Anisole-(H2O)n (n=1-3) complexes:: An experimental and theoretical investigation of the modulation of optimal structures, binding energies, and vibrational spectra in both the ground and first excited states

We present the results obtained from spectroscopic investigations and quantum chemical calculations of the interaction of anisole (methoxybenzene) with small water clusters. The experiments have been carried out using resonant two-photon ionization (R2PI) and IR-UV double-resonance vibrational spectroscopy (IR/R2PI) in the region of the OH stretches. Apart from the vibrational spectra of the water moiety in the clusters, their intermolecular vibrations in the electronically excited S-1 state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the S-1 state and compared with the vibrations calculated for the S-0 state. The calculations for the S-0 state were carried out at the second order Moller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets and for the S-1 state at the configuration interaction singles (CIS) level with the 6-31+G(*) basis set. In the electronic ground state (S-0), the interaction of a water monomer to anisole is mediated through its oxygen atom, and that of a water dimer both through the oxygen atom (sigma type of interaction) and the arene ring (pi type of interaction). Thus in contrast to the interaction of fluorinated benzenes with water clusters, wherein a conformational transition from an in-plane sigma to a on-top pi bonding emerges starting with a water trimer, this conformational transition appears in case of anisole already with a water dimer. In the excited state (S-1) of the investigated systems, there is a pronounced weakening of the interaction of the water cluster with the aromatic chromophore, which is also responsible for the blue shift of the electronic transitions. Consequently, the structures of the complexes of anisole with a water monomer or dimer are very different in both states. The weakening results from a diminished electron density of the oxygen atom and of the pi system of anisole in the excited state. The calculated binding energies of the ground-state conformers indicate that these small water clusters are bound more strongly to anisole than to other pi systems like benzene, toluene, fluorobenzene, and p-difluorobenzene. The many-body decomposition of the binding energy (S-0) reveals that a progressive increase in the size of the water clusters results in a weakening of the sigma O...H interaction and a concurrent strengthening of the pi-H interaction. The complex containing a cyclic water trimer exhibits in the excited state also a pi-type H-bonding interaction, but its stability emerges from a delocalization of the electron density from the water trimer to the anisole oxygen. Excepting the water dimer complexes, there is a good agreement between the calculated OH vibrational frequencies and the experimental IR spectra recorded for the ground state. The surprisingly good agreement of the calculated and the experimentally observed intermolecular modes in the excited state of these complexes provides convincing evidence that the experimental spectra emerge from structures similar to those predicted at the CIS/6-31+G* level. (C) 2002 American Institute of Physics.