Pnicogen bonded complexes of PO2X (X = F, Cl) with nitrogen bases

σ-Hole, lone-pair-hole, and π-hole site-based interactions in aerogen-comprising complexes: a comparative study

Herein, the potential of ZO3 and ZF2 aerogen-comprising molecules (where Z = Ar, Kr, and Xe) to engage in σ-, lp-, and π-hole site-based interactions was comparatively studied using various ab initio computations. For the first time, a premier in-depth elucidation of the external electric field (EEF) influence on the strength of the σ-, lp-, and π-hole site-based interactions within the ZO3/ZF2⋯NH3 and ⋯NCH complexes was addressed using oriented EEF with disparate magnitude. Upon the energetic features, σ-hole site-based interactions were noticed with the most prominent preferability in comparison to lp- and π-hole analogs. This finding was ensured by the negative interaction energy values of -11.65, -3.50, and -2.74 kcal mol-1 in the case of σ-, lp-, and π-hole site-based interactions within the XeO3⋯ and XeF2⋯NH3 complexes, respectively. Detailedly, the strength of the σ- and lp-hole site-based interactions directly correlated with the atomic size of the aerogen atoms and the magnitude of the positively oriented EEF. Unexpectedly, an irregular correlation was noticed for the interaction energies of the π-hole site-based interactions with the size of the π-hole. Interestingly, the π-hole site-based interactions within Kr-comprising complexes exhibited higher negative interaction energies than the Ar- and Xe-comprising counterparts. Notwithstanding, a direct proportion between the interaction energies of the π-hole site-based interactions and π-hole size was obtained by employing EEF along the positive orientation with high strength. The present outcomes would be a fundamental basis for forthcoming progress in studying the σ-, lp-, and π-hole site-based interactions within aerogen-comprising complexes and their pertinent applications in materials science and crystal engineering.

Chalcogen Noncovalent Interactions between Diazines and Sulfur Oxides in Supramolecular Circular Chains

The noncovalent chalcogen interaction between SO2/SO3 and diazines was studied through a dispersion-corrected DFT Kohn-Sham molecular orbital together with quantitative energy decomposition analyses. For this, supramolecular circular chains of up to 12 molecules were built with the aim of checking the capability of diazine molecules to detect SO2/SO3 compounds within the atmosphere. Trends in the interaction energies with the increasing number of molecules are mainly determined by the Pauli steric repulsion involved in these σ-hole/π-hole interactions. But more importantly, despite the assumed electrostatic nature of the involved interactions, the covalent component also plays a determinant role in its strength in the involved chalcogen bonds. Noticeably, π-hole interactions are supported by the charge transfer from diazines to SO2/SO3 molecules. Interaction energies in these supramolecular complexes are not only determined by the S···N bond lengths but attractive electrostatic and orbital interactions also determine the trends. These results should allow us to establish the fundamental characteristics of chalcogen bonding based on its strength and nature, which is of relevance for the capture of sulfur oxides.

Pnictogen bond-driven control of the molecular interaction between organophosphorus and acetylcholinesterase enzyme

This study addresses a comprehensive assessment of the interaction between chemical warfare agents (CWA) and acetylcholinesterase (AChE) systems, focus on the intriguing pnictogen-bond interaction (PnB). Utilizing the crystallographic data from the Protein Data Bank pertaining to the AChE-CWA complex involving Sarin (GB), Cyclosarin (GF), 2-[fluoro(methyl)phosphoryl]oxy-1,1-dimethylcyclopentane (GP) and venomous agent X (VX) agents, the CWA is systematically displaced by increments of 0.1 Å along the PO bond axis, extending its distance by 4 Å from the original position. The AIM analysis was carried out and consistently revealed the presence of a significant interaction along the PO bond. Investigating the intrinsic nature of the PnB, the NBO and the EDA analysis unearthed the contribution of orbital factors to the overall energy of the system. Strikingly, this observation challenges the conventional σ-hole explanation commonly associated with such interactions. This finding adds a layer of complexity to understanding of PnB, encouraging further exploration into the underlying mechanisms governing these intriguing chemical phenomena.

Experimental and Theoretical Evidence of a Pb⋅⋅⋅Pb Ditetrel Bond Without a σ-Hole

The crystal structure of a newly synthesized compound, [PbL(Ac)]₂ , (where L=2 (amino(pyrazin-2-yl) methylene) hydrazinecarbothioamide, Ac=acetate anion) exhibits a close contact between pairs of Pb atoms, suggesting a ditetrel bond, in addition to two Pb⋅⋅⋅O tetrel bonds, and two C-H⋅⋅⋅O H-bonds. The presence of this ditetrel bond as an attractive component is confirmed by various quantum chemical methods. This novelty of this particular bond is its existence even in the absence of a σ-hole on the Pb atom, which is typically considered a prerequisite for a bond of this type. From a wider perspective, a survey of the Cambridge Structural Database suggests this bond may be more common than was hitherto thought, with 44 examples of Pb⋅⋅⋅Pb contacts amongst a total number of 219 examples of T⋅⋅⋅T interactions in general (T=Si, Ge, Sn, Pb).

The Pnictogen Bond: The Covalently Bound Arsenic Atom in Molecular Entities in Crystals as a Pnictogen Bond Donor

In chemical systems, the arsenic-centered pnictogen bond, or simply the arsenic bond, occurs when there is evidence of a net attractive interaction between the electrophilic region associated with a covalently or coordinately bound arsenic atom in a molecular entity and a nucleophile in another or the same molecular entity. It is the third member of the family of pnictogen bonds formed by the third atom of the pnictogen family, Group 15 of the periodic table, and is an inter- or intramolecular noncovalent interaction. In this overview, we present several illustrative crystal structures deposited into the Cambridge Structure Database (CSD) and the Inorganic Chemistry Structural Database (ICSD) during the last and current centuries to demonstrate that the arsenic atom in molecular entities has a significant ability to act as an electrophilic agent to make an attractive engagement with nucleophiles when in close vicinity, thereby forming σ-hole or π-hole interactions, and hence driving (in part, at least) the overall stability of the system’s crystalline phase. This overview does not include results from theoretical simulations reported by others as none of them address the signatory details of As-centered pnictogen bonds. Rather, we aimed at highlighting the interaction modes of arsenic-centered σ- and π-holes in the rationale design of crystal lattices to demonstrate that such interactions are abundant in crystalline materials, but care has to be taken to identify them as is usually done with the much more widely known noncovalent interactions in chemical systems, halogen bonding and hydrogen bonding. We also demonstrate that As-centered pnictogen bonds are usually accompanied by other primary and secondary interactions, which reinforce their occurrence and strength in most of the crystal structures illustrated. A statistical analysis of structures deposited into the CSD was performed for each interaction type As···D (D = N, O, S, Se, Te, F, Cl, Br, I, arene’s π system), thus providing insight into the typical nature of As···D interaction distances and ∠R–As···D bond angles of these interactions in crystals, where R is the remainder of the molecular entity.

σ-Hole and LP-Hole Interactions of Pnicogen···Pnicogen Homodimers under the External Electric Field Effect: A Quantum Mechanical Study

σ-Hole and lone-pair (lp)-hole interactions within σ-hole···σ-hole, σ-hole···lp-hole, and lp-hole···lp-hole configurations were comparatively investigated on the pnicogen···pnicogen homodimers (PCl₃)₂, for the first time, under field-free conditions and the influence of the external electric field (EEF). The electrostatic potential calculations emphasized the impressive versatility of the examined PCl₃ monomers to participate in σ-hole and lp-hole pnicogen interactions. Crucially, the sizes of σ-hole and lp-hole were enlarged under the influence of the positively directed EEF and decreased in the case of reverse direction. Interestingly, the energetic quantities unveiled more favorability of the σ-hole···lp-hole configuration of the pnicogen···pnicogen homodimers, with significant negative interaction energies, than σ-hole···σ-hole and lp-hole···lp-hole configurations. Quantum theory of atoms in molecules and noncovalent interaction index analyses were adopted to elucidate the nature and origin of the considered interactions, ensuring their closed shell nature and the occurrence of attractive forces within the studied homodimers. Symmetry-adapted perturbation theory-based energy decomposition analysis alluded to the dispersion force as the main physical component beyond the occurrence of the examined interactions. The obtained findings would be considered as a fundamental underpinning for forthcoming studies pertinent to chemistry, materials science, and crystal engineering.

Tetrel Bonds between Phenyltrifluorosilane and Dimethyl Sulfoxide: Influence of Basis Sets, Substitution and Competition

Ab initio calculations have been performed for the complexes of DMSO and phenyltrifluorosilane (PTS) and its derivatives with a substituent of NH₃, OCH₃, CH₃, OH, F, CHO, CN, NO₂, and SO₃H. It is necessary to use sufficiently flexible basis sets, such as aug’-cc-pVTZ, to get reliable results for the Si···O tetrel bonds. The tetrel bond in these complexes has been characterized in views of geometries, interaction energies, orbital interactions and topological parameters. The electron-donating group in PTS weakens this interaction and the electron-withdrawing group prominently strengthens it to the point where it exceeds that of the majority of hydrogen bonds. The largest interaction energy occurs in the p-HO₃S-PhSiF₃···DMSO complex, amounting to −122 kJ/mol. The strong Si···O tetrel bond depends to a large extent on the charge transfer from the O lone pair into the empty p orbital of Si, although it has a dominant electrostatic character. For the PTS derivatives of NH₂, OH, CHO and NO₂, the hydrogen bonded complex is favorable to the tetrel bonded complex for the NH₂ and OH derivatives, while the σ-hole interaction prefers the π-hole interaction for the CHO and NO₂ derivatives.

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile's lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.

The pnictogen bond: a quantitative molecular orbital picture

We have analyzed the structure and stability of archetypal pnictogen-bonded model complexes D3PnA- (Pn = N, P, As, Sb; D, A = F, Cl, Br) using state-of-the-art relativistic density functional calculations at the ZORA-M06/QZ4P level. We have accomplished two tasks: (i) to compute accurate trends in pnictogen-bond strength based on a set of consistent data; and (ii) to rationalize these trends in terms of detailed analyses of the bonding mechanism based on quantitative Kohn-Sham molecular orbital (KS-MO) theory in combination with a canonical energy decomposition analysis (EDA) and Voronoi deformation density (VDD) analyses of the charge distribution. We have found that pnictogen bonds have a significant covalent character stemming from strong HOMO-LUMO interactions between the lone pair of A- and σ* of D3Pn. As such, the underlying mechanism of the pnictogen bond is similar to that of hydrogen, halogen, and chalcogen bonds.

CHAL336 Benchmark Set: How Well Do Quantum-Chemical Methods Describe Chalcogen-Bonding Interactions?

We present the CHAL336 benchmark set-the most comprehensive database for the assessment of chalcogen-bonding (CB) interactions. After careful selection of suitable systems and identification of three high-level reference methods, the set comprises 336 dimers each consisting of up to 49 atoms and covers both σ- and π-hole interactions across four categories: chalcogen-chalcogen, chalcogen-π, chalcogen-halogen, and chalcogen-nitrogen interactions. In a subsequent study of DFT methods, we re-emphasize the need for using proper London dispersion corrections when treating noncovalent interactions. We also point out that the deterioration of results and systematic overestimation of interaction energies for some dispersion-corrected DFT methods does not hint at problems with the chosen dispersion correction but is a consequence of large density-driven errors. We conclude this work by performing the most detailed DFT benchmark study for CB interactions to date. We assess 109 variations of dispersion-corrected and dispersion-uncorrected DFT methods and carry out a detailed analysis of 80 of them. Double-hybrid functionals are the most reliable approaches for CB interactions, and they should be used whenever computationally feasible. The best three double hybrids are SOS0-PBE0-2-D3(BJ), revDSD-PBEP86-D3(BJ), and B2NCPLYP-D3(BJ). The best hybrids in this study are ωB97M-V, PW6B95-D3(0), and PW6B95-D3(BJ). We do not recommend using the popular B3LYP functional nor the MP2 approach, which have both been frequently used to describe CB interactions in the past. We hope to inspire a change in computational protocols surrounding CB interactions that leads away from the commonly used, popular methods to the more robust and accurate ones recommended herein. We would also like to encourage method developers to use our set for the investigation and reduction of density-driven errors in new density functional approximations.

Noncovalent Bonds through Sigma and Pi-Hole Located on the Same Molecule. Guiding Principles and Comparisons

Over the last years, scientific interest in noncovalent interactions based on the presence of electron-depleted regions called σ-holes or π-holes has markedly accelerated. Their high directionality and strength, comparable to hydrogen bonds, has been documented in many fields of modern chemistry. The current review gathers and digests recent results concerning these bonds, with a focus on those systems where both σ and π-holes are present on the same molecule. The underlying principles guiding the bonding in both sorts of interactions are discussed, and the trends that emerge from recent work offer a guide as to how one might design systems that allow multiple noncovalent bonds to occur simultaneously, or that prefer one bond type over another.

Nitrogen as a pnicogen?: evidence for π-hole driven novel pnicogen bonding interactions in nitromethane-ammonia aggregates using matrix isolation infrared spectroscopy and ab initio computations

The role of nitrogen, the first member of the pnicogen group, as an electron donor in hypervalent non-covalent interactions has been established long ago, while observation of its electron accepting capability is still elusive experimentally, and remains quite intriguing, conceptually. In the light of minimal computational exploration of this novel class of pnicogen bonding so far, the present work provides experimental proof with unprecedented clarity, for the existence of N(acceptor)N(donor) interaction using the model nitromethane (NM) molecule with ammonia (AM) as a Lewis base in NM-AM aggregates. The NM-AM dimer, in which the nitrogen atom of NM (as a unique pnicogen) accepts electrons from AM (the traditional electron donor), was synthesized at low temperatures under isolated conditions within inert gas matrixes and was characterized using infrared spectroscopy. The experimental generation of the NM-AM dimer stabilized via NN interaction has strong corroboration from ab initio calculations. Furthermore, confirmation regarding the directional prevalence of this NN interaction over C-HN and N-HO hydrogen bonding is elucidated quantitatively by quantum theory of atoms in molecules (QTAIM), electrostatic potential mapping (ESP), natural bond orbital (NBO), non-covalent interaction (NCI) and energy decomposition (ED) analyses. The present study also allows the extension of σ-hole/π-hole driven interactions to the atoms of the second period, in spite of their low polarizability.

Unusual blue to red shifting of C-H stretching frequency of CHCl3 in co-operatively P⋯Cl phosphorus bonded POCl3-CHCl3 heterodimers at low temperature inert matrixes

Heterodimers of POCl₃-CHCl₃ were generated in Ne, Ar, and Kr matrixes at low temperatures and were studied using infrared spectroscopy. The remarkable role of co-operative pentavalent phosphorus bonding in the stabilization of the structure dictated by hydrogen bonding is deciphered. The complete potential energy surface of the heterodimer was scanned by ab initio and density functional theory computational methodologies. The hydrogen bond between the phosphoryl oxygen of POCl₃ and C-H group of CHCl₃ in heterodimers induces a blue-shift in the C-H stretching frequency within the Ne matrix. However, in Ar and Kr matrixes, the C-H stretching frequency is exceptionally red-shifted in stark contrast with Ne. The plausibility of the Fermi resonance by the C-H stretching vibrational mode with higher order modes in the heterodimers has been eliminated as a possible cause within Ar and Kr matrixes by isotopic substitution (CDCl₃) experiments. To evaluate the influence of matrixes as a possible cause of red-shift, self-consistent Iso-density polarized continuum reaction field model was applied. This conveyed the important role of the dielectric matrixes in inducing the fascinating vibrational shift from blue (Ne) to red (Ar and Kr) due to the matrix specific transmutation of the POCl₃-CHCl₃ structure. The heterodimer produced in the Ne matrix possesses a cyclic structure stabilized by hydrogen bonding with co-operative phosphorus bonding, while in Ar and Kr the generation of an acyclic open structure stabilized solely by hydrogen bonding is promoted. Compelling justification regarding the dispersion force based influence of matrix environments in addition to the well-known dielectric influence is presented.

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

A quantum chemical perspective on the potency of electron donors and acceptors in pnicogen bonds (AS...N, P...N, N...N)

A quantum chemical perspective of 31 structures contains electron acceptors: ASCl₃ (arsenic trichloride), PCl₃ (phosphorous trichloride) and NCl₃ (nitrogen trichloride); forming non-covalent bond with various nitrogen-based electron donors that resulted in pnicogen bonds, AS...N, P...N and N...N were calculated at M062X/def2-QZVP level of theory. Besides the above method, MP2/def2-QZVP and CCSD(T)/def2-QZVP level of theories have also been analysed to have in depth knowledge about the bonds formed. The nature of the bonds was assumed from the electrostatic potential evaluated for all the monomers, where σ hole is positive for all the monomers. The strongest pnicogen bonds are ASCl₃-NF₂H, PCl₃-NCH₃CH₃CH₃ and NCl₃-NCH₃CH₃CH₃ having interaction energies as -4.15, -11.58 and -3.25 kcal/mol, respectively, at MP2/def2-QZVP level of theory. Further at CCSD(T)/def2-QZVP level of theory, ASCl₃-NF₂H and NCl₃-NCH₃CH₃CH₃ are found to be the most stable with interaction energies as -3.53 and -2.45 kcal/mol, respectively. The potential energy surface scan was performed for all the stable complexes in order to confirm the existences of energies are true minima. Moreover to confirm the halogen and pnicogen bonds, AIM analysis was carried out. The results from the above factors of pnicogen bond will help crystal growth, material science and engineering community to explore novel materials, which abide for modernization. Graphical abstract PCl₃-NCH₃CH₃CH₃ complex with 2.61 Å and pnicogen angle of 178.54° is strong, and interaction energy is -11.58 kcal/mol. Electron donors - ASCl₃, PCl₃ and NCl₃ and electron acceptors -NCH₃CH₃CH₃, NH₃C₂ and NHCO have strong electrostatic contribution. High and low values of (ρ) ∇²(ρ) reveal the strong and weak pnicogen bond. Schematic representation of acceptors surrounded by its donors and Electrostatic Potential map.

On the ability of pnicogen atoms to engage in both σ and π-hole complexes. Heterodimers of ZF2C6H5 (Z = P, As, Sb, Bi) and NH3

When bound to a pair of F atoms and a phenyl ring, a pyramidal pnicogen (Z) atom can form a pnicogen bond wherein an NH₃ base lies opposite one F atom. In addition to this σ-hole complex, the ZF₂C₆H₅ molecule can distort in such a way that the NH₃ approaches on the opposite side to the lone pair on Z, where there is a so-called π-hole. The interaction energies of these π-hole dimers are roughly 30 kcal/mol, much larger than the equivalent quantities for the σ-hole complexes, which are only 4-13 kcal/mol. On the other hand, this large interaction energy is countered by the considerable deformation energy required for the Lewis acid to adopt the geometry necessary to form the π-hole complex. The overall energetics of the complexation reaction are thus more exothermic for the σ-hole dimers than for the π-hole dimers.

Local-structure effects on 31P NMR chemical shift tensors in solid state

The effect of the local structure on the ³¹P NMR chemical shift tensor (CST) has been studied experimentally and simulated theoretically using the density functional theory gauge-independent-atomic-orbital approach. It has been shown that the dominating impact comes from a small number of noncovalent interactions between the phosphorus-containing group under question and the atoms of adjacent molecules. These interactions can be unambiguously identified using the Bader analysis of the electronic density. A robust and computationally effective approach designed to attribute a given experimental ³¹P CST to a certain local morphology has been elaborated. This approach can be useful in studies of surfaces, complex molecular systems, and amorphous materials.

Elusive hypervalent phosphorus⋯π interactions: evidence for paradigm transformation from hydrogen to phosphorus bonding at low temperatures

A paradigm transformation from hydrogen to phosphorus bonding is found to depend on the proton affinity of the interacting π-systems.

Influence of monomer deformation on the competition between two types of σ-holes in tetrel bonds

Competition between two competing sites on a tetrel atom is explained by balance between structural deformation and σ-hole intensity.

Strong Tetrel Bonds: Theoretical Aspects and Experimental Evidence

In recent years, noncovalent interactions involving group-14 elements of the periodic table acting as a Lewis acid center (or tetrel-bonding interactions) have attracted considerable attention due to their potential applications in supramolecular chemistry, material science and so on. The aim of the present study is to characterize the geometry, strength and bonding properties of strong tetrel-bond interactions in some charge-assisted tetrel-bonded complexes. Ab initio calculations are performed, and the results are supported by the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) approaches. The interaction energies of the anionic tetrel-bonded complexes formed between XF₃M molecule (X=F, CN; M=Si, Ge and Sn) and A- anions (A-=F-, Cl-, Br-, CN-, NC- and N₃-) vary between -16.35 and -96.30 kcal/mol. The M atom in these complexes is generally characterized by pentavalency, i.e., is hypervalent. Moreover, the QTAIM analysis confirms that the anionic tetrel-bonding interaction in these systems could be classified as a strong interaction with some covalent character. On the other hand, it is found that the tetrel-bond interactions in cationic tetrel-bonded [p-NH₃(C₆H₄)MH₃]⁺···Z and [p-NH₃(C₆F₄)MH₃]⁺···Z complexes (M=Si, Ge, Sn and Z=NH₃, NH₂CH₃, NH₂OH and NH₂NH₂) are characterized by a strong orbital interaction between the filled lone-pair orbital of the Lewis base and empty BD*M-C orbital of the Lewis base. The substitution of the F atoms in the benzene ring provides a strong orbital interaction, and hence improved tetrel-bond interaction. For all charge-assisted tetrel-bonded complexes, it is seen that the formation of tetrel-bond interaction is accompanied bysignificant electron density redistribution over the interacting subunits. Finally, we provide some experimental evidence for the existence of such charge-assisted tetrel-bond interactions in crystalline phase.

Competition between tetrel bond and pnicogen bond in complexes of TX3-ZX2 and NH3

The complexes formed between TX₃-ZX₂ (T = C, Si, Ge; Z = P, As, Sb; X = F, Cl) and NH₃ were studied at the MP2/aug-cc-pVTZ(PP) level. For each TX₃-ZX₂, two types of complex were obtained. For CX₃-ZX₂, NH₃ is inclined to approach the σ-hole on the Z atom, forming a pnicogen bond. For TX₃-ZX₂ (T = Si and Ge), however, the base favors engaging in a tetrel bond with the σ-hole on the T atom although the corresponding pnicogen-bonded complex is also stable. When NH₃ approaches the CX₃ terminal of CX₃-ZX₂, weak interactions are observed that may be classified as van der Waals interactions. The relative stability of both types of complexes is not affected by the substituent X. The tetrel bond is very strong and the largest interaction energy is up to -144 kJ mol^-1. Dispersion is dominant in the weak van der Waals complexes, while tetrel- and pnicogen-bonded complexes are dominated by electrostatic interactions, with comparable contributions from polarization.

The potential of pnicogen bonding for catalysis - a computational study

Pnicogen bonding is a noncovalent interaction between the electrophilic region of a phosphorus atom and a Lewis base. Although this interaction can be comparable in strength to other noncovalent interactions, no systematic application in organic synthesis or catalysis is known so far. To identify the potential of this interaction for organocatalysis, we have now analysed different pnicogen-bond donors as catalysts for the activation of three different model reactions employing density functional theory. Our calculations suggest rate accelerations of several orders of magnitude for all cases indicating that synthetic applications should be feasible. Furthermore, our results indicate that pnicogen-bond donors can be comparable to halogen-bond-based catalysts in these reactions.

Aerogen bonds formed between AeOF2 (Ae = Kr, Xe) and diazines: comparisons between σ-hole and π-hole complexes

The interaction between KrOF₂ or XeOF₂ and the 1,2, 1,3, and 1,4 diazines is characterized chiefly by a Kr/XeN aerogen bond, as deduced from ab initio calculations. The most stable dimers take advantage of the σ-hole on the aerogen atom, wherein the two molecules lie in the same plane. The interaction is quite strong, as much as 18 kcal mol^-1. A second class of dimer geometry utilizes the π-hole above the aerogen atom in an approximate perpendicular arrangement of the two monomers; these structures are not as strongly bound: 6-8 kcal mol^-1. Both sorts of dimers contain auxiliary CHF H-bonds which contribute to their stability, but even with their removal, the aerogen bond energy remains as high as 14 kcal mol^-1. The nature and strength of each specific interaction is confirmed and quantified by AIM, NCI, NBO, and electron density shift patterns. There is not a great deal of sensitivity to the identity of either the aerogen atom or the position of the two N atoms in the diazine.

Pentavalent phosphorus as a unique phosphorus donor in POCl3 homodimer and POCl3–H2O heterodimer: matrix isolation infrared spectroscopic and computational studies

Phosphorus, an important element among the pnicogen group, opens up avenues for experimental and computational explorations of its interaction in a variety of compounds.

The effect of anions on noncovalent interactions in model clusters of chalcogen-containing (CH3)2X (X = O, S, Se) molecules

A computational study of F⁻⋯(CH₃)₂O⋯CH₃F with F⁻ bound to the protons of the two methyl groups, found significant enhancement of the O⋯C interaction relative to the neutral (CH₃)₂O⋯CH₃F dyad.

Halogen Bonding Involving CO and CS with Carbon as the Electron Donor

MP2/aug'-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH₂. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC:ClNC and SC:ClCl surfaces. SC:ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC:ClY complexes. The largest binding energies are found for the ion-pair SC-Cl⁺:-Y complexes. The transition structures which connect the complex and the ion pair on SC:ClNC and SC:ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC:ClY and SC:ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY. This secondary interaction assumes increased importance in the SC:ClNH₂ complex, and is a factor leading to its unusual structure. C-O and C-S stretching frequencies and 13C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC:ClY complexes and SC-Cl⁺:-Y ion pairs. Spin-spin coupling constants 1xJ(C-Cl) for OC:ClY complexes increase with decreasing distance. As a function of the C-Cl distance, 1xJ(C-Cl) and ¹J(C-Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.

Trapping CO2 by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO₂ at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to -77 kJ mol^-1 for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.