Calculation of boron-isotope fractionation between B(OH)3(aq) and B(OH)4-(aq)

Density functional and correlated molecular orbital calculations (MP2) are carried out on B(OH)3·nH2O clusters (n = 0, 6, 32), and B(OH)4-·nH2O (n = 0, 8, 11, 32) to estimate the equilibrium distribution of 10B and 11B isotopes between boric acid and borate in aqueous solution. For the large 32-water clusters, multiple conformations are generated from ab initio molecular dynamics simulations to account for the effect of solvent fluctuations on the isotopic fractionation. We provide an extrapolated value of the equilibrium constant α34 for the isotope exchange reaction 10B(OH)3(aq) + 11B(OH)4- (aq) = 11B(OH)3(aq) + 11B(OH)4- (aq) of 1.026–1.028 near the MP2 complete basis set limit with 32 explicit waters of solvation. With some exchange-correlation functionals we find potentially important contributions from a tetrahedral neutral B(OH)3·H2O Lewis acid–base complex. The extrapolations presented here suggest that DFT calculations give a value for 103lnα34 about 15% higher than the MP2 calculations.