Question

What factors determine the IR frequency of a chemical bond according to Hooke’s Law? Based on your answer, explain why carbon-carbon triple bonds absorb at higher frequencies than carbon-carbon double bonds, and double bonds in turn absorb at higher frequencies than single bonds.

Answer 1

According to Hooke's law:

- stronger is the bond higher is the frequencies absorption.
- weaker is the bonds lower is frequencies absorption.
- bonds between lighter atoms absorb at higher frequencies.
- bonds between heavier atoms absorb at lower frequencies.

Triple bond is composed of a 2 pi bond and a sigma bond. Double bond is composed of 1 pi and 1 sigma bond and single bond contains only sigma bond. So, greater energy is to break a triple bond than a double bond than single bond. So, strongness of the bond lies in the order

triple bonds > double bond > single bond

So, according to Hooke’s law they follow the same order in abosrption of IR frequency ie

Triple bond absorbs at higher frequency than double than and double bonds in turn absorbs at higher frequency than single bond.