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 approximati on for the evaluation of the relativistic contribution. Upon combining these results with microwave spectroscopic data for nuclear quadrupol e 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 m oments need to be revised.