Knowledge Bank

The 1+1 resonance ionization (REMPI) spectra of benzotriazole (BT) and its hydrogen bonded water clusters benzotriazole($H2O)n$ n=1-3 have been measured in a molecular beam between $34919cm-1$ and $36100cm-1$. The electronic origin of the monomer can only be determined by 1+1 two color REMPI, as the difference between $S1$- and ionization energy is slightly larger than the $S0$ - $S1$ energy difference. Due to the low ionization excess energy, fragmentation of the clusters is small. Certainty about the existence of more than one conformer (for both monomer and clusters) in the molecular beam can only be obtained by spectral hole burning. Stability and vibrational frequencies of the two possible tautomers (1H and 2H) of BT are calculated by ab initio and density functional calculations. Many possible structures had to be calculated because both water and BT can act as proton donor and acceptor. Cyclic structures for BT($H20)1$ were found to be nearly as stable as a trans-linear arrangement of the hydrogen bond. The transition state structure, which is a saddle point at the potential surface, is calculated for the BT($H2O)n$ clusters and a mechanism for the H- bond subsidized tautomerisation is proposed. For a complete assignment of the cluster spectra, low frequency intermolecular vibrations and torsional motion of the water molecule have to be discussed.