Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

Degradation Product-Promoted Depolymerization Strategy for Chemical Recycling of Poly(bisphenol A carbonate)

The accumulation of waste plastics has a severe impact on the environment, and therefore, the development of efficient chemical recycling methods has become an extremely important task. In this regard, a new strategy of degradation product-promoted depolymerization process was proposed. Using N,N'-dimethyl-ethylenediamine (DMEDA) as a depolymerization reagent, an efficient chemical recycling of poly(bisphenol A carbonate) (BPA-PC or PC) material was achieved under mild conditions. The degradation product 1,3-dimethyl-2-imidazolidinone (DMI) was proven to be a critical factor in facilitating the depolymerization process. This strategy does not require catalysts or auxiliary solvents, making it a truly green process. This method improves the recycling efficiency of PC and promotes the development of plastic reutilization.

Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and 1H NMR Spectra Using DFT Calculations

Intramolecular hydrogen bonding (HB) is a complex phenomenon that extends beyond a simple valence event, affecting the core electrons of a molecule. Salicylic acid (SA) and its conformers provide an excellent model compound for studying intramolecular HB as the proton donor (H) and acceptor (O) can be toggled by rotating the C-O and C-C bonds to form up to seven potential conformers through various HB. In this study, we computationally investigated intramolecular interactions in SA conformers with and without such HB, by examining their calculated O 1s core electron-binding energy (CEBE) and ¹H NMR chemical shifts validated using recent measurements. The quantum mechanically stable SA conformers are fully defined by three rotatable bonds in the compound, which are abstracted as SA(η₁η₂η₃) digital structures, where η_i = 0 if the η_i angles match the most stable SA conformer (000) and η_i = 1 otherwise. Our findings suggest that the stability is dominated by the appearance of the intergroup intramolecular HB of H_p···O (where O is in the carboxylic acid functional group and H_p is the phenolic proton in -OH_p), and η₃ serves as a switch of such HB. As a result, the (η₁η₂0) SA conformers containing such H_p···O HB are more stable than other SA conformers (η₁η₂1) without such the H_p···O HB. The present density functional theory calculations reveal that this H_p···O HB results in splitting of the O 1s CEBEs of two hydroxyl groups (-OH) by up to 1 eV and deshielding the H_p proton ¹H NMR (δH_p) up to 11.68 ppm for the (η₁η₂0) conformers. Without such H_p···O HB, the O 1s XPS binding energies of two -OH groups will be closely located in the same band, and the ¹H NMR chemical shift of the H_p atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The present study indicates that the O 1s CEBE splitting between two -OH groups serves as an indicator of the presence of the H_p···O HB in SA conformers, which is also supported by the ¹H NMR results.

Synthesis of the Prototypical Cyclopropyl Dipeptide Mimic and Evaluation of Its Turn-Inducing Capability

The (+) and (-) enantiomers of a new turn-inducing cyclopropyl dipeptide mimic have been synthesized and evaluated. The mimic derives its turn-inducing capabilities solely from the cyclopropyl group and without the conformational biasing that would be provided by side-chain substituents. The mimic and peptide-mimic hybrids prepared from it have been studied using a combination of spectroscopic techniques (NMR, IR, and CD). The dipeptide mimic itself displays intramolecular hydrogen bonding in organic solvents, which differs from that observed in natural peptide turns. In contrast, more elaborate peptide-mimic hybrids exhibit hydrogen bonding characteristics that vary with solvent but are consistent with structures found in the tetrapeptide portion (i → i + 3) of a native β-turn.

Solvation and Ion-Pairing Effects of Choline Acetate Electrolyte in Protic and Aprotic Solvents Studied by NMR Titrations

Choline‐based electrolytes have been proposed as environmentally friendly and low‐cost alternatives for secondary zinc air batteries. Choline acetate [Ch]⁺[OAc]⁻ in protic (D₂O) and aprotic (DMSO‐d₆) solvents has been studied by means of concentration‐dependent ¹H NMR, viscosity, and density measurements. The viscosities have been calculated on the basis of the Jones‐Dole equation and showed that the dominant contribution originates from short‐range ion‐solvent interactions. Site‐specific association affinities were assigned from NMR chemical shift titrations. In DMSO‐d₆, the hydroxyl group of choline was found to have the smallest dissociation constant followed by the methyl group of acetate. The corresponding Gibbs energies at low concentration were found to be in agreement with a solvent‐separated ion pair (2SIP) configuration, whereas at concentrations above 300 mM, a solvent‐shared ion pair (SIP) configuration was assigned. For [Ch]⁺[OAc]⁻ in D₂O, association effects were found to be weaker, attributed to the high dielectric constant of the solvent. On time scales on the order of 100 ms, NMR linewidth perturbations indicated a change in the local rotational dynamics of the ions, attributed to short‐range cation‐solvent interactions and not to solvent viscosity. At 184 mM, ∼ 40 % of the cations in DMSO‐d₆ and ∼ 10 % in D₂O were found to exhibit short‐range interactions, as indicated by the linewidth perturbations. It was found that at about 300 mM, the ions in DMSO‐d₆ exhibit a transition from free to collective translational dynamics on time scales on the order of 400 ms. In DMSO‐d₆, both ions were found to be almost equally solvated, whereas in D₂O solvation of acetate was stronger, as indicated by the obtained effective hydrodynamic radii. For [Ch]⁺[OAc]⁻ in DMSO‐d₆, the results suggest a solvent‐shared ion association with weak H‐bonding interactions for concentrations between 0.3–1 M. Overall, the extent of ion association in solvents such as DMSO is not expected to significantly limit charge transport and hinder the performance of choline‐based electrolytes.

Factors Affecting Preparation of Molecularly Imprinted Polymer and Methods on Finding Template-Monomer Interaction as the Key of Selective Properties of the Materials

Molecular imprinting is a technique for creating artificial recognition sites on polymer matrices that complement the template in terms of size, shape, and spatial arrangement of functional groups. The main advantage of Molecularly Imprinted Polymers (MIP) as the polymer for use with a molecular imprinting technique is that they have high selectivity and affinity for the target molecules used in the molding process. The components of a Molecularly Imprinted Polymer are template, functional monomer, cross-linker, solvent, and initiator. Many things determine the success of a Molecularly Imprinted Polymer, but the Molecularly Imprinted Polymer component and the interaction between template-monomers are the most critical factors. This review will discuss how to find the interaction between template and monomer in Molecularly Imprinted Polymer before polymerization and after polymerization and choose the suitable component for MIP development. Computer simulation, UV-Vis spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Proton-Nuclear Magnetic Resonance (¹H-NMR) are generally used to determine the type and strength of intermolecular interaction on pre-polymerization stage. In turn, Suspended State Saturation Transfer Difference High Resolution/Magic Angle Spinning (STD HR/MAS) NMR, Raman Spectroscopy, and Surface-Enhanced Raman Scattering (SERS) and Fluorescence Spectroscopy are used to detect chemical interaction after polymerization. Hydrogen bonding is the type of interaction that is becoming a focus to find on all methods as this interaction strongly contributes to the affinity of molecularly imprinted polymers (MIPs).

Synthesis and crystal structure of N,N′-bis(4-chlorophenyl)thiourea N,N-dimethylformamide

This study is about the synthesis of N,N′-bis(4-chlorophenyl)thiourea N,N-dimethylformamide (C16H17Cl2N3OS) compound. Single crystals of the compound were obtained by slow evaporation of N,N′-bis(4-chlorophenyl)thiourea (C13H10Cl2N2S) in N,N-dimethylformamide (C3H7NO; DMF) through recrystallization under mild condition. Important classical N–H⋯O links the two molecules together. Results revealed that C16H17Cl2N3OS crystallized in the monoclinic space group P21/c with the respective cell parameters of a = 92,360 (4) Å, b = 7.2232 (3) Å, 25.2555 (11) Å, β = 91.376 (3), α = γ = 90°, V = 1684.40 (12) Å3, T = 119.94 (13) K and Z = 4 and Z′ = 1.

A Spectroscopic Overview of Intramolecular Hydrogen Bonds of NH…O,S,N Type

Intramolecular NH…O,S,N interactions in non-tautomeric systems are reviewed in a broad range of compounds covering a variety of NH donors and hydrogen bond acceptors. ¹H chemical shifts of NH donors are good tools to study intramolecular hydrogen bonding. However in some cases they have to be corrected for ring current effects. Deuterium isotope effects on ¹³C and ¹⁵N chemical shifts and primary isotope effects are usually used to judge the strength of hydrogen bonds. Primary isotope effects are investigated in a new range of magnitudes. Isotope ratios of NH stretching frequencies, νNH/ND, are revisited. Hydrogen bond energies are reviewed and two-bond deuterium isotope effects on ¹³C chemical shifts are investigated as a possible means of estimating hydrogen bond energies.

Assessment of O-H⋯O and O-H⋯S intramolecular hydrogen bond energies in some substituted pyrimidines using quantum chemistry methods

Resonance-assisted hydrogen bond (RAHB) theory was studied in some substituted pyrimidines in which encompass O-H⋯Y unit (Y= O and S). Alteration of substituents (R ₁, R₂, R₃ = H, C₂H, C₂F) on pyrimidine ring changes properties of electron charge density at this ring and influences indirectly on strength of intramolecular hydrogen bond (IHB) interactions in the mentioned structures. Then, IHB energies were estimated using cis-trans method (CTM), related rotamers method (RRM), Espinosa' method (EM), and a viewpoint based on properties of electron charge densities at ring critical point (RCP) of RAHB ring. Moreover, the estimated IHB energies with these methods were compared with those obtained using modified Espinosa' method (MEM), Iogansen's relationship, and chemical shift-based method to find more consistent method with the proposed viewpoint based on RCP characteristics. The linear correlations between the all estimated IHB energies and some hydrogen bonding descriptors such as geometrical, spectroscopic, topological, and molecular orbital factors were examined. Results indicated that the IHB energies that obtained by way of MEM and Iogansen's relationship have better correlations with hydrogen bonding descriptors. Also, there are good consistencies between results of these two methods with viewpoint based on properties of RCPs. Therefore, IHB energies can suitably estimate using properties of RCPs in heterocyclic molecular systems especially in cases that rotation around C-C/CC bonds makes additional interactions in isomers and influences on accuracy of calculated IHB energies using approaches such as CTM and RRM.

Theoretical investigation on hydrogen bond interaction between adrenaline and hydrogen sulfide

In this study, we elucidated the formation of hydrogen bond between adrenaline (AD) and hydrogen sulfide utilizing computational studies. Six potential complexes were studied including geometrical parameters, energy, vibrational frequency, topological analysis, natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), and NMR analysis. Moreover, these calculations were examined through DFT/ωB97XD/6-311G++(d,p) level. It was found that there are no indication on formation on hydrogen bonding between the two catecholic OHs where the one formed between the amino group and the hydroxyl oxygen atom of adrenaline monomer was broken in AS₁ to form two new interactions namely SH^...N and O₁H₁^...S, while it retained in other complexes. Furthermore, the bond became stronger due to cooperativity in AS₃ and AS₆, for the presence of withdrawing effect of the phenyl ring, the H-bonds formed with the side chain oxygen atom. The adrenaline and H₂S interaction was experimentally examined via FT-IR spectrometry and thin layer chromatography for confirmation of our theoretical study. Graphical abstract.

A Critical Overview of Current Theoretical Methods of Estimating the Energy of Intramolecular Interactions

This article is probably the first such comprehensive review of theoretical methods for estimating the energy of intramolecular hydrogen bonds or other interactions that are frequently the subject of scientific research. Rather than on a plethora of numerical data, the main focus is on discussing the theoretical rationale of each method. Additionally, attention is paid to the fact that it is very often possible to use several variants of a particular method. Both of the methods themselves and their variants often give wide ranges of the obtained estimates. Attention is drawn to the fact that the applicability of a particular method may be significantly limited by various factors that disturb the reliability of the estimation, such as considerable structural changes or new important interactions in the reference system.

Relationships between NMR shifts and interaction energies in biphenyls, alkanes, aza-alkanes, and oxa-alkanes with X─H… H─Y and X─H… Z (X, Y = C or N; Z = N or O) hydrogen bonding

Hydrogen-hydrogen C─H^… H─C bonding between the bay-area hydrogens in biphenyls, and more generally in congested alkanes, very strained polycyclic alkanes, and cis-2-butene, has been investigated by calculation of proton nuclear magnetic resonance (NMR) shifts and atom-atom interaction energies. Computed NMR shifts for all protons in the biphenyl derivatives correlate very well with experimental data, with zero intercept, unit slope, and a root mean square deviation of 0.06 ppm. For some congested alkanes, there is generally good agreement between computed values for a selected conformer and the experimental data, when it is available. In both cases, the shift of a given proton or pair of protons tends to increase with the corresponding interaction energy. Computed NMR shift differences for methylene protons in polycyclic alkanes, where one is involved in a very short contact ("in") and the other is not ("out"), show a rough correlation with the corresponding C─H^… H─C exchange energies. The "in" and "in,in" isomers of selected aza- and diaza-cycloalkanes, respectively, are X─H^… H─N hydrogen bonded, whereas the "out" and "in,out" isomers display X─H^… N hydrogen bonds (X = C or N). Oxa-alkanes and the "in" isomers of aza-oxa-alkanes are X─H^… O hydrogen bonded. There is a very good general correlation, including both N─H^… H─Y (Y = C or N) and N─H^… Z (Z = N or O) interactions, for NH proton shifts against the exchange energy. For "in" CH protons, the data for the different C─H^… H─Y and C─H^… Z interactions are much more dispersed and the overall shift/exchange energy correlation is less satisfactory.

Magnetic descriptors of hydrogen bonds in malonaldehyde and its derivatives

The nature of the hydrogen bond, HB, as such is still unknown, though a few of its most fundamental features has been uncovered during the last few decades. At the moment, it is possible to obtain reliable results for only a few of its broadest properties, like magnetic properties. They could give new insights into the physics underlying the strength and features of HBs. In this article we analyze the electronic origin of the NMR spectroscopic parameters of malonaldehyde, MA, and some substituted MAs. These substituted MAs are such that the H-bonds are assisted by one of two phenomena: resonance, RAHB, or charge, CAHB. We have studied the dependences of these parameters on two of the main factors which contribute the most to both phenomena, the geometrical and electronic factors, and found out how they can be used to characterize RAHB or CAHB by means of reliable theoretical calculations. We show that in the set of compounds analyzed here (i) the shielding of the proton of the H-bond can be used as a measure of the strength of the HB and (ii) the relation between the contact and non-contact mechanisms of J-couplings between donor and acceptor atoms is a reliable descriptor of whether the H-bond is resonance assisted or charge assisted.

Preparation and Characterization of Cellulose Acetate Propionate Films Functionalized with Reactive Ionic Liquids

1-(1,3-diethoxy-1,3-dioxopropan-2-ylo)-3-methylimidazolium bromide (RIL1_Br), 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium bromide (RIL2_Br), 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium tetrafluoroborate (RIL3_BF4) ionic liquids were synthesized. Subsequently, the dense cellulose acetate propionate (CAP)-based materials containing from 9 to 28.6 wt % of these reactive ionic liquids were elaborated. Reactive ionic liquids (RILs) were immobilized in CAP as a result of the transesterification reaction. The yield of this reaction was over 90% with respect to the used RIL. The physicochemical properties of resultant films were studied using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM), and thermogravimetric analysis (TGA). The RIL incorporation influenced the morphology of films by increasing their surface roughness with the rise of RIL content. The thermal stability of CAP-based membranes was dependent on the nature of the ionic liquid. Nevertheless, it was proven that CAP films containing RILs were stable up to 120-150 °C. Transport properties were characterized by water permeation tests. It was found that the type and the amount of the ionic liquid in the CAP matrix substantially influenced the transport properties of the prepared hybrid materials.

The Dawn of Thiol-Yne Triazine Triones Thermosets as a New Material Platform Suited for Hard Tissue Repair

The identification of a unique set of advanced materials that can bear extraordinary loads for use in bone and tooth repair will inevitably unlock unlimited opportunities for clinical use. Herein, the design of high-performance thermosets is reported based on triazine-trione (TATO) monomers using light-initiated thiol-yne coupling (TYC) chemistry as a polymerization strategy. In comparison to traditional thiol-ene coupling (TEC) systems, TYC chemistry has yielded highly dense networks with unprecedented mechanical properties. The most promising system notes 4.6 GPa in flexural modulus and 160 MPa in flexural strength, an increase of 84% in modulus and 191% in strength when compared to the corresponding TATO system based on TEC chemistry. Remarkably, the mechanical properties exceed those of polylactide (PLA) and challenge poly(ether ether ketone) PEEK and today's methacrylate-based dental resin composites. All the materials display excellent biocompatibility, in vitro, and are successfully: i) molded into medical devices for fracture repair, and ii) used as bone adhesive for fracture fixation and as tooth fillers with the outstanding bond strength that outperform methacrylate systems used today in dental restoration application. Collectively, a new era of advanced TYC materials is unfolded that can fulfill the preconditions as bone fixating implants and for tooth restorations.

A Secondary Structural Element in a Wide Range of Fucosylated Glycoepitopes

The increasing understanding of the essential role of carbohydrates in development, and in a wide range of diseases fuels a rapidly growing interest in the basic principles governing carbohydrate-protein interactions. A still heavily debated issue regarding the recognition process is the degree of flexibility or rigidity of oligosaccharides. Combining NMR structure determination based on extensive experimental data with DFT and database searches, we have identified a set of trisaccharide motifs with a similar conformation that is characterized by a non-conventional C-H⋅⋅⋅O hydrogen bond. These motifs are present in numerous classes of oligosaccharides, found in everything from bacteria to mammals, including Lewis blood group antigens but also unusual motifs from amphibians and marine invertebrates. The set of trisaccharide motifs can be summarized with the consensus motifs X-β1,4-[Fucα1,3]-Y and X-β1,3-[Fucα1,4]-Y-a secondary structure we name [3,4]F-branch. The wide spectrum of possible modifications of this scaffold points toward a large variety of glycoepitopes, which nature generated using the same underlying architecture.

A Study about Regioisomeric Hydroquinones with Multiple Intramolecular Hydrogen Bonding

A theoretical exploration about hydrogen bonding in a series of synthetic regioisomeric antitumor tricyclic hydroquinones is presented. The stabilization energy for the intramolecular hydrogen bond (IHB) formation in four structurally different situations were evaluated: (a) IHB between the proton of a phenolic hydroxyl group and an ortho-carbonyl group (forming a six-membered ring); (b) between the oxygen atom of a phenolic hydroxyl group and the proton of an hydroxyalkyl group (seven membered ring); (c) between the proton of a phenolic hydroxyl group with the oxygen atom of the hydroxyl group of a hydroxyalkyl moiety (seven-membered ring); and (d) between the proton of a phenolic hydroxyl group and an oxygen atom directly bonded to the aromatic ring in ortho position (five-membered ring). A conformational analysis for the rotation around the hydroxyalkyl substituent is also performed. It is observed that there is a correspondence between the conformational energies and the IHB. The strongest intramolecular hydrogen bonds are those involving a phenolic proton and a carbonyl oxygen atom, forming a six-membered ring, and the weakest are those involving a phenolic proton with the oxygen atom of the chromenone, forming five-membered rings. Additionally, the synthesis and structural assignment of two pairs of regioisomeric hydroquinones, by 2D-NMR experiments, are reported. These results can be useful in the design of biologically-active molecules.

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, νOH, and OH chemical shifts, δOH (in the latter case, after correction for ring current effects). Limits for O-H···Y systems are taken as 2800 > νOH > 1800 cm-1, and 19 ppm > δOH > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as β-diketone enols, β-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O···O distance has long been used as a parameter for hydrogen bond strength in O-H···O systems. On a broad scale, a correlation between OH stretching wavenumbers and O···O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted β-diketone enols this correlation is relatively weak.

Energy of Intramolecular Hydrogen Bonding in ortho-Hydroxybenzaldehydes, Phenones and Quinones. Transfer of Aromaticity from ipso-Benzene Ring to the Enol System(s)

Intramolecular hydrogen bonding (HB) is one of the most studied noncovalent interactions of molecules. Many physical, spectral, and topological properties of compounds are under the influence of HB, and there are many parameters used to notice and to describe these changes. Hitherto, no general method of measurement of the energy of intramolecular hydrogen bond (EHB) has been put into effect. We propose the molecular tailoring approach (MTA) for EHB calculation, modified to apply it to Ar-O-H∙∙∙O=C systems. The method, based on quantum calculations, was checked earlier for hydroxycarbonyl-saturated compounds, and for structures with resonance-assisted hydrogen bonding (RAHB). For phenolic compounds, the accuracy, repeatability, and applicability of the method is now confirmed for nearly 140 structures. For each structure its aromaticity HOMA indices were calculated for the central (ipso) ring and for the quasiaromatic rings given by intramolecular HB. The comparison of calculated HB energies and values of estimated aromaticity indices allowed us to observe, in some substituted phenols and quinones, the phenomenon of transfer of aromaticity from the ipso-ring to the H-bonded ring via the effect of electron delocalization.

Hydrogen Atomic Positions of O-H···O Hydrogen Bonds in Solution and in the Solid State: The Synergy of Quantum Chemical Calculations with ¹H-NMR Chemical Shifts and X-ray Diffraction Methods

The exact knowledge of hydrogen atomic positions of O-H···O hydrogen bonds in solution and in the solid state has been a major challenge in structural and physical organic chemistry. The objective of this review article is to summarize recent developments in the refinement of labile hydrogen positions with the use of: (i) density functional theory (DFT) calculations after a structure has been determined by X-ray from single crystals or from powders; (ii) ¹H-NMR chemical shifts as constraints in DFT calculations, and (iii) use of root-mean-square deviation between experimentally determined and DFT calculated ¹H-NMR chemical shifts considering the great sensitivity of ¹H-NMR shielding to hydrogen bonding properties.