On the supramolecular interactions into a pH- and Metal-Actuated Molecular Shuttle: some insights from QTAIM modeling.

Supramolecular contacts responsible for chemical interaction of cucurbit[7]uril (CB[7]) macrocycle on a Tolyl-Viologen-Phenylene-Imidazole (T-VPI) molecular thread, at acid pH (T-VPI-H) or after Ag cation addition (T-VPI-Ag), are analytically addressed in a computational framework combining Quantum Theory of Atoms in Molecules (QTAIM) with Density Functional Theory (DFT). In this respect, the crystallographic structure (CCDC number 2217466) is taken as reference condition for addressing the nature of the chemical interactions driving the shuttling of the CB[7] between T and P stations recently observed in dilute water solutions. Beside the host(CB[7]) vs guest(T-VPI-H or T-VPI-Ag) complexation, the coordination sphere of the Ag cation is also investigated by means of local electronic energy density - H(r) - descriptors.

Optical and Supramolecular Properties of Cyclometalated Palladium Nanostructures with Tumor Targeting Properties in Living Mice: A Quantum Mechanics Interpretation.

The recent discovery that metallophilic interactions between cyclometalated palladium supramolecular nanostructures - with efficient tumour accumulation rate in a skin melanoma model - maintain excellent photodynamic properties even in a hypoxic microenvironment has inspired the present study focused on the theoretical predictions of optical properties of the bis-cyclometalated palladium compound in different contexts. More specifically, structural and UV/Vis absorption properties of both monomeric and dimeric forms of this anticancer drug are well reproduced with a Time-Dependent Density Functional Theoretical (TD-DFT) approach based on Exchange-Correlation (XC) hybrid functionals in conjunction with conductor-like and polarization solvation effects. A further novelty is represented by a fine investigation of the supramolecular interactions between the different subunits of the drug via dispersion force correction and Quantum Theory of Atoms in Molecules (QTAIM).

Noble Gas Anions: An Overview of Strategies and Bonding Motifs.

This review article aims to provide an overview of the strategies employed to prepare noble gas anions under different environments and experimental conditions, and of the bonding motifs typically occurring in these species. Observed systems include anions fixed into synthesized salts, detected in the gas phase or in high-pressure devices. The major role of the theoretical calculations is also highlighted, not only in support of the experiments, but also as effective in predicting still unreported species.

Noble gas hydrides: theoretical prediction of the first group of anionic species.

The first group of anionic noble-gas hydrides with the general formula HNgBeO (Ng = Ar, Kr, Xe, Rn) is predicted through MP2, Coupled-Cluster, and Density Functional Theory computations employing correlation-consistent atomic basis sets. We derive that these species are stable with respect to the loss of H, H, BeO, and BeO, but unstable with respect to Ng + HBeO. The energy barriers of the latter process are, however, high enough to suggest the conceivable existence of the heaviest HNgBeO species as metastable in nature.

Noble Gas-Silicon Cations: Theoretical Insights into the Nature of the Bond.

The structure, stability, and bonding situation of some exemplary noble gas-silicon cations were investigated at the MP2/aVTZ level of theory. The explored species include the mono-coordinated NgSiX3+ (Ng = He-Rn; X = H, F, Cl) and NgSiF22+ (Ng = He-Rn), the di-coordinated Ar2SiX3+ (X = H, F, Cl), and the “inserted” FNgSiF2+ (Ng = Kr, Xe, Rn). The bonding analysis was accomplished by the method that we recently proposed to assay the bonding situation of noblegas compounds.

Noble-gas compounds: A general procedure of bonding analysis.

This paper accounts for a general procedure of bonding analysis that is, expectedly, adequate to describe any type of interaction involving the noble-gas (Ng) atoms. Building on our recently proposed classification of the Ng-X bonds (X = binding partner) [New J. Chem.

Concerning the Role of -Hole in Non-Covalent Interactions: Insights from the Study of the Complexes of ArBeO with Simple Ligands.

The structure, stability, and bonding character of some exemplary Ar and -ArBeO ( = He, Ne, Ar, N, CO, F, Cl, ClF, HF, HCl, NH) were investigated by MP2 and coupled-cluster calculations, and by symmetry-adapted perturbation theory. The nature of the stabilizing interactions was also assayed by the method recently proposed by the authors to classify the chemical bonds in noble-gas compounds. The comparative analysis of the Ar and -ArBeO unraveled geometric and bonding effects peculiarly related to the -hole at the Ar atom of ArBeO, including the major stabilizing/destabilizing role of the electrostatic interactionensuing from the negative/positive molecular electrostatic potential of at the contact zone with ArBeO.

On the Proton-Bound Noble Gas Dimers (Ng-H-Ng) and (Ng-H-Ng') (Ng, Ng'= He-Xe): Relationships betweenStructure, Stability, and Bonding Character.

The structure, stability, and bonding character of fifteen (Ng-H-Ng) and (Ng-H-Ng') (Ng, Ng' = He-Xe) compounds were explored by theoretical calculations performed at the coupled cluster level of theory. The nature of the stabilizing interactions was, in particular, assayed using a method recently proposed by the authors to classify the chemical bonds involving the noble-gas atoms. The bond distances and dissociation energies of the investigated ions fall in rather large intervals, and follow regular periodic trends, clearly referable to the difference between the proton affinity (PA) of the various Ng and Ng'.

Cationic Noble-Gas Hydrides: From Ion Sources to Outer Space.

Cationic species with noble gas (Ng)-hydrogen bonds play a major role in the gas-phase ion chemistry of the group 18 elements. These species first emerged more than 90 years ago, when the simplest HeH and HeH were detected from ionized He/H mixtures. Over the years, the family has considerably expanded and currently includes various bonding motifs that are investigated with intense experimental and theoretical interest.

Complexes of helium with neutral molecules: Progress toward a quantitative scale of bonding character.

The complexes of helium with nearly 30 neutral molecules (M) were investigated by various techniques of bonding analysis and symmetry-adapted perturbation theory (SAPT). The main investigated function was the local electron energy density H(r), analyzed, in particular, so to estimate the degree of polarization (DoP) of He in the various He(M). As we showed recently (Borocci et al.

Noncovalent Complexes of the Noble-Gas Atoms: Analyzing the Transition from Physical to Chemical Interactions.

The bonding character of the noncovalent complexes of the noble-gas (Ng) atoms ranges from nearly purely dispersive contacts to interactions featuring appreciable contributions of induction and charge transfer. In this study, we discuss a new quantitative index that seems peculiarly informative about these diverse bonding situations. This index was termed as the degree of polarization (DoP) of Ng, as it measures, in essence, the Ng polarization promoted by the binding partner.

Helium Accepts Back-Donation In Highly Polar Complexes: New Insights into the Weak Chemical Bond.

We studied the puzzling stability and short distances predicted by theory for helium adducts with some highly polar molecules, such as BeO or AuF. On the basis of high-level quantum-chemical calculations, we carried out a detailed analysis of the charge displacement occurring upon adduct formation. For the first time we have unambiguously ascertained that helium is able not only to donate electron density, but also, unexpectedly, to accept electron density in the formation of weakly bound adducts with highly polar substrates.

Bimolecular Homolytic Substitutions at Nitrogen: An Experimental and Theoretical Study on the Gas-Phase Reactions of Alkyl Radicals with NF3.

The X-ray irradiation of binary mixtures of alkyl iodides R-I (R=CH3 , C2 H5 , or i-C3 H7 radicals) and NF3 produces R-NF2 and R-F. Based on calculations performed at the CCSD(T), MRCI(SD+Q), G3B3, and G3 levels of theory, the former product arises from a bimolecular homolytic substitution reaction (SH 2) by the alkyl radicals R, which attack the N atom of NF3 . This mechanism is consistent with the suppression of R-NF2 by addition of O2 (an efficient alkyl radical scavenger) to the reaction mixture.

Complexes of the noble gases with H3O+: a theoretical investigation on Ng(H3O+) (Ng = He-Xe).

The geometries, harmonic vibrational frequencies, and binding energies (Bes) of the Ng(H(3)O(+)) complexes (Ng = He-Xe) were investigated at the coupled cluster level of theory, and their bonding situation was assayed by various methods of bonding analysis. The effects of Ng on H(3)O(+) progressively increase from He to Xe, and only He can be regarded as an essentially "innocent" ligand. The binding energies also increase in the same periodic order, and are by far dominated by the "noncovalent" ion-induced dipole interaction arising from the H(3)O(+)-induced polarization of Ng.

Bonding Motifs of Noble-Gas Compounds As Described by the Local Electron Energy Density.

The bonding situation of some exemplary noble-gas (Ng) compounds, including HNg(+), HNgF, FNgO(-), Ng-HF, and NgBeO (Ng = He-Xe) was assayed by examining their local electron energy density H(r). In general, this function partitions the space of atomic species (neutral and ionic) into inner regions of negative values and outer regions of positive values. In the formation of chemical bonds, these atomic regions combine so to form a molecular H(r), Hmol(r), whose plotted form naturally shows the "covalent" and "noncovalent" regions of the molecular species and allows also the recognition of different types of noncovalent interactions such van der Waals, hydrogen, and ionic or partially ionic bonds.

Catching the role of anisotropic electronic distribution and charge transfer in halogen bonded complexes of noble gases.

The systems studied in this work are gas-phase weakly bound adducts of the noble-gas (Ng) atoms with CCl4 and CF4. Their investigation was motivated by the widespread current interest for the intermolecular halogen bonding (XB), a structural motif recognized to play a role in fields ranging from elementary processes to biochemistry. The simulation of the static and dynamic behaviors of complex systems featuring XB requires the formulation of reliable and accurate model potentials, whose development relies on the detailed characterization of strength and nature of the interactions occurring in simple exemplary halogenated systems.

Electronic structure and conformational flexibility of D-cycloserine.

The first comprehensive investigation of the effect of conformational flexibility of gaseous D-cycloserine on the valence and core electronic structures is reported here. The seven most stable conformers among the twelve structures calculated at the MP2/6-311++G** level of theory were assumed to properly describe the properties of the investigated compound. Taking into account the contribution of these isomers, the valence photoelectron spectrum (UPS) was simulated by the Outer Valence Green' s Function (OVGF) method.

Experimental evidence of chemical components in the bonding of helium and neon with neutral molecules.

The complexes of helium and neon with gaseous neutral molecules are generally perceived to be van der Waals adducts held together by physical (non-covalent) forces, owing to the combination of size (exchange) repulsion with dispersion/induction attraction. Molecular beam experiments confirm that this is the case for He-CF4 , Ne-CF4 adducts, but revealed that the interaction of He and Ne with CCl4 features an appreciable contribution of chemical components that arise from the anisotropy of the electron density of CCl4 that enhances a charge transfer from Ng (Ng=He, Ne). These findings furnish a novel assay of the bonding capabilities of helium and neon, and invite to revisit the neutral complexes of these elements as systems of chemical relevance.

Complexes of XeHXe⁺ with simple ligands: a theoretical investigation on (XeHXe⁺)L (L = N₂, CO, H₂O, NH₃).

The structure, stability, and harmonic frequencies of the (XeHXe(+))L complexes (L = N2, CO, H2O, NH3) were investigated by ab initio and density functional theory (DFT) calculations. Their bonding situation was also assayed by natural bond orbital (NBO), atoms-in-molecules (AIM), and energy decomposition (EDA) analyses. For any L, we located a linear and a T-shaped isomer, whose energy difference progressively increases in the order N2 < CO < H2O < NH3 and ranges from nearly 0 to 4.

Neutral compounds with xenon-germanium bonds: a theoretical investigation on FXeGeF and FXeGeF₃.

The structure and stability of FXeGeF and FXeGeF3 were investigated by MP2, CCSD(T), and B3LYP calculations, and their bonding situation was examined by NBO and AIM analysis. These molecules are thermochemically stable with respect to dissociation into F + Xe + GeF(n) (n = 1, 3), and kinetically stable with respect to dissociation into Xe + GeF(n+1), thus suggesting their conceivable existence as metastable species. FXeGeF and FXeGeF3 are best described by the resonance structures F(-)(Xe-GeF(+)) and F(-)(Xe-GeF3(+)), and feature essentially ionic xenon-fluorine interactions.

Germyl cations with Ge-S bond: an experimental and theoretical study on the gaseous F(n)Ge(SH)(3-n)+ (n=0-2).

The germyl cations F(2)Ge(SH)(+), FGe(SH)(2)(+) and Ge(SH)(3)(+) were obtained from ionized mixtures of GeF(4) and H(2)S. Ion trap mass spectrometry revealed the occurrence of three consecutive addition-HF elimination reactions between GeF(3)(+), F(2)Ge(SH)(+) and FGe(SH)(2)(+) and H(2)S. The structure and the mechanism of formation of the observed F(n)Ge(SH)(3-n)(+) (n = 0-2) were investigated by ab initio calculations performed at the MP2 and coupled cluster level of theory.

Gas-phase reactions of SiH(n)+ (n = 1,2) with NF3: a computational investigation on the detailed mechanistic aspects.

The mechanism of the gas-phase reactions of SiH(n)(+) (n = 1,2) with NF(3) were investigated by ab initio calculations at the MP2 and CAS-MCSCF level of theory. In the reaction of SiH(+), the kinetically relevant intermediates are the two isomeric forms of fluorine-coordinated intermediate HSi-F-NF(2)(+). These species arise from the exoergic attack of SiH(+) to one of the F atoms of NF(3) and undergo two competitive processes, namely an isomerization and subsequent dissociation into SiF(+) + HNF(2) , and a singlet-triplet crossing so to form the spin-forbidden products HSiF(+) + NF(2).

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives.

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.