O=[N+]([O-])c1ccc(C2CC(c3ccc(NCc4nc5ccccc5[nH]4)cc3)=NO2)cc1
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?