Ab initio molecular dynamics calculations are used here to calculate vibration frequencies for B(OH)(3)(aq) and B(OH)(4)(-)(aq). We show that previous calculations have either underestimated or omitted altogether a major fractionating vibrational mode. The new results indicate that the B-11 partitions into B(OH)(4)(-) in water, in contrast to recent experimental measurement of the fractionation factor. The discrepancy appears to result from using finite-temperature vibrational frequencies in the standard harmonic expression for the fractionation factor. While our results connect the measured spectrum to previous harmonic electronic structure calculations, they indicate that harmonic frequencies must be extracted from experimental vibrational spectra before they can be used in the standard expressions.