Massimiliano Pasquini

The gas phase anisole dimer: a combined high-resolution spectroscopy and computational study of a stacked molecular system.

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.

A study on the anisole–water complex by molecular beam–electronic spectroscopy and molecular mechanics calculations

An experimental and theoretical study is made on the anisole-water complex. It is the first van der Waals complex studied by high resolution electronic spectroscopy in which the water is seen acting as an acid. Vibronically and rotationally resolved electronic spectroscopy experiments and molecular mechanics calculations are used to elucidate the structure of the complex in the ground and first electronic excited state. Some internal dynamics in the system is revealed by high resolution spectroscopy.

Noncovalent interactions in the gas phase: the anisole-phenol complex.

The present paper reports on an integrated spectroscopic study of the anisole-phenol complex in a molecular beam environment. Combining REMPI and HR-LIF spectroscopy experimental data with density functional computations (TD-M05-2X/M05-2X//N07D) and first principle spectra simulations, it was possible to locate the band origin of the S(1) ← S(0) electronic transition and determine the equilibrium structure of the complex, both in the S(0) and S(1) electronic states. Experimental and computational evidence indicates that the observed band origin is due to an electronic transition localized on the phenol frame, while it was not possible to localize experimentally another band origin due to the electronic transition localized on the anisole molecule. The observed structure of the complex is stabilized by a hydrogen bond between the phenol, acting as a proton donor, and the anisole molecule, acting as an acceptor through the lone pairs of the oxygen atom. A secondary interaction involving the hydrogen atoms of the anisole methyl group and the π electron system of the phenol molecule stabilizes the complex in a nonplanar configuration. Additional insights about the landscapes of the potential energy surfaces governing the ground and first excited electronic states of the anisole-phenol complex, with the issuing implications on the system photodynamic, can be extracted from the combined experimental and computational studies.

On the properties of microsolvated molecules in the ground (S0) and excited (S1) states: The anisole-ammonia 1:1 complex

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.

Binding Energies of the π‐Stacked Anisole Dimer: New Molecular Beam—Laser Spectroscopy Experiments and CCSD(T) Calculations

Among noncovalent interactions, π-π stacking is a very important binding motif governed mainly by London dispersion. Despite its importance, for instance, for the structure of bio-macromolecules, the direct experimental measurement of binding energies in π-π stacked complexes has been elusive for a long time. Only recently, an experimental value for the binding energy of the anisole dimer was presented, determined by velocity mapping ion imaging in a two-photon resonant ionisation molecular beam experiment. However, in that paper, a discrepancy was already noted between the obtained experimental value and a theoretical estimate. Here, we present an accurate recalculation of the binding energy based on the combination of the CCSD(T)/CBS interaction energy and a DFT-D3 vibrational analysis. This proves unambiguously that the previously reported experimental value is too high and a new series of measurements with a different, more sensitive apparatus was performed. The new experimental value of 1800±100 cm(-1) (5.15±0.29 kcal mol(-1)) is close to the present theoretical prediction of 5.04±0.40 kcal mol(-1). Additional calculations of the properties of the cationic and excited states involved in the photodissociation of the dimer were used to identify and rationalise the difficulties encountered in the experimental work.

A study on the anisole–carbon dioxide van der Waals complex by high resolution electronic spectroscopy

The van der Waals complex formed by anisole and carbon dioxide was studied in a supersonic beam by means of high resolution electronic spectroscopy. The spectrum was assigned by means of a rigid rotor Hamiltonian model. A planar complex structure was obtained and described with a simple three-parameter geometrical model. It is shown that the carbon dioxide molecule lies to the side of the aromatic ring close to the methoxy group. This geometry is quite different from those resulting from previous studies on similar systems like phenol–carbon monoxide or phenol–nitrogen where the hydroxyl group directly bonds to the N2 and CO molecules.

Variable gain detection strategy for time-of-flight multiphoton ionization spectroscopy experiments

This note presents a new, simple approach to measure time-of-flight mass spectra containing ions in a wide concentration range as generated by laser photoionization studies of van der Waals complexes. Real-time control of the gain of the microchannel plate detector, applying to it fast-rising high-voltage pulses like in slice imaging experiments, is suggested. Results are presented for a model system study.

Microsolvation in molecular complexes

In this paper, we report the results of our study of the microsolvation process involving the anisole molecule. We are able to study bimolecular complexes of different compositions. Changing the second partner molecule bound to anisole, we observed complexes of different geometries, because of the large variety of interactions possible for the anisole. High-resolution electronic spectroscopy is the best tool to reveal the correct vibrationally (zero-point) averaged geometry of the complex. That is done by analysing the rovibronic structure of the electronic spectra, which are related to the equilibrium geometry of the complex as well as dynamical processes, both in the ground and in the excited state. The interpretation of the experimental results is supported by high-level quantum calculations.

Structural Determinations and Dynamics on Floppy Molecular Systems

We discuss on the central role of very high resolution spectroscopy for the study of molecular systems weakly bonded or flexible. It will appear evident how the lack of high resolution results can lead to wrong conclusions. The paper will focalize the attention in particular on two different cases: one involving the hydrogen bonded complex anisole‐water, the other involving the very floppy 1,3‐benzodioxole (BDO) molecule. In the first case the issue is the determination of the structure of the complex that cannot be correctly inferred from resonance enhanced multi photon ionization (REMPI) data and ab initio calculations. In the second case the non‐rigidity of the molecule and the possibility of the interaction of two low frequency modes (ring‐puckering and ring‐butterfly) have lead to a wrong assignment of the laser induced fluorescence (LIF) vibronic spectrum.