HR: 0800h
AN: PP31B-1528    [Abstracts]
TI: Accurate Ab Initio Calculation of the Isotopic Exchange Equilibrium 10B(OH)3 + 11B(OH)4- = 11B(OH)3 + 10B(OH)4- In Aqueous Solution
AU: * Tossell, J A
EM: tossell@chem.umd.edu
AF: University of Maryland, Dept. of Chemistry and Biochemistry Univ. of Maryland, College Park, MD 20782 United States
AB: For more than a decade the B isotopic compositions of marine carbonates have been used as paleo-pH proxies for seawater and to reconstruct paleo-[CO2] concentrations in the atmosphere. A necessary step is this process is the accurate determination of the equilibrium constant, K, for the reaction shown in the title above. This equilibrium constant has been recently calculated using ab initio quantum chemical methods applied to nanoclusters containing the solutes B(OH)3 and B(OH)4- coordinated by large numbers of explicit solvent molecules, a computationally difficult procedure. To obtain the most accurate possible value for K the calculated vibrational frequencies were scaled to best fit the limited experimental data available. The value of K obtained (@ 25°C) was 1.027 (significantly larger than the long used value of 1.0194). Even more recently a purely experimental value of K= 1.0265 ± 0.0015 has been obtained through an accurate spectrophotometric determination of the difference of pKa's of commercially available bulk samples of >99% enriched 10B(OH)3(s) and 11B(OH)3 (s). Since we now know the correct experimental value and have a calculation, admittedly a difficult and slightly parameterized one, which matches the experimental result (which was obtained after the calculation), it is worthwhile to analyze the steps in the theoretical calculation of K in more detail. We need to establish a general procedure which can yield accurate K values for other similar aqueous species even if we have no accurate experimental value for K and no vibrational spectral data. To this end we will examine the dependence of the calculated values of vibrational frequencies, isotopomer frequency differences and K values on a number of factors, including (a) the quantum mechanical level (basis set and treatment of electron correlation) used for the free solutes, (b) the incorporation of aqueous medium effects, (c) the effects of vibrational anharmonicity, (d) incorporation of the effects of counterions or of ionic strength and (e) the proper microscopic descriptions of reactants and products. We will also discuss the relationship between bond strength and isotopomer free energy differences. Our goal is to move the calculation of isotopic exchange K values from its present ad hoc basis to a reliable general methodology and to quantitatively connect K values with other known properties.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 4870 Stable isotopes (0454, 1041)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005