O=C(C[N+]1=C2SC=C(c3ccccc3)N2CC1)c1ccc(Br)cc1.[Br-]
The negative base-10 logarithmic of dissociation constant (pKa) for an acid is a direct consequence of the underlying thermodynamics of the dissociation reaction; the negative pKa value is directly proportional to the standard Gibbs free energy change for the reaction. What is the negative pKa value of this molecule?