Structure and molecular interactions of anti-thyroid drugs. Part 1. Dipole moments of carbimazole and methimazole, and conformation of carbimazole

Abstract

The dipole moments of carbimazole and methimazole are respectively 4.30 D (in benzene) and 5.43 D (in dioxane). The 4.30 D value and the solvent variation of the two carbonyl bands of carbimazole in the IR spectrum are consistent with a Z⇌E conformational equilibrium. Polar solvents stabilize the more polar Z conformer and, whereas carbimazole exists about 90% in the E form in heptane and CCl₄, it is almost entirely in the Z form in water. In CCl₄ the energy difference between the Z and E isomers, determined from the ratio of the intensities of the two carbonyl bands, is found to be 9.6 kJ mol^–1, and is in good agreement with 6-31G** calculations.