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|Title: ||Comparison between implicit and hybrid solvation methods for the determination of pK (a) of mono-protonated form of 13(2)-(demethoxycarbonyl) pheophytin a in methanol|
|Authors: ||MEHTA, N|
|Issue Date: ||2009|
|Publisher: ||INDIAN ACAD SCIENCES|
|Citation: ||JOURNAL OF CHEMICAL SCIENCES, 121(5), 881-886|
|Abstract: ||Both implicit solvation method (dielectric polarizable continuum model, DPCM) and hybrid solvation method (cluster-continuum model) were adopted to calculate the pK (a) of mono-protonated form of 13(2)-(demethoxycarbonyl) pheophytin a (Pheo) in methanol. In the cluster-continuum model calculations, we considered only 1 solvent molecule attached explicitly and others treated implicitly whereas in the DPCM calculations all the solvent molecules were treated implicitly. DPCM calculations were carried out on Pheo, PheoH(+), Pheo-CH(3)OH and PheoH(+)-CH(3)OH in methanol solution. The aim of these calculations was to determine the free energy changes involved in the deprotonation of PheoH(+) (Delta G (sol)) and finally to obtain the corresponding pK (a) value. DPCM calculations were carried out employing the restricted open-shell density functional treatment (ROB3LYP) using the 6-31G(d) basis set to determine the free energy of solvation of bare Pheo and PheoH(+) and of the clusters, Pheo-CH(3)OH and PheoH(+)-CH(3)OH in methanol. In-vacuo geometries of all the species were obtained by performing optimizations at ROB3LYP level using the 6-31G(d) basis. Electronic energies of all the species were then obtained by carrying out single point DFT calculations using 6-311+G(2d, 2p) basis set on the respective optimized geometries. Differences in thermal energy and molecular entropy were calculated by carrying out frequency calculations at ROB3LYP/STO-3G level on the optimized geometries of the truncated models. The optimized geometries of the clusters display intermolecular hydrogen bonding interactions. The pK (a) values of PheoH(+) calculated by DFT-DPCM and cluster-continuum methods are 6 center dot 12 and 4 center dot 70 respectively while the observed value is 4 center dot 14. The hydrogen bonding interaction between the solute and the solvent can be attributed for the good performance of the cluster-continuum model over pure continuum model.|
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