Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering

Anisotropic circular dichroism of jet-cooled chiral molecules

Anisotropic circular dichroism (CD) refers to the CD of oriented molecules, which varies with the direction of light propagation toward the molecules. Thus, anisotropic CD spectroscopy has been used to investigate the orientations of molecules in anisotropic media such as liquid crystals and thin films. However, it is unclear whether anisotropic CD results from isolated chromophores or their intermolecular interactions with other atoms or molecules that form anisotropically aligned structures. Herein, anisotropic CD of isolated chiral molecules was observed for the first time. The resonant two-photon ionization CD spectra of jet-cooled pseudoephedrine and styrene oxide indicated a difference between the CD values of the P/R and Q branches of the origin bands of the S0-S1 transition. This difference may have resulted from the anisotropic CD phenomena of these molecules, which are oriented via photoselection. Although jet-cooled molecules may have nearly random orientations, those excited to the P/R or Q branch become oriented because the transition probability to these branches depends on the molecular orientation relative to the direction of light propagation. These results demonstrate that the CD spectra of cold, isolated molecules, such as those in an interstellar medium, may exhibit anisotropic CD values.

Quantum-assisted diamagnetic deflection of molecules

We measure the diamagnetic deflection of anthracene and adamantane in a long-baseline matter-wave interferometer.

Stereodynamical Effects by Anisotropic Intermolecular Forces

Electric and magnetic field gradients, arising from sufficiently strong anisotropic intermolecular forces, tend to induce molecular polarization which can often modify substantially the results of molecular collisions, especially at low rotational temperatures and low collision energies. The knowledge of these phenomena, today still not fully understood, is of general relevance for the control of the stereo-dynamics of elementary chemical-physical processes, involving neutral and ionic species under a variety of conditions. This paper reports on results obtained by combining information from scattering, spectroscopic and reactivity experiments, within a collaboration between the research groups in Perugia and Trento. We addressed particular attention to the reactions of small atomic ions with polar neutrals for their relevance in several environments, including interstellar medium, planetary atmospheres, and laboratory plasmas. In the case of ion-molecule reactions, alignment/orientation is a general phenomenon due to the electric field generated by the charged particle. Such phenomenon originates critical stereo-dynamic effects that can either suppress (when the orientation drives the collision complex into non-reactive or less reactive configurations), or enhance the reactivity (when orientation confines reagents in the most appropriate configuration for reaction). The associated rate coefficients show the propensity to follow an Arrhenius and a non-Arrhenius behavior, respectively.

Chirality in molecular collision dynamics

Chirality is a phenomenon that permeates the natural world, with implications for atomic and molecular physics, for fundamental forces and for the mechanisms at the origin of the early evolution of life and biomolecular homochirality. The manifestations of chirality in chemistry and biochemistry are numerous, the striking ones being chiral recognition and asymmetric synthesis with important applications in molecular sciences and in industrial and pharmaceutical chemistry. Chiral discrimination phenomena, due to the existence of two enantiomeric forms, very well known in the case of interaction with light, but still nearly disregarded in molecular collision studies. Here we review some ideas and recent advances about the role of chirality in molecular collisions, designing and illustrating molecular beam experiments for the demonstration of chiral effects and suggesting a scenario for a stereo-directional origin of chiral selection.

On the state selection of linear triatomic molecules by electrostatic hexapole fields

Electrostatic hexapole state-selector is a versatile tool in experimental stereodynamics. The requirement of appropriate models to correctly predict the behavior of molecules in the hexapole motivated us to realize a treatment that predicts the Stark effect of linear triatomic molecules with rotational doublet states. Various perturbative approximations are conventionally adopted to obtain analytic Stark energy derivatives of a truncated Hamiltonian matrix, without utilizing numerical diagonalization of the full Hamiltonian matrix. By including both the low and high field effects, which were alternatively ignored in the analytical formulae of such approximate approaches, herein we demonstrate that the performance of hexapole state selector to linear triatomic molecules can be appropriately predicted via Van Vleck transformation. This method can provide analytic Stark energy derivatives that are acceptably in consistent with the ones obtained via numerical diagonalization of the full Hamiltonian matrix. Particularly, this work is suitable for v2 = 1 level of linear triatomic molecules, due to the following reasons: (1) the Stark energy derivative and the molecular orientation as a function of the electric field are expressed in analytical formulae, hence it is suitable for implementation without involving numerical diagonalization of the full Hamiltonian matrix; (2) a better prediction of the focusing curves with respect to conventional analytical treatments is provided, allowing a reliable determination of the selected state compositions and molecular orientation.

H2O–CH4and H2S–CH4complexes: a direct comparison through molecular beam experiments and ab initio calculations

Electron density redistribution upon the formation of the water–methane complex arises from polarisation and charge transfer effects.

Quantum chemistry of C(3)H(6)O 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.

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.

Ultrafast angular momentum orientation by linearly polarized laser fields

We theoretically show and experimentally verify that a pair of linearly polarized intense femtosecond pulses can create molecular ensembles with oriented rotational angular momentum on an ultrafast (approximately ps) time scale, when the delay and the mutual polarization between them are appropriately arranged. An asymmetric distribution for +M and -M sublevels relies on quantum interference between rotational wave packets created in stimulated Raman excitation by the first and second pulses. The present approach provides spatiotemporally propagating ensembles, of which the classical perspective is molecules rotating in a clockwise or counterclockwise direction.

Observed molecular alignment in gaseous streams and possible chiral effects in vortices and in surface scattering

Extensive work in this laboratory has been devoted to the study of intermolecular interactions from scattering experiments, in order to provide ingredients for modelling forces acting in systems involving hydrocarbons, the components of atmospheres, and water. Our detection of aligned oxygen in gaseous streams and further evidence on simple molecules has been extended to benzene and various hydrocarbons. Chiral effects can be seen in the differential scattering of oriented molecules, in particular from surfaces. It is pointed out that it may be of pre-biotical interest that we focus on possible mechanisms for chiral bio-stereochemistry of oriented reactants, 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 are in progress.

Alignment of molecules in pulsed resonant laser fields

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.