Infrared Cavity Ringdown Spectroscopy of Jet-Cooled Nucleotide Base Clusters and Water Complexes

We present the first direct infrared absorption measurements of gas phase nucleotide base clusters and complexes with water. Spectra in the NH stretching region indicate the presence of several doubly H-bonded isomers of both thymine and uracil dimers, as well as both larger base clusters and uracil−water complexes, but no singly H-bonded species are observed. Three NH stretching bands are assigned to three different uracil−water dimer isomers. Bands due to larger complexes are also observed, but due to spectral congestion, assignment to specific clusters is tentative. The IR cavity ringdown laser absorption spectroscopy results show unusually large bonded OH stretch red shifts for uracil−water clusters, providing evidence of the extremely strong hydrogen bonds formed between these molecules predicted in ab initio calculations.