Giangaetano Pietraperzia

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.

Vibrational spectrum of 4-fluoraniline

The Raman spectrum of 4-fluoraniline (4FA) has been recorded, the quadratic force field has been calculated at RHF/631 1 G p level of theory and then scaled to reproduce the experimental frequencies, by using Pulay’s scaled quantum mechanical force field (SQMFF) methodology. Likewise, DFT force field has been calculated at the B3LYP/6-31 1 G p level. On the basis of all these results, a general assignment of the vibrational spectra of 4FA has been proposed and compared with that of aniline in order to determine the substitution effect of the fluorine atom. q 2001 Elsevier Science B.V. All rights reserved.

High-resolution absorption, excitation, and microwave-UV double resonance spectroscopy on a molecular beam: S1 aniline

Abstract A high-resolution, molecular beam UV spectrometer was constructed that uses both traditional laser induced fluorescence and optothermal detection of the energy content of the molecular beam. Microwave-UV double resonance capabilities were implemented to assist in the assignment of the UV spectra of medium-sized molecules with the ultimate goal to unravel their S1/T1 dynamics. It is shown that high quality optothermal spectra can be obtained in the 300 nm region, by recording the high-resolution, optothermally detected, S1 ← S0 0-0 absorption spectrum of aniline with a signal-to-noise ratio comparable to that of the LIF spectrum of the same band. The latter is used to obtain accurate rotational constants for the vibrationless S1 state of this molecule. Double resonance is demonstrated by confirming several rotational assignments in this way.

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.

The high resolution spectrum of the S1←S0 transition of anisole

The high resolution electronic excitation spectrum of the band origin in the S1←S0 electronic excitation of anisole has been measured in a molecular beam experiment. High accuracy upper state rotational constants have been determined from the fit of more than 400 rovibronic transitions (with a 10 MHZ standard deviation). The planarity of anisole in both the ground and the excited state has been clearly demonstrated and additional information gained about the changes in the structure with electronic excitation.

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.

High-resolution spectroscopy of aniline-rare gas Van der Waals complexes: results and comparison with theoretical predictions

Abstract Laser-induced fluorescence spectra are presented for the first allowed electronic transitions in the near-UV of the aniline-argon and aniline-neon 1:1 Van der Waals complexes, formed in a molecular beam. The experimental linewidth was of the order of 10−3 cm−1 due to residual Doppler broadening and lifetime contributions. Single rotational eigenstates were resolved and complete sets of spectral assignments obtained using a rigid-rotor Hamiltonian model. From the rotational constants we extract both structural and dynamical information on the different clusters. This information is better understood in the light of a comparison with the results of quantum calculations.

Path-breaking schemes for nonequilibrium free energy calculations

We propose a path-breaking route to the enhancement of unidirectional nonequilibrium simulations for the calculation of free energy differences via Jarzynskis equality [C. Jarzynski, Phys. Rev. Lett. 78, 2690 (1997)]. One of the most important limitations of unidirectional nonequilibrium simulations is the amount of realizations necessary to reach suitable convergence of the work exponential average featuring the Jarzynskis relationship. In this respect, a significant improvement of the performances could be obtained by finding a way of stopping trajectories with negligible contribution to the work exponential average, before their normal end. This is achieved using path-breaking schemes which are essentially based on periodic checks of the work dissipated during the pulling trajectories. Such schemes can be based either on breaking trajectories whose dissipated work exceeds a given threshold or on breaking trajectories with a probability increasing with the dissipated work. In both cases, the computer time needed to carry out a series of nonequilibrium trajectories is reduced up to a factor ranging from 2 to more than 10, at least for the processes under consideration in the present study. The efficiency depends on several aspects, such as the type of process, the number of check-points along the pathway and the pulling rate as well. The method is illustrated through radically different processes, i.e., the helix-coil transition of deca-alanine and the pulling of the distance between two methane molecules in water solution.

Vibrational predissociation dynamics in the vibronic states of the aniline–neon van der Waals complex: New features revealed by complementary spectroscopic approaches

We report two independent sets of experimental spectroscopic data which both contain information about the vibrational dynamics occurring in the aniline–neon van der Waals complex in its S1 electronically excited state. The high resolution excitation spectra of the three vibronic bands, 6a01¯, I02¯, and 101¯, of the S1←S0 transition, exhibit lifetime broadening with respect to transitions to the corresponding states in the aniline monomer. The dispersed emission spectra taken under excitation of the same three vibronic bands give access to both the distribution of aniline monomer states produced by vibrational predissociation of the complex and to the rates at which this dynamics proceeds. The overall results are discussed in a consistent way, with emphasis being given to the role of the coupling between the intramolecular and the intermolecular vibrational states. In the case of I02¯ excitation, it is shown that this coupling is reflected in the shape of the van der Waals wavefunction, as accessed throug...