The gas phase structures of anisole dimer in the ground and first singlet electronic excited states have been characterized by a combined experimental and computational study. The dimer, formed in a molecular beam, has been studied by resonance-enhanced multiphoton ionization and high-resolution laser-induced fluorescence techniques. The assignment of the rotational fine structure of the S(1) <-- S(0) electronic transition origin has provided important structural information on the parallel orientation of aromatic rings of anisole moieties. By comparison with the DFT/TD-DFT computational results, it has been possible to infer the detailed equilibrium structure of the complex. The analysis of the equilibrium structure and interaction energy confirms that the anisole dimer is stabilized by dispersive interaction in the gas phase. This is, to the best of our knowledge, the first detailed work (reporting both theoretical and high-resolution experimental data) on an isolated cluster in the pi-stacking configuration.

State-of-the-art spectroscopic and theoretical methods have been exploited in a joint effort to elucidate the subtle features of the structure and the energetics of the anisole-ammonia 1:1 complex, a prototype of microsolvation processes. Resonance enhanced multiphoton ionization and laser-induced fluorescence spectra are discussed and compared to high-level first-principles theoretical models, based on density functional, many body second order perturbation, and coupled cluster theories. In the most stable nonplanar structure of the complex, the ammonia interacts with the delocalized pi electron density of the anisole ring: hydrogen bonding and dispersive forces provide a comparable stabilization energy in the ground state, whereas in the excited state the dispersion term is negligible because of electron density transfer from the oxygen to the aromatic ring. Ground and excited state geometrical parameters deduced from experimental data and computed by quantum mechanical methods are in very good agreement and allow us to unambiguously determine the molecular structure of the anisole-ammonia complex.

Resonance enhanced multiphoton ionization and rotationally resolved S1<--S0 electronic spectra of the anisole-2H2O complex have been obtained. The experimental results are compared with high level quantum mechanical calculations and with data already available in the literature. Quite surprisingly, the equilibrium structure of the anisole-2H2O complex in the S0 state shows some non-negligible differences from that of the isotopomer anisole-1H2O complex. Actually, the structure of the deuterated complex is more similar to the corresponding structure of the anisole-1H2O complex in the S1 state. In anisole-water, two equivalent H(D) atoms exist as revealed by line splitting in the rotationally resolved spectra. It is possible to suggest a mechanism for the proton/deuteron exchange ruled by a bifurcated transition state for the exchange reaction, with both water hydrogen atoms interacting with the anisole oxygen atom. From the analysis of all of the available experimental data and of computational results, we can demonstrate that in the S1 excited state the hydrogen bond in which the water molecule acts as an acid is weaker than in the electronic ground state but is still the principal interaction between water and the anisole molecules.

Integrated experimental and computational results help to clarify the nature of the intermolecular interactions in a simple, isolated π-stacked dimer prepared in a molecular beam. The properties of bimolecular anisole complexes are examined and discussed in terms of the local/supramolecular nature of the electronic wavefunctions. Experimental resonance-enhanced multi-photon ionization spectra of clusters with different isotopic compositions confirmed the fundamentally localized nature of the S(1)←S(0) electronic transition. A detail analysis of the experimental results however shows the existence of non-negligible excitonic coupling for the excited-state wavefunctions leading to the doubling of the single-molecule vibronic levels in the S(1) state, with a splitting of about 30 cm(-1). Theoretical simulation of the vibrationally resolved electronic spectra and computations of the excitonic coupling convincingly support the experimental findings. The overall combined experimental/theoretical study allows a detailed description of the stacking interaction in the anisole dimer.