Asymmetry of the “Strongest” OHO Hydrogen Bond, in the Monoanion of (±)-α,α′-Di-tert-butylsuccinate

Symmetry of Hydrogen Bonds: Application of NMR Method of Isotopic Perturbation and Relevance of Solvatomers

Short, strong, symmetric, low-barrier hydrogen bonds (H-bonds) are thought to be of special significance. We have been searching for symmetric H-bonds by using the NMR technique of isotopic perturbation. Various dicarboxylate monoanions, aldehyde enols, diamines, enamines, acid-base complexes, and two sterically encumbered enols have been investigated. Among all of these, we have found only one example of a symmetric H-bond, in nitromalonamide enol, and all of the others are equilibrating mixtures of tautomers. The nearly universal lack of symmetry is attributed to the presence of these H-bonded species as a mixture of solvatomers, meaning isomers (or stereoisomers or tautomers) that differ in their solvation environment. The disorder of solvation renders the two donor atoms instantaneously inequivalent, whereupon the hydrogen attaches to the less well solvated donor. We therefore conclude that there is no special significance to short, strong, symmetric, low-barrier H-bonds. Moreover, they have no heightened stability or else they would have been more prevalent.

Large transition state stabilization from a weak hydrogen bond

A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H⋯O Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol⁻¹) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar O–H⋯OC hydrogen bond (1.5 kcal mol⁻¹). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms.

A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond.

The Structure of the "Vibration Hole" around an Isotopic Substitution-Implications for the Calculation of Nuclear Magnetic Resonance (NMR) Isotopic Shifts

Calculations of nuclear magnetic resonance (NMR) isotopic shifts often rest on the unverified assumption that the “vibration hole”, that is, the change of the vibration motif upon an isotopic substitution, is strongly localized around the substitution site. Using our recently developed difference-dedicated (DD) second-order vibrational perturbation theory (VPT2) method, we test this assumption for a variety of molecules. The vibration hole turns out to be well localized in many cases but not in the interesting case where the H/D substitution site is involved in an intra-molecular hydrogen bond. For a series of salicylaldehyde derivatives recently studied by Hansen and co-workers (Molecules2019, 24, 4533), the vibrational hole was found to stretch over the whole hydrogen-bond moiety, including the bonds to the neighbouring C atoms, and to be sensitive to substituent effects. We discuss consequences of this finding for the accurate calculation of NMR isotopic shifts and point out directions for the further improvement of our DD-VPT2 method.

From zero- to three-dimensional heterobimetallic coordination polymers with the [Pt{SSC-N(CH2COO)2}2]4− metalloligand

A series of novel, structurally manifold heterobimetallic dithiocabamato-carboxylates (DTCCs) were efficiently prepared by a metalloligand approach.

Efficient calculation of NMR isotopic shifts: Difference-dedicated vibrational perturbation theory

We present difference-dedicated second-order vibrational perturbation theory (VPT2) as an efficient method for the computation of nuclear magnetic resonance (NMR) isotopic shifts, which reflect the geometry dependence of the NMR property in combination with different vibration patterns of two isotopologues. Conventional calculations of isotopic shifts, e.g., by standard VPT2, require scanning the geometry dependence over the whole molecule, which becomes expensive rapidly as the molecule size increases. In DD-VPT2, this scan can be restricted to a small region around the substitution site. At the heart of DD-VPT2 is a set of localized vibration modes common to the two isotopologues and designed such that the difference between the vibration patterns is caught by a small subset of them (usually fewer than 10). We tested the DD-VPT2 method for a series of molecules with increasing size and found that this method provides results with the same quality as VPT2 and in good agreement with the experiment, with computational savings up to 95% and less numerical instabilities. The method is easy to automatize and straightforward to generalize to other molecular properties.

Isotopic-Perturbation NMR Study of Hydrogen-Bond Symmetry in Solution: Temperature Dependence and Comparison of OHO and ODO Hydrogen Bonds

Is a hydrogen bond symmetric, with the hydrogen centered between two donor atoms, or is it asymmetric, with the hydrogen closer to one but jumping to the other? The NMR method of isotopic perturbation has been used to distinguish these. Previous evidence from isotope shifts implies that a wide variety of dicarboxylate monanions are asymmetric, present as a rapidly equilibrating mixture of tautomers. However, calculations of hydrogen trajectories across an anharmonic potential-energy surface could reproduce the observed isotope shifts in a phthalate monoanion. Therefore, it was concluded that those isotope shifts are instead consistent with isotope-induced desymmetrization on a symmetric potential-energy surface. To distinguish between these two interpretations, the ¹⁸O-induced isotope effects on the ¹³C NMR chemical shifts of cyclohexene-1,2-dicarboxylate monoanion in chloroform-d and on the ¹⁹F NMR chemical shifts of difluoromaleate monoanion in D₂O have been investigated. In both cases the isotope effects are larger at lower temperature and also with deuterium in the hydrogen bond. It is concluded that these behaviors are consistent with the perturbation of an equilibrium between asymmetric tautomers and inconsistent with isotope-induced desymmetrization on a symmetric potential-energy surface.

Double salt ionic liquids based on 1-ethyl-3-methylimidazolium acetate and hydroxyl-functionalized ammonium acetates: strong effects of weak interactions

The weaker N–H⋯O interactions between hydroxyl-functionalized ammonium acetates are more important than the stronger O–H⋯O interactions in determining solubility in the ionic liquid 1-ethyl-3-methylimidazolium acetate.

Isotope effects on chemical shifts in the study of intramolecular hydrogen bonds

The paper deals with the use of isotope effects on chemical shifts in characterizing intramolecular hydrogen bonds. Both so-called resonance-assisted (RAHB) and non-RAHB systems are treated. The importance of RAHB will be discussed. Another very important issue is the borderline between “static” and tautomeric systems. Isotope effects on chemical shifts are particularly useful in such studies. All kinds of intramolecular hydrogen bonded systems will be treated, typical hydrogen bond donors: OH, NH, SH and NH⁺, typical acceptors C=O, C=N, C=S C=N⁻. The paper will be deal with both secondary and primary isotope effects on chemical shifts. These two types of isotope effects monitor the same hydrogen bond, but from different angles.

Measurement of proton tunneling in short hydrogen bonds in single crystals of 3,5 pyridinedicarboxylic acid using nuclear magnetic resonance spectroscopy

In this Letter, we present NMR spin-lattice and relaxometry data for proton transfer in one of the shortest known N-H⋯O hydrogen bonds in a single crystal of 3,5 pyridinedicarboxylic acid (35PDCA). It is widely believed that proton transfer by quantum tunneling does not occur in short hydrogen bonds since the ground state energy level lies above the potential barrier, yet these data show a temperature independent, proton tunneling rate below 77 K and a clear deviation from classical dynamics below 91 K. This study therefore suggests that proton tunneling occurs in all hydrogen bonds at low temperature and the crossover temperature to classical hopping must be determined when evaluating whether proton tunneling persists at higher temperature, for example in enzyme catalysis under physiological conditions.

Variable-temperature study of hydrogen-bond symmetry in cyclohexene-1,2-dicarboxylate monoanion in chloroform-d

The symmetry of the hydrogen bond in hydrogen cyclohexene-1,2-dicarboxylate monoanion was determined in chloroform using the NMR method of isotopic perturbation. As the temperature decreases, the (18)O-induced (13)C chemical-shift separations increase not only at carboxyl carbons but also at ipso (alkene) carbons. The magnitude of the ipso increase is consistent with an (18)O isotope effect on carboxylic acid acidity. Therefore it is concluded that this monoanion is a mixture of tautomers in rapid equilibrium, rather than a single symmetric structure in which a chemical-shift separation arises from coupling between a desymmetrizing vibration and anharmonic isotope-dependent vibrations, which is expected to show the opposite temperature dependence.

Hydrogen-bond symmetry in difluoromaleate monoanion

The symmetry of the hydrogen bond in hydrogen difluoromaleate monoanion is probed by X-ray crystallography and by the NMR method of isotopic perturbation in water, in two aprotic organic solvents, and in an isotropic liquid crystal. The X-ray crystal structure of potassium hydrogen difluoromaleate shows a remarkably short O-O distance of 2.41 Å and equal O-H distances of 1.206 Å, consistent with a strong and symmetric hydrogen bond. Incorporation of (18)O into one carboxyl group allows investigation of the symmetry of the H-bond in solution by the method of isotopic perturbation. The (19)F NMR spectra of the mono-(18)O-substituted monoanion in water, CD(2)Cl(2), and CD(3)CN show an AB spin system, corresponding to fluorines in different environments. The difference is attributed to the perturbation of the acidity of a carboxylic acid by (18)O, not to the mere presence of the (18)O, because the mono-(18)O dianion shows equivalent fluorines. Therefore, it is concluded that the monoanion exists as an equilibrating pair of interconverting tautomers and not as a single symmetric structure not only in water but also in organic solvents. However, in the isotropic liquid crystal phase of 4-cyanophenyl 4-heptylbenzoate, tetrabutylammonium hydrogen difluoromaleate-(18)O shows equivalent fluorines, consistent with a single symmetric structure. These results support earlier studies, which suggested that the symmetry of hydrogen bonds can be determined by the local environment.

On the Intramolecular Hydrogen Bond in Solution: Car-Parrinello and Path Integral Molecular Dynamics Perspective

The issue of the symmetry of short, low-barrier hydrogen bonds in solution is addressed here with advanced ab initio simulations of a hydrogen maleate anion in different environments, starting with the isolated anion, going through two crystal structures (sodium and potassium salts), then to an aqueous solution, and finally in the presence of counterions. By Car-Parrinello and path integral molecular dynamics simulations, it is demonstrated that the position of the proton in the intramolecular hydrogen bond of an aqueous hydrogen maleate anion is entirely related to the solvation pattern around the oxygen atoms of the intramolecular hydrogen bond. In particular, this anion has an asymmetric hydrogen bond, with the proton always located on the oxygen atom that is less solvated, owing to the instantaneous solvation environment. Simulations of water solutions of hydrogen maleate ion with two different counterions, K(+) and Na(+), surprisingly show that the intramolecular hydrogen-bond potential in the case of the Na(+) salt is always asymmetric, regardless of the hydrogen bonds to water, whereas for the K(+) salt, the potential for H motion depends on the location of the K(+). It is proposed that repulsion by the larger and more hydrated K(+) is weaker than that by Na(+) and competitive with solvation by water.

NMR study of conformational exchange and double-well proton potential in intramolecular hydrogen bonds in monoanions of succinic acid and derivatives

We present a (1)H, (2)H, and (13)C NMR study of the monoanions of succinic (1), meso- and rac-dimethylsuccinic (2, 3), and methylsuccinic (4) acids (with tetraalkylammonium as the counterion) dissolved in CDF(3)/CDF(2)Cl at 300-120 K. In all four monoanions, the carboxylic groups are linked by a short intramolecular OHO hydrogen bond revealed by the bridging-proton chemical shift of about 20 ppm. We show that the flexibility of the carbon skeleton allows for two gauche isomers in monoanions 1, 2, and 4, interconverting through experimental energy barriers of 10-15 kcal/mol (the process itself and the energy barrier are also reproduced in MP2/6-311++G** calculations). In 3, one of the gauche forms is absent because of the steric repulsion of the methyl groups. In all four monoanions, the bridging proton is located in a double-well potential and subject, at least to some extent, to proton tautomerism, for which we estimate the two proton positions to be separated by ca. 0.2 Å. In 1 and 3, the proton potential is symmetric. In 2, slowing the conformational interconversion introduces an asymmetry to the proton potential, an effect that might be strong enough even to synchronize the proton tautomerism with the interconversion of the two gauche forms. In 4, the asymmetry of the proton potential is due to the asymmetric substitution. The intramolecular H-bond is likely to remain intact during the interconversion of the gauche forms in 1, 3, and 4, whereas the situation in 2 is less clear.

Adsorption of fatty acids on iron (hydr)oxides from aqueous solutions

The interaction of iron (hydr)oxides with fatty acids is related to many industrial and natural processes. To resolve current controversies about the adsorption configurations of fatty acids and the conditions of the maximum hydrophobicity of the minerals, we perform a detailed study of the adsorption of sodium laurate (dodecanoate) on 150 nm hematite (α-Fe(2)O(3)) particles as a model system. The methods used include in situ FTIR spectroscopy, ex situ X-ray photoelectron spectroscopy (XPS), measurements of the adsorption isotherm and contact angle, as well as the density functional theory (DFT) calculations. We found that the laurate adlayer is present as a mixture of inner-sphere monodentate mononuclear (ISMM) and outer-sphere (OS) hydration shared complexes independent of the solution pH. Protonation of the OS complexes does not influence the conformational order of the surfactant tails. One monolayer, which is filled through the growth of domains and is reached at the micellization/precipitation edge of laurate, makes the particles superhydrophobic. These results contradict previous models of the fatty acid adsorption and suggest new interpretation of literature data. Finally, we discovered that the fractions of both the OS laurate and its molecular form increase in D(2)O, which can be used for interpreting complex spectra. We discuss shortcomings of vibrational spectroscopy in determining the interfacial coordination of carboxylate groups. This work advances the current understanding of the oxide-carboxylate interactions and the research toward improving performance of fatty acids as surfactants, dispersants, lubricants, and anticorrosion reagents.

Are short, low-barrier hydrogen bonds unusually strong?

In a symmetric hydrogen bond (H-bond), the hydrogen atom is perfectly centered between the two donor atoms. The energy diagram for hydrogen motion is thus a single-well potential, rather than the double-well potential of a more typical H-bond, in which the hydrogen is covalently bonded to one atom and H-bonded to the other. Examples of symmetric H-bonds are often found in crystal structures, and they exhibit the distinctive feature of unusually short length: for example, the O-O distance in symmetric OHO H-bonds is found to be less than 2.5 Å. In comparison, the O-O distance in a typical asymmetric H-bond, such as ROH···OR(2), ranges from about 2.7 to 3.0 Å. In this Account, we briefly review and update our use of the method of isotopic perturbation to search for a symmetric, centered, or single-well-potential H-bond in solution. Such low-barrier H-bonds are thought to be unusually strong, owing perhaps to the resonance stabilization of two identical resonance forms [A-H···B ↔ A···H-B]. This presumptive bond strength has been invoked to explain some enzyme-catalyzed reactions. Yet in solution, a wide variety of OHO, OHN, and NHN H-bonds have all been found to be asymmetric, in double-well potentials. Examples include the monoanion of (±)-2,3-di-tert-butylsuccinic acid and a protonated tetramethylnaphthalenediamine, even though these two ions are often considered prototypes of species with strong H-bonds. In fact, all of the purported examples of strong, symmetric H-bonds have been found to exist in solution as pairs of asymmetric tautomers, in contrast to their symmetry in some crystals. The asymmetry can be attributed to the disorder of the local solvation environment, which leads to an equilibrium among solvatomers (that is, isomers that differ in solvation). If the disorder of the local environment is sufficient to break symmetry, then symmetry itself is not sufficient to stabilize the H-bond, and symmetric H-bonds do not have an enhanced stability or an unusual strength. Nor are short H-bonds unusually strong. We discuss previous evidence for "short, strong, low-barrier" H-bonds and show it to be based on ambiguous comparisons. The role of such H-bonds in enzyme-catalyzed reactions is then ascribed not to any unusual strength of the H-bond itself but to relief of "strain."