Hydrogen Bonds Are Never of an "Anti-electrostatic" Nature: A Brief Tour of a Misleading Nomenclature.

A large amount of scientific works have contributed through the years to rigorously reflect the different forces leading to the formation of hydrogen bonds, the electrostatic and polarization ones being the most important among them. However, we have witnessed lately with the emergence of a new terminology, anti-electrostatic hydrogen bonds (AEHBs), that seems to contradict this reality. This nomenclature is used in the literature to describe hydrogen bonds between equally charged systems to justify the existence of these species, despite numerous proofs showing that AEHBs are, as any other hydrogen bond between neutral species, mostly due to electrostatic forces. In this Viewpoint, we summarize the state of the art regarding this issue, try to explain why this terminology is very misleading, and strongly recommend avoiding its use based on the hydrogen bond physical grounds.

Metastable Charged Dimers in Organometallic Species: A Look into Hydrogen Bonding between Metallocene Derivatives.

Multiply charged complexes bound by noncovalent interactions have been previously described in the literature, although they were mostly focused on organic and main group inorganic systems. In this work, we show that similar complexes can also be found for organometallic systems containing transition metals and deepen in the reasons behind the existence of these species. We have studied the structures, binding energies, and dissociation profiles in the gas phase of a series of charged hydrogen-bonded dimers of metallocene (Ru, Co, Rh, and Mn) derivatives isoelectronic with the ferrocene dimer. Our results indicate that the carboxylic acid-containing dimers are more strongly bonded and present larger barriers to dissociation than the amide ones and that the cationic complexes tend to be more stable than the anionic ones. Additionally, we describe for the first time the symmetric proton transfer that can occur while in the metastable phase. Finally, we use a density-based energy decomposition analysis to shine light on the nature of the interaction between the dimers.

Stand up for Electrostatics: The Disiloxane Case.

The basicity of the simplest silicone, disiloxane (H3 Si-O-SiH3 ), 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.

On the Use of Normalized Metrics for Density Sensitivity Analysis in DFT.

In the past years, there has been a discussion about how the errors in density functional theory might be related to errors in the self-consistent densities obtained from different density functional approximations. This, in turn, brings up the discussion about the different ways in which we can measure such errors and develop metrics that assess the sensitivity of calculated energies to changes in the density. It is important to realize that there cannot be a unique metric in order to look at this density sensitivity, simultaneously needing size-extensive and size-intensive metrics. In this study, we report two metrics that are widely applicable to any density functional approximation. We also show how they can be used to classify different chemical systems of interest with respect to their sensitivity to small variations in the density.

Energetics of non-heme iron reactivity: can ab initio calculations provide the right answer?

We use a variety of computational methods to characterize and compare the hydrogen atom transfer (HAT) and epoxidation reaction pathways for oxidation of cyclohexene by an iron(iv)-oxo complex. Previous B3LYP calculations have led to predictions that both alcohol (from the HAT route) and epoxide should be formed in similar amounts, which was not in agreement with experiment where only the HAT product was observed. We show here that ab initio calculations which can take both static and dynamic correlation into account are needed to explain the experimentally observed dominance of the HAT process. Since these systems do not have very strong multireference character we have also tested different flavours of local coupled cluster methods. We suggest that further improvements are necessary before they can provide highly accurate results for these systems.

Ab Initio Calculations for Spin-Gaps of Non-Heme Iron Complexes.

We employed our recently proposed multireference approach CASPT2/CC to calculate the quintet-triplet gaps ΔETQ of a series of non-heme FeIV═O species and subsequently used these results to benchmark density functional theory (DFT) as well as two variants of local coupled-cluster approaches (DLPNO-CCSD(T) and LUCCSD(T0)). We showed that current implementations of the local coupled-cluster method are not sufficiently accurate. DLPNO-CCSD(T) systematically overstabilizes the quintet state, whereas LUCCSD(T0) overestimates the triplet one. This sort of systematic bias may be helpful in improving local correlation methods and can also be used as the basis for a simple correction scheme.

Hydrogen-Bonding Acceptor Character of Be3, the Beryllium Three-Membered Ring.

The ability of Be3 as a hydrogen bond acceptor has been explored by studying the potential complexes between this molecule and a set of hydrogen bond donors (HF, HCl, HNC, HCN, H2O, and HCCH). The electronic structure calculations for these complexes were carried out at the MP2 and CCSD(T) computational levels together with an extensive NBO, ELF, AIM, and electrostatic potential characterization of the isolated Be3 system. In all the complexes, the Be-Be σ bond acts as electron donor, with binding energies between 19 and 6 kJ mol-1. A comparison with the analogous cyclopropane:HX complexes shows similar binding energies and contributions of the DFT-SAPT energetic terms. A blue-shift of the harmonic frequencies of Be3 is observed upon complexation.

Large Proton-Affinity Enhancements Triggered by Noncovalent Interactions.

High affinity: The proton affinity (PA) of the OH group of YHx OH compounds is always increased by noncovalent interaction (NCI) with a Lewis base (LB; see figure). This PA enhancement depends on the type of NCI. Weak NCIs can give rise to PA enhancements equal to or even larger than strong NCIs. The binding energies of protonated species play a major role in the case of sigma-hole interactions.

Modulating the Proton Affinity of Silanol and Siloxane Derivatives by Tetrel Bonds.

The proton affinity (PA) on the oxygen atom in silanol and siloxane derivatives is enhanced by the formation of tetrel bonds with small Lewis bases [B···R3SiOH, B···R3SiOSiR3, B···R3SiOSiR3···B; B = H2O, CO, NH3, HCN, H2S; R = H, Me], as shown by MP2/jul-cc-pVTZ calculations. The complexed systems become more basic than ether and other carbon-related compounds, and even more basic than pyridine in some specific cases, reaching values up to 959.4 kJ/mol (H3N···H3SiOSiH3···NH3 complex). Changes on PAs are directly related to very large binding energies for the protonated species. Topological methods and the natural bond orbital scheme are used to rationalize the observed trends. The PA enhancement should be taken into account when dealing with silanols and siloxanes in different environments.