The bromine nuclear quadrupole moment revisited

For the bromine atom and the hydrogen bromide molecule, we report results for the electric-field gradient at the bromine nucleus based on quantum-chemical calculations. Highly accurate values are obtained by using coupled-cluster methods for the treatment of electron correlation, by minimising remaining basis-set effects through the use of large atomic-orbital sets, and by taking into account relativistic effects. For hydrogen bromide, zero-point vibrational corrections are considered as well. The obtained results for the bromine electric-field gradients are used to derive values for the Br-79 quadrupole moment: 308.1 and 309.3 mb based on data for the bromine atom and hydrogen bromide, respectively. As these values deviate from those reported in recent compilations of nuclear quadrupole moments (see, for example, P. Pyykko, Mol. Phys.106, 1965 (2008)), we suggest to replace the currently accepted value of 313(3) mb for Br-79 by the mean value from the present investigation, i.e., 308.7(20) mb. The remaining uncertainty is estimated in a conservative manner to about 2 mb. The accuracy of the proposed value for the bromine quadrupole moment is demonstrated by performing calculations for the bromine quadrupole-coupling tensors of CH3Br, CH2FBr, and CHF2Br. Good agreement with experimental numbers is obtained provided electron correlation, basis-set issues, and scalar-relativistic effects are treated in an adequate manner.