O=C(NCCCCNC(=O)c1nc[nH]c1C(=O)Nc1cccc(Cl)c1)c1nc[nH]c1C(=O)Nc1cccc(Cl)c1
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?