Monday, November 5, 2007 - 1:26 PM
114d

New Insights Into Solvation Effects, Resolves An Apparent Paradox

David A. Gallagher, CAChe Research, 13690 SW Otter Lane, Beaverton, OR 97008

Until recently, theoretical studies on transition metal complexes, including metallo-enzymes, have been limited to gas-phase calculations. However, gas-phase calculations do not always reflect observed behavior in solution or in vivo. Recent algorithm developments now facilitate the accurate prediction of the thermodynamic effects of specific solvents and so, have opened up the condensed phase chemistry of organometallics to theoretical study. Calculations in simulated solvent now offer new insights that contradict previous rationalizations of the properties of bisglycinato copper(II) in aqueous solution.

Background: Bisglycinato copper(II) exists as two stable isomers in the solid state at room temperature, cis and trans. When crystals of the cis monohydrate are heated, they first loose water of crystallization then isomerize to the trans isomer. This is in agreement with high-level quantum mechanics DFT gas-phase calculations, which indicate that the trans isomer is more thermodynamically stable than the cis, by over 10 kcal/mol. The same high-level calculations also suggest that the gas-phase energy barrier for the cis to trans isomerization is so low that the trans isomer is likely to be formed spontaneously above 100 K.

The fact that crystals of the cis isomer are apparently stable at room temperature, has been attributed to the crystal packing forces being sufficiently strong to prevent isomerization occurring in the solid state.

Paradoxically, the cis isomer is the product from synthesis in aqueous solution. It is reportedly because the cis isomer is the kinetic product, i.e. cis crystallizes from solution before it can isomerize to the trans isomer. However, as there should be no significant crystal packing forces in solution, this is inconsistent with the calculated low energy barrier that should lead to spontaneous isomerization to trans above 100 K.

New Insights: Calculations with the COSMOtherm SW program reveal a different explanation. While DFT gas phase calculations agree that the trans isomer is more thermodynamically stable in the gas-phase, the presence of a high-dielectric water solvent field, simulated by COSMOtherm, appears to stabilize the cis isomer more than the trans isomer, to the extent that their relative thermodynamic stabilities (free energies) are reversed in aqueous solution. As a consequence, the author will propose a new hypothesis for the mechanism of cis-bisglycinato copper(II) synthesis.