Calculations Of PK_a And Henry’s Law Constant Values For Some Organic Moleculars Using DFT

In this paper the relative Gibbs free energy changes and quantum mechanical descriptors with density functional theory at the level of B3LYP/6-31+G(d,p) along with a polarizable continuum model (PCM) are employed to predict the pKa values of106carboxylic acids and70alcohols respectively. In addition an effective approach was established to estimate molecules Henry’ law constant values (H) with density functional theory at the level of B3LYP/6-311+G(d,p) along with a polarizable continuum model (PCM). The319H values of43alcohols,50amines,24aldehydes,32ketones,33organic fluorides,72organic halogenides,32organic bromides and33organic nitrates, are estimated, respectively. The (?) results indicate that:(1) Two methods above mentioned can all been used to calculate pKa values of these molecules. The results using relative Gibbs free energy change of acid dissociations which take a lot calculation cost is more precisely than using quantum mechanical descriptors such as molecular electrostatic potential (MEP, NAO). As for carboxylic acid, relative Gibbs free energy change in gas-phase and solution calculates pKa values of molecules with correlation coefficient R are0.87,0.92, standard deviation down to0.58,0.48pKa units, respectively. Quantum mechanical descriptors MEPo in gas-phase and solution calculate pKa values of molecules with correlation coefficient R are0.82,-0.76, standard deviation are0.67,0.78pKa units, respectively. Quantum mechanical descriptors MEPH in gas-phase and solution calculate pKa values of molecules with correlation coefficient R0.83,-0.82, standard deviation are0.65,0.68pKa units, respectively. As for alcohols and phenols, relative Gibbs free energy change in gas-phase and solution calculates pKa values of molecules with correlation coefficient R are0.97,0.99, standard deviation down to0.80,0.48pKa units, respectively. Quantum mechanical descriptors MEPo in gas-phase and solution calculate pKa values of molecules with correlation coefficient R are-0.96,0.94, standard deviation are0.95,1.13pKa units, respectively. Quantum mechanical descriptors MEPH in gas-phase and solution calculate pKa values of molecules with correlation coefficient are-0.96,-0.91, standard deviation are1.01,1.43pKa units, respectively. It is known from the comparison that the results using relative Gibbs energy method is more precise than quantum mechanical descriptors, which can been used to calculate pKa values of both the second ionization for dicarboxylic acid and molecules with strong polar groups, but this method need more calculation cost. When the precision of pKa values needed is not high, one can choose gas-phase quantum mechanical descriptors which can not only save a lot calculation cost but also meet the demand of experiment. Therefore, the precision of pKa values needed is very high and these molecules with strong polarity, one can choose using aqueous solution relative Gibbs energy method.(2)The Henry’law constant values estimated by this procedure are in good agreement with the experimental results with the standard deviations of1.381nH unit for alcohols,1.941nH unit for amines,1.55lnH unit for aldehydes,1.181nH unit for ketones,1.241nH unit for organic fluorides,1.44lnH unit for organic chlorides,149lnH unit for organic bromides and0.95lnH unit for organic nitrates, respectively.