Gas-phase Acidity and Liquid Phase pKa Calculations of Some Cyclic Oxocarbon Acids (CnOnH2 (n = 3, 4, 5, 6)): A Theoretical Investigation.
The gas-phase acidity and the pKa values of the organic acids have been attracting a lot of attention from organic and analytical chemists. The gas-phase acidity and pKa calculations for some oxocarbon acids were investigated under B3LYP/6-311+G(d,p) level of theory. In this study, four oxocarbon acids, CnOnH2 were selected, namely deltic acid (n = 3), squaric acid (n = 4), croconic acid (n = 5) and rhodizonic acids (n = 6). The calculations in gas phase show that the gas-phase acidity of the acids increases as the ring size increases due to the formation of resonance-stabilized monoanions and dianions. Solvation free energy is required in pKa calculation. However, there is significant uncertainty in calculating the solvation free energy using theoretical methods. Therefore, three methods to calculate the free energy of solvation were adopted for comparison, which are the (i) solvent reaction field (SCRF) method using the Simple United Atom Topological Model (UA0); (ii) SCRF using the United Atom Topological Model (UAHF) optimized for HF/6-31G(d) level of theory; and (iii) Langevin dipoles (LD) method. From the results obtained, it was shown that SCRF method with UAHF atomic radii has the potential of reproducing the experimental values. The calculated pKa values were in good agreement with experimental results and have shown that the croconic acid, C5O5H2 is found to be the most acidic in aqueous phase.
Zhi Xiang
ZX
Wong
Hassan H
HH
Abdallah