Stereodynamical Effects by Anisotropic Intermolecular Forces

REVIEW published: 31 May 2019 doi: 10.3389/fchem.2019.00390 Stereodynamical Effects by Anisotropic Intermolecular Forces Daniela Ascenzi 1*, Mario Scotoni 1 , Paolo Tosi 1 , David Cappelletti 2 and Fernando Pirani 2 1 Dipartimento di Fisica, Università di Trento, Trento, Italy, 2 Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia, Italy Edited by: Alkwin Slenczka, University of Regensburg, Germany Reviewed by: Fuminori Misaizu, Tohoku University, Japan Sonia Melandri, University of Bologna, Italy *Correspondence: Daniela Ascenzi Specialty section: This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry Received: 03 February 2019 Accepted: 15 May 2019 Published: 31 May 2019 Citation: Ascenzi D, Scotoni M, Tosi P, Cappelletti D and Pirani F (2019) Stereodynamical Effects by Anisotropic Intermolecular Forces. Front. Chem. 7:390. doi: 10.3389/fchem.2019.00390 Frontiers in Chemistry | www.frontiersin.org 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. Keywords: alignment, orientation, stereo-dynamics, ion-molecule reactions, astrochemistry INTRODUCTION The focus of the present work is on investigating the role of electric and magnetic field gradients, arising from anisotropic intermolecular forces, which can induce molecular polarization (i.e. alignment/orientation of rotational angular momentum / bond direction of a molecule along a preferential axis) as a consequence of collisions with other atoms or molecules. Deep knowledge of these phenomena, today still not fully understood, is of general relevance to control the stereodynamics of elementary chemical-physical processes, occurring both in gaseous and condensed phases under a variety of conditions (Vattuone et al., 2004, 2009, 2010; Gerbi et al., 2006). In particular, understanding the mode-specificity in reaction dynamics of open-shell atoms, free radicals, molecules, atomic and molecular ions, under hyper-thermal, thermal, and subthermal conditions is of fundamental importance for catalysis, plasmas, photodynamics as well as interstellar, and low-temperature chemistry (see for instance Aquilanti et al., 2005; Chang et al., 2013; Li et al., 2014; Rösch et al., 2014; Balucani et al., 2015). The possibility of aligning or orienting molecules by collisions in gaseous streams may also have some implications in unraveling the origin of chiral discrimination and chiral selectivity emerging in vortices formed both in the liquid and in the gas phase (Lombardi and Palazzetti, 2018, and references therein; Su et al., 2013). 1 May 2019 | Volume 7 | Article 390 Ascenzi et al. Stereodynamical Effects by Anisotropic Intermolecular Forces On the basis of the experimental findings, achieved in the last 25 years by the authors, it is proper to distinguish: Molecular alignment determined by weak van der Waals forces: It arises as a combined effect of several elastic/inelastic collisions occurring along preferential directions in environments where anisotropic velocity distributions are operative; Molecular orientation controlled by anisotropic intermolecular forces of intermediate strength: Such phenomenon manifests even during single collision events, when the molecules are in low rotational states; Molecular orientation induced by anisotropic intermolecular forces of high strength: It becomes dominant in each collision event under an ample variety of conditions. The abovementioned classification is proposed on the basis of the results obtained exploiting different but complementary experimental techniques, in the Perugia and Trento laboratories, as well as an integrated experimental-theoretical approach. This paper focuses on selected results highlighting the role of molecular polarization, induced in a natural way by weak, intermediate and strongly anisotropic forces, on the reaction stereo-dynamics under a variety of conditions, including those of applied interest. FIGURE 1 | Illustration of dynamical processes (focusing in the forward direction of the expansion, bending of the rotational plane, rotational relaxation, acceleration) that a diatomic molecule, seeded in a lighter carrier gas, experiences during the formation of a supersonic molecular beam (MBS, Molecular Beam Source; SK1 and SK2, skimmers; QMS, quadrupole mass spectrometer). In this conceptual scheme a common MBS and velocity selection is exploited for the successive (A) magnetic analysis or (B) scattering experiment. In the magnetic analysis (A) the MB transmittance IB /I0 varies with the molecular velocity and paramagnetism. In the scattering experiments (B) the beam attenuation I/I0 varies with the molecular velocity and intermolecular forces that drive the scattering. RESULTS AND DISCUSSION Molecular Alignment by Weak Anisotropic Forces Molecular alignment induced by weak anisotropic van der Waals (vdW) forces emerges in supersonic expansions leading to the formation of seeded molecular beams, where hundreds of collisions between seeded molecules and lighter (hence faster) carrier atoms occur preferentially in the forward direction of the expansion. Value and direction of the relative collision velocity, also defined as velocity slip, play a crucial role in determining important selectivities in the involved elastic and inelastic collisions. To correctly identify the microscopic phenomena, it is useful to distinguish two different regions of the expansion zone. The first one is confined in the proximity of the source nozzle, (...truncated)