O=C(CSc1nc2c([nH]c3ccccc32)c(=O)n1-c1ccc(Cl)cc1)NCc1ccccc1
An acid's negative base-10 logarithmic dissociation constant (pka) follows directly from its underlying thermodynamics; pKa is directly proportional to its standard Gibbs free energy change. What is the pka value of this molecule?