Dielectric relaxation and dipole moments of substituted pyrroles

Abstract

The electric permittivities of 17 pyrroles have been measured in benzene and 1,4-dioxan solutions at one radio and two microwave frequencies. Their dielectric relaxation is discussed in terms of the size and possible internal rotations of the solute molecules. One conclusion drawn, that internal rotation in 2-acylpyrroles is negligible, is supported by n.m.r. evidence. Their electric dipole moments in benzene solutions at 25·0°C are analysed in terms of component moments and it is concluded that 2-acylpyrroles and similar molecules exist in a syn-conformation. In 1,4-dioxan solutions the apparent increments in the electric dipole moments of pyrroles with a free N–H indicate the presence of hydrogen bonding to the solvent.