Question

A chemist is undertaking a multi-step synthesis of a dye molecule. An intermediate compound contains a chloride functional group singly bonded to the aromatic carbon core of the molecule. The IR spectrum of this intermediate shows a strong absorption due to a stretching vibrational mode involving the =C–Cl bond at 726 cm-1. What property of the =C–Cl bond is mostly responsible for the stretching mode of the bond showing the strong signal in the mid-IR at 726 cm-1? Calculate the force constant for this bond assuming the carbon atom is 12C and the chlorine atom is a 35Cl