Adsorption of 2-mercaptopyrimidine (2-MP), a prototype of biologically important N-heterocyclic aromatic thiols, has been examined on 100-120 nm silver particles in aqueous solution. Density functional calculations (UBPW91/6-31+G**) on various thione and thiol tautomers of the molecule have been performed to augment experimental data on reference vibrational frequencies. Criteria for determining the structural form of the molecule on the metal-water interface and its binding site and strength have been developed. The frequencies of the ring stretching Wilson modes 8a and 8b and their separation (Deltanu(8a-8b)) are good indicators of the protonation state of the molecule. The experimental Deltanu(8a-8b) for the Ag-bound 2-MP (20(+/-2) cm(-1) in the pH range 4-11) identifies the structural form with the neutral thiol (C4H3N2S-), rather than thiolate (C4H3N2S-). The molecule bonds with the Ag surface at its S site. A noticeable change in the SERS intensity pattern occurs around pH 6.5 due to the change in the surface environment. In the presence of chloride ion on the surface the ring breathing frequency increases by 10(+/-2) cm(-1) and the relative enhancement of the 8a and 8b bands changes dramatically, indicating reorientation of the molecule, from a vertical configuration of the ring to a slightly twisted horizontal configuration.