Federico Palazzetti

Quantum Chemistry of C3H6O Molecules: Structure and Stability, Isomerization Pathways, and Chirality Changing Mechanisms

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.

A quantum chemical study of H2S2: Intramolecular torsional mode and intermolecular interactions with rare gases

The structural and energetic properties of the H(2)S(2) molecule have been studied using density functional theory, second-order Moller-Plesset method, and coupled cluster theory with several basis sets. In order to extend previous work on intra- and intermolecular dynamics of the chirality changing modes for H(2)O(2) and its derivatives, our focus has been on the torsion around the S-S bond, along with an extensive characterization of the intermolecular potentials of H(2)S(2) with the rare gases (He, Ne, Ar, and Kr). Use is made of previously defined coordinates and expansion formulas for the potentials which allow for a faithful representation of geometrical and symmetry properties of these systems that involve the interaction of an atom with a floppy molecule. The potential energy surfaces obtained in this work are useful for classical and quantum mechanical simulations of molecular collisions responsible for chirality changing processes of possible interest in the modeling of prebiotic phenomena.

The origin of chiral discrimination: supersonic molecular beam experiments and molecular dynamics simulations of collisional mechanisms

The target of the present paper is the study of chirality effects in molecular dynamics from both a theoretical and an experimental point of view under the hypothesis of a molecular dynamics mechanism as the origin of chiral discrimination. This is a fundamental problem per se, and of possible relevance for the problem of the intriguing homochirality in Nature, so far lacking satisfactory explanations. We outline the steps that have been taken so far toward this direction, motivated by various experimental studies of supersonic molecular beams carried out in this laboratory, such as the detection of aligned oxygen in gaseous streams and further evidence on nitrogen, benzene and various hydrocarbons, showing the insurgence of molecular orientation in the dynamics of molecules in flows and in molecular collisions. Chiral effects are theoretically demonstrated to show up in the differential scattering of oriented molecules, also when impinging on surfaces. Focus on possible mechanisms for chiral bio-stereochemistry of oriented reactants may be of pre-biotical interest, for example when flowing in atmospheres of rotating bodies, specifically the planet Earth, as well as in vortex motions of celestial objects. Molecular dynamics simulations and experimental verifications of the hypothesis are reviewed and objectives of future research activity proposed.

Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide†

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.

Aligned molecules: chirality discrimination in photodissociation and in molecular dynamics

Emergence of biochemical homochirality is an intriguing topic, and none of the proposed scenarios has encountered a unanimous consensus. Candidates for naturally occurring processes, which may originate chiral selection, involve interaction of matter with light and molecular collisions. We performed and report here: (1) simulations of photodissociation of an oriented chiral molecule by linearly polarized (achiral) light observing that the angular distribution of the photofragments is characteristic of each enantiomer and both differ from the racemic mixture; and (2) molecular dynamics simulations (elastic collisions of oriented hydrogen peroxide, one of the most simple chiral molecules, with Ne atom) demonstrating that the scattering and the recoil angles are specific of the enantiomeric form. The efficacy of non-chiral light (in the case of photodissociation) and of non-chiral projectile (in the case of collisions) is due to the molecular orientation, as an essential requirement to observe chiral effects. The results of the simulations, that we report in this article, provide the background for the perspective realization of experiments which go beyond the well-documented ones involving interaction of circularly polarized laser (chiral light) with the matter, specifically by making use of non-chiral, i.e. linearly polarized or unpolarized light sources, and also by obtaining chiral effects with no use at all of light, but simply inducing them by molecular collisions. The case of vortices is discussed in a companion paper.

Molecular alignment and chirality in gaseous streams and vortices

The discovery of collisional mechanisms, leading to aligned molecules in gaseous streams, opened the way to the research of possible origin of chiral discrimination involving aligned reactants in molecular dynamics. Experimental evidence is being provided that translational-rotational motions (such as those occurring in vortices), in the liquid phase first and in the gas phase more recently, show manifestations of chiral effects, revealing interesting relations with scenarios on the emergence of chiral selectivity. In this account, we also report the results of experiments of enantiomeric enrichment induced on chiral rotamers by rotational-translational motion. Several chiral molecules were expanded in supersonic seeded molecular beams and the vortex motion was generated by two molecular drag pumps with a right- and a left-handed screw motion, respectively. Connections can be established with many processes occurring in the Universe (and thus possibly related to a prebiotic context), like the stellar accretion of low-mass stars like the Sun and the flowing in atmospheres of rotating bodies.

Hexapole-Oriented Asymmetric-Top Molecules and Their Stereodirectional Photodissociation Dynamics

Molecular orientation is a fundamental requisite in the study of stereodirected dynamics of collisional and photoinitiated processes. In this past decade, variable hexapolar electric filters have been developed and employed for the rotational-state selection and the alignment of molecules of increasing complexity, for which the main difficulties are their mass, their low symmetry, and the very dense rotational manifold. In this work, for the first time, a complex molecule such as 2-bromobutane, an asymmetric top containing a heavy atom (the bromine), was successfully oriented by a weak homogeneous field placed downstream from the hexapolar filter. Efficiency of the orientation was characterized experimentally, by combining time-of-flight measurements and a slice-ion-imaging detection technique. The application is described to the photodissociation dynamics of the oriented 2-bromobutane, which was carried out at a laser wavelength of 234 nm, corresponding to the breaking of the C-Br bond. The Br photofragment is produced in both the ground Br ((2)P3/2) and the excited Br ((2)P1/2) electronic states, and both channels are studied by the slice imaging technique, revealing new features in the velocity and angular distributions with respect to previous investigations on nonoriented molecules.

Carbon Oxides in Gas Flows and Earth and Planetary Atmospheres: State-to-State Simulations of Energy Transfer and Dissociation Reactions

In this paper we illustrate an approach to the study of the molecular collision dynamics, suited for massive calculations of vibrational state-specific collision cross sections and rate constants of elementary gas phase processes involving carbon oxides. These data are used in the theoretical modeling of the Earth and planetary atmospheres and of non-equilibrium reactive gas flows containing the CO2 and CO molecules. The approach is based on classical trajectory simulations of the collision dynamics and on the bond-bond semi-empirical description of the intermolecular interaction potential, that allows the formulation of full dimension potential energy surfaces (the main input of simulations) for small and medium size systems. The bond-bond potential energy surfaces account for the dependence of the intermolecular interaction on some basic physical properties of the colliding partners, including modulations induced by the monomer deformation. The approach has been incorporated into a Grid empowered simulator able to handle the modeling of the CO2 + CO2 collisions, while extensions to other processes relevant for the modeling of gaseous flows and atmospheres, such as CO + CO → C + CO2 and CO2 + N2, are object of current work. Here the case of CO2 + CO2 collisions will be illustrated in detail to exemplify an application of the method.

Potential Energy Surface for the H2O−H2 System†

In the present paper, we introduce a representation of the potential energy surface for the H(2)O...H(2) system based on orthogonal vectors, assuming that the two molecules are rigid. We represent the interaction potential by an expansion in real hyperspherical harmonics depending on the distance between the centers of mass of the two molecules and on four angles, which account for two contributions: an external one depending on the three angle variables which define the mutual orientation of the two molecules and an internal one expressed by the angle which describes the position of the oxygen atom in H(2)O with respect to the H(2)O...H(2) system. The surface was generated in the framework of the supermolecular approach, using the counterpoise-corrected interaction energies at the MP2/aug-cc-pVQZ level. Comparisons with other recent work are presented and features of the representation discussed.

Rovibrationally Excited Molecules on the Verge of a Triple Breakdown: Molecular and Roaming Mechanisms in the Photodecomposition of Methyl Formate

For the photodissociation of the simplest of esters, methyl formate HCOOCH3, the energy threshold for triple fragmentation into H, CH3O, and CO was measured by previous ion-imaging experiments at a sequence of wavelengths. The translational energy features of product CO in the ground vibrational level (υ = 0) and for selected rotational states were characterized. In this integrated experimental and theoretical approach (i) the focus is at a laser energy barely below that threshold; (ii) Fourier-transform infrared emission spectroscopy measurements probe the rovibrational energy deposition in CO(υ) for υ > 0 and the emergence of the roaming phenomenon; (iii) accompanying quantum chemical calculations describe the selective rupture of bonds; and (iv) molecular dynamics simulations of dissociation are performed, introducing an approach explicitly involving outcomes from paths originated nonadiabatically through conical intersections. Quantitative information on energy disposal is provided: we found extensive vibrational excitation of CO, while rotational bands are colder and bimodal, due to contributions from direct and roaming modes.