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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. The derived non-covalent interaction patterns are found to support diagnostic 1H NMR signals used for detecting the mutual position of the CB[7] along the axle. This work highlights the potentialities of a QTAIM based approach in the characterization of supramolecular and metal-complexation effects in molecular aggregates such as not-interlocked synthetic molecular shuttles.

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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). This contribution while supporting the photoexcitation properties derived in laboratory following the self-assembly of monomeric units when passing from dimethyl sulfoxide (DMSO) to a H2O/DMSO mixture at 298K, shed some light on the nature of the chemical interactions modulating the formation of nano-size aggregates.

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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. The chemistry of noble gas anions overall appears as a varied and rich paint, offering fascinating opportunities for both experimentalists and theoreticians.

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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. Their stability arises from the interaction of the H- moiety with the positively-charged Ng atoms, particularly with the σ-hole ensuing from their ligation to BeO. This actually promotes relatively tight Ng-H bonds featuring a partially-covalent character, whose degree progressively increases when going from HArBeO- to HRnBeO-. The HNgBeO- compounds are also briefly compared with other noble-gas anions observed in the gas phase or isolated in crystal lattices.

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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. The Ng-Si bonds are generally tight and feature a partial contribution of covalency. In the NgSiX3+, the degree of the Ng-Si interaction mirrors the trends of two factors, namely the polarizability of Ng that increases when going from Ng = He to Ng = Rn, and the Lewis acidity of SiX3+ that decreases in the order SiF3+ > SiH3+ > SiCl3+. For the HeSiX3+, it was also possible to catch peculiar effects referable to the small size of He. When going from the NgSiF3+ to the NgSiF22+, the increased charge on Si promotes an appreciable increase inthe Ng-Si interaction, which becomes truly covalent for the heaviest Ng. The strength of the bond also increases when going from the NgSiF3+ to the "inserted" FNgSiF2+, likely due to the cooperative effect of the adjacent F atom. On the other hand, the ligation of a second Ar atom to ArSiX3+ (X = H, F, Cl), as to form Ar2(SiX3+), produces a weakening of the bond. Our obtained data were compared with previous findings already available in the literature.

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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. 44, 15536 (2020)], these contacts are first distinguished into three types, namely, A, B, or C, based on the topology of the electron energy density H(r) and on the shape of its plotted form. Bonds of type B or C are, then, further assigned as B-loose (Bl) or B-tight (Bt) and C-loose (Cl) or C-tight (Ct) depending on the sign that H(r) takes along the Ng-X bond path located from the topological analysis of ρ(r), particularly at around the bond critical point (BCP). Any bond of type A, Bl/Bt, or Cl/Ct is, finally, assayed in terms of contribution of covalency. This is accomplished by studying the maximum, minimum, and average value of H(r) over the volume enclosed by the low-density reduced density gradient (RDG) isosurface associated with the bond (typically, the RDG isosurface including the BCP) and the average ρ(r) over the same volume. The bond assignment is also corroborated by calculating the values of quantitative indices specifically defined for the various types of interactions (A, B, or C). The generality of our taken approach should encourage its wide application to the study of Ng compounds.

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The structure, stability, and bonding character of some exemplary LAr and L-ArBeO (L = He, Ne, Ar, N2, CO, F2, Cl2, ClF, HF, HCl, NH3) 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 LAr and L-ArBeO unraveled geometric and bonding effects peculiarly related to the σ-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 L at the contact zone with ArBeO. The role of the inductive and dispersive components was also assayed, making it possible to discern the factors governing the transition from the (mainly) dispersive domain of the LAr, to the σ-hole domain of the L-ArBeO. Our conclusions could be valid for various types of non-covalent interactions, especially those involving σ-holes of respectable strength such as those occurring in ArBeO.

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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'. These variations are nicely correlated with the bonding situation of the (Ng-H-Ng)+ and (Ng-H-Ng')+. The Ng-H and Ng'-H contacts range, in fact, between strong covalent bonds to weak, non-covalent interactions, and their regular variability clearly illustrates the peculiar capability of the noble gases to undergo interactions covering the entire spectrum of the chemical bond.

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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 2 + were detected from ionized He/H2 mixtures. Over the years, the family has considerably expanded and currently includes various bonding motifs that are investigated with intense experimental and theoretical interest. Quite recently, the results of these studies acquired new and fascinating implications. The diatomic ArH+ and HeH+ were, in fact, detected in various galactic and extragalactic regions, and this stimulates intriguing questions concerning the actual role in the outer space of the Ng-H cations observed in the laboratory. The aim of this review is to briefly summarize the most relevant information currently available on the structure, stability, and routes of formation of these fascinating systems.

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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., J. Comput. Chem., 2019, 40, 2318-2328), the DoP is a quantitative index that is generally informative about the role of polarization (induction plus charge transfer [CT]) and dispersion in noncovalent noble gas complexes. As further evidence in this regard, we presently ascertained quantitative correlations between the DoP(He) of the He(M) and indices based on the electron density ρ(r), including the molecular electrostatic potential at the HeM bond critical point, as well as the percentage contributions of induction and dispersion to the SAPT binding energies. Based also on the explicit evaluation of the CT, accomplished through the study of the charge-displacement function, we derived a quantitative scale that ranks the He(M) according to their dispersive, inductive, and CT bonding character. Our taken approach could be conceivably extended to other types of noncovalent complexes.

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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. The definition of the DoP(Ng) relies on the analysis of the local electron energy density H(r), and its physical meaning was best appreciated by studying also the charge-displacement function and the molecular electrostatic potential of the investigated benchmark species, that include nearly 60 Ngs complexes of different bonding character. The DoP(Ng) appears of general applicability, and is also positively correlated with other bonding character indices. © 2019 Wiley Periodicals, Inc.

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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. The presence of a large dipole moment induces a large electric field at He, which lowers its 2p orbital energy and enables receipt of π electron density. These findings offer unprecedented important clues toward the design and synthesis of stable helium compounds.

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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. The R-F product arises from the attack of R to the F atom of NF3 , but additional contributing channels are conceivably involved. The F-atom abstraction is, indeed, considerably more exothermic than the SH 2 reaction, but the involved energy barriers are comparable, and the two processes are comparably fast.

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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. For Ne, Ar, Kr, and Xe, this term has a larger contribution from the p orbital lying on the bond axis, and two smaller contributions from the p orbitals perpendicular to the bond axis. For the heaviest Ar(H(3)O(+)), Kr(H(3)O(+)), and Xe(H(3)O(+)), BE also has a "covalent" component, which is ascribed to the relatively-appreciable charge transfer from Ng to H(3)O(+).

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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. We thus selected the prototypical Ng-CCl4 and Ng-CF4 and performed high-resolution molecular beam scattering experiments to measure the absolute scale of their intermolecular potentials, with high sensitivity. In general, we expected to probe typical van der Waals interactions, consisting of a combination of size (exchange) repulsion with dispersion/induction attraction. For the He/Ne-CF4, the analysis of the glory quantum interference pattern, observable in the velocity dependence of the integral cross section, confirmed indeed this expectation. On the other hand, for the He/Ne/Ar-CCl4, the scattering data unravelled much deeper potential wells, particularly for certain configurations of the interacting partners. The experimental data can be properly reproduced only including a shifting of the repulsive wall at shorter distances, accompanied by an increased role of the dispersion attraction, and an additional short-range stabilization component. To put these findings on a firmer ground, we performed, for selected geometries of the interacting complexes, accurate theoretical calculations aimed to evaluate the intermolecular interaction and the effects of the complex formation on the electron charge density of the constituting moieties. It was thus ascertained that the adjustments of the potential suggested by the analysis of the experiments actually reflect two chemically meaningful contributions, namely, a stabilizing interaction arising from the anisotropy of the charge distribution around the Cl atom in CCl4 and a stereospecific electron transfer that occurs at the intermolecular distances mainly probed by the experiments. Our model calculations suggest that the largest effect is for the vertex geometry of CCl4 while other geometries appear to play a minor to negligible role.

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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. A different sensitivity towards the conformational flexibility of the outermost photoelectron bands was exhibited in the simulated spectrum. The comparison of the theoretical UPS with the experimental one allowed a detailed assignment of the outermost valence spectral region. The composition and bonding properties of the relevant MOs of the most stable conformers were analyzed in terms of leading Natural Bond Orbital (NBO) contributions to the HF/6-311++G** canonical MOs. The C1s, N1s, and O1s photoelectron spectra (XPS) were theoretically simulated by calculating the vertical Ionization Energies (IEs) of the relevant conformers using the ΔSCF approach. The different IE chemical shift spread of the XPS components associated with various conformers, which is expected to affect the experimental spectra, could be evaluated by simulated XPS, thus providing a new insight into the core electronic structure. The comparison of the theoretical results with the experimental ones unraveled that the atomic XPS components are not mixed by conformational flexibility of D-cycloserine, and that the specific vibronic structure of different spectral components should play a crucial role in determining different relative intensities and band shapes observed in the experiment.

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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. The qualitative assignment of the various bonding motifs is corroborated by the topological analysis of Hmol(r), which typically includes several critical points of rank 3 and variable signature. These points are, in particular, characterized here in terms of their bond degree (BD). From a previous definition (Espinosa J. Chem. Phys. 2002, 117, 5529-5542), this quantity is taken as the ratio between the energy density calculated at the critical point of H(r), H(rc), and the corresponding electron density ρ(rc): BD = -H(rc)/ρ(rc). Thus, the BD is positive for covalent interactions (H(rc) < 0) and negative for noncovalent interactions (H(rc) > 0). For structurally related species, the BD result, in general, positively correlated with the binding energies and is, therefore, a semiquantitative index of stability. The present study suggests the general validity of the Hmol(r) to effectively assay the bonding motifs of noble-gas compounds.

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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. The CCl4 -Ng are also peculiar examples of halogen bonds, a group of interactions of major current concern. Finally, this investigation is a prelude to the development of semi-empirical models for force fields aimed to the unified description of static and dynamical properties of systems of comparable or higher complexity.

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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 4.5 kcal mol(-1). The absolute complexation energies of both the linear and the T-shaped isomers also increase in the same order, and their EDA analysis revealed the prevailing contribution of electrostatic interactions. The noncovalent character of the bonding between XeHXe(+) and L was confirmed by the AIM analysis. In particular, we based our investigation on the joint use of numerical AIM indices and graphic examination of the local Hamiltonian kinetic energy density, K(r). Interestingly, this function visually identifies the "covalent" regions occupied by XeHXe(+) and L and the "noncovalent" zones existing between them, which include, in particular, the bond critical point located on the Xe-L bond paths. Only for the linear (XeHXe(+))NH3 did the AIM analysis suggest an onset of covalency in the xenon-nitrogen interaction. Further work is in progress to examine the effectiveness of K(r), and its plotted forms, as a function of the bonding situation of noble-gas compounds.