The topology of the reaction stereo-dynamics in chemi-ionizations

Details on the stereo-dynamic topology of chemi-ionizations highlight the role of the centrifugal barrier of colliding reactants: it acts as a selector of the orbital quantum number effective for reaction in a state-to-state treatment. Here, an accurate internally consistent formulation of the Optical interaction potentials, obtained by the combined analysis of scattering and spectroscopic experimental findings, casts light on structure, energy and angular momentum couplings of the precursor (pre-reactive) state controlling the stereo-dynamics of prototypical chemi-ionization reactions. The closest approach (turning point) of reagents, is found to control the relative weight of two different reaction mechanisms: (i) A direct mechanism stimulated by exchange chemical forces mainly acting at short separation distances and high collision energy; (ii) An indirect mechanism, caused by the combination of weak chemical and physical forces dominant at larger distances, mainly probed at low collision energy, that can be triggered by a virtual photon exchange between reagents.

Selectivity of weak intermolecular forces and precursor state of elementary oxidation reactions, a new insight on Ne* + N2 chemiionization

This paper reports on the collision dynamics of N2 with metastable Ne* promoting chemiionizations, prototype of barrier-less oxidation reactions of great interest for fundamental and applied research. Extending guidelines presented in previous papers for the atom-atom case, an innovative treatment of the reaction stereodynamics involving molecules in a quantum state-to-state resolution conditions is proposed that emphasizes the role of structure and stability of the precursor that is here the reaction transition state. A critical test of such treatment, carried out exploiting a new formulation both of real and imaginary parts of the optical potential driving the reaction dynamics, is represented by the detailed-combined description of all relevant findings, provided by high resolution molecular beam scattering experiments carried out in our and other laboratories. The present analysis casts light on basic electronic rearrangements of such prototype oxidation reaction which are expected to be of fundamental interest for many other reactions involving open shell atoms and free radicals.

Chemi-Ionization Reactions and Basic Stereodynamical Effects in Collisions of Atom-Molecule Reagents

A new theoretical method, developed by our laboratory to describe the microscopic dynamics of gas-phase elementary chemi-ionization reactions, has been applied recently to study prototype atom-atom processes involving reactions between electronically excited metastable Ne*(3P2,0) and heavier noble gas atoms. Important aspects of electronic rearrangement selectivity have been emphasized that suggested the existence of two fundamental microscopic reaction mechanisms. The distinct mechanisms, which are controlled by intermolecular forces of chemical and noncovalent nature respectively, emerge under different conditions, and their balance depends on the collision energy regime investigated. The present paper provides the first step for the extension of the method to cases involving molecules of increasing complexity, whose chemi-ionization reactions are of relevance in several fields of basic and applied researches. The focus is here on the reactions of Ne* with simple inorganic molecules as Cl2 and NH3, and the application of the method discloses relevant features of the reaction microscopic evolution. In particular, this study shows that the balance of two fundamental reaction mechanisms depends not only on the collision energy and on the relative orientation of reagents but also on the orbital angular momentum of each collision complex. The additional insights so emphasized are of general relevance to assess in detail the stereodynamics of many other elementary processes.

Electronic Rearrangements and Angular Momentum Couplings in Quantum State-to-State Channels of Prototype Oxidation Processes

An innovative theoretical method to describe the microscopic dynamics of chemi-ionization reactions as prototype oxidation processes driven by selective electronic rearrangements has been recently published. It was developed and applied to reactions of Ne* atoms excited in their metastable 3PJ state, and here, its physical background is extensively described in order to provide a clear description of the microscopic phenomenon underlying the chemical reactivity of the oxidative processes under study. It overcomes theoretical models previously proposed and reproduces experimental results obtained in different laboratories. Two basic reaction mechanisms have been identified: (i) at low collision energies, a weakly bounded transition state is formed which spontaneously ionizes through a radiative physical mechanism (photoionization); (ii) in the hyperthermal regime, an elementary oxidation process occurs. In this paper, the selectivity of the electronic rearrangements triggering the two mechanisms has been related to the angular momentum couplings by Hund's cases, casting further light on fundamental aspects of the reaction stereodynamics of general interest. The obtained results allow peculiar characteristics and differences of the terrestrial oxidizing chemistry compared to that of astrochemical environments to be highlighted.

Quantum-State Controlled Reaction Channels in Chemi-ionization Processes: Radiative (Optical-Physical) and Exchange (Oxidative-Chemical) Mechanisms

ConspectusMost chemical processes are triggered by electron or charge transfer phenomena (CT). An important class of processes involving CT are chemi-ionization reactions. Such processes are very common in nature, involving neutral species in ground or excited electronic states with sufficient energy (X*) to yield ionic products, and are considered as the primary initial step in flames. They are characterized by pronounced electronic rearrangements that take place within the collisional complex (X···M)* formed by approaching reagents, as shown by the following scheme, where M is an atomic or molecular target: X* + M → (X···M)* → [(X+···M) ↔ (X···M+)]e- → via ⁡ e - ⁡ CT (X···M)+ + e- → final ions.Despite their important role in fundamental and applied research, combustion, plasmas, and astrochemistry, a unifying description of these basic processes is still lacking. This Account describes a new general theoretical methodology that demonstrates, for the first time, that chemi-ionization reactions are prototypes of gas phase oxidation processes occurring via two different microscopic mechanisms whose relative importance varies with collision energy, Ec, and separation distance, R. These mechanisms are illustrated for simple collisions involving Ne*(3P2,0) and noble gases (Ng). In thermal and hyperthermal collisions probing interactions at intermediate and short R, the transition state [(Ne···Ng)+]e- is a molecular species described as a molecular ion core with an orbiting Rydberg electron in which the neon reagent behaves as a halogen atom (i.e., F) with high electron affinity promoting chemical oxidation. Conversely, subthermal collisions favor a different reaction mechanism: Ng chemi-ionization proceeds through another transition state [Ne*······Ng], a weakly bound diatomic-lengthened complex where Ne* reagent, behaving as a Na atom, loses its metastability and stimulates an electron ejection from M by a concerted emission-absorption of a "virtual" photon. This is a physical radiative mechanism promoting an effective photoionization. In the thermal regime of Ec, there is a competition between these two mechanisms. The proposed method overcomes previous approaches for the following reasons: (1) it is consistent with all assumptions invoked in previous theoretical descriptions dating back to 1970; (2) it provides a simple and general description able to reproduce the main experimental results from our and other laboratories during last 40 years; (3) it demonstrates that the two "exchange" and "radiative" mechanisms are simultaneously present with relative weights that change with Ec (this viewpoint highlights the fact that the "canonical" chemical oxidation process, dominant at high Ec, changes its nature in the subthermal regime to a direct photoionization process; therefore, it clarifies differences between the cold chemistry of terrestrial and interstellar environments and the energetic one of combustion and flames); (4) the proposed method explicitly accounts for the influence of the degree of valence orbital alignment on the selective role of each reaction channel as a function of Ec and also permits a description of the collision complex, a rotating adduct, in terms of different Hund's cases of angular momentum couplings that are specific for each reaction channel; (5) finally, the method can be extended to reaction mechanisms of redox, acid-base, and other important condensed phase reactions.

General treatment for stereo-dynamics of state-to-state chemi-ionization reactions

The investigation of chemi-ionization processes provides unique information on how the reaction dynamics depend on the energy and structure of the transition state which relate to the symmetry, relative orientation of reagent/product valence electron orbitals, and selectivity of electronic rearrangements. Here we propose a theoretical approach to formulate the optical potential for Ne*(³P_2,0) noble gas atom chemi-ionizations as prototype oxidation processes. We include the selective role of atomic alignment and of the electron transfer mechanism. The state-to-state reaction probability is evaluated and a unifying description of the main experimental findings is obtained. Further, we reproduce the results of recent and advanced molecular beam experiments with a state selected Ne* beam.The selective role of electronic rearrangements within the transition state, quantified through the use of suitable operative relations, could cast light on many other chemical processes more difficult to characterize.

Energy and orientation independence of the channel branching in Ne* + ND3 chemi-ionisation

Collisions of excited neon atoms with ammonia molecules can lead to two reaction processes, dissociative ionisation and Penning ionisation. Both processes result in the ionisation of the ammonia molecule and redistribution of the electronic energy into the internal ammonia ion rovibrational modes. We performed energy dependent, crossed-beam stereodynamics studies of the branching ratio between the two ionisation processes. It was found that the branching ratio is totally and completely insensitive to both the neon orientation and the collision energy across the range we sampled, 370-520 cm^-1. The total lack of stereodynamics can be explained by the structure of the ammonia and that its orientation, which we do not attempt to control, is the critical factor in the reaction outcome.

A New Insight on Stereo-Dynamics of Penning Ionization Reactions

Recent developments in the experimental study of Penning ionization reactions are presented here to cast light on basic aspects of the stereo-dynamics of the microscopic mechanisms involved. They concern the dependence of the reaction probability on the relative orientation of the atomic and molecular orbitals of reagents and products. The focus is on collisions between metastable Ne^*(³P_{2, 0}) atoms with other noble gas atoms or molecules, for which play a crucial role both the inner open-shell structure of Ne^* and the HOMO orbitals of the partner. Their mutual orientation with respect to the intermolecular axis controls the characteristics of the intermolecular potential, which drives the collision dynamics and the reaction probability. The investigation of ionization processes of water, the prototype of hydrogenated molecules, suggested that the ground state of water ion is produced when Ne^* approaches H₂O perpendicularly to its plane. Conversely, collisions addressed toward the lone pair, aligned along the water C_2v symmetry axis, generates electronically excited water ions. However, obtained results refer to a statistical/random orientation of the open shell ionic core of Ne^*. Recently, the attention focused on the ionization of Kr or Xe by Ne^*, for which we have been able to characterize the dependence on the collision energy of the branching ratio between probabilities of spin orbit resolved elementary processes. The combined analysis of measured PIES spectra suggested the occurrence of contributions from four different reaction channels, assigned to two distinct spin-orbit states of the Ne^*(³P_{2, 0}) reagent and two different spin-orbit states of the ionic M⁺(²P_{3/2, 1/2}) products (M = Kr, Xe). The obtained results emphasized the reactivity change of ³P₀ atoms with respect to ³P₂, in producing ions in ²P_3/2 and ²P_1/2 sublevels, as a function of the collision energy. These findings have been assumed to arise from a critical balance of adiabatic and non-adiabatic effects that control formation and electronic rearrangement of the collision complex, respectively. From these results we are able to characterize for the first time, according to our knowledge, the state to state reaction probability for the ionization of Kr and Xe by Ne^* in both ³P₂ and ³P₀ sublevels.

The electron couplings in the transition states: The stereodynamics of state to state autoionization processes

Measurements of the kinetic energy distribution of electrons, emitted in collision between Ne^*(³P_2,0) and Kr(¹S₀) and Xe(¹S₀), have been performed in a crossed molecular beam apparatus which employs a mass spectrometer and a hemispherical electron analyzer as detectors. The analysis of the obtained experimental results provides new insights on electronic rearrangements and electronic angular momentum coupling effects that determine relevant properties of the transition state of autoionization processes, and that we have found useful to classify as adiabatic and non-adiabatic effects. In particular, while the adiabatic effects control sequence, energy, and symmetry of quantum states accessible to both reagents and products in the probed collision energy range, the non-adiabatic ones trigger the passage from entrance to exit channels. The obtained results are important not only to compact previous theoretical schemes of autoionization reactions in a unified representation but also to cast light on the role of electronic rearrangements within the transition state of many other types of chemical processes that are more difficult to characterize.

Adiabatic and Nonadiabatic Effects in the Transition States of State to State Autoionization Processes

The energy distribution of electrons, emitted from collisions between Ne^{*}(^{3}P_{2,0}) and Kr(^{1}S_{0}), have been measured under high resolution conditions in a crossed molecular beam apparatus containing a hemispherical electron analyzer as detector. The experimental results provide new insights on the electronic adiabatic and nonadiabatic effects in the stereodynamics of state to state atomic and molecular collisions, controlling relevant properties of the transition state of autoionization processes. In particular, while the adiabatic effects determine sequence, energy, and symmetry of quantum states accessible both to reagents and products, the nonadiabatic effects trigger the passage from entrance to exit channels.

Effect of Magnesium Bond on the Competition Between Hydrogen and Halogen Bonds and the Induction of Proton and Halogen Transfer

HOX (X=Cl, Br, I, and At) can engage in either a H-bond (HB) or halogen bond (XB) with a base-like HCN, NH₃ , and imidazole. Although the former is energetically preferred for X=Cl and Br, it is the XB that is more stable for At, with I showing little preference. MgY₂ forms a Mg-bond with the O atom of HOX, which grows stronger in the order X=Cl<Br<I<At and Y=F<Cl<Br. When all three molecules are combined, both the Mg and the H/X bonds are cooperatively strengthened to a large degree. Rather than causing a reversal in the HB/XB competition, the Mg-bond acts primarily to amplify the natural preference within the dimer. The Mg-bond induces a certain degree of transfer from O to N of the bridging atom in the H/X bond. Comparison is also made with the effects of a Be-bond.

May halogen bonding interactions compete with Cu⋯Cl semi-coordinate bonds? Structural, magnetic and theoretical studies of two polymorphs of trans-bis(5-bromo-2-chloro pyridine)dichlorocopper(ii) and trans-bis(2,5-dichloropyridine)dichlorocopper(ii)

Interaction of the negative potential area from one molecule with the positive areas I and II from two different molecules produces polymorphs 1 and 2.

Chemical Characterization of Lodoicea maldivica Fruit

In the present study, we report the attempt to characterize the chemical composition of fruit kernel of Lodoicea maldivica coco nucifera palm (commonly named as 'Coco de mer') by gas chromatographic method. The analysis was performed by HS-SPME and GC/MS techniques to determine volatile aroma, sterol, and fatty acid composition profiles in the internal and external pulp of two distinct coconuts. Although no qualitative differences in flavour composition were observed between the two analysed coconuts and the relative two pulp parts, variations in the abundance levels of the prominent compounds have been recorded. The averaged quantity of total phytosterols, resulting from the two analysed 'Coco de mer' samples, was almost constant in both kernels coconut, being 24.5 μg/g (of dry net matter) for the external, and 26.9 μg/g (of dry net matter) for the internal portion. In both coconuts, the fatty acid pattern composition was characterized by seven saturated acids ranged from C14:0 (myristic) to C20:0 (arachidic) and two monounsaturated acids, the palmitoleic (C16:1, ω7) and the oleic (C18:1, ω9). Palmitic acid (C16:0) was the predominant one with an average contribution of about 49.0%, followed by pentadecanoic 16.5%, stearic (C18:0) 11.6%, and myristic (C14:0) 9.9% acids in all two examined kernel portions.

The role of charge transfer in the stability and reactivity of chemical systems from experimental findings

Phenomena are described within a unifying picture, by isolating charge/electron transfer as an interaction component triggering chemical reactivity.

Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Measurements of Ionization Cross Sections by Molecular Beam Experiments: Information Content on the Imaginary Part of the Optical Potential

In this work, we present and analyze in detail new and recent ionization cross section and mass spectrum determinations, collected in the case of He*, Ne*-H2O, -H2S, and -NH3 ionizing collisions. These sets of data, obtained under the same experimental conditions, are relevant to identify differences in the autoionization stereodynamics of the three hydrogenated molecules and on the selective role of the imaginary part of the optical potential. We demonstrate that in these autoionization processes hydrogen and halogen bonds are competing because they are controlling both real and imaginary components of the optical potential that drives the complete reaction dynamics. In particular, we found that both components critically depend on the angular and radial approach between the reagent partners in determining the collision dynamics.