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

Transition from covalent to noncovalent bonding between tetrel atoms

The strength and nature of the bonding between tetrel (T) atoms in R2T⋯TR2 is examined by quantum calculations. T atoms cover the range of Group 14 atoms from C to Pb, and substituents R include Cl, F, and NH2. Systems vary from electrically neutral to both positive and negative overall charged radicals. There is a steady weakening progression in T-T bond strength as the tetrel atom grows larger, transitioning smoothly from a strong covalent to a much weaker noncovalent bond for the larger T atoms. The latter have some of the characteristics of a ditetrel bond, but there are also significant deviations from a classic bond of this type. The T2Cl4- anions are more strongly bonded than the corresponding cations, which are in turn stronger than the neutrals.

Cooperativity between H-bonds and tetrel bonds. Transformation of a noncovalent C⋯N tetrel bond to a covalent bond

The dimers and trimers formed by imidazole (IM) and F2TO (T = C, Si, Ge) are studied by ab initio calculations. IM can engage in either a NH⋯O H-bond with F2TO or a T⋯N tetrel bond (TB) with the π-hole above the T atom. The latter is a true noncovalent TB for T = C but is a much shorter and stronger covalent bond with F2SiO or F2GeO. When a second IM is added, the cooperativity emerging from its H-bond with the first IM makes it a stronger nucleophile, leading to two minima with F2CO. The first structure contains a long noncovalent C⋯O TB and there is a much shorter covalent bond in the other, with a small energy barrier separating them. The same sort of double minimum occurs when the two IM units are situated parallel to one another in a stacked geometry.

C∙∙∙O and Si∙∙∙O Tetrel Bonds: Substituent Effects and Transfer of the SiF3 Group

The tetrel bond (TB) between 1,2-benzisothiazol-3-one-2-TF3-1,1-dioxide (T = C, Si) and the O atom of pyridine-1-oxide (PO) and its derivatives (PO-X, X = H, NO2, CN, F, CH3, OH, OCH3, NH2, and Li) is examined by quantum chemical means. The Si∙∙∙O TB is quite strong, with interaction energies approaching a maximum of nearly 70 kcal/mol, while the C∙∙∙O TB is an order of magnitude weaker, with interaction energies between 2.0 and 2.6 kcal/mol. An electron-withdrawing substituent on the Lewis base weakens this TB, while an electron-donating group has the opposite effect. The SiF3 group transfers roughly halfway between the N of the acid and the O of the base without the aid of cooperative effects from a third entity.

Does a halogen bond require positive potential on the acid and negative potential on the base?

It is usually expected that formation of a halogen bond (XB) requires that a region of positive electrostatic potential associated with a σ or π-hole on the Lewis acid will interact with the negative potential of the base, either a lone pair or π-bond region. Quantum calculations of model systems suggest this not to be necessary. The placement of electron-withdrawing substituents on the base can reverse the sign of the potential in its lone pair or π-bond region to positive, and this base can nonetheless engage in a XB with the positive σ-hole of a Lewis acid. The reverse scenario is also possible in certain circumstances, as a negatively charged σ-hole can form a XB with the negative lone pair region of a base. Despite these classical Coulombic repulsions, the overall electrostatic interaction is attractive in these XBs, albeit only weakly so. The strengths of these bonds are surprisingly insensitive to changes in the partner molecule. For example, even a wide range in the depth of the σ-hole of the approaching acid yields only a minimal change in the strength of the XB to a base with a positive potential.

Intermolecular interactions between the heavy-atom analogues of acetylene T2H2 (T = Si, Ge, Sn, Pb) and HCN

METHODS

The intermolecular interactions between the heavy-atom analogues of acetylene T₂H₂ (T = Si, Ge, Sn, Pb) and HCN have been investigated by theoretical calculations at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level.

RESULTS

The global energy minimum of T₂H₂ is the butterfly structure A, and another energy minimum is the planar structure B. Both structures A and B exhibit the dual behavior when binding with HCN. The various hydrogen bond (HB), dihydrogen bond (DB) and tetrel bond (TB) complexes can be found according to the MEP maps of T₂H₂. One TB and three HB complexes formed between structure A and HCN can be located for Si₂H₂ and Ge₂H₂. One TB, two HB and one DB complexes formed between structure A and HCN can be located for Sn₂H₂ and Pb₂H₂. Four TB and one HB complexes formed between structure B and HCN can be located for all the T₂H₂. The geometries and binding strengths of the complexes are compared and analyzed.

CONCLUSIONS

The interactions in these complexes are generally weak, and the interaction energies of these complexes range from -0.53 to -8.23 kcal/mol. The interaction energies of the TB complexes are larger than those of the corresponding HB and DB complexes for structure A···HCN systems. The relative binding strength of the four TB complexes exhibits different order for different structure B···HCN systems, which is consistent with the MEP maps of the isolated monomers.

Strongly Bound π-Hole Tetrel Bonded Complexes between H2SiO and Substituted Pyridines. Influence of Substituents

Ab initio calculation at the MP2/aug-cc-pVTZ level has been performed on the π-hole based N … Si tetrel bonded complexes between substituted pyridines and H 2 SiO. The primary aim of the study is to find out the effect of substitution on the strength and nature of this tetrel bond, and its similarity/difference with the N … C tetrel bond. Correlation between the strength of the N … Si bond and several molecular properties of the Lewis acid (H 2 SiO) and base (pyridines) are explored. The properties of the tetrel bond are analyzed using AIM, NBO, and symmetry-adapted perturbation theory calculations. The complexes are characterized with short N … Si intermolecular distances and high binding energies ranging between -142.72 and -115.37 kJ/mol. The high value of deformation energy indicates significant geometrical distortion of the monomer units. The AIM and NBO analysis reveal significant coordinate covalent  bond character of the N … Si π-hole bond.  Sharp differences are also noticed in the orbital interactions present in the N … Si and N … C tetrel bonds.

Influence of Substituents in the Benzene Ring on the Halogen Bond of Iodobenzene with Ammonia

The effects on the CI··N halogen bond between iodobenzene and NH3 of placing various substituents on the phenyl ring are monitored by quantum calculations. Substituents R = N(CH3)2, NH2, CH3, OCH3, COCH3, Cl, F, COH, CN, and NO2 were each placed ortho, meta, and para to the I. The depth of the σ-hole on I is deepened as R became more electron-withdrawing which is reflected in a strengthening of the halogen bond, which varied between 3.3 and 5.5 kcal/mol. In most cases, the ortho placement yields the largest perturbation, followed by meta and then para, but this trend is not universal. Parallel to these substituent effects is a progressive lengthening of the covalent C-I bond. Formation of the halogen bond reduces the NMR chemical shielding of all three nuclei directly involved in the C-I··N interaction. The deshielding of the electron donor N is most closely correlated with the strength of the bond, as is the coupling constant between I and N, so both have potential use as spectroscopic measures of halogen bond strength.

Maximal occupation by bases of π-hole bands surrounding linear molecules

Linear molecules such as CO₂ contain a positive π-hole ring that surrounds C on the molecule's equator. Quantum calculations examine the question as to how many bases can simultaneously bind to this ring. Linear molecules examined are TO₂ , where T = C, Si, Ge, Sn; bases are NCH and NH₃ . CO₂ engages in the weakest of the tetrel bonds, and can bind up to three NCH and two NH₃ . Unlike σ-hole tetrel bonds, Si forms the strongest tetrel bonds, with interaction energies as high as 43 kcal/mol with NH₃ . But like GeO₂ , SiO₂ can sustain only two bases in its equatorial ring. The π-hole ring of SnO₂ can engage in up to four tetrel bonds with either NCH or NH₃ , even though these bonds are weaker than those with GeO₂ or SiO₂ . As all of these complexes cast TO₂ in the role of multiple electron acceptor, the resulting negative cooperativity makes each successive bond weaker than its predecessor as bases are added, as well as reducing the magnitude of the central molecule's π-hole.

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.

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.