Determination of the quadrupole moment of the halogen nuclei (Cl, Br, I) from molecular data

The electric field gradients at the halogen nuclei in hydrogen halides are calculated by using the coupled cluster method for the treatment of the electron correlation, and the spin-averaged no-pair approximation for the evaluation of the relativistic contribution. Upon combining these results with microwave spectroscopic data for nuclear quadrupole coupling constants in HCl, HBr, and HI, the following values of the nuclear quadrupole moment of the halide nuclei are obtained: Q(Cl-35) = 0.0816 barn, Q(Br-79) = 0.2989 barn, and Q(I-127) = -0.651 barn. The present 'molecular' value of Q(Cl-35) agrees with the latest 'atomic' data. The results obtained for Br-79 and I-127 strongly indicate that the current recommended standard values of their nuclear quadrupole moments need to be revised.