Ab initio calculation of Br-18 nuclear quadrupole resonance transition frequencies for brominated aromatics (flame retardants)

Brominated aromatics, used extensively as flame retardants, have been studi ed with Br-81 nuclear quadrupole resonance (NQR) spectroscopy. NQR requires lengthy frequency searches because Br-81 NQR transition frequencies in bro minated aromatics are spread over a wide (40 MHz) range. We investigate the ability of ab initio calculations to narrow this search range by predictin g Br-81 NQR transition frequencies for a series of brominated aromatics, us ing restricted Hartree-Fock (RHF) and Becke's three-parameter Lee-Yang-Parr density functional theory hybrid method (B3LYP). Basis sets used are of do uble and triple-zeta quality with varying degrees of polarization included on bromine. Geometries are the isolated molecules, with coordinates optimiz ed for lowest energy. The results of calculations for nine simple brominate d aromatics are fit against experimental frequencies and the fit is subsequ ently used to predict frequencies of larger, two-ring brominated aromatics (one is sold commercially as a flame retardant). Comparison to experiment s hows the accuracy of this approach to be approximately 5 MHz, reflecting a significant, 8-fold decrease in the spectral range to be searched by experi ment.