The hydrogen-bonding topologies of indole–(water)n clusters from resonant ion-dip infrared spectroscopy

A combination of resonant two-photon ionization, infrared-ultraviolet hole burning, and resonant ion-dip infrared spectroscopies are used to assign and selectively probe the hydrogen bonding topologies of indole–${\mathrm{(water)}}_n$ clusters with $\text{n=1,2.}$ The indole–(water)${}_1$ complex is confirmed to possess the ${\mathrm{N-H\cdots OH}}_{\mathrm{2}}$ structure surmised from previous studies. However, the bands in the ultraviolet previously assigned to a π H-bound indole–water complex are shown to be due instead to the indole–(water)${}_2$ cluster in which the water dimer forms a H-bonded bridge between the N–H and aromatic π clouds of indole. The implications of this reassignment for our understanding of the influence of H-bonding solvents on indole’s fluorescence properties are discussed.