On the Nature of Blueshifting Hydrogen Bonds

The block‐localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F₃CH with NH₃ and OH₂ shows that blueshifting is a long‐range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long‐range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short‐range n(Y)→σ*(X?H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F₃CH ??? NH₃ and blueshifting in F₃CH ??? OH₂. This mechanism works well in the series F_nCl_3?nCH ??? Y (n=0–3, Y=NH₃, OH₂, SH₂) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O?H bond of water even with the electron transfer between benzene and water completely quenched. A distance‐dependent analysis for this system reveals that the long‐range electrostatic interaction is again responsible for the initial lengthening and redshifting.     

Controlling the Subtle Energy Balance in Protic Ionic Liquids: Dispersion Forces Compete with Hydrogen Bonds

The properties of ionic liquids are determined by the energy‐balance between Coulomb‐interaction, hydrogen‐bonding, and dispersion forces. Out of a set of protic ionic liquids (PILs), including trialkylammonium cations and methylsulfonate and triflate anions we could detect the transfer from hydrogen‐bonding to dispersion‐dominated interaction between cation and anion in the PIL [(C₆H₁₃)₃NH][CF₃SO₃]. The characteristic vibrational features for both ion‐pair species can be detected and assigned in the far‐infrared spectra. Our approach gives direct access to the relative strength of hydrogen‐bonding and dispersion forces in a Coulomb‐dominated system. Dispersion‐corrected density functional theory (DFT) calculations support the experimental findings. The dispersion forces could be quantified to contribute about 2.3 kJ mol^?1 per additional methylene group in the alkyl chains of the ammonium cation.     

SiH4与HX(X=F,Cl,Br,I)形成的二氢键复合物的结构特征及本质

本文就SiH4与HX形成的二氢键复合物的结构特征及本质进行了探讨。在MP2/6-311++G(3d,3p)水平优化、频率验证得到复合物的分子结构,通过分子间距离及电子密度等值线图,我们确认SiH4与卤化氢已形成了二氢键复合物。MP2/6-311++G(3d,3p)水平下进行BSSE校正后的结合能为2.703－4.439 KJ/mol。用对称匹配微绕理论(SAPT)对结合能进行分解,分解结果显示,SiH4匟X(X=F,Cl,Br,I)二氢键复合物中静电能对总吸引能的贡献小于28％,并且相对稳定,这就是说SiH4匟X二氢键复合物的本质并非静电作用,而是静电能、诱导能、色散能、交换能对总结合能的贡献都非常重要。     

Bimetallic Cooperativity and Hydrogen Bonding Allow Efficient Reduction of CO2

Reducing CO₂ selectively to one of the several C1 products is challenging, as the thermodynamic reduction potentials for the different n e⁻/n H⁺ reductions of CO₂ are similar and so is the reduction potential for H⁺ reduction. Recently, Halime, Aukauloo, and co-workers have taken inspiration from the active site of nickel CO dehydrogenase (Ni-CODH) to design bimetallic iron porphyrins bridged by a urea moiety. These complexes show fast and selective reduction of CO₂ to CO and the results suggest a Ni-CODH type mechanism at play where one of the two metals binds and reduces the CO₂ while the other stabilizes the reduced species by forming a bridged complex, facilitating the C−O bond cleavage.     

Properties of Sulfur-Sulfur Bonds

SS bonds are extraordinarily flexible and have properties that are observed only on isolated occasions for other homonuclear bonds: the bond lengths very between 1.8 and 3.0Å, the bond angles between 90 and 180° and the dihedral angles between 0 and 180°; the bond energies amount to up to 430 kJ/mol. The SS stretching frequencies can appear over the range 177–820 cm^?1 and force constants of 1.4 to 6.3 mdyne/Å have been calculated. This variability is illustrated with examples containing isolated and cumulated SS bonds.     

Covalent versus Dative Bonds to Main Group Metals,a Useful Distinction

Textbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine-boranes, X₃N → BY₃, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H 3 N → BH₃ and the covalent bond in H₃C?CH₃ are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp³ type atomic orbitals on the bonded atoms; the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physiocochemical properties of covalent and dative bonds may be – and often are – quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.     

Oxidative addition to main group versus transition metals: Insights from the Activation Strain model

We have studied the oxidative addition of the methane C-H and chloromethane C-Cl bonds to a number of main group (Be, Mg and Ca) and transition metals (Pd, Zn and Cd), using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. The purpose is to better understand what causes the characteristic differences in reactivity between main group and transition metals towards oxidative addition. Thus, we have analyzed our model reactions using the Activation Strain model in which the activation energy ΔE^≠ is decomposed into the activation strain of and the stabilizing TS interaction between the reactants in the activated complex: . Activation of the C-H bond goes with higher barriers than activation of the C-Cl bond because the higher bond strength of the former translates into a higher activation strain . The barriers for bond activation increase along Pd < Be, Ca < Mg < Zn, Cd. This can be straightforwardly understood through the TS-interaction , that is, in terms of the bonding capabilities of the metals. Pd yields the lowest barriers because it achieves the most stabilizing . This is the result of the small HOMO-LUMO gap between its occupied 4d and unfilled 5s AOs, which makes Pd both a good electron donor and acceptor. Zn and Cd yield the highest barriers because the large HOMO-LUMO gap between the occupied valence ns and unfilled valence np AOs makes them both poor donors and poor acceptors of electronic charge.     

Theoretical Investigation of Normal to Strong Hydrogen Bonds

We review our theoretical work done on a variety of different chemical systems, which show different H-bonding characteristics. The systems include water clusters, its interactions with polar molecules and π-systems, organic nanotubes, enzymes, and ionophores/receptors. Special features of normal, short, short strong, and π-type H-bonding interactions in these systems are discussed in terms of structures, interaction energies, and spectra.     

Charge-Assisted Hydrogen Bonds and Halogen-Halogen Interactions in Organic Salts: Benzylammonium Benzoates and Pentafluorobenzoates

The enhanced influence of charge-assisted hydrogen bonds upon the resulting crystal structure in the presence of competing noncovalent interactions has been examined using single-crystal X-ray diffraction. The crystal structures of six organic salts are presented: 4-chlorobenzylammonium 2,4-dichlorobenzoate, 3-chlorobenzylammonium 2,4-dichlorobenzoate, 4-methylbenzylammonium pentafluorobenzoate, 4-chlorobenzylammonium pentafluorobenzoate, 3-methylbenzylammonium benzoate, and 4-chlorobenzylammonium benzoate monohydrate. All structures are dominated by charge-assisted hydrogen bonds that generate either one-dimensional ribbons or two-dimensional sheets. The structural influence of weaker intermolecular interactions is limited as there are very few short, halogen-halogen, -, or distances in this series of compounds.     

Strong intramolecular hydrogen bonds. Part I. Vibrational frequencies of the OH group in some picolinic acid N-oxides predicted from DFT calculated potentials and located in the infrared spectra

Hydrogen bonding in picolinic acid N-oxide (I), its 4-nitro (III), 4-methoxy (IV), 4-amino (V) derivatives and in quinaldic acid N-oxide (II) was characterized by calculations (B3LYP/6-31G(d)) of metric parameters, H-bond energies and one-dimensional proton potential functions with vibrational energy levels. Solvent effects were estimated by the SCRF PCM method of Tomasi and coworkers (J. Tomasi, M. Persico, Chem. Rev. 94 (1994) 2027). The potential functions are strongly asymmetric with the energy minimum placed near the carboxylic oxygen. The inflection near the NO oxygen develops into a second, shallower minimum under the SCRF.

Empirical assignments of the OH stretching and bending modes were made for (I)–(IV). The stretchings of (I, II) and (IV) in various solvents are observed in the region 1600–1300 cm⁻¹, but near 2600 cm⁻¹ for (III). The calculated and observed frequencies are in fairly good agreement with theoretical predictions reflecting the electronic effects of the substituents upon the H-bond strength. The observed trends in the solvent effects upon various parameters characterizing the H-bonding also correspond to predictions.     

Hydrogen bonding networks in gabapentin protic pharmaceutical salts: NMR and in silico studies

Hydrogen bonds (HBs) play a key role in the supramolecular arrangement of crystalline solids and, although they have been extensively studied, the influence of their strength and geometry on crystal packing remains poorly understood. Here we describe the crystal structures of two novel protic gabapentin (GBP) pharmaceutical salts prepared with the coformers methanesulfonic acid (GBP:METHA) and ethanesulfonic acid (GBP:ETHA). This study encompasses experimental and computational electronic structure analyses of ¹H NMR chemical shifts (CSs), upon in silico HB cleavage. GBP:METHA and GBP:ETHA crystal packing comprise two main structural domains: an ionic layer (characterized by the presence of charge-assisted ⁺NH_GBP⋯ O⁻_METHA/ETHA HB interactions) and a neutral layer generated in a different way for each salt, mainly due to the presence of bifurcated HB interactions. A comprehensive study of HB networks is presented for GBP:METHA, by isolating molecular fragments involved in distinct HB types (NH⋯ O, OH⋯ O, and CH⋯ O) obtained from in silico disassembling of an optimized three-dimensional packing structure. Formation of HB leads to calculated ¹H NMR CS changes from 0.4 to ~5.8 ppm. This study further attempts to assess how ¹H NMR CS of protons engaged in certain HB are affected when other nearby HB, involving bifurcated or geminal/vicinal hydrogen atoms, are removed.     

Strength of Intramolecular Hydrogen Bonds

The concept of resonance-assisted hydrogen bonds(RAHBs)highlights the synergistic interplay between theπ-resonance and hydrogen bonding interactions.This concept has been well-accepted in academia and is widely used in practice.However,it has been argued that the seemingly enhanced intramolecular hydrogen bonding(IMHB)in unsaturated compounds may simply be a result of the constraints imposed by theσ-skeleton framework.Thus,it is crucial to estimate the strength of IMHBs.In this work,we used two approaches to probe the resonance effect and estimate the strength of the IMHBs in the two exemplary cases of the enol forms of acetylacetone and o-hydroxyacetophenone.One approach is the block-localized wavefunction(BLW)method,which is a variant of the ab initio valence bond(VB)theory.Using this approach,it is possible to derive the geometries and energetics with resonance shut down.The other approach is Edmiston’s truncated localized molecular orbital(TLMO)technique,which monitors the energy changes by removing the delocalization tails from localized molecular orbitals.The integrated BLW and TLMO studies confirmed that the hydrogen bonding in these two molecules is indeed enhanced byπ-resonance,and that this enhancement is not a result ofσconstraints.     

Exploring Orthogonality between Halogen and Hydrogen Bonding Involving Benzene

The concept of orthogonality between halogen and hydrogen bonding, brought out by Ho and coworkers some years ago, has become a widely accepted idea within the chemists’ community. While the original work was based on a common carbonyl oxygen as acceptor for both interactions, we explore here, by means of M06-2X, M11, ωB97X, and ωB97XD/aug-cc-PVTZ DFT calculations, the interdependence of halogen and hydrogen bonding with a shared π-electron system of benzene. The donor groups (specifically NCBr and H₂O) were placed on either or the same side of the ring, according to a double T-shaped or a perpendicular geometry, respectively. The results demonstrate that the two interactions with benzene are not strictly independent on each other, therefore outlining that the orthogonality between halogen and hydrogen bonding, intended as energetical independence between the two interactions, should be carefully evaluated according to the specific acceptor group.     

_f-A_aD_d氢键体系的统计特征

基于氢键的形成和缩聚反应机理在统计意义下的相似性,利用高分子反应统计理论和反应动力学理论对氢键溶液的一个模型体系进行了相关讨论.给出了体系的溶胶分数和发生溶胶-凝胶相变的条件,指出质子受体基团间的竞争作用对溶胶凝胶相变点的影响,进而讨论了体系的数量分布函数和相关问题.     

Modulating Cation Migration and Deposition with Xylitol Additive and Oriented Reconstruction of Hydrogen Bonds for Stable Zinc Anodes

Highly reversible plating/stripping in aqueous electrolytes is one of the critical processes determining the performance of Zn-ion batteries, but it is severely impeded by the parasitic side reaction and dendrite growth. Herein, a novel electrolyte engineering strategy is first proposed based on the usage of 100 mM xylitol additive, which inhibits hydrogen evolution reaction and accelerates cations migration by expelling active H₂O molecules and weakening electrostatic interaction through oriented reconstruction of hydrogen bonds. Concomitantly, xylitol molecules are preferentially adsorbed by Zn surface, which provides a shielding buffer layer to retard the sedimentation and suppress the planar diffusion of Zn²⁺ ions. Zn²⁺ transference number and cycling lifespan of Zn ∥ Zn cells have been significantly elevated, overwhelmingly larger than bare ZnSO₄. The cell coupled with a NaV₃O₈ cathode still behaves much better than the additive-free device in terms of capacity retention.     

The Nature of Chemical Bonds from PNOF5 Calculations

Natural orbital functional theory (NOFT) is used for the first time in the analysis of different types of chemical bonds. Concretely, the Piris natural orbital functional PNOF5 is used. It provides a localization scheme that yields an orbital picture which agrees very well with the empirical valence shell electron pair repulsion theory (VSEPR) and Bent’s rule, as well as with other theoretical pictures provided by valence bond (VB) or linear combination of atomic orbitals–molecular orbital (LCAO‐MO) methods. In this context, PNOF5 provides a novel tool for chemical bond analysis. In this work, PNOF5 is applied to selected molecules that have ionic, polar covalent, covalent, multiple (σ and π), 3c–2e, and 3c–4e bonds.     

Hydrogen Bonds in Coal -The Influence of Coal Rank and the Recognition of a New Hydrogen Bond in Coal

By means of in-situ diffuse reflectance FTIR.The IR spectra of 6 coals with different ranks were obtained from room temperature to 230℃.A new curve fitting method was used to recognize the different hydrogen bonds in the coals.and the influence of coal ranks on the distribution of hydrogen bonds(HBs) in the coals and their thermal stability were discussed.The results show that there is another new HB(around 2514cm^-1) between the-SH in mercaptans or thiophenols and the nitrogen in the pyridine-like compounds in the coals.and the evidence for that was provided.The controversial band of the HB between hydroxyl and the nitrogen of the pyridine-like compounds was determined in the range of 3028-2984cm^-1,and the result is comsistent with but more specific than that of Painter et al.It was ound that the stability of different HBs in the coals is influenced by both coal rank and temperature,For some HBs.the higher the coal rank,the higher the stability of them.Within the temperature range of our research,the stability of the HB between the hydroxyl and the π bond increases to some extent for some coals at temperatures higher than 110 or 140℃.     

Force Constants and Bond Orders of Nitrogen Bonds

The force constants and the corresponding bond orders of nitrogen bonds have been calculated from the vibrational spectra (infrared and Raman spectra) of a great number of nitrogen compounds. Plotting the maximum bond order of stable nitrogen bonds against the sum of Pauling's electronegativities of the bonding partners (Σx) leads to one continuous curve for the N? X bonds where X represents elements of the first and the second short period of the periodic table. Furthermore, when the bonds formed between these elements are arranged in a coordinate system in such a way that the position of each bond is determined by the difference between the electronegativities of the bonding partners (Δx along the ordinate) and the sum of the electronegativities of the bonding partners (Σx along the abscissa), the bonding partners capable of forming multiple bonds all lie within a closed domain, where their position can be correlated with their polymerizability and other reactivities of the multiple bonds. Also discussed are the orders of bonds between nitrogen and some transition elements. In an appendix, the present methods used to calculate force constants and bond orders are surveyed.