Hyperfine constants of bromine and iodine monofluoride

The J′−J″=1−0 rotational transitions of 79BrF, 81BrF, and IF have been reinvestigated in the ground and first excited vibrational states using microwave Fourier transform spectroscopy. For IF, lines were also detected in the second excited vibrational state. Bromine and iodine nuclear quadrupole coupling constants, spin–rotation constants for all nuclei, and tensor and scalar spin–spin coupling constants (S and J), have been determined along with the rotational constants. Antishielding occurs at the fluorine nucleus for both BrF and IF, as has been found earlier for ClF. The antishielding has been rationalized following a model developed for ClF. The ratio 1.197 0514 (32) has been found for the equilibrium bromine quadrupole coupling constants of 79/81BrF.