Cooperativity between Intermolecular Hydrogen and Carbon Bonds in ZY···CH3CN/CH3NC···HX Trimers (ZY = H2O, H2S, HF, HCl, HBr, NH3, and H2CO; HX = HF, HCl, and HBr)

Hydrogen-bonding and carbon-bonding interactions are widespread in nature. We studied the cooperativity between these interactions in 42 trimeric complexes ZY···CH3CN/CH3NC···HX, where ZY molecules are H2O, H2S, HF, HCl, HBr, NH3, and H2CO, and HX molecules are HF, HCl, and HBr. Acetonitrile (CH3CN) and isoacetonitrile (CH3NC) act as hydrogen bond acceptors as well as carbon bond donors in these trimers. Various theoretical methods, such as electronic structure calculations, quantum theory of atoms in molecule (QTAIM), natural bond orbital (NBO), and reduced density gradient analysis, are employed to study these trimers, and the results are compared with the corresponding ZY···CH3CN/CH3NC and CH3CN/CH3NC···HX dimers. Electronic structure calculations are performed at the second-order Mo̷ller-Plesset perturbation theory using the 6-311++G(2d,2p) basis set. We show that both the interactions act synergistically in these trimers leading to an increase in their bond strength as compared to the strength in the individual dimers. The cooperative energies for these trimers are in the range of 0.69 to 3.22 kJ/mol. It is seen that the carbon bonds benefit more from the cooperativity than the hydrogen bonds. The trends of cooperativity and correlations of interaction energies and cooperative energies with relevant QTAIM and NBO parameters are reported.

The Tetrel Bonds of Hypervalent Halogen Compounds

The tetrel bond between PhXF2Y(TF3) (T = C and Si; X = Cl, Br, and I; Y = F and Cl) and the electron donor MCN (M = Li and Na) was investigated at the M06-2X/aug-cc-pVDZ level of theory. As the electronegativity of the halogen atom X increases, the strength of the tetrel bond also increases, but as the electronegativity of the halogen atom Y increases, the strength of the tetrel bond decreases. The magnitude of the interaction energy in most -CF3 complexes was found to be less than 10 kcal/mol, but to exceed 11 kcal/mol for PhClF2Cl(CF3)⋯NCNa. The tetrel bond is greatly enhanced when the -SiF3 group interacts with LiCN or NaCN, with the largest interaction energy approaching 100 kcal/mol and displaying a covalent Si⋯N interaction. Along with this enhancement, the Si⋯N distance was found to be less than the X-Si bond length, the -SiF3 group to be closer to the N atom, and in most -SiF3 systems, the X-Si-F angle to be less than 90°; the -SiF3 group therefore undergoes inversion and complete transfer in some systems.

Adjusting the balance between hydrogen and chalcogen bonds

A complex is assembled which pairs a carboxyl group of X₁COOH with a 1,2,5-chalcogenadiazole ring containing substituents on its C atoms. The OH of the carboxyl group donates a proton to a N atom of the ring to form a OH⋯N H-bond (HB), while its carbonyl O engages in a Y⋯O chalcogen bond (ChB) with the ring in which Y = S, Se, Te. The ChB is strengthened by enlarging the size of the Y atom from S to Se to Te. Placement of an electron-withdrawing group (EWG) X₁ on the acid strengthens the HB while weakening the ChB; the reverse occurs when EWGs are placed on the ring. By selection of the proper substituents on the two units, it is possible to achieve a near perfect balance between the strengths of these two bonds. These bond strengths are also reflected in the NMR spectroscopic properties of the chemical shielding of the various atoms and the coupling between the nuclei directly involved in each bond.

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I···I or I···X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I···F rings are generally more stable than the corresponding I···I cycles as the I···F bond is stronger than I···I even in the simple dimer. However, I···I rings are comparable in energy to I···Cl. It is possible to construct I···I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I···X rings is 6. Red shifts are observed in the I-X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I···I chains but diminishes when I···Cl bonds are present.

Various Sorts of Chalcogen Bonds Formed by an Aromatic System

The chalcogen Y atom in the aromatic ring of thiophene and its derivatives YC₄H₄ (Y = S, Se, Te) can engage in a number of different interactions with another such unit within the homodimer. Quantum calculations show that the two rings can be oriented perpendicular to one another in a T-shaped dimer in which the Y atom accepts electron density from the π-system of the other unit in a Y···π chalcogen bond (ChB). This geometry best takes advantage of attractions between the electrostatic potentials surrounding the two monomers. There are two other geometries in which the two Y atoms engage in a ChB with one another. However, instead of a simple interaction between a σ-hole on one Y and the lone pair of its neighbor, the interaction is better described as a pair of symmetrically equivalent Y···Y interactions, in which charge is transferred in both directions simultaneously, thereby effectively doubling the strength of the bond. These geometries differ from what might be expected based simply on the juxtaposition of the electrostatic potentials of the two monomers.

Malonaldehyde-like Systems: BeF2 Clusters—A Subtle Balance between Hydrogen Bonds, Beryllium Bonds, and Resonance

The stability of malonaldehyde is governed by intramolecular hydrogen bonds (IMHBs) as well as in malonaldehyde-like systems where oxygen is replaced by N or S at any of the basic sites. As beryllium bonds have been shown to strongly cooperate with hydrogen bonds, this work explores at the high level ab initio G4 level of theory the effect of including this non-covalent interaction in the system through its association with BeF2. Although malonaldehyde follows the expected trends, where the formation of a pseudocyclic form is favored also when IMHB and Be bonds are present, the subtle balance between both non-covalent interactions leads to some surprising results when other heteroatoms are involved, to the point that interaction energies can be much larger than expected or even cyclization is not favored. A complete analysis using different computational tools gives an answer to those cases escaping the predictable trends.

Cooperative Effect of Noncovalent Interactions on Tetrel Bonding in Halogenated Silanes

Tetrel bond, a weak noncovalent interaction between the  σ -hole of a Group IV element (Silicon in our case) and the cloud of an electronegative element (Oxygen in our case) is the focus of current work. The percentage strengthening of tetrel bond has been investigated by optimising 16 binary complexes of halogenated silane and water of general formula SiX n H 4 - n  - H 2 O and 16 ternary complexes, of general formula NaX - SiX n H 4 - n  -H 2 O, where X =F, Cl, Br and I and n = 1, 2, 3 and 4 at various levels of theory defined within the formalism of density functional theory (DFT). With the addition of NaX, tetrel bond between Si and O in SiX n H 4 - n  - H 2 O gets strengthened up to 49%, owing to cooperativity effect exerted by hydrogen bonding between X and H in the ternary complex NaX - SiX n H 4 - n  - H 2 O. In the series of complexes studied here, overall stabilization due to cooperativity lies between 10 kJ/mol to 170 kJ/mol. This large extent of reinforcement due to cooperativity has never been showcased before. The exceptional stabilization and reinforcement owe its genesis to the transformation of the ternary complex into a cluster orchestrated by the H-bonding in most of the cases and covalent bonding in few of the cases.

Ability of Lewis Acids with Shallow σ-Holes to Engage in Chalcogen Bonds in Different Environments

Molecules of the type XYT = Ch (T = C, Si, Ge; Ch = S, Se; X,Y = H, CH₃, Cl, Br, I) contain a σ-hole along the T = Ch bond extension. This hole can engage with the N lone pair of NCH and NCCH₃ so as to form a chalcogen bond. In the case of T = C, these bonds are rather weak, less than 3 kcal/mol, and are slightly weakened in acetone or water. They owe their stability to attractive electrostatic energy, supplemented by dispersion, and a much smaller polarization term. Immersion in solvent reverses the electrostatic interaction to repulsive, while amplifying the polarization energy. The σ-holes are smaller for T = Si and Ge, even negative in many cases. These Lewis acids can nonetheless engage in a weak chalcogen bond. This bond owes its stability to dispersion in the gas phase, but it is polarization that dominates in solution.

Proximity Effects of Substituents on Halogen Bond Strength

It is well known that the presence of an electron-withdrawing substituent (EWS) placed near the halogen (X) atom on a Lewis acid molecule amplifies the ability of this unit to engage in a halogen bond with a base. Quantum calculations are applied to examine how quickly these effects fade as the EWS is moved further and further from the X atom. Conjugated alkene and alkyne chains of varying lengths with a terminal C-I first facilitate analysis as to how the number of these multiple bonds affects the strength of CI··N XB to NH₃. Then, electron-withdrawing F and C≡N substituents are placed on the opposite end of the chain, and their effects on the XB properties are monitored as a function of their distance from I. These same EWSs are added to the ortho, meta, and para positions of aromatic iodobenzene. It is found that the XB grows in strength as more triple bonds are added to the alkyne, but there is little change caused by elongating an alkene. The cyano group has a much stronger effect than does F. While F strengthens the XB, its effects are quickly attenuated as it is moved further from I. The consequences of C≡N substitution are stronger and extend over a longer distance. Placement of an EWS on the phenyl ring diminishes with distance: o > m > p, and the effects of disubstitution are nearly additive. These trends apply not only to energetics but also to geometries, properties of the wave function, σ-hole depth, and NMR shielding.

Competition between Inter and Intramolecular Tetrel Bonds: Theoretical Studies Complemented by CSD Survey

Crystal structures document the ability of a TF₃ group (T=Si, Ge, Sn, Pb) situated on a naphthalene system to engage in an intramolecular tetrel bond (TB) with an amino group on the adjoining ring. Ab initio calculations evaluate the strength of this bond and evaluate whether it can influence the ability of the T atom to engage in a second, intermolecular TB with another nucleophile. A very strong CN^- anionic base can approach the T either along the extension of a T-C or T-F bond and form a strong TB with an interaction energy approaching 100 kcal/mol, although this bond is weakened a bit by the presence of the internal T⋅⋅⋅N bond. The much less potent NCH base engages in a correspondingly longer and weaker TB, less than 10 kcal/mol. Such an intermolecular TB is weakened by the presence of the internal TB, to the point that it only occurs for the two heavier tetrel atoms Sn and Pb.

Anion-Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

Ab initio calculations are applied to the question as to whether a AeX₅^- anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F^-, Cl^-, or CN^-. The latter approaches the central Ae atom from above the molecular plane, along its C₅ axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier.

Relative Strengths of a Pnicogen and a Tetrel Bond and Their Mutual Effects upon One Another

The ability of the T and Z atoms of TR₃ZR₂ to engage in a noncovalent interaction with NH₃ is assessed by DFT calculations, where the T atom refers to C, Si, and Ge; Z = As, Sb, and P; and substituents R = H and F. In most instances, the tetrel bond (TB) is both stronger and shorter than the pnicogen bond (ZB). These two bond strengths can be equalized, or preference shifted to the ZB, if F substituents are placed on the Z and H on the T atoms. Employing C as the T atom results in a very weak TB, with the ZB clearly favored energetically. The simultaneous formation of both TB and ZB weakens both, particularly the latter, but both bonds survive intact. Geometric and spectroscopic perturbations of the subunits reflect the two types of noncovalent bonds.

Origins and properties of the tetrel bond

The tetrel bond (TB) recruits an element drawn from the C, Si, Ge, Sn, Pb family as electron acceptor in an interaction with a partner Lewis base. The underlying principles that explain this attractive interaction are described in terms of occupied and vacant orbitals, total electron density, and electrostatic potential. These principles facilitate a delineation of the factors that feed into a strong TB. The geometric deformation that occurs within the tetrel-bearing Lewis acid monomer is a particularly important issue, with both primary and secondary effects. As a first-row atom of low polarizability, C is a reluctant participant in TBs, but its preponderance in organic and biochemistry make it extremely important that its potential in this regard be thoroughly understood. The IR and NMR manifestations of tetrel bonding are explored as spectroscopy offers a bridge to experimental examination of this phenomenon. In addition to the most common σ-hole type TBs, discussion is provided of π-hole interactions which are a result of a common alternate covalent bonding pattern of tetrel atoms.

High-energetic and low-sensitive 1,3,5-triamino 2,4,6-trinitrobenzene (TATB) crystal: first principles investigation and Hirshfeld surface analysis

The N–H⋯O hydrogen bonding between neighboring TATB molecules within a planar layer cooperates with π⋯π interaction between two adjacent molecules from two adjacent layers in the TATB crystal.

Coordination of a Central Atom by Multiple Intramolecular Pnicogen Bonds

A central pnicogen Z atom (Z = Sb, As) is covalently attached to the O atom of three -O(CH₂)_nX chains where X represents either an aldehyde or amine group. The chain can fold around so that the basic X group can engage in a noncovalent pnicogen bond with the central Z. The formation of up to three pnicogen bonds is energetically favored. The amine appears to engage in stronger pnicogen bonds than does the aldehyde, and bonds to Sb are favored over As, but there is little dependence on the length of the chain. The formation of each successive pnicogen bond reduces the magnitude of the σ-holes surrounding the Z atom, which tends to weaken the attraction for the basic end of the chain.

The ditetrel bond: noncovalent bond between neutral tetrel atoms

The ability of a tetrel atom to serve in the capacity of electron donor in a σ-hole noncovalent bond is tested by quantum calculations.