The experimental techniques and interpretative tools of modern physical chemistry have been applied to specific issues concerning the broad topic of molecular chirality. This project is inspired by the study on homochirality, that is the phenomenon whereby classes of molecules have a single mirror shape (all aminoacids in proteins are left-handed - type while sugars are right-handed) and by the high efficiency with which the selectivity of enantiomers in the processes, where molecules of biological interest are involved, in order to better understand the mechanisms of chiral recognition.
The control of the translational, internal and orientational molecular states in the physico-chemical processes in the gas phase allows one to obtain important information on the geometric characteristics of the molecules and on the aspects of the dynamics, which would otherwise be hidden by the chaotic movement of the molecules and constitute an important challenge both for the construction of models and for the applications of physical chemistry. An important aspect is that represented by chiral selection mechanisms.
The aim of ORCHID is to characterize the enantioselective mechanisms, verify the role of molecular orientation in the processes leading to chiral discrimination and establish the stereodynamic nature of chirality. We define molecular alignment and orientation phenomena of non-statistical distribution of the rotational angular momentum with respect to a quantization axis. Alignment concerns the polarization of the direction of the angular momentum vector, while orientation also considers its direction.
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The mechanical speed selector is a device that allows to control the translational degrees of freedom of the molecules through the speed selection of the beam. Furthermore, the speed selection allows to control the degree of natural alignment, which occurs in the molecular beams, exploiting the "seeding" effect, that is the collisions induced by lighter and faster gases, which transfer momentum to the molecule, determining the phenomenon of alignment. It is therefore necessary to develop an apparatus for the speed selection of the supersonic molecular beams which allows to obtain an optimal control of the degrees of freedom of the system. This allows to study the non-binding interactions involving chiral molecules, in order to be able to establish a phenomenology. The aim is to build models of potential energy surfaces that allow to characterize the interactions that intervene in the collisional processes between homo- and heterochiral molecules.
(2) The hexapolar orientation technique consists in coupling the aligning electric field of the hexapole, i.e. a non-uniform electric field arranged along the propagation axis of the molecular beam, to the orienting electric field of a second element located downstream of the hexapole. The development of an experimental apparatus for the electrostatic hexapolar orientation to be coupled to an apparatus for molecular beams is therefore necessary for the verification of the chiral effects in photodissociation processes (interaction between linearly polarized light and chiral molecule) and for collisional ones ( interaction between linearly polarized light and noble gas atom). The use of a non-chiral component, linearly polarized light rather than circularly polarized light in the photoinduced process, and a noble gas atom rather than a second chiral molecule in the collisional process, aims to verify the role played by orientation molecular in the chiral discrimination mechanism. For a full understanding of this phenomenon it is necessary to shed light on the way in which the three vectors that characterize the interacting system, velocity of the dissociating atom v, permanent electric dipole moment d and transition dipole moment are correlated, giving chirality to the system itself (the diagram of the vectors and the cover of the magazine dedicated to our experiment are shown in the Figure).
(3) Construction of theoretical models for the characterization of chiral selection mechanisms and intermolecular forces, as mentioned in point (1), in order to understand the kinetic and dynamic aspects of collisional and photoinduced processes.
Computational applications that allow to predict the course of enantioselective processes.
Development of methods that allow to compact and classify large quantities of data obtained both experimentally and computationally, of structural, chemical-physical, kinetic and dynamic properties of chiral molecules, in order to evaluate the accuracy of the methods and their consistency and possibly suggest improvements to the methods under consideration.