Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Energetics and Bonding in Aluminosilicate Rings with Alkali Metal and Alkaline-Earth Metal Charge-Compensating Cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered <i>faujausite</i>-like rings has been calculated in terms of Kohn–Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na⁺ and K⁺ were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by −3.6 ± 0.4, −3.9 ± 0.5, −7.3 ± 0.1, and −7.6 ± 0.2 eV by K⁺, Na⁺, Ca²⁺, and Mg²⁺ adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain −3.7 ± 0.1, −4.1 ± 0.1, −8.1 ± 0.1, and −9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si–O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling’s double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures

Understanding the Reorientational Dynamics of Solid-State MBH₄ (M = Li–Cs)

The reorientational dynamics of crystalline MBH₄ (M = Li–Cs) have been characterized with the interacting quantum atom theory. This interpretive approach enables an atomistic deciphering of the energetic features involved in BH₄^– reorientation using easily graspable chemical terms. It reveals a complex construction of the activation energy that extends beyond interatomic distances and chemical interactions. BH₄^– reorientations are in LiBH₄ and NaBH₄ regulated by their interaction with the nearest metal cation; however, higher metal electronic polarizability and more covalent M···H interactions shift the source of destabilization to internal deformations in the heavier systems. Underlying electrostatic contributions cease abruptly at CsBH₄, triggering a departure in the otherwise monotonically increasing activation energy. Such knowledge concurs to the fundamental understanding and advancement of energy solutions in the field of hydrogen storage and solid-state batteries

Revealing Electron Delocalization through the Source Function

The source function (SF) introduced in late 90s by Bader and Gatti quantifies the <i>influence</i> of each atom in a system in determining the amount of electron density at a given point, regardless of the atom’s remote or close location with respect to the point. The SF may thus be attractive for studying directly in the real space somewhat elusive molecular properties, such as “electron conjugation” and “aromaticity”, that lack rigorous definitions as they are not directly associated to quantum-mechanical observables. In this work, the results of a preliminary test aimed at understanding whether the SF descriptor is capable to reveal electron delocalization effects are corroborated by further examination of the previously investigated benzene, 1,3-cyclohexadiene, and cyclohexene series and by extending the analysis to some benchmark organic systems with different unsaturated bond patterns. The SF can actually reveal, order, and quantify π-electron delocalization effects for formal double, single conjugated, and allylic bonds, in terms of the influence of distant atoms on the electron density at given bond critical points. In polycyclic aromatic hydrocarbons, the SF neatly reveals the mutual influence of the benzenoid subunits. In naphthalene it provides a rationale for the changes observed in the local aromatic character of one ring when the other is partially hydrogenated. The SF analysis describes instead biphenyl as made up by two weakly interacting benzene rings, only slightly perturbed by the combination of mutual steric and electronic effects. Eventually, a new SF-based indicator of local aromaticity is introduced, which shows excellent correlation with the aromatic index developed by Matta and Hernández-Trujillo, based on the delocalization indices. At variance with this latter and other commonly employed quantum-mechanical (local) aromaticity descriptors, the SF-based indicator does not require the knowledge of the pair density, nor the system wave function, being therefore promising for applications to experimentally derived charge density distributions

A Theoretical Study on the Rotational Motion and Interactions in the Disordered Phase of MBH₄ (M = Li, Na, K, Rb, Cs)

The rotational motion in the high-temperature disordered phase of MBH₄ (M = Li, Na, K, Rb, Cs) is investigated utilizing two complementary theoretical approaches. The first one consists of high-level periodic DFT calculations which systematically consider several instantaneous representations of the structural disorder. The second approach is based on a series of in vacuo calculations on molecular complexes suitably extracted from the crystal and chosen as to possibly disentangle the energetic factors leading to the observed rotational barriers. The results of the first part demonstrate that the motion of the BH₄^– anion is dominated by 90° reorientations around the 4-fold symmetry axes of the cubic crystal, and depending on the instantaneous structural disorder activation energies are found to be between 0.00 and 0.31 eV for LiBH₄, 0.05 and 0.26 eV for NaBH₄, 0.16 and 0.27 eV for KBH₄, 0.22 and 0.31 eV for RbBH₄, and 0.21 and 0.32 eV for CsBH₄. The increasing rotational barriers as well as the movement of the transition state from 7° to 44° observed along the series of alkaline metals, M = Li–Rb, appear to be simply accounted for by an analysis of the energy profiles for the <i>C</i>₂ rotation of a BH₄^– group in M⁺–BH₄^– and BH₄^––BH₄^– in vacuo complexes. The energy gained from the introduction of disorder shows a trend opposite to that of the rotational barriers as it decreases along the Li–Rb series. Similar considerations apply to the <i>C</i>₃ rotational motion of the BH₄^– anion, which likewise has been studied in the crystal and in the in vacuo molecular complexes. CsBH₄ deviates from the systematic trends observed for LiBH₄–RbBH₄. Depending on the structural starting point of the rotation, its <i>C</i>₂ rotational barriers are found to be slightly higher or slightly lower than for RbBH₄, whereas its energy gain due to the introduction of disorder is found to be positioned between that of KBH₄ and RbBH₄. The <i>C</i>₃ rotational barriers of CsBH₄ are instead significantly smaller compared to those of RbBH₄ and even marginally below those of KBH₄

Intermolecular Recognition of the Antimalarial Drug Chloroquine: A Quantum Theory of Atoms in Molecules–Density Functional Theory Investigation of the Hydrated Dihydrogen Phosphate Salt from the 103 K X‑ray Structure

The relevant noncovalent interaction patterns responsible for intermolecular recognition of the antiplasmodial chloroquine (CQ) in its bioactive diprotonated form, CQH₂²⁺, are investigated. Chloroquine dihydrogen phosphate hydrated salt (<i>P</i>2₁/<i>c</i>) was crystallized by gel diffusion. A high-resolution single-crystal X-ray diffraction experiment was performed at 103(2) K, and a density functional theory model for the in-crystal electron density was derived, allowing the estimation of the interaction energies in relevant molecular pairs. H₂PO₄^– ions form infinite chains parallel to the monoclinic axis, setting up strong NH···O charge-assisted hydrogen bonds (CAHBs) with CQH₂²⁺. Couples of facing protonated quinoline rings are packed in a π···π stacked arrangement, whose contribution to the interaction energy is very low in the crystal and completely overwhelmed by Coulomb repulsion between positive aromatic rings. This questions the ability of CQ in setting up similar stacking interactions with the positively charged Fe-protoporphyrin moiety of the heme substrate in solution. When the heme/CQ adduct incorporates a Fe–N coordinative bond, stronger π···π interactions are instead established due to the lacking of net electrostatic repulsions. Yet, CAHBs among the protonated tertiary amine of CQ and the propionate group of heme still provide the leading stabilizing effect. Implications on possible modifications/improvements of the CQ pharmacophore are discussed