Pnicogen–pnicogen interactions in O2XP:PH2Y complexes (X=H, F, CN; Y=H, OH, OCH3, CH3, NH2)

External Electric Field Effect on the Strength of σ-Hole Interactions: A Theoretical Perspective in Like⋯Like Carbon-Containing Complexes

For the first time, σ-hole interactions within like⋯like carbon-containing complexes were investigated, in both the absence and presence of the external electric field (EEF). The effects of the directionality and strength of the utilized EEF were thoroughly unveiled in the (F-C-F₃)₂, (F-C-H₃)₂, and (H-C-F₃)₂ complexes. In the absence of the EEF, favorable interaction energies, with negative values, are denoted for the (F-C-F₃)₂ and (H-C-F₃)₂ complexes, whereas the (F-C-H₃)₂ complex exhibits unfavorable interactions. Remarkably, the strength of the applied EEF exhibits a prominent role in turning the repulsive forces within the latter complex into attractive ones. The symmetrical nature of the considered like⋯like carbon-containing complexes eradicated the effect of directionality of the EEF. The quantum theory of atoms in molecules (QTAIM), and the noncovalent interaction (NCI) index, ensured the occurrence of the attractive forces, and also outlined the substantial contributions of the three coplanar atoms to the total strength of the studied complexes. Symmetry-adapted perturbation theory (SAPT) results show the dispersion-driven nature of the interactions.

Not Only Hydrogen Bonds: Other Noncovalent Interactions

In this review, we provide a consistent description of noncovalent interactions, covering most groups of the Periodic Table. Different types of bonds are discussed using their trivial names. Moreover, the new name “Spodium bonds” is proposed for group 12 since noncovalent interactions involving this group of elements as electron acceptors have not yet been named. Excluding hydrogen bonds, the following noncovalent interactions will be discussed: alkali, alkaline earth, regium, spodium, triel, tetrel, pnictogen, chalcogen, halogen, and aerogen, which almost covers the Periodic Table entirely. Other interactions, such as orthogonal interactions and π-π stacking, will also be considered. Research and applications of σ-hole and π-hole interactions involving the p-block element is growing exponentially. The important applications include supramolecular chemistry, crystal engineering, catalysis, enzymatic chemistry molecular machines, membrane ion transport, etc. Despite the fact that this review is not intended to be comprehensive, a number of representative works for each type of interaction is provided. The possibility of modeling the dissociation energies of the complexes using different models (HSAB, ECW, Alkorta-Legon) was analyzed. Finally, the extension of Cahn-Ingold-Prelog priority rules to noncovalent is proposed.

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.

The chalcogen bond in F2P(S)N⋅⋅⋅SX2, F2PNS⋅⋅⋅SX2, F2PSN⋅⋅⋅SX2 (X = F, Cl, Br, OH, CH3, NH2) complexes

As a kind of intermolecular noncovalent interaction, chalcogen bonding plays a critical role in the fields of chemistry and biology. In this paper, S⋅⋅⋅S chalcogen bonds in three groups of complexes, F₂P(S)N⋅⋅⋅SX₂, F₂PNS⋅⋅⋅SX₂, and F₂PSN⋅⋅⋅SX₂ (X = F, Cl, Br, OH, CH₃, NH₂), were investigated at the MP2/aug-cc-pVTZ level of theory. The calculated results show that the formation of S⋅⋅⋅S chalcogen bond is in the manner of attraction between the positive molecular electrostatic potential (V_S,max) of chalcogen bond donator and the negative V_S,min of chalcogen bond acceptor. It is found that a good correlation exists between the S⋅⋅⋅S bond length and the interaction energy. The energy decomposition indicates the electrostatic energy and polarization energy are closely correlated with the total interaction energy. NBO analysis reveals that the charge transfer is rather closely correlated with the polarization, and the charge transfer has a similar behavior as the polarization in the formation of complex. Our results provide a new example for interpreting the noncovalent interaction based on the σ-hole concept. Graphical abstract The chalcogen bonds in the studied binary complexes are Coulombic in nature, and the charge transfer has a similar behavior as the polarization in the formation of the complex.

Theoretical Study on the Aza-Diels-Alder Reaction Catalyzed by PHCl2 Lewis Acid via Pnicogen Bonding

The reaction mechanism of the Aza-Diels-Alder (A-D-A) cycloaddition reaction between X₂C═NNH₂, where X = H, F, Cl, Br, and 1,3-butadiene catalyzed by a PHCl₂ Lewis acid was characterized using density functional theory calculations. The influences of various substituents of X on the studied reaction were analyzed using the activation strain model (ASM), which is also termed as the distortion-interaction model. Calculations showed that the smallest and largest values of the activation energies belong to the substituents of F and Br, respectively. The activation energy of the studied reactions was decreased within 8.6 kcal·mol^-1 in the presence of PHCl₂ catalyst. Investigations showed that the pnicogen bonding is adequately capable of activating the A-D-A reaction. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis were implemented to understand the nature of C_4,Cbut···C_XIm and C_1,Cbut···N_XIm bonds at the TS structures. Additionally, the energy decomposition analysis (EDA) based on the ETS-NOCV scheme was used to characterize the nature of C_4,Cbut···C_XIm and C_1,Cbut···N_XIm bond. The results of the study mirror the fact that the PHCl₂ Lewis acid may be suggested as a simple suitable catalyst for experimental studies on the A-D-A reactions.

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.

Cooperativity between the hydrogen bonding and σ-hole interaction in linear NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 complexes (X = Cl, Br; Z = Ar, Kr): a comparative study

Quantum chemical calculations are performed to investigate the cooperativity of hydrogen bonding with halogen or aerogen bonding interactions in linear NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 clusters, where X = Cl, Br and Z = Ar, Kr. To understand the cooperativity mechanism in these systems, the corresponding binary NCX···NCH and O3Z···NCH complexes are also considered. The binding distances, interaction energies, and bonding properties of the NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 clusters are analyzed in detail. It is found that the cooperative effects in the hydrogen bonding tend to strengthen X···N and Z···N interactions. For both NCX···(NCH)n and O3Z···(NCH)n clusters, a small bond shrinkage is observed from n = 4 to n = 5, which suggests that the cooperativity effects are almost saturated in the larger clusters (n > 5). As the size of the X or Z atom is increased, the magnitude of the cooperative energy in these systems is also increased, which is mainly ascribed to changes in electrostatic potentials and orbital interactions. Our results indicate that the cooperative effects lead to a substantial change in the 14N nuclear quadrupole coupling constants of the NCH molecule.

Modulating of the pnicogen-bonding by a H⋯π interaction: An ab initio study

An ab initio study of the cooperativity in XH₂P⋯NCH⋯Z and XH₂P⋯CNH⋯Z complexes (X=F, Cl, Br, CN, NC; Z=C₂H₂,C₆H₆) connected by pnicogen-bonding and H⋯π interactions is carried out by means of MP2 computational method. A detailed analysis of the structures, interaction energies and bonding properties is performed on these systems. For each set of the complexes considered, a favorable cooperativity is observed, especially in X=F and CN complexes. However, for a given X or Z, the amount of cooperativity effects in XH₂P⋯CNH⋯Z complexes are more important than XH₂P⋯NCH⋯Z counterparts. Besides, the influence of a H⋯π interaction on a P⋯N (C) bond is more pronounced than that of a P⋯N (C) bond on a H⋯π bond. The quantum theory of atoms in molecules shows that ternary complexes have increased electron densities at their bond critical points relative to the corresponding binary systems. The results also indicate that the strength of the P⋯N(C) and H⋯π interactions increases in the presence of the solvent.

Modulating intramolecular P⋯N pnictogen interactions

The strength of P⋯N intramolecular pnictogen interactions can be modulated, enhanced or diminished upon substitution of different electron withdrawing or donor groups.

Comparison of π-hole tetrel bonding with σ-hole halogen bonds in complexes of XCN (X = F, Cl, Br, I) and NH3

In addition to the standard halogen bond formed when NH₃approaches XCN (X = F, Cl, Br, I) along its molecular axis, a perpendicular approach is also possible, toward a π-hole that is present above the X–C bond.