Boron Isotope Fractionation Between B(OH)3 and B(OH)4− in Aqueous Solution: A Theoretical Investigation Beyond the Harmonic and Born−Oppenheimer Approximations

Boron isotope fractionation between boric acid and borate ion in aqueous solution (hereafter, alpha(3-4)) is a key coefficient for reconstructing paleo-pH and paleo-pCO(2) records, and a valuable parameter for understanding boron isotope geochemistry related to mineral-water interactions. Although boron isotopes have wide geologic application, the effect of temperature and ion pairing on boron isotope fractionation is poorly described. Moreover, conventional density functional theory (DFT) calculations do not provide an accurate estimate of the boron isotope fractionation factor. Here, we provide a new strategy to accurately calculate alpha(3-4) values in aqueous solution by evaluating harmonic frequencies, higher-order energy terms, and solvation effects. Using benchmark coupled cluster (CCSD(T)) calculations, our 25 degrees C results for alpha(3-4) ranges from 1.0259 to 1.0275, which is in good agreement with recent experimental data. Solvation effects reduced the alpha(3-4) value by similar to 6 parts per thousand, while high-order corrections raised the alpha(3-4) value by similar to 2 parts per thousand. Our calculations evaluating the effect of aqueous ion pairs suggest that the alpha(3-4) values in seawater and pure water should be almost identical. Our results provide a reasonable estimation of the partition function ratios of dissolved boron species, which will benefit studies of boron isotope fractionation in aqueous environments.