Hydrogen bonding in the indole-water complex: A high resolution UV study of the hydrogen donor conformer

The nature of the intermolecular bonding in the indole-water complex is investigated by highly resolved UV-RER IPI spectra with rotational fine structure. First, we determine the coordinates of the amino hydrogen atom by isotope substituting it with deuterium for the S-0 electronic ground and S-1 excited state of indole. In a second step, we calculate the rotational fine structure resulting from possible cluster structures and compare it with the experimental results. A fit routine adapts the spectroscopic constants within the condition of a planar complex which is a reasonable assumption for the observed rotational a,b-type band structure. The best agreement between simulation and experiment is found when the water molecule oxygen is located 2.93(5) Angstrom away from the amino hydrogen of indole. This clearly demonstrates that in the indole-water complex a hydrogen bond is formed between the hydrogen donor indole and the oxygen atom of the water molecule. Upon electronic excitation to S-1 this bond length is nearly constant.