Quantum Mechanical Resonance and Internal Rotation

The effect of single bond—double bond resonance upon the internal rotation was discussed for carboxylic and their derivatives. In acids and ester the strong carboxylic resonance imparts the double bond character to the C–O bond the actual “mesomeric” molecule to such an extent that practically the complete inhibition of the internal motion is realized at all ordinary temperatures, while the replacement of hydrogen of H–COOR by chlorine or methoxyl group greatly reduces this double bond character, so that the internal oscillatory rotation of finite amplitude can take place about the C–O bond as axis.