H-bond Strength Research Articles

Nitrofurantoin-melamine monohydrate (cocrystal hydrate): Probing the role of H-bonding on the structure and properties using quantum chemical calculations and vibrational spectroscopy

Cocrystal monohydrate of nitrofurantoin (NF) with melamine (MELA) has been studied as NF is an antibacterial drug used for the treatment of urinary tract infections. The structure of nitrofurantoin-melamine-monohydrate (NF-MELA-H2O) is characterized by FT-IR and FT-Raman spectroscopy. The energies and vibrational frequencies of the optimized structures calculated using quantum chemical calculations. Supported by normal coordinate analyses and potential energy distributions (PEDs), the complete vibrational assignments recommended for the observed fundamentals of cocrystal hydrate. With the aim of inclusion of all the H-bond interactions, dimer of NF-MELA-H2O has been studied as only two molecules of cocrystal hydrate are present in the unit cell. By the study of dimeric model consistent assignment of the FT-IR and FT-Raman spectrum obtained. H-bonds are of essential importance in an extensive range of molecular sciences. The vibrational analyses depict existence of H-bonding (O-H⋯N) between water O-H and pyridyl N atom of MELA in both monomer and dimer. To probe the strength and nature of H-bonding in monomer and dimer, topological parameters such as electron density (ρBCP), Laplacian of electron density (∇2ρBCP), total electron energy density (HBCP) and H-bond energy (EHB) at bond critical points (BCP) are evaluated by quantum theory of atoms in molecules (QTAIM). Natural bond orbitals (NBOs) analyses are carried out to study especially the intra and intermolecular H-bonding and their second order stabilization energy (E(2)). The value of HOMO-LUMO energy band gap for NF-MELA-H2O (monomer and dimer both) is less than NF, showing more chemical reactivity for NF-MELA-H2O. Chemical reactivity has been described with the assistance of electronic descriptors. Global electrophilicity index (ω = 7.3992 eV) shows that NF-MELA-H2O behaves as a strong electrophile than NF. The local reactivity descriptors analyses such as Fukui functions, local softnesses and electrophilicity indices performed to determine the reactive sites within NF-MELA-H2O. In MEP map of NF-MELA (monomer and dimer) electronegative regions are about NO2 and C=O group of NF, although the electropositive regions are around NH2, N-H group and H2O molecule. Molar refractivity (MR) value of NF-MELA-H2O (monomer and dimer) lies within the range set by Lipinski's modified rules. This study could set as an example to study the H-bond interactions in pharmaceutical cocrystals.

Studying structural and local dynamics in model H-bonded active ingredient — Curcumin in the supercooled and glassy states at various thermodynamic conditions

Different experimental techniques were applied to study thermal and structural properties, strength of H-bonds, possible keto-enol tautomerism and molecular dynamics at various thermodynamic conditions in the H-bonded active substance, curcumin (CRM). Dielectric measurements revealed dynamical features of examined compound that are uncharacteristic for the associated systems. This includes enormously large pressure coefficient of the glass transition temperature and prominent drop of the fragility with compression. Simultaneously, the shape of α-process slightly broadened at elevated pressures. Infrared investigations demonstrated that this effect is related to the variation in the population of H-bonds. Moreover, we studied the changes in the structural and dynamical properties of the glasses prepared upon cooling of the melt (ordinary glass, OG) and the one obtained after compression of CRM in the liquid phase and decompression at T = 293 K (dense glass, DG). Interestingly, during the aging of the latter sample, a clear shift of the β-relaxation towards higher frequencies was noted. This unexpected result indicated that the density of DG is probably getting smaller with time. Complementary X-ray diffraction experiments confirmed this supposition. Additionally, they showed that in DG there are traces of polymorph II of CRM that has a higher density than initial crystals (polymorph I). Finally, infrared studies demonstrated that H-bond pattern in DG is slightly different with respect to OG. Furthermore, Raman investigations suggested that probably keto-enol tautomerism might be shifted towards diketo form in the glass obtained at high compression. Our investigations are very interesting in the context of better understanding of the behavior of associated systems at high compression as well as provide a better insight into dynamics of higher density glasses produced at elevated pressures.

Electrochemically-Controlled Dimerization in Ferrocene Ureidopyrimidone Derivatives. the Effect of Ferrocene Position

Ureidopyrimidones (UPy’s) are well-known to dimerize in weakly polar solvents such as CH2Cl2, via the formation of four strong, linear H-bonds. This, coupled with their relative ease of synthesis, has led to their use as a linker in supramolecular polymers and gels. The attraction of such materials is their inherent self-healing properties, whereby a defect can be repaired by using heat or mechanical stress to reversibly break the polymer at the H-bond sites. This increases the fluidity of the material, allowing the defect to fill in. Upon cooling or relief of stress, the H-bonds reform and the defect is repaired. However, while the use of heat or mechanical stress as stimuli are clearly useful, neither is inherently very selective. On-going research in our lab is focused on increasing the versatility of the UPy system by creating electroactive UPy’s in which dimerization strength can be controlled in a more selective manner through oxidation and reduction. Previously, our group has shown that dimers based on the ferrocene-UPy derivative, 1, break apart upon oxidation of the ferrocene to the ferrocenium form at mM concentrations in CH2Cl2. This could be due both to the creation of electrostatic repulsion and a decrease in H-bond strength due to the reduced H-donor ability of the O and N on the pyrimidone side. In this project, another Fc-UPy has been prepared, 2, in which the ferrocene is attached to the urea side of the molecule. In contrast to 1, oxidation of 2 shows a single reversible ferrocene CV wave in CH2Cl2 at mM concentrations. Under these conditions, 1H NMR indicates that 2 is fully dimerized, thus, it appears that, unlike with 1, oxidation has no significant effect on dimerization of 2. In both cases, oxidation would increase electrostatic repulsion. However, unlike in 1, oxidation of 2 should actually increase H-bond strength by making the urea NH a stronger H-donor. Therefore, it appears that the effect of oxidation on the H-donating or accepting ability is a more crucial factor for dimerization control than electrostatics in these systems. Figure 1

THEORETICAL STUDY OF VARIOUS SOLVENTS EFFECT ON 5-FLUOROURACIL-VITAMIN B3 COMPLEX USING PCM METHOD

In this article, a detailed study of the solvent effects on the stability order, binding energy and hydrogen bond strength in 5-Fluorouracil-Vitamin B3 (FU–VB) complex is performed using M05-2X method with 6-311++G(d,p) basis set. Based on the average of the calculated H-bond energies, the H-bond strength of FU-VB complex in gas phase is more than solution phase. The binding energy in solution phase is also lower than the gas phase. Therefore, the stability of the studied complex increases in solution phase with respect to the gas phase. Furthermore, the topological properties of the electron density distribution are analyzed in term of the Bader Quantum Theory of “Atoms in Molecules” (QTAIM). The natural bond orbital (NBO) method is also applied to get a more precise insight into the nature of intermolecular interactions. Finally, the solvent effect on the frontier molecular orbital energies (HOMO and LUMO), chemical potential and hardness of FU–VB complex is investigated.

Ring-Stacking Water Clusters: Morphology and Stabilities.

The structures and interaction energies of water clusters with ring stacking motifs are studied by using ab initio calculations. The structures of the water clusters are constructed by stacking either single rings or multi‐rings of tetramer, pentamer, and hexamer. We found that, in the single‐ring‐stacking motif, the most stable isomers exhibit an alternative clockwise‐anticlockwise stacking pattern. We also show that four‐layer single‐ring‐stacking isomers are not energetically favorable in comparison with those of two‐layer multi‐ring‐stacking isomers. The relative stability of the isomers is also analyzed in terms of H‐bond strength and elastic distortions of the water molecules.

Infrared Spectra of Gas Hydrates from First-Principles.

The infrared spectra of sII gas hydrates have been computed using density functional theory for the first time, at equilibrium, and under pressure. It is also the first account of a full vibrational analysis (both guest and host vibrations) for gas hydrates with hydrocarbon guest molecules. Five hydrate structures were investigated: empty, propane, isobutane, ethane-methane, and propane-methane sII hydrates. The computed IR spectra are in good agreement with available experimental and theoretical results. The OH stretching frequencies were found to increase, while the H-bond stretching and H2O libration frequencies decreased with an increase in guest size and cage occupancy and with a decrease in pressure. The H2O bending vibrations are relatively independent of guest size, cage occupancy, pressure, temperature, and crystal structure. The guest vibrational modes, especially the bending modes, also have minimal pressure dependence. We have also provided more quantitative evidence that gas hydrate material properties are defined by their hydrogen bond properties, by linking H-bond strength to Young's modulus. The results and ensuing vibrational analysis presented in this paper are a valuable contribution to the ongoing efforts into developing more accurate gas hydrate identification and characterization methods in the laboratory, in industry/nature, and even in outer space.

Water absorption by deep eutectic solvents.

Deep eutectic solvents (DESs) are one type of green solvents. Most of the DESs could absorb water from air. However, even a trace amount of water can affect the chemical structure and physical properties of DESs. To date, no study has been reported on the hygroscopicity of DESs. Consequently, in this study, a comprehensive investigation was performed on the capacity, kinetics, mechanism, and furthermore the dynamic process (by PCMW2D-COS IR spectra) of atmospheric water absorption from air by DESs. The results show that most DESs are highly hygroscopic. Surface absorption enhances the overall water absorption capacity by DESs in spite of decreasing the initial water absorption rate. In the beginning, the water absorption increases with an increase in the number of hydrophilic groups in DESs due to the retained DES nanostructure during this period. Therefore, DESs with more hydrophilic groups (ChCl:glucose than ChCl:xylitol) possess a higher water absorption initial rate. However, when the water absorption capacity is high, the hindrance from the H-bond strength from inner DESs needs to be overcome for the absorption of more water. In this case, DESs with stronger H-bonds (ChCl:glucose than ChCl:xylitol) have a lower steady-state water absorption capacity and an easier equilibrium.

Rationalizing the diversity of amide-amide H-bonding in peptides using the natural bond orbital method.

Natural bond orbital (NBO) analysis of electron delocalization in a series of capped isolated peptides is used to diagnose amide-amide H-bonding and backbone-induced hyperconjugative interactions, and to rationalize their spectral effects. The sum of the stabilization energies corresponding to the interactions between NBOs that are involved in the H-bonding is demonstrated as an insightful indicator for the H-bond strength. It is then used to decouple the effect of the H-bond distance from that, intrinsic, of the donor/acceptor relative orientation, i.e., the geometrical approach. The diversity of the approaches given by the series of peptides studied enables us to illustrate the crucial importance of the approach when the acceptor is a carbonyl group, and emphasizes that efficient approaches can be achieved despite not matching the usual picture of a proton donor directly facing a lone pair of the proton acceptor, i.e., that encountered in intermolecular H-bonds. The study also illustrates the role of backbone flexibility, partly controlled by backbone-amide hyperconjugative interactions, in influencing the equilibrium structures, in particular by frustrating or enhancing the HB for a given geometrical approach. Finally, the presently used NBO-based HB strength indicator enables a fair prediction of the frequency of the proton donor amide NH stretching mode, but this simple picture is blurred by ubiquitous hyperconjugative effects between the backbone and amide groups, whose magnitude can be comparable to that of the weakest H-bonds.

Ab Initio Investigation of the Interionic Interactions in Triethylammonium-Based Protic Ionic Liquids: The Role of Anions in the Formation of Ion Pair and Hydrogen Bonded Structure.

The results of structural analysis and cation-anion interactions of 10 ion pairs and their relevance for the physicochemical properties of triethylammonium-based protic ionic liquids are reported. The calculations were mainly performed by dispersion corrected density functional theory method (B3LYP-GD3). It is shown that the dispersion correction is important in the evaluation of the interaction energies of these compounds. The role of anions in the formation of ion pairs and hydrogen-bonded structure are analyzed. To obtain a quantitative measure of the strength of H-bonds, the Bader's theory of atoms in molecule (AIM) was applied. The correlations between hydrogen bond lengths, their energies, and electron-topological parameters at the H···O bond critical point are presented.

Transition from Water Wires to Bifurcated H-Bond Networks in 2-Pyridone·(H2O) n, n = 1-4 Clusters.

Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O) n clusters with n = 1-4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm-1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1-4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

Synergistic Effect of Distal Polar Interactions in Myoglobin and Their Structural Consequences.

In the L29F variant of myoglobin (Mb), the coordination of oxygen (O2) to the heme Fe atom is stabilized by favorable electrostatic interactions between the polar Fe-O2 moiety and the multipole of the phenyl ring of the Phe29 side chain (Phe29 interaction), in addition to the well-known hydrogen bond (H-bond) between the Fe-bound O2 and the 64th residue (distal H-bond; Carver, T. E.; Brantley, R. E., Jr.; Singleton, E. W.; Arduini, R. M.; Quillin, M. L.; Phillips, G. N., Jr.; Olson, J. S. J. Biol. Chem. 1992, 267, 14443-14450). The O2 and carbon monoxide (CO) binding properties and autoxidation of the L29F/H64L and L29F/H64Q variants reconstituted with a series of chemically modified heme cofactors were analyzed and then compared with those of native Mb, and the L29F, H64Q, and H64L variants similarly reconstituted with the chemically modified heme cofactors in order to elucidate the relationship between the Phe29 interaction and the distal H-bond that critically contributes to stabilization of Fe-bound O2. We found that the Phe29 interaction and distal H-bond act cooperatively to stabilize the Fe-bound O2 in such a manner that the Phe29 interaction strengthens with increasing strength of the distal H-bond. Comparison of the functional properties between the L29F and H64L variants indicated that the synergistic effect of the two interactions decreases the O2 dissociation and autoxidation rate constants of the protein by factors of ∼1/2000 and ∼1/400, respectively. Although the CO binding properties of the proteins were not greatly affected by the distal polar interactions, their synergistic effects were clearly and sharply manifested in the vibrational frequencies of the Fe-bound C-O stretching of the proteins.

The Role of Chain-End Association Lifetime in Segmental and Chain Dynamics of Telechelic Polymers

Broadband dielectric spectroscopy, differential scanning calorimetry, and rheology were employed to study the impact of hydrogen (H)-bonding end-groups on segmental and chain dynamics of telechelic polypropylene glycol (PPG) and poly(dimethylsiloxane) (PDMS). Polymer chains with three types of H-bonding end-groups possessing different interaction strengths and a non-H-bonding end-group as reference were compared. The glass transition temperature (Tg) of H-bonding PPG systems with low molecular weight increases compared to the reference, and the Tg difference varies with chain-end interaction strength. In contrast, their shear viscosities (for Tg-scaled temperature, i.e., when the shift in Tg is accounted for) are similar to that one of the reference. This is in strong contrast to the behavior of telechelic PDMS with the same set of end-groups, where the Tg increase of all H-bonding systems is independent of H-bond strengths, while shear viscosity increases significantly only for the strongest H-bonding en...

Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays.

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the TyrZ-His190 redox relay in photosystem II. These compounds provide a platform to theoretically and experimentally explore and expand proton-coupled electron transfer (PCET) processes. The models feature H-bonds between the phenol and the nitrogen at the 3-position of the benzimidazole and between the 1 H-benzimidazole proton and the imine nitrogen. Protonation of the benzimidazole and the imine can be unambiguously detected by infrared spectroelectrochemistry (IRSEC) upon oxidation of the phenol. DFT calculations and IRSEC results demonstrate that with sufficiently strong electron-donating groups at the para-position of the N-phenylimine group (e.g., -OCH3 substitution), proton transfer to the imine is exergonic upon phenol oxidation, leading to a one-electron, two-proton (E2PT) product with the imidazole acting as a proton relay. When transfer of the second proton is not sufficiently exergonic (e.g., -CN substitution), a one-electron, one-proton transfer (EPT) product is dominant. Thus, the extent of proton translocation along the H-bond network, either ∼1.6 Å or ∼6.4 Å, can be controlled through imine substitution. Moreover, the H-bond strength between the benzimidazole NH and the imine nitrogen, which is a function of their relative p Ka values, and the redox potential of the phenoxyl radical/phenol couple are linearly correlated with the Hammett constants of the substituents. In all cases, a high potential (∼1 V vs SCE) is observed for the phenoxyl radical/phenol couple. Designing and tuning redox-coupled proton wires is important for understanding bioenergetics and developing novel artificial photosynthetic systems.

Correlation among Hydrogen Bond, Excited-State Intramolecular Proton-Transfer Kinetics and Thermodynamics for −OH Type Proton-Donor Molecules

A series of new molecules bearing alkyl-substitutes on the parent molecule 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) has been designed and synthesized, which possesses an intramolecular hydrogen bond (H-bond) between −OH proton donor and carbonyl proton acceptor. All studied molecules present an equilibrium type of excited-state intramolecular proton transfer (ESIPT) at 298 K. The alkyl-substitutions at various positions subtly alter the intramolecular H-bond strength, which then fine-tune the excited-state equilibrium and hence thermodynamics between normal and tautomer species. These, in combination with finite rates of ESIPT resolved by femtosecond fluorescence upconversion techniques, lead to the establishment of an empirical relationship among H-bond strengths, ESIPT kinetics, and thermodynamics for HBF series of molecules, in which the stronger H-bond is, the faster and more exergonic ESIPT is. The intensity ratio for normal versus proton-transfer tautomer emission can be systematically tuned, demo...

A reassessment of entropy convergence in solvation thermodynamics

Entropy convergence is a well-established phenomenon for the hydration of noble gases, hydrocarbons and n-alcohols, whose theoretical explanation has been associated with cavity creation thermodynamics in water. In the last years, computer simulation studies have cast doubts on the occurrence of entropy convergence on increasing the cavity size. In the present work, classic scaled particle theory calculations are used to show that: (a) entropy convergence occurs in water, but not in carbon tetrachloride for spherical, prolate and oblate spherocylindrical cavities, regardless of their size; (b) its occurrence in water in mainly caused by the special temperature dependence of water density that, in turn, reflects the strength of H-bonds and the plasticity of the 3D H-bonding tetrahedral network.

The role of chlorine substituents in lichexanthones properties: the ionic and halogen bond interactions

The effects of chlorine substituents (Cl substituents) on the structural and electronic properties of lichexanthones (LXs) such as aromaticity of rings, the intramolecular interactions and the complexation of LXs with Mg2+ and NH3 (the ionic and halogen bond interactions) have been investigated by the quantum mechanical calculations at the M06-2X/6-311++G(d,p) level of theory. The effects of Cl substituents on the intramolecular H-bond interactions strongly depend on their positions on the ring. The Cl substituents increase the E2 values of nX → π*CC interactions, obtained from the natural bond orbital analysis, reduce the electron densities on the center of the substituted ring, ρRCP in the results of atoms in molecules analysis, and decrease the aromaticity of the rings with respect to the aromatic fluctuation index. The stabilities of compounds depend on the aromaticity of rings, the O···H-bond strength, and especially the Cl···H interaction. Also, the effects of substituents on the local minimum values of electrostatic potential around the O atoms (Vmin) and therefore the binding energies of [LX···Mg]2+ and [LX···Mg·4H2O]2+ complexes strongly depend on the positions of the substituents. The aromaticity of rings A and C decrease and that of B increases after complexation with the Mg2+ ion. The electronic effects (and in some cases the steric effects) play a special role in the formation and stability of LX···NH3 complexes. The stabilities of the LX···NH3 complexes increase by increasing the natural charges and the local maximum electrostatic potential (Vmax−Cl) calculated, respectively, on and around the Cl atoms, and the ρCl−N values calculated at the Cl···N BCPs.

H/D Isotope Effects on Nuclear Shielding As a Tool for the Study of Interionic Hydrogen Bonds in Ionic Liquids

H/D isotope effects on nuclear shielding (IENS) have found applications in the study of reaction kinetics, chemical and conformational equilibria, and non-bonded interactions. In particular, they are especially well-suited to investigate hydrogen bonded (H-bonded) systems. In this presentation, we demonstrate that H/D IENS are indeed a powerful tool to scrutinize interionic H-bonds in ionic liquids (ILs), using 1-n-alkyl-3-methylimidazolium salts as examples. Results obtained with protonated and deuterated isotopologues of 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl), tetrafluroborate ([C4mim]BF4), and hexafluorofosphate ([C4mim]PF6) indicate that IENS on the 35/37Cl and 19F resonances of the anions, or D35/37Cl(H,D) and D19F(H,D), allow to identify and differentiate H-bonds between the anion and the different aromatic protons on the cation. Furthermore, IENS measured on sequentially deuterated [C4mim]BF4 and [C4mim]PF6 isotopologues reveal the formation of aliphatic C-H⋅⋅⋅X hydrogen bonds between the fluorinated anions and hydrogens along the N-alkyl sidechains of the cations. Our findings also indicate that the interactions between the fluorinated anions and hydrogens on the C-1’ and C-1” position of the N-alkyl sidechains are comparable to, and in some cases stronger than, those involving protons on the aromatic ring, underscoring the role that these weak interionic forces have on the local ordering of imidazolium salts in the liquid state. The methodology also shows that the length of the N-alkyl sidechain anchored on the N-1 position has no sizable effect on the strength of interionic H-bonds in this type of ILs. Current studies involving variations on the sidechain structure, was well as applications of the approach to the study of other systems bearing NMR-active nuclei, are discussed. Figure 1

Comparison of Redox-Dependent H-Bonding in Simple, Electroactive Ureas Containing Either Ferrocene or Phenylenediamine Redox Couples – Similar Results with Different Mechanisms

An important goal in supramolecular chemistry is the design of stimuli-responsive supramolecular systems in which binding between components can be controlled by external signals. Supramolecular chemistry research in our group focuses in particular on using electron transfer to control the strength of H-Bonding interactions. In this presentation the redox-dependent H-bonding behavior of two simple phenylureas containing either a phenylenediamine redox couple, Figure 1a, or a ferrocene redox couple, FcUHH, Figure 1b, will be compared. The urea functional group provides two good H-donors (a DD array) which can be paired with two good H-acceptors (an AA array) in the PZD guest. Oxidation of either the ferrocene or phenylenediamine would be expected to increase the acidity of the attached urea NH, increasing the H-Bonding strength to PZD. Experimentally, this should result in a negative shift in the E1/2 of the cyclic voltammetric wave upon addition of PZD, with little change in wave shape or height. This behavior is indeed observed for both ureas in 0.1 M NBu4PF6/CH2Cl2. Plots of the ΔE1/2 of FcUHH vs [PZD] are fit well using a 1:1 binding isotherm, with initial results giving estimates of the binding constants to the oxidized and reduced forms as Kred = 80 M−1 and Kox = 500 M−1. The same type of experiments have been run with the phenylenediamine urea, UHH. The data also fits well to a simple 1:1 binding isotherm and gives similar values for the binding constants, Kred = 30 M−1 and Kox = 700 M−1. This is interesting because from our previous work1 we know that the electrochemistry is quite a bit more complex for UHH than FcUHH. In the former case the reversible, apparent 1 e− cyclic voltammetric (CV) wave observed in CH2Cl2 actually corresponds to 2 e− oxidation of half of the UHH’s to a quinoidal cation accompanied by proton transfer and electro-deactivation of the other half, Figure 1a. Given this, it is somewhat surprising that any potential shift is observed upon addition of PZD, since the oxidation product can only form a single H-bond with PZD. However, the key realization was that the other product, the protonated urea, can still form a 2 H-bond complex, and that should be stronger than the one formed with the starting urea due to the increased acidity of one of the urea NH’s. From this perspective, it is not surprising that we see such similar behavior between FcUHH and UHH. In the former, oxidation creates a +1 charge on one side of the urea (ferrocenium), Figure 1b, and, in the latter, proton transfer creates +1 charge, Figure 1a. This conclusion suggests an entirely new strategy to perturb H-bonding through electron transfer. Rather than doing this directly through oxidation/reduction introduced charges, it can also be done indirectly through oxidation-reduction induced proton transfer! 1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society 2013, 135, 18930-18941. Figure 1

How Does the Hydrogen Bonding Interaction Influence the Properties of Polybenzoxazine? An Experimental Study Combined with Computer Simulation

The formation of intra- or intermolecular hydrogen bonding and their influences on polybenzoxazine’s properties were investigated after the model benzoxazine monomers (o-AB-fbz, o-AF-fbz, o-AP-fbz, p-AB-fbz, p-AF-fbz, p-AP-fbz) were synthesized. Because of the different electron-donating abilities of bridging units (benzene, furan and pyridine) in isophthalic acid (IPA), 2,5-furandicarboxylic acid (2,5-FDCA) and 2,6-pyridinedicarboxylic acid (2,6-PDCA), the strength of intramolecular H-bonding involved with the oxygen atom in oxazine ring in o-AB-fbz, o-AF-fbz and o-AP-fbz followed the order of o-AB-fbz > o-AF-fbz > o-AP-fbz, and the strength of the overall H-bonding was arranged as follows: o-AB-fbz < o-AF-fbz < o-AP-fbz. While more intermolecular H-bonding was formed in p-AB-fbz and p-AF-fbz as well as p-AP-fbz. DSC and FT-IR results discovered the relationship between the H-bonding involved with the oxygen atom in oxazine ring and the curing activities of benzoxazines. After curing reaction, the cured ...

Investigation of the conformation and hydrogen bonds in adamantylthiacalix[4]arene by IR spectroscopy and DFT

Thiacalix [4]arene (TC4A), p-tert-butylthiacalix [4]arene (t-BuTC4A) and p-(1-adamantyl)thiacalix [4]arene (1-AdTC4A) IR spectra were studied. Calculated energies and IR spectra of the various conformers (cone, partial cone, 1,2-alternate, and 1,3-alternate) were compared. The cone is the most stable conformation for all studied thiacalix [4]arenes. Theoretical IR spectrum of adamantylthiacalix [4]arene molecule in the form of a cone is consistent with experiment. Classification of bands in the IR spectrum of adamantylthiacalix [4]arene has been implemented. The strength of H-bonding in the thiacalix [4]arenes depends on the type of the substituent. In the conformation of the cone cyclic system of H-bonds are implemented for all the studied thiacalix [4]arenes. Introduction of adamantyl substituents leads to convergence of oxygen atoms in molecules of thiacalix [4]arene and H-bonds are strengthened. Conformation cone of molecules of thiacalix [4]arenes does not change when heated to a temperature of 180 °C and dissolving in a neutral solvent.