Rotational spectra and nuclear quadrupole coupling constants of 4- and 2-iodoimidazole

Pure rotational spectra of 4-iodoimidazole and 2-iodoimidazole have been measured and assigned to determine rotational, centrifugal distortion and nuclear quadrupole coupling constants. Samples of each isomer were laser-vaporised from the surface of a solid target rod using the focussed pulse of a Nd:YAG laser prior to cooling via supersonic co-expansion with argon buffer gas. Microwave spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Inertial defects of 4-iodoimidazole and 2-iodoimidazole were determined to be 0.0906(1) a.m.u. Å2 and 0.0814(2) a.m.u. Å2 respectively, consistent with rigid planar geometries for these molecules. The evaluated nuclear quadrupole coupling constants of iodine atoms imply that approximately 1% of an electron is transferred from the iodine lone pair into aromatic π orbitals when iodine is substituted for a hydrogen atom in either the 4- or 2-position. Results are compared with data for other halogenated aromatic molecules and discussed in context of the extent of conjugation across the range