Strong Short-Range Cooperativity in Hydrogen-Bond Chains

Dissecting Solvent Effects on Hydrogen Bonding

The experimental isolation of H-bond energetics from the typically dominant influence of the solvent remains challenging. Here we use synthetic molecular balances to quantify amine/amide H-bonds in competitive solvents. Over 200 conformational free energy differences were determined using 24 H-bonding balances in 9 solvents spanning a wide polarity range. The correlations between experimental interaction energies and gas-phase computed energies exhibited wild solvent-dependent variation. However, excellent correlations were found between the same computed energies and the experimental data following empirical dissection of solvent effects using Hunter's α/β solvation model. In addition to facilitating the direct comparison of experimental and computational data, changes in the fitted donor and acceptor constants reveal the energetics of secondary local interactions such as competing H-bonds.

Energetic Ordering of Hydrogen Bond Strengths in Methanol-Water Clusters: Insights via Molecular Tailoring Approach

In this work, we examine the strength of various types of individual hydrogen bond (HB) in mixed methanol-water M_n W_m , (n+m=2 to 7) clusters, with an aim to understand the relative order of their strength, using our recently proposed molecular tailoring-based approach (MTA). Among all the types of HB, it is observed that the O_M -H…O_W HBs are the strongest (6.9 to 12.4 kcal mol^-1 ). The next ones are O_M -H…O_M HBs (6.5 to 11.6 kcal mol^-1 ). The O_W -H…O_W (0.2 to 10.9 kcal mol^-1 ) and O_W -H…O_M HBs (0.3 to 10.3 kcal mol^-1 ) are the weakest ones. This energetic ordering of HBs is seen to be different from the respective HB energies in the dimer i. e., O_M -H…O_M (5.0 to 6.0 kcal mol^-1 )>O_W -H…O_M (1.5 to 6.0 kcal mol^-1 )>O_M -H…O_W (3.8 to 5.6 kcal mol^-1 )>O_W -H…O_W (1.2 to 5.0 kcal mol^-1 ). The plausible reason for the difference in the HB energy ordering may be attributed to the increase or decrease in HB strengths due to the formation of cooperative or anti-cooperative HB networks. For instance, the cooperativity contribution towards the different types of HB follows: O_M -H…O_W (2.4 to 8.6 kcal mol^-1 )>O_M -H…O_M (1.3 to 6.3 kcal mol^-1 )>O_W -H…O_W (-1.0 to 6.5 kcal mol^-1 )>O_W -H…O_M (-1.2 to 5.3 kcal mol^-1 ). This ordering of cooperativity contribution is similar to the HB energy ordering obtained by the MTA-based method. It is emphasized here that, the interplay between the cooperative and anti-cooperative contributions are indispensable for the correct energetic ordering of these HBs.

Promotion of TH3 (T = Si and Ge) group transfer within a tetrel bond by a cation-π interaction

The possibility of the transfer of the TH₃ group across a tetrel bond is considered by ab initio calculations. The TB is constructed by pairing PhTH₃ (Ph = phenyl; T = Si and Ge) with bases NH₃, NHCH₂, and the C₃N₂H₄ carbene. The TH₃ moves toward the base but only by a small amount in these dimers. However, when a Be²⁺ or Mg²⁺ dication is placed above the phenyl ring, the tetrel bond strength is greatly magnified reaching up to nearly 100 kcal mol^-1. This dication also induces a much higher degree of transfer which can be best categorized as half-transfer for the two N-bases and a near complete transfer for the carbene.

Activated Self-Resolution and Error-Correction in Catalytic Reaction Networks*

Understanding the emergence of function in complex reaction networks is a primary goal of systems chemistry and origin-of-life studies. Especially challenging is to create systems that simultaneously exhibit several emergent functions that can be independently tuned. In this work, a multifunctional complex reaction network of nucleophilic small molecule catalysts for the Morita-Baylis-Hillman (MBH) reaction is demonstrated. The dynamic system exhibited triggered self-resolution, preferentially amplifying a specific catalyst/product set out of a many potential alternatives. By utilizing selective reversibility of the products of the reaction set, systemic thermodynamically driven error-correction could also be introduced. To achieve this, a dynamic covalent MBH reaction based on adducts with internal H-transfer capabilities was developed. By careful tuning of the substituents, rate accelerations of retro-MBH reactions of up to four orders of magnitude could be obtained. This study thus demonstrates how efficient self-sorting of catalytic systems can be achieved through an interplay of several complex emergent functionalities.

Quantifying Through-Space Substituent Effects

The description of substituents as electron donating or withdrawing leads to a perceived dominance of through-bond influences. The situation is compounded by the challenge of separating through-bond and through-space contributions. Here, we probe the experimental significance of through-space substituent effects in molecular interactions and reaction kinetics. Conformational equilibrium constants were transposed onto the Hammett substituent constant scale revealing dominant through-space substituent effects that cannot be described in classic terms. For example, NO2 groups positioned over a biaryl bond exhibited similar influences as resonant electron donors. Meanwhile, the electro-enhancing influence of OMe/OH groups could be switched off or inverted by conformational twisting. 267 conformational equilibrium constants measured across eleven solvents were found to be better predictors of reaction kinetics than calculated electrostatic potentials, suggesting utility in other contexts and for benchmarking theoretical solvation models.

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance-dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short-range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O⋅⋅⋅C=O interactions.