Sulphoxides group frequencies

The position of the S=0 band is dependent on the electronegativity of the attached group. Electronegative substituents tend to raise the frequency since they tend to stabilize the form S=0 rather than S+-0". Hence, the frequency of the S=0 stretching vibration increases in the following order sulphoxides < sulphinic acids < sulphinic acid esters sulphinyl chlorides... 

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