The bromine nuclear quadrupole moment revisited

Abstract

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 ⁷⁹Br 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. Pyykkö, Mol. Phys. 106, 1965 (2008)), we suggest to replace the currently accepted value of 313(3) mb for ⁷⁹Br 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.

Keywords:

• quantum chemistry
• rotational spectroscopy
• bromine quadrupole moment
• coupled-cluster calculations
• relativistic effects

Acknowledgements

The authors thank Dage Sundholm (Helsinki) for helpful discussions concerning the accuracy of the calculations reported in Ref. [39]. This work has been supported in Mainz by the Deutsche Forschungsgemeinschaft (DFG GA 370/5-1) and the Fonds der Chemischen Industrie as well as in Bologna by MIUR (PRIN 2009 funds) and by the University of Bologna (RFO funds).

Notes

^a Computed by means of VPT2 at the MP2/cc-pCVTZ level.

^b Experimental values for CH₃Br were taken from Ref. [97], for CH₂FBr from Ref. [84], and for CHF₂Br from Ref. [85].