Ab initio molecular orbital calculations for boron isotope fractionations on boric acids and borates

Ab initio quantum chemistry calculations have been performed on the isotopic exchange reaction between B(OH)3 and B(OH)4−. Several calculation methods have been carefully compared and evaluated. The “water-droplet” method is chosen to investigate this isotope exchange reaction using cluster models with up to 34 water molecules surrounding the solute. HF/6-31G* level calculations coupled with a 0.920 scaling factor are used for the frequency calculations. A larger K value (1.027) is obtained from this study compared to the commonly used 1.0194 (Kakihana et al., 1977).The fractionations for several boric acid polymers and boron minerals are also studied. Our results suggest that assuming the BO4 bonding in B(OH)4− is identical to that in borosilicates is wrong. Tetrahedral boron in silicates has a significantly smaller reduced isotopic partition function ratio (RPFR) and hence will be much isotopically lighter than in B(OH)4−.The new theoretical curve of pH vs. δ11B composition of B(OH)4− using our calculated 1.027 can be used to predict pH values for equilibrium cases such as incorporation into inorganic calcite. We also find that the shape of this curve is very sensitive to both K and pKa value, giving the possibility of also predicting salinity from the different shapes of the curve.