American Journal of Chemistry 2013, 3(3): 37-43 DOI: 10.5923/j.chemistry.20130303.01 Comparative Study of Calculated and Experimental pKa Values for Some Carbon and Alcoholic Compounds M. Abul Kashem Liton 1,* , U. Salma 1 , M. Babul Hossain 2 1 Department of Chemistry, Mawlana Bhashani Science and Technology University, Santosh, Tangail-1902, Bangladesh 2 Department of Mathematics, Mawlana Bhashani Science and Technology University, Santosh, Tangail-1902, Bangladesh Abstract Our attempt is to develop an effective computational procedure for predicting the accurate pKa for some carbon and alcoholic compounds within 5 to 50. The experimental determination of these values is an arduous and challenging for the low water solubility compare to the inorganic molecules. As a result theoretical calculations may keep an important role for determining pKa values. An excellent linear regression (R 2 = 0.99, SD=0.15) is obtained between experimental and calculated pKa at the B3LYP/6-311+G(2d,2p) and MPW1PW91/6-311+G(2d,2p) level of theory for gas phase Gibb’s free energy combined with the solvation energy at B3LYP/6-311++G(2d,2p)-CPCM and HF/6-31G(d,p)-CPCM methods for some carbon and oxygen acids respectively. Keywords pKa, B3LYP, MPW1PW91, CPCM, ∆Gº 1. Introduction The important role played a part by proton transfer reactions in chemistry, bioorganic chemistry, biology, and material science[1,2] makes the assessment of reliable pKa values, a topic of continuing connotation. The deprotonation energies of organic compounds and the proton affinities of the corresponding conjugate bases are widely used for the prediction of gas-phase and aqueous phase Bronsted acidities[3–10]. Strong acids have small values of deprotonotion energy while strong bases have large values of proton affinity. Several works on the prediction of the acidity of organic compounds can be found in the literature. For instance, Smith and Radom[11,12] have shown that the G2 and G2(MP2) methods provide excellent results for both deprotonation enthalpies and proton affinities of small molecules and Liton et al. have successfully calculated pKa values of trimethylaminium ion using G2(MP2) gas phase Gibb’s free energy together with B3LYP/6-311G(d)-CPCM solvation energy[13]. Catalan and Palo mar[14] have investigated gas phase acidities of a number of species and have shown that calculations at the B3LYP method with 6-311+G(d) and 6-311+G(3df,3pd) basis sets correlate well with the experimental data. Good correlations have been obtained between experimental pKa values of a wide range of organic compounds and their calculated gas-phase deprotonation enthalpies[15]. * Corresponding author: litonchem@hotmail.com (M. Abul Kashem Liton) Published online at http://journal.sapub.org/chemistry Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved Correlations between theoretical predictions and gas-phase acid-base equilibrium constants of organic compounds have been reported for amines, alcohols and thiols[16–18]. An excellent correlation was obtained between the aqueous-phase acidity calculated with the HF/3-21G(d) method and experimental pKa values for a series of nitrogen bases[19]. Recently, various theoretical descriptors were used to investigate their correlation with the carbon acidity of compounds. A reasonable correlation was obtained between the deprotonation energies of the compounds calculated at the HF/3-21G(d) and B3LYP/6-31G(d) levels of theory and their aqueous pKa values[20,21]. The ability to predict acidity using a coherent, well-defined theoretical approach, without external approximation or fitting to experimental data would be very useful to chemists. However, the current situation is less satisfactory in solution, mostly due to the difficulty of calculating solvation energies with adequate accuracy. The aim of this work is to establish the theoretical calculation without any external approximation or fitting for determining the pKa values of some organic compounds that exhibit pKa values from 5 to 50. 2. Method and Theoretical Calculations 2.1. Thermodynamic Cycle The gas-phase Gibb’s free energy change ∆Gº gas and aqueous phase Gibb’s free energy change ∆Gº aq in Figure 1 are calculated using Eq. (2) and Eq. (4)[23] respectively. The equilibrium constant of reaction (1) is Ka and the pKa is -logKa for this reaction.