Nonconventional Hydrogen Bonds between Silver Anion and Nucleobases: Size-Selected Anion Photoelectron Spectroscopy and Density Functional Calculations.

We conducted combined gas-phase anion photoelectron spectroscopy and density functional theory studies on nucleobase-silver complexes. The most probable structures of the nucleobase-Ag complexes were determined by comparing the theoretical calculations with the experimental measurements. The vertical detachment energies (VDEs) of uracil-Ag, thymine-Ag, cytosine-Ag, and guanine-Ag were estimated to be 2.18 ± 0.08, 2.11 ± 0.08, 2.04 ± 0.08, and 2.20 ± 0.08 eV, respectively, based on their photoelectron spectra. Adenine-Ag has two isomers coexisting in the experiment; the experimental VDEs of the two isomers are 2.18 and 2.53 eV, respectively. In the most probable isomers of nucleobases-Ag, uracil, thymine, and cytosine interact with Ag anion via N-H···Ag and C-H···Ag hydrogen bonds, while adenine and guanine interact with Ag anion through two N-H···Ag hydrogen bonds. The N-H···Ag hydrogen bonds can be characterized as medium or strong hydrogen bonds. It is found that binding sites of the Ag anion to the nucleobases are affected by the deprotonation energies and the steric effects of two adjacent X-H groups.