Characterization of Three Positional Isomers Adopted by the Monohydrated 4-Aminobenzoic Acid O-Protomer Using Two-Color, IR-IR Photobleaching of the H2-Tagged Ions and Electronic Structure Theory

Protonation of 4-aminobenzoic acid (4ABAH+) can occur on the carboxylic acid (O) or on the amino (N) functional groups. The structures of isomers generated upon complexation of a water molecule to O (O•H2O) are determined by analysis of their vibrational band patterns with electronic structure calculations. Isomer-specific vibrational spectra were obtained using a two-color, IR-IR photobleaching scheme augmented by an IR-UV configuration that serves to isolate all O•H2O spectral contributions from those associated with the N•H2O tautomer. The 4ABAH+ ions were generated by electrospray ionization, and the hydrates were formed by condensation of water onto 4ABAH+ in a 200 K ion trap. H2-tagging was carried out by transferring the ions to a cryogenically cooled (∼20 K) ion trap. Of the four potential O•H2O isomers (which are based on the E vs Z orientations of the two OH groups within the −C(OH)2 motif), three are generated using this protocol as evidenced by their distinct vibrational spectra. Two of these are based on the trans E,Z scaffold but differ in the attachment site of the water molecule. The third and most interesting motif, which has been reported as the global minimum, features symmetrical attachment of the water molecule to the two OH groups in a cis, Z,Z configuration. The Z,Z isomer uniquely exhibits very diffuse bands in the OH stretching region. This behavior is traced to combination bands involving soft ion–molecule modes that are activated upon excitation of the OH stretching fundamentals. The extent of this anharmonic coupling is addressed with a vibrationally adiabatic model involving separation of the slow translational and rotational motions of the tethered water molecule and the much higher frequency OH stretches.