Theoretical Calculation of Separation Factors for Boron Isotopic Exchange between BF3 and BF3·C6H5OCH3

The separation factors (or equilibrium constants) for boron isotopic exchange reaction (10)BF(3) + (11)BF(3) x C(6)H(5)OCH(3) <==> (11)BF(3) + (10)BF(3) x C(6)H(5)OCH(3) were obtained from MP2/6-31+G(d,p) and B3LYP/6-31+G(d,p) calculations. New scaling factors, single and multiple, were derived from the harmonic frequencies through the least-squares fit for BF(3) and C(6)H(5)OCH(3). The use of multiple scaling factors in the case of C(6)H(5)OCH(3) led to significant improvement in the calculated frequencies over using a single scaling factor. There exists a negligible difference in the separation factors obtained by using the harmonic and the scaled frequencies of the same method, and in those obtained by using different methods. The calculated separation factors for the boron isotopic exchange reaction at 273.15, 293.15, and 298.15K are 1.039, 1.036, and 1.035, respectively, which are in excellent agreement with the experimental values, 1.041 +/- 0.002, 1.030 +/- 0.002, and 1.035 +/- 0.003. This study demonstrated the promise of using DFT (B3LYP) to obtain separation factors for the reactions where the interactions are weaker than covalent bonds but stronger than van der Waals interactions, and consequently to search for better complexation agents than C(6)H(5)OCH(3) for the isotopic separation of BF(3).