Infrared and ultraviolet spectroscopy of water-containing clusters of indole, 1-methylindole, and 3-methylindole

A combination of resonant two-photon ionization (R2PI), resonant ion-dip infrared spectroscopy (RIDIRS), and infrared-ultraviolet (IR-UV) hole-burning spectroscopy is used to characterize the hydrogen-bonding topologies of indole-(water)(1,2), 1-methylindole-(water)(1-3), and 3-methylindole-(water)(1) clusters formed and cooled in a supersonic expansion. The combination of methods provides a means of disentangling R2PI spectra that contain contributions from more than one species in the same mass channel due either to fragmentation or to the presence of conformational isomers. Density functional theory calculations (DFT Becke3LYP/6-31+G*) of the structures, harmonic vibrational frequencies, and infrared intensities provide a basis for distinguishing which structures are observed experimentally. The clusters studied exhibit a range of solvation structures around indole. In the indole-(water)(1) and 3-methylindole-(water)(1) complexes, the RIDIR spectra provide a benchmark frequency shift for the N-H ... OH2 H-bonds in these structurally well-characterized complexes. In indole-(water)(2), the two water molecules form a water dimer bridge between the N-H H-bond donor site and the indole pi cloud acceptor site. For 1-methylindole-(water)(n) clusters, the N-H H-bonding site is blocked, favoring structures in which water acts as an H-bond donor to the indole pi cloud. The RIDIR spectra show water to be pi-bound either as a single molecule (n = 1), a water dimer (n = 2), or a water trimer cycle (n = 3). Two isomers of 1-methylindole-(water)(3) with similar but highly entangled UV spectra are distinguished and assigned using IR-UV hole-burning spectroscopy. The isomers differ in the orientation of the H-bonds (clockwise or counterclockwise) in the water trimer cycle relative to 1-methylindole, effectively freezing out the two chiral structures of the cyclic water trimer. After the H-bonding topologies of these clusters are assigned, their electronic frequency shifts and Franck-Condon profiles are reevaluated in terms of the L-1(a)-L-1(b) character of the observed transitions. A coherent explanation of these data can be made without invoking L-1(a)-L-1(b) energy reversal.