STRUCTURE AND MOLECULAR-INTERACTIONS OF ANTITHYROID DRUGS - PART 1 - DIPOLE-MOMENTS OF CARBIMAZOLE AND METHIMAZOLE, AND CONFORMATION OF CARBIMAZOLE

The dipole moments of carbimazole and methimazole are respectively 4.3 0 D (in benzene) and 5.43 D (in dioxane). The 4.30 D value and the sol vent variation of the two carbonyl bands of carbimazole in the IR spec trum are consistent with a Z reversible arrow E conformational equilib rium. 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 intensi ties of the two carbonyl bands, is found to be 9.6 kJ mol(-1), and is in good agreement with 6-31G* calculations.