Non‐Ideal Behaviour and Solution Interactions in Binary DMSO Solutions

The structure and dynamics of hydrogen‐bonded structures are of significant importance in understanding many binary mixtures. Since self‐diffusion is very sensitive to changes in the molecular weight and shape of the diffusing species, hydrogen‐bonded associated structures in dimethylsulfoxide–methanol (DMSO–MeOH) and DMSO–ethanol (DMSO–EtOH) mixtures are investigated using nuclear magnetic resonance (NMR) diffusion experiments and molecular dynamics (MD) simulations over the entire composition range at 298 K. The self‐diffusion coefficients of DMSO–MeOH and DMSO–EtOH mixtures decrease by up to 15% and 10%, respectively, with DMSO concentration, indicating weaker association as compared to DMSO–water mixtures. The calculated heat of mixing and radial distribution functions reveal that the intermolecular structures of DMSO–MeOH and DMSO–EtOH mixtures do not change on mixing. DMSO–alcohol hydrogen‐bonded dimers are the dominant species in mixtures. Direct comparison of the simulated and experimental data afford greater insights into the structural properties of binary mixtures.     

A Halogen‐Bond Donor Catalyst for Templated Macrocyclization

A halogen‐bond templated 1:1 macrocyclization in solution is reported. Tetra(iodoperfluorophenyl) ethers were used as halogen‐bonded exotemplates in a substoichiometric amount (5 mol %). Pyridine‐containing macrocyclic architectures were formed by ruthenium‐catalyzed tandem metathesis/transfer hydrogenation sequence using sodium borohydride and methanol as non‐dihydrogen hydrogen source. The halogen‐bonded stabilization energies were analyzed relying on density functional theory.     

A polarizable dipole–dipole interaction model for evaluation of the interaction energies for NH···OC and CH···OC hydrogen‐bonded complexes

In this article, a polarizable dipole–dipole interaction model is established to estimate the equilibrium hydrogen bond distances and the interaction energies for hydrogen‐bonded complexes containing peptide amides and nucleic acid bases. We regard the chemical bonds N? H, C?O, and C? H as bond dipoles. The magnitude of the bond dipole moment varies according to its environment. We apply this polarizable dipole–dipole interaction model to a series of hydrogen‐bonded complexes containing the N? H···O?C and C? H···O?C hydrogen bonds, such as simple amide‐amide dimers, base‐base dimers, peptide‐base dimers, and β‐sheet models. We find that a simple two‐term function, only containing the permanent dipole–dipole interactions and the van der Waals interactions, can produce the equilibrium hydrogen bond distances compared favorably with those produced by the MP2/6‐31G(d) method, whereas the high‐quality counterpoise‐corrected (CP‐corrected) MP2/aug‐cc‐pVTZ interaction energies for the hydrogen‐bonded complexes can be well‐reproduced by a four‐term function which involves the permanent dipole–dipole interactions, the van der Waals interactions, the polarization contributions, and a corrected term. Based on the calculation results obtained from this polarizable dipole–dipole interaction model, the natures of the hydrogen bonding interactions in these hydrogen‐bonded complexes are further discussed. © 2013 Wiley Periodicals, Inc.     

How cryoprotectants work: hydrogen-bonding in low-temperature vitrified solutions

Dimethyl sulfoxide (DMSO) increases cell and tissue viability at low temperatures and is commonly used as a cryoprotectant for cryogenic storage of biological materials. DMSO disorders the water hydrogen-bond networks and inhibits ice-crystal growth, though the specific DMSO interactions with water are difficult to characterize. In this study, we use a combination of Fourier Transform infrared spectroscopy (FTIR), molecular dynamics simulations, and vibrational frequency maps to characterize the temperature-dependent hydrogen bonding interactions of DMSO with water from 30 °C to −80 °C. Specifically, broad peaks in O–D stretch vibrational spectra of DMSO and deuterated water (HDO) cosolvent systems show that the hydrogen bond networks become increasingly disrupted compared to pure water. Simulations demonstrated that these disrupted hydrogen bond networks remain largely localized to the first hydration shell of DMSO, which explains the high DMSO concentrations needed to prevent ice crystal formation in cryopreservation applications.

Dimethyl sulfoxide (DMSO) increases cell and tissue viability at low temperatures and is commonly used as a cryoprotectant for cryogenic storage of biological materials.     

Microsolvation of DMSO: Density functional study on the structure and polaraizabilities

The structure and stability for the association of water with dimethyl sulfoxide (DMSO) are investigated using the density functional M06‐2X level theory. Stable complexes are formed by the formation of hydrogen bonding between water and oxygen atom of DMSO molecule, while the electrostatic force between water and DMSO plays a vital role in deciding the structure. The water‐DMSO interactions are stronger than the interwater hydrogen bonds, which can be inferred from the shorter DMSO‐water bond distance compared with the water–water bond distance. The calculated solvent association energy does not saturate, and it remains favorable to attach additional water molecules to the existing water network. The calculated IR spectra shifts supports the formation stronger hydrogen bonding, while the electrostatic potential (ESP) plot supports the existence of weaker electrostatic interaction in the studied clusters. The polarizabilities for the ground state clusters were found to increase monotonically with the cluster size. The presence of additional electrostatic bonding between water and DMSO, devastates the linear hydrogen‐bonding network. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants—Comparison with Salt Bridges and Molecular Systems

We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.     

An AAA–DDD Triply Hydrogen‐Bonded Complex Easily Accessible for Supramolecular Polymers

For a complementary hydrogen‐bonded complex, when every hydrogen‐bond acceptor is on one side and every hydrogen‐bond donor is on the other, all secondary interactions are attractive and the complex is highly stable. AAA–DDD (A=acceptor, D=donor) is considered to be the most stable among triply hydrogen‐bonded sequences. The easily synthesized and further derivatized AAA–DDD system is very desirable for hydrogen‐bonded functional materials. In this case, AAA and DDD, starting from 4‐methoxybenzaldehyde, were synthesized with the Hantzsch pyridine synthesis and Friedländer annulation reaction. The association constant determined by fluorescence titration in chloroform at room temperature is 2.09×10⁷ M ^?1. The AAA and DDD components are not coplanar, but form a V shape in the solid state. Supramolecular polymers based on AAA–DDD triply hydrogen bonded have also been developed. This work may make AAA–DDD triply hydrogen‐bonded sequences easily accessible for stimuli‐responsive materials.     

Spectroscopic analysis and kinetics of intermolecular hydrogen bond formation in poly‐pyridobisimidazole (M5) fiber

Poly‐pyridobisimiazole (M5) single filaments subjected to varying degrees of heat treatment have been analyzed using Fourier Transform Infrared (FTIR) microspectroscopy in transmission mode to detect changes in the state of intermolecular hydrogen bonding as a function of fiber annealing conditions. The FTIR absorbance bands associated with hydrogen bonding in M5 fiber have been identified, and the integrated molar absorption coefficients for the bands of interest have been determined experimentally, which allows to quantify the concentration of N? H vibration groups hydrogen‐bonded (H‐bonded) to water molecules, and the concentration of N? H vibration groups H‐bonded to adjacent polymer chains in the fiber. A dual mechanism kinetic rate expression is used to describe intermolecular H‐bond formation in M5 fiber as a function of annealing conditions, from which an activation energy for H‐bond formation of 14.8 kJ/mol is obtained. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1809–1824, 2009     

Competition between hydrogen and halogen bonding in halogenated 1‐methyluracil: Water systems

The competition between hydrogen‐ and halogen‐bonding interactions in complexes of 5‐halogenated 1‐methyluracil (XmU; X = F, Cl, Br, I, or At) with one or two water molecules in the binding region between C5‐X and C4?O4 is investigated with M06‐2X/6‐31+G(d). In the singly‐hydrated systems, the water molecule forms a hydrogen bond with C4?O4 for all halogens, whereas structures with a halogen bond between the water oxygen and C5‐X exist only for X = Br, I, and At. Structures with two waters forming a bridge between C4?O and C5‐X (through hydrogen‐ and halogen‐bonding interactions) exist for all halogens except F. The absence of a halogen‐bonded structure in singly‐hydrated ClmU is therefore attributed to the competing hydrogen‐bonding interaction with C4?O4. The halogen‐bond angle in the doubly‐hydrated structures (150–160°) is far from the expected linearity of halogen bonds, indicating that significantly non‐linear halogen bonds may exist in complex environments with competing interactions. © 2016 Wiley Periodicals, Inc.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts

A near‐IR spectral study on pure water and aqueous salt solutions is used to investigate stoichiometric concentrations of different types of hydrogen‐bonded water species in liquid water and in water comprising the hydration shell of salts. Analysis of the thermodynamics of hydrogen‐bond formation signifies that hydrogen‐bond making and breaking processes are dominated by enthalpy with non‐negligible heat capacity effects, as revealed by the temperature dependence of standard molar enthalpies of hydrogen‐bond formation and from analysis of the linear enthalpy–entropy compensation effects. A generalized method is proposed for the simultaneous calculation of the spectrum of water in the hydration shell and hydration number of solutes. Resolved spectra of water in the hydration shell of different salts clearly differentiate hydrogen bonding of water in the hydration shell around cations and anions. A comparison of resolved liquid water spectra and resolved hydration‐shell spectra of ions highlights that the ordering of absorption frequencies of different kinds of hydrogen‐bonded water species is also preserved in the bound state with significant changes in band position, band width, and band intensity because of the polarization of water molecules in the vicinity of ions.     

Does Urea Alter the Collective Hydrogen‐Bond Dynamics in Water? A Dielectric Relaxation Study in the Terahertz‐Frequency Region

We report the ultrafast collective hydrogen‐bond dynamics of water in the extended hydration layer of urea by using terahertz time‐domain spectroscopy in the frequency region of 0.3–2.0 THz. The complex dielectric function has been fitted using a Debye relaxation model, and the timescales obtained are in the order of approximately 9 ps and 200 fs for bulk water; this exhibits a considerable acceleration beyond the 4 M urea concentration and indicates a possible disruption in the collective hydrogen‐bonded water‐network structure, which, in turn, provides an indirect support for the water “structure‐breaking” ability of urea. With 5 M urea in the presence of different concentrations of trimethylamine‐N‐oxide (TMAO), it was found that these parameters essentially follow the trend observed for TMAO itself, which signifies that any possible disruption of the water structure by urea is outdone by the strong hydrogen‐bonding ability of TMAO, which explains its ability to revive urea‐denatured proteins to their respective native states.     

Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy

Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm^?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.     

C(Ar)-H···O hydrogen bonds in substituted isobenzofuranone derivatives: geometric, topological, and NMR characterization

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).     

Rapid evaluation of the binding energies in hydrogen-bonded amide-thymine and amide-uracil dimers in gas phase

The binding energies and the equilibrium hydrogen bond distances as well as the potential energy curves of 48 hydrogen‐bonded amide–thymine and amide–uracil dimers are evaluated from the analytic potential energy function established in our lab recently. The calculation results show that the potential energy curves obtained from the analytic potential energy function are in good agreement with those obtained from MP2/6‐311+G** calculations by including the BSSE correction. For all the 48 dimers, the analytic potential energy function yields the binding energies of the MP2/6‐311+G** with BSSE correction within the error limits of 0.50 kcal/mol for 46 dimers, only two differences are larger than 0.50 kcal/mol and the largest one is only 0.60 kcal/mol. The analytic potential energy function produces the equilibrium hydrogen bond distances of the MP2/6‐311+G** with BSSE correction within the error limits of 0.050 Å for all the 48 dimers. The analytic potential energy function is further applied to four more complicated hydrogen‐bonded amide–base systems involving amino acid side chain and β‐sheet. The values of the binding energies and equilibrium hydrogen bond distances obtained from the analytic potential energy function are also in good agreement with those obtained from MP2 calculations with the BSSE correction. These results demonstrate that the analytic potential energy function can be used to evaluate the binding energies in hydrogen‐bonded amide–base dimers quickly and accurately. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Molecular Structure and Dynamics of Water at the Water–Air Interface Studied with Surface‐Specific Vibrational Spectroscopy

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water–air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water–vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen‐bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen‐bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water–air interface using laser‐based surface‐specific vibrational spectroscopy.     

Hydrogen‐bonded complexes of nicotine with simple alcohols

Ab initio and density functional theory studies have been performed on the hydrogen‐bonded complexes of neutral and protonated nicotine with ethanol, methanol, and trifluromethanol to explore their relative stability in a systematic way. Among all the hydrogen‐bonded nicotine complexes considered here, protonated forms in nicotine–ethanol and nicotine–methanol, and neutral form in nicotine–trifluromethanol complexes have been found to be the most stable. In the former two complexes, the proton attached to the pyrrolidine nitrogen acts as a strong hydrogen bond donor, whereas the pyrrolidine nitrogen atom acts as a hydrogen bond acceptor in the latter case. Neutral complex of nicotine with trifluromethanol has been found to possess a very short hydrogen bond (1.57 Å) and basis set superposition error corrected hydrogen bond energy value of 19 kcal/mol. The nature of the various hydrogen bonds formed has been investigated through topological aspects using Bader's atoms in molecules theory. From the calculated topological results, excellent linear correlation is shown to exist among the hydrogen bond length, electron density, and its Laplacian at the bond critical points for all the complexes considered. The natural bond orbital analysis has been carried out to investigate the charge transfer in the nicotine alcohol complexes. In contrast to the blue shifting behavior that is generally exhibited by other C? H···O hydrogen bonds involving sp³ carbon atom, the C? H···O hydrogen bond in the protonated nicotine–ethanol and methanol complexes has been found to be proper with red shifting in nature. © 2011 Wiley Periodicals, Inc.     

Femtosecond Hydrogen Bond Dynamics of Bulk‐like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD‐VSFG Spectroscopy

Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular‐level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two‐dimensional heterodyne‐detected vibrational sum frequency generation (2D HD‐VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<～100 fs) component, followed by the minor sub‐picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub‐picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid–water interface are attributable to the bulk‐like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen‐bonded to the hydrophilic head group.     

Time‐dependent density functional theory study on the electronic excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in aqueous solution: 4AP and 4AP–(H2O)1,2 clusters

The time‐dependent density functional theory (TDDFT) method has been carried out to investigate the excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in hydrogen‐donating water solvent. The infrared spectra of the hydrogen‐bonded solute?solvent complexes in electronically excited state have been calculated using the TDDFT method. We have demonstrated that the intermolecular hydrogen bond C? O···H? O and N? H···O? H in the hydrogen‐bonded 4AP?(H₂O)₂ trimer are significantly strengthened in the electronically excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen‐bonding groups in different electronic states. The hydrogen bonds strengthening in the electronically excited state are confirmed because the calculated stretching vibrational modes of the hydrogen bonding C?O, amino N? H, and H? O groups are markedly red‐shifted upon photoexcitation. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electroniclly excited state of chromophores in hydrogen‐donating solvents exists in many other systems in solution. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Instantaneous and Selective Bare Eye Detection of Inorganic Fluoride Ion by Coumarin–Pyrazole‐Based Receptors

A series of rationally designed coumarin–pyrazole‐based scaffolds, equipped with N―H and C―H hydrogen bond donors ( R1–R5 ) and containing various electron‐withdrawing groups at key positions, are synthesized and characterized in order to investigate their inorganic fluoride binding properties in highly competitive media (1:1 DMSO–water). Only one, 3‐{4‐[(2,4‐dinitrophenyl)‐hydrazonomethyl]‐1‐phenyl‐1H‐pyrazol‐3‐yl}‐chromen‐2‐one ( R1 ), of the five compounds synthesized, is found to be capable to selectively detect inorganic fluoride via naked eye amongst other anionic species in aqueous media. Qualitative and spectroscopic studies exhibit that receptor R1 has the potential of showing instantaneous change of color from yellow to pink upon addition of sodium fluoride (0.95 ppm) in aqueous media, at concentration lower than that recommended by World Health Organization (1 ppm). Intensity of color increases with increasing fluoride concentration till 5 ppm, beyond which intensity of color change becomes saturated. This has established the applicability of this receptor for assessment of the level of fluoride in water. Anion binding studies carried out by UV–visible titration portrayed substantial bathochromic peak shift from 410 to 495 nm, upon addition of varying concentrations of aqueous sodium fluoride solution, which has validated the color change. Jobs plot data confirmed 1:1 stoichiometry between R1 and fluoride ion. ¹H‐NMR investigation reveals that the deprotonation of N―H hydrogen donor group of receptor R1 and its interaction with fluoride ion is responsible for the observed color change.