the IR spectrum. Infrared frequencies correspond to vibrational modes in specific chemical bonds. The stronger the bond like C-O, the higher the frequency (shorter the wavelength, higher the energy) the atoms vibrate at. High transmittance means there are few bonds, low transmittance means there is a high population of bonds.

In your case low transmittance (high population of bonds) is responsible for readings reach further down (means very high intensity peaks due to stronger bonds) on the graph than others.

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