Simultaneous Hydrogen Bonds with Different Binding Modes: The Acceptor "Rules" but the Donor "Chooses"

This computational work studies the different hydrogen bond (HB) binding modes that can be established between neighbouring HB donors and acceptors in structures with relevance in catalysis and biology. To analyse the electronic effect on the σ-hole, unsubstituted HB donors and ones with two different substituents, an electron withdrawing (EWG), and an electron donating (EDG) group, were studied. Upon complexation, three different binding modes were observed: bifurcated, parallel, and zigzag. It was found that, as a general trend, HBs within a parallel pattern are the strongest followed by those within bifurcated and zigzag binding modes, leading to a "competition" between the last two. Similar patterns and trends have been found in experimental structures found in a search within the CSD. In conclusion, even though the HB acceptors "rule" the pattern and strength of the HB interactions within the dimers, when there is an option for different binding modes within a particular dimer, the HB donors "choose" the type of binding established.

Stand up for Electrostatics: The Disiloxane Case

The basicity of the simplest silicone, disiloxane (H 3 Si-O-SiH 3 ), is strongly affected by the Si-O-Si angle (α). We use high-level ab initio MP2/aug'-cc-pVTZ calculations and the molecular electrostatic potential (MEP) to analyze the relationship between the increase in basicity and the reduction of α. Our results clearly point out that this increase can be explained through the MEP, as the interactions between oxygen from disiloxane and the acceptors are mostly electrostatic. Furthermore, the effect of α on the tetrel bond between disiloxane and several Lewis bases can again be rationalized using the MEP. Finally, we explore the cooperativity throughout α for ternary complexes where disiloxane simultaneously interacts with a Lewis acid and a Lewis base. Both non-covalent interactions remain cooperative for all α values, although the largest cooperativity effects are not always those maximizing the binding energy in the binary complexes. Overall, the MEP remains a powerful predictor for noncovalent interactions.

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.

Significant bonding rearrangements triggered by Mg4 clusters

The structure, stability, and bonding of the complexes formed by the interaction of Mg₄ clusters and first row Lewis bases, namely, ammonia, water, and hydrogen fluoride, have been investigated through the use of high-level G4 single-reference and CASPT2 multireference formalisms. The adducts formed reflect the high electrophilicity of the Mg₄ cluster through electron density holes in the neighborhood of each metallic center. After the adduct formation, the metallic bonding of the Mg₄ moiety is not significantly altered so that the hydrogen shifts from the Lewis base toward the Mg atoms lead to new local minima with enhanced stability. For the particular case of ammonia and water, the global minima obtained when all the hydrogens of the Lewis base are shifted to the Mg₄ moiety have in common a very stable scaffold with a N or an O center covalently tetracoordinated to the four Mg atoms, so the initial bonding arrangements of both reactants have completely disappeared. The reactivity features exhibited by these Mg₄ clusters suggest that nanostructures of this metal might have an interesting catalytic behavior.

Molecular designs for expanding the limits of ultralong C-C bonds and ultrashort HH non-bonded contacts

Recent experiments have reported the formation of very long C-C bonds (d_C-C > 1.80 Å) and very short HH non-bonded contacts (d_HH < 1.5 Å) in several sets of molecules. Both these rare phenomena arise due to specific donor-acceptor interactions and London dispersion interactions respectively. Favorable negative hyperconjugation, namely H₂N(lone-pair) →σ*(C-C), creates an ultralong C-C bond in diamino-o-carborane with d_C-C > 1.829 Å and a planar amine reminiscent of a transition-state like structure for ammonia inversion. The small and narrow barrier favours rapid inversion through quantum mechanical tunnelling (QMT) and produces a translationally averaged planar amine as observed in the experiments. On the other hand, designing specific confined molecular cavities or chambers like in,in-bis(hydrosilane) or its germanane analogs furnishes an ultrashort HH distance = 1.47 Å and 1.38 Å respectively. The predisposition of such closely placed HH contacts arises from the rather effective attractive dispersion interactions between them. Controlling the strength of the dispersion interactions provides a rich landscape for realizing such close HH distances. Molecular design ably assisted by computational modeling to further tune these interactions provides new avenues to break the glass-ceilings of ultralong C-C bonds or ultrashort HH contacts. Dispersion-corrected DFT calculations and ab initio molecular dynamics simulations generate a large library of such unique features in a diverse class of molecules. This feature article highlights the design principles to realize hitherto longest C-C bonds/shortest HH contacts.

Beryllium bonding: insights from the σ- and π-hole analysis

Beryllium bonding is actually a subclass of secondary bonding. Similar to the case of halogen bonding, the σ- and π-holes on the Be atom of the monomers give in zeroth approximation the direction of electrophilic attack favorable to the formation of beryllium bonds. The nature of beryllium bonding is purely electrostatic so that the symmetry-adapted perturbation theory energy decomposition perfectly explains the relevance of the polarization and dispersion contribution on the formation of the beryllium bond.

Weak Interactions Get Strong: Synergy between Tetrel and Alkaline-Earth Bonds

Weak and strong noncovalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH₃F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug'-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies, ²⁹Si NMR chemical shifts, and topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the ²⁹Si chemical shifts in all the ternary complexes. At the same time the ternary complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

Tetrel bonding interaction: an analysis with the block-localized wavefunction (BLW) approach

In this study, fifty-one iconic tetrel bonding complexes were studied using the block localized wave function (BLW) method which can derive the self-consistent wavefunction for an electron-localized (diabatic) state where charge transfer is strictly deactivated.