Showing papers on "Hydrogen bond published in 2010"
TL;DR: Experimental as well as computational studies indicate that halogen and other sigma-hole interactions can be competitive with hydrogen bonding, which itself can be viewed as a subset of s Sigma-hole bonding.
Abstract: A halogen bond is a highly directional, electrostatically-driven noncovalent interaction between a region of positive electrostatic potential on the outer side of the halogen X in a molecule R–X and a negative site B, such as a lone pair of a Lewis base or the π-electrons of an unsaturated system. The positive region on X corresponds to the electronically-depleted outer lobe of the half-filled p-type orbital of X that is involved in forming the covalent bond to R. This depletion is labeled a σ-hole. The resulting positive electrostatic potential is along the extension of the R–X bond, which accounts for the directionality of halogen bonding. Positive σ-holes can also be found on covalently-bonded Group IV–VI atoms, which can similarly interact electrostatically with negative sites. Since positive σ-holes often exist in conjunction with negative potentials on other portions of the atom's surface, such atoms can interact electrostatically with both nucleophiles and electrophiles, as has been observed in surveys of crystallographic structures. Experimental as well as computational studies indicate that halogen and other σ-hole interactions can be competitive with hydrogen bonding, which itself can be viewed as a subset of σ-hole bonding.
1,332 citations
TL;DR: These studies suggest the possibility of tuning the properties of GO composites by altering the density of functional groups on individual platelets, the water content, and possibly the functional groups participating in hydrogen bonding with interlayer water molecules.
Abstract: A multilayered composite structure formed by a random stacking of graphene oxide (GO) platelets is an attractive candidate for novel applications in nanoelectromechanical systems and paper-like composites. We employ molecular dynamics simulations with reactive force fields to elucidate the structural and mechanical properties of GO paper-like materials. We find that the large-scale properties of these composites are controlled by hydrogen bond networks that involve functional groups on individual GO platelets and water molecules within the interlayer cavities. Water content controls both the extent and collective strength of these interlayer hydrogen bond networks, thereby affecting the interlayer spacing and elastic moduli of the composite. Additionally, the chemical composition of the individual GO platelets also plays a critical role in establishing the mechanical properties of the composite—a higher density of functional groups leads to increased hydrogen bonding and a corresponding increase in stiffn...
674 citations
TL;DR: This tutorial review will focus on the privileged C-H hydrogen bond donor of the 1,2,3-triazole ring systems as elucidated from anion-binding studies with macrocyclic triazolophanes and other receptors.
Abstract: The supramolecular chemistry of anions provides a means to sense and manipulate anions in their many chemical and biological roles. For this purpose, Click chemistry facilitated the synthetic creation of new receptors and thus, an opportunity to aid in the recent re-examination of CH⋯anion hydrogen bonding. This tutorial review will focus on the privileged C–H hydrogen bond donor of the 1,2,3-triazole ring systems as elucidated from anion-binding studies with macrocyclic triazolophanes and other receptors. Triazolophanes are shape-persistent and planar macrocycles that direct four triazole and four phenylene CH groups into a 3.7 A cavity. They display strong (log K(Cl−) = 7), size-dependent halide binding (Cl− > Br− ≫ F− ≫ I−) and a rich set of binding equilibria. For instance, the too large iodide (4.4 A) can be sandwiched between two pyridyl-based triazolophanes with extreme positive cooperativity. Computational studies verify the triazole's hydrogen bond strength indicating it approaches the traditional NH donors from pyrrole. These examples, those of transport, sensing (e.g., ion-selective electrodes), templation, and versatile synthesis herald the use of triazoles in anion-receptor chemistry.
554 citations
TL;DR: It is found that changes in these properties depend on a subtle balance between the strength of the hydrogen bond interaction, geometry of the newly formed ring system, and the relative energies of the open and closed conformations in polar and unpolar environments.
Abstract: The formation of intramolecular hydrogen bonds has a very pronounced effect on molecular structure and properties. We study both aspects in detail with the aim of enabling a more rational use of this class of interactions in medicinal chemistry. On the basis of exhaustive searches in crystal structure databases, we derive propensities for intramolecular hydrogen bond formation of five- to eight-membered ring systems of relevance in drug discovery. A number of motifs, several of which are clearly underutilized in drug discovery, are analyzed in more detail by comparing small molecule and protein-ligand X-ray structures. To investigate effects on physicochemical properties, sets of closely related structures with and without the ability to form intramolecular hydrogen bonds were designed, synthesized, and characterized with respect to membrane permeability, water solubility, and lipophilicity. We find that changes in these properties depend on a subtle balance between the strength of the hydrogen bond interaction, geometry of the newly formed ring system, and the relative energies of the open and closed conformations in polar and unpolar environments. A number of general guidelines for medicinal chemists emerge from this study.
453 citations
TL;DR: A detailed study of the thermodynamics of the halogen-bonding interaction in organic solution is presented in this paper, where 19F NMR titrations are used to determine association constants for the interactions of a variety of Lewis bases with fluorinated iodoalkanes and iodoarenes.
Abstract: A detailed study of the thermodynamics of the halogen-bonding interaction in organic solution is presented. 19F NMR titrations are used to determine association constants for the interactions of a variety of Lewis bases with fluorinated iodoalkanes and iodoarenes. Linear free energy relationships for the halogen bond donor ability of substituted iodoperfluoroarenes XC6F4I are described, demonstrating that both substituent constants (σ) and calculated molecular electrostatic potential surfaces are useful for constructing such relationships. An electrostatic model is, however, limited in its ability to provide correlation with a more comprehensive data set in which both halogen bond donor and acceptor abilities are varied: the ability of computationally derived binding energies to accurately model such data is elucidated. Solvent effects also reveal limitations of a purely electrostatic depiction of halogen bonding and point to important differences between halogen bonding and hydrogen bonding.
318 citations
TL;DR: It is shown that an intimate interaction between backbone amides likewise arises from the delocalization of a lone pair of electrons from an oxygen atom to the antibonding orbital of the subsequent carbonyl group.
Abstract: Hydrogen bonds between backbone amides are common in folded proteins. Here, we show that an intimate interaction between backbone amides likewise arises from the delocalization of a lone pair of electrons (n) from an oxygen atom to the antibonding orbital (π*) of the subsequent carbonyl group. Natural bond orbital analysis predicted significant n→π* interactions in certain regions of the Ramachandran plot. These predictions were validated by a statistical analysis of a large, non-redundant subset of protein structures determined to high resolution. The correlation between these two independent studies is striking. Moreover, the n→π* interactions are abundant, and especially prevalent in common secondary structures such as α-, 310-, and polyproline II helices, and twisted β-sheets. In addition to their evident effects on protein structure and stability, n→π* interactions could play important roles in protein folding and function, and merit inclusion in computational force fields.
302 citations
TL;DR: The organic molecular porous material 1 obtained by recrystallization of cucurbit[6]uril (CB[6]) from HCl shows a high CO(2) sorption capacity at 298 K, 1 bar and shows the highest selectivity of CO( 2) over CO among the known porous materials so far.
Abstract: The organic molecular porous material 1 obtained by recrystallization of cucurbit[6]uril (CB[6]) from HCl shows a high CO2 sorption capacity at 298 K, 1 bar. Most interestingly, 1 showed the highest selectivity of CO2 over CO among the known porous materials so far. The remarkable selectivity of CO2 may be attributed to the exceptionally high enthalpy of adsorption (33.0 kJ/mol). X-ray crystal structure analysis of CO2 adsorbed 1 revealed three independent CO2 sorption sites: two in the 1D channels (A and B) and one in the molecular cavities (C). The CO2 molecules adsorbed at sorption site A near the wall of the 1D channels interact with 1 through hydrogen bonding and at the same time interact with those at site B mainly through quadrupole−quadrupole interaction in a T-shaped arrangement. Interestingly, two CO2 molecules are included in the CB[6] cavity (site C), interacting not only with the carbonyl groups of CB[6] but also with each other in a slipped-parallel geometry. The exceptionally selective CO2 ...
267 citations
TL;DR: In this paper, the effect of CN groups and solvent on the enthalpies of homolytic and heterolytic N-H bond cleavage was studied using DFT/UB3LYP/6-31++G∗∗ method.
Abstract: In this article, we have studied p-phenylenediamine (PPD) and tetracyano-p-phenylenediamine (TCPPD) molecules in order to study the effect of CN groups and the solvent effect on the enthalpies of homolytic and heterolytic N–H bond cleavage. Geometries of the molecules and reaction enthalpies related to hydrogen atom transfer, single electron transfer–proton transfer (SET–PT) mechanism and sequential proton loss electron transfer (SPLET) mechanisms were studied using DFT/UB3LYP/6-31++G∗∗ method. Ab initio MP2/6-31++G∗∗ method was used as the reference for the geometry calculation of the two molecules in vacuum. Solvent contribution to the enthalpies was computed employing integral equation formalism IEF-PCM method. Obtained results show that solvent is able to cause significant change in the reaction enthalpies of the stepwise SET–PT and SPLET mechanisms of hydrogen splitting-off from NH2 group. This may result in the change in thermodynamically preferred mechanism. Solvents also attenuate the CN-substituent effect in the case of SET–PT and SPLET mechanisms.
257 citations
TL;DR: Ultrafast infrared polarization- and wavelength-selective pump-probe experiments can measure the orientational relaxation of water and distinguish water at an interface from water removed from an interface, providing quantitative determination of the dynamics of water as a function of the size and nature of the confining structure.
Abstract: The properties of water depend on its extended hydrogen bond network and the continual picosecond-time scale structural evolution of the network. Water molecules in confined environments with pools a few nanometers in diameter or at interfaces undergo hydrogen bond structural dynamics that differ drastically from the dynamics they undergo in bulk water. Orientational motions of water require hydrogen bond network rearrangement. Therefore, observations of orientational relaxation in nanoscopic water systems provide information about the influence of confinement and interfaces on hydrogen bond dynamics. Ultrafast infrared polarization- and wavelength-selective pump-probe experiments can measure the orientational relaxation of water and distinguish water at an interface from water removed from an interface. These experiments can be applied to water in reverse micelles (spherical nanopools). The results provide quantitative determination of the dynamics of water as a function of the size and nature of the confining structure.
249 citations
TL;DR: NMR data of the model system clearly suggest hydrogen bonding is formed between hydroxyls of cellobiose and both anion and cation of EmimAc, and in situ and variable-temperature NMR spectra suggest the above mechanism also works in the real system.
Abstract: The dissolution mechanism of cellulose in ionic liquids has been investigated by using cellobiose and 1-ethyl-3-methylimidazolium acetate (EmimAc) as a model system under various conditions with conventional and variable-temperature NMR spectroscopy. In DMSO-d6 solution, NMR data of the model system clearly suggest that hydrogen bonding is formed between hydroxyls of cellobiose and both anion and cation of EmimAc. The CH3COO− anion favors the formation of hydrogen bonds with hydrogen atoms of hydroxyls, and the aromatic protons in bulky cation [Emim]+, especially the most acidic H2, prefer to associate with the oxygen atoms of hydroxyls with less steric hindrance, while after acetylation of all hydroxyls in cellobiose the interactions between cellobiose octaacetate and EmimAc become very weak, implying that hydrogen bonding is the major reason of cellobiose solvation in EmimAc. Meanwhile the stoichiometric ratio of EmimAc/hydroxyl is estimated to be between 3:4 and 1:1 in the primary solvation shell, suggesting that there should be one anion or cation to form hydrogen bonds with two hydroxyl groups simultaneously. In situ and variable-temperature NMR spectra suggest the above mechanism also works in the real system.
247 citations
TL;DR: A program of direct spectroscopic observation of hydrogen bonds is initiated by successively increasing hydrogen bond abilities in a set of well-chosen imidazolium-based ionic liquids by choosing the same anion for all the ILs and successfully increasing the H-bond abilities of the depicted cations.
Abstract: Ionic liquids (ILs) have many valuable applications in chemistry and technology. They are understood as liquids consisting entirely of ions and having melting points below 100 8C. The interesting properties of ionic liquids are governed by the type and strength of interaction between its constituents. It is assumed that hydrogen bonding plays an important role for the properties and reaction dynamics of these Coulomb systems. The presence of hydrogen bonding in the structure of 1-alkyl-3-methylimidazolium salt was first reported by Seddon et al. in 1986. Since then, evidence for hydrogen bonding has been obtained from X-ray diffraction and mid-infrared and NMR spectroscopy. Local and directional interactions, such as hydrogen bonds, in imidazoliumbased ILs are indicated by shorter C H···anion distances, redshifted C H frequencies, and downfield-shifted C H proton chemical shifts. Indications of hydrogen bonding is also provided by theoretical studies. Recently however, some authors have strongly challenged the presence of hydrogen bonds in ionic liquids, and claimed that hydrogen bonding need not be invoked for explaining IL properties. For this reason, we initiated a program of direct spectroscopic observation of hydrogen bonds by successively increasing hydrogen bond abilities in a set of well-chosen imidazolium-based ionic liquids. Studying and understanding these interactions is a real challenge, and in particular for ILs. For imidazolium-based ILs, hydrogen bonding in infrared spectroscopy has been primarily concerned with the shift (Dns) of the C H stretching frequency in the mid-infrared region. However, it is more pertinent to observe the stretching (ns) and bending (nb) frequencies of hydrogen bonds themselves in far-infrared (FIR) spectra. These modes are shown and assigned in Scheme 1. The force constants obtained from these frequencies provide information about the hydrogen-bonding potential function as well as being a measure of the bond strength. Unambiguous assignment of the hydrogen-bond frequencies has provided a major difficulty in the FIR investigations. In particular, the low-frequency spectrum is surprisingly rich in information. Even for light molecules, low-energy intramolecular vibrations, such as torsions and certain skeletal motions, fall in the FIR region. Recently, we presented the low-frequency vibrational spectra of imidazolium-based ionic liquids in the range 30–300 cm 1 obtained by FIR spectroscopy. We could show that the wavenumbers above 150 cm 1 can be assigned to intramolecular bending and wagging modes of cations and anions in the ionic liquid. The contributions below 150 cm 1 were assigned to the bending and stretching vibrational modes of the intermolecular anion– cation interactions. This assignment was supported by DFT calculations, which gave wavenumbers for the bending and stretching modes of ion pairs and ion-pair aggregates in this frequency range. We also suggested that the frequencies and intensities of the FIR vibrational bands may contribute to the development of forcefields in molecular dynamics simulations. However, important issues could not be clarified definitively. To what extend does the intermolecular vibrational band stem from localized short-ranged H-bonds and/or from non-localized long-ranged Coulomb forces? This question is addressed herein by choosing the same anion for all the ILs and successively increasing the H-bond abilities of the depicted cations. The second unclear point concerns the origin of the frequency shifts for the intermolecular vibrational bands. Following the solution of the equation for simple harmonic oscillators, w = (k/m), those shifts can occur either from the changing force constant and/or the reduced masses. This problem is addressed herein by choosing cations that give comparable or even the same reduced masses in combination with the same anion. If that is the case, the frequency shifts can be attributed to changing force constants and thus the changing strength of cation–anion interaction only. Scheme 1. The stretching (ns) and bending (nb) frequencies of a hydrogen bond shown for the C2 H···A interaction in a 1,3-dimethylimidazolium cation.
TL;DR: In this paper, the properties of halogen and hydrogen σ-hole interactions with negative sites B and negative sites C were investigated. And they were found to correlate well with the positive electrostatic potentials created at the negative sites by halogens and hydrogens.
Abstract: Halogen bonding, RX···B, and hydrogen bonding, RH···B, are electrostatically-driven noncovalent interactions of covalently-bonded halogens RX and hydrogens RH with negative sites B. A significant difference between halogen and hydrogen bonding is that the former is typically near-linear (the R-X-B angles are close to 180°), whereas the latter is more likely to be nonlinear (R-H-B angles sometimes considerably less than 180°). In this work, we have looked at the properties of several RBr···B and RH···B complexes as functions of these angles. The differences in the directional tendencies in these interactions can be attributed to the presence of nonbonding valence electrons on the bromines, and their absence on hydrogen. We also found that for a given negative site, the halogen and hydrogen bonding interaction energies correlate very well with the positive electrostatic potentials created at it by RX and RH. This attests to the electrostatically-driven nature of these interactions. Overall, this study provides support for regarding both halogen and hydrogen bonding as subsets of σ-hole interactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010
TL;DR: DFT calculations have uncovered the additional role of the pivalate additive as a promoter of phosphine dissociation from the Pd(II) intermediate, enabling the CMD transition state and kinetic studies performed provide rare experimental support for a concerted metalation-deprotonation (CMD) transition state.
Abstract: The reactivity of C(sp3)−H bonds adjacent to a nitrogen atom can be tuned to allow intramolecular alkane arylation under Pd(0) catalysis. Diminishing the Lewis basicity of the nitrogen lone pair is crucial for this catalytic activity. A range of N-methylamides and sulfonamides react exclusively at primary C(sp3)−H bonds to afford the products of alkane arylation in good yields. The isolation of a Pd(II) reaction intermediate has enabled an evaluation of the reaction mechanism with a focus on the role of the bases in the C(sp3)−H bond cleaving step. The results of these stoichiometric studies, together with kinetic isotope effect experiments, provide rare experimental support for a concerted metalation−deprotonation (CMD) transition state, which has previously been proposed in alkane C(sp3)−H arylation. Moreover, DFT calculations have uncovered the additional role of the pivalate additive as a promoter of phosphine dissociation from the Pd(II) intermediate, enabling the CMD transition state. Finally, kinet...
TL;DR: It is demonstrated that the fluorescent state of compound 3 can be assigned as a metal-to-ligand charge transfer (MLCT) state and a new fluorescence modulation mechanism by hydrogen bonding is proposed to explain different fluorescence emissions of 3 in hydrogen-bonding solvents and nonhydrogen-b Bonding Solvents.
Abstract: The bimetallic platinum complexes are known as unique building blocks and arewidely utilized in the coordination-driven self-assembly of functionalized supramolecular metallacycles Hence, photophysical study of the bimetallic platinum complexes will be very helpful for the understanding on the optical properties and further applications of coordination-driven self-assembled supramolecular metallacycles Herein, we report steady-state and time-resolved spectroscopic experiments as well as quantum chemistry calculations to investigate the significant intermolecular hydrogen bonding effects on the intramolecular charge transfer (ICT) fluorescence of a bimetallic platinum compound 4,4'-bis(trans-Pt(PEt(3))(2)OTf)benzophenone 3 in solution We demonstrated that the fluorescent state of compound 3 can be assigned as a metal-to-ligand charge transfer (MLCT) state Moreover, it was observed that the formation of intermolecular hydrogen bonds can effectively lengthen the fluorescence lifetime of 3 in alcoholic solvents compared with that in hexane solvent At the same time, the electronically excited states of 3 in solution are definitely changed by intermolecular hydrogen bonding interactions As a consequence, we propose a new fluorescence modulation mechanism by hydrogen bonding to explain different fluorescence emissions of 3 in hydrogen-bonding solvents and nonhydrogen-bonding solvents
TL;DR: In this article, the authors used ATR-FTIR spectroscopy to study cellulose II fabrics with aqueous sodium hydroxide solution under varying condition parameters, and found that maximum molecular reorganisation occurs in the amorphous and quasi-crystalline phases of the cellulose-II polymer.
Abstract: Cellulose is a linear 1,4-β-glucan polymer where the units are able to form highly ordered structures, as a result of extensive interaction through intra- and intermolecular hydrogen bonding of the three hydroxyl groups in each cellulose unit. Alkali has a substantial influence on morphological, molecular and supramolecular properties of cellulose II polymer fibres causing changes in crystallinity. These physical changes were observed herein using ATR-FTIR spectroscopy, following continuous treatment of the cellulose II fabrics with aqueous sodium hydroxide solution under varying condition parameters. Post-treatment, maxima for total crystallinity index and lateral order index, and minima for hydrogen bond intensity, were observed at concentrations of 3.3 and 4.5 mol dm−3 NaOH, when treated at 25 °C and 40 °C, respectively. Under these treatment conditions, it is proposed that maximum molecular reorganisation occurs in the amorphous and quasi-crystalline phases of the cellulose II polymer.
TL;DR: Investigation of molecular interactions between water and EMIES showed that the hydrogen bond involving the -SO(3) group in the ethyl sulfate anion (ES) was enhanced, while those involving the aromatic C-H groups of 1-ethyl-3-methylimidazolium cation (EMI) were weakened in the presence of water.
Abstract: 1-Ethyl-3-methylimidazolium ethyl sulfate (EMIES) is a novel ionic liquid with potential industrial applications. Attenuated total reflectance infrared spectroscopy, 1H NMR spectroscopy, and quantum chemical calculations were employed to investigate the molecular interactions between water and EMIES. The infrared spectra were analyzed by two methods: excess spectroscopy and two-dimensional correlation spectroscopy. This showed that the hydrogen bond involving the −SO3 group in the ethyl sulfate anion (ES) was enhanced, while those involving the aromatic C−H groups of 1-ethyl-3-methylimidazolium cation (EMI) were weakened in the presence of water. During the process of increasing water concentration, the hydrogen bonding interaction between H2O and SO is prior to that between H2O and the C−H group on the imidazolium ring. At low concentrations, water interacts selectively with −SO3 in the ethyl sulfate anion, while, at high concentrations (mole fraction of water equal or greater than 0.6), it can also form...
TL;DR: In this paper, the authors studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy.
Abstract: Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly.
TL;DR: This review discusses the different classes of synthetic receptors for organic ammonium ion recognition and illustrates the scope and limitations of each class with selected examples from the recent literature.
Abstract: Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation–π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.
TL;DR: In this paper, a method for numerical calculation of accurate references for the kinetic energy component of the embedding potential is presented. But the method is limited to a set of model systems, where the subsystems are connected by hydrogen bonds of various strength.
Abstract: The frozen-density embedding (FDE) scheme [Wesolowski and Warshel, J. Phys. Chem. 97, 8050 (1993)] relies on the use of approximations for the kinetic-energy component vT[ρ1,ρ2] of the embedding potential. While with approximations derived from generalized-gradient approximation kinetic-energy density functional weak interactions between subsystems such as hydrogen bonds can be described rather accurately, these approximations break down for bonds with a covalent character. Thus, to be able to directly apply the FDE scheme to subsystems connected by covalent bonds, improved approximations to vT are needed. As a first step toward this goal, we have implemented a method for the numerical calculation of accurate references for vT. We present accurate embedding potentials for a selected set of model systems, in which the subsystems are connected by hydrogen bonds of various strength (water dimer and F–H–F−), a coordination bond (ammonia borane), and a prototypical covalent bond (ethane). These accurate potent...
TL;DR: It is demonstrated that unsaturated acyl phosphonates are excellent hydrogen-bond acceptors in enantioselective organocatalysis by employing chiral thioureas or squaramides as catalysts, thereby providing successful relay of the chirality from the catalyst to the substrate.
Abstract: This study demonstrates that unsaturated acyl phosphonates are excellent hydrogen-bond acceptors in enantioselective organocatalysis. By employing chiral thioureas or squaramides as catalysts, the acyl phosphonates are effectively coordinated and activated by hydrogen bonding, thereby providing successful relay of the chirality from the catalyst to the substrate. A variety of highly stereoselective conjugate additions to α,β-unsaturated acyl phosphonates were performed, using different carbon-based nucleophiles such as oxazolones, indoles, and 1,3-dicarbonyl compounds. The reaction concept has been developed to be a double nucleophilic reaction, and it is shown that the acyl phosphonates serve as masked ester or amide equivalents, which upon quenching with the second nucleophile generate the parent structures in situ. Accordingly, formal C−C bond formation reactions of ester and amide substrates are achieved, affording a broad spectrum of optically active conjugate adducts in good yields and excellent ena...
TL;DR: The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO4 solution.
Abstract: The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the H-bond exchange mechanism in aqueous NaClO4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extracted an average jump angle of 49 +/- 4 degrees, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO4 solution.
TL;DR: The rhodium-catalyzed synthesis of silafluorenes from biphenylhydrosilanes is described, and a ladder-type bis-silicon-bridged p-terphenyl could also be synthesized.
Abstract: The rhodium-catalyzed synthesis of silafluorenes from biphenylhydrosilanes is described. This highly efficient reaction proceeds via both Si−H and C−H bond activation, producing only H2 as a side product. Using this method, a ladder-type bis-silicon-bridged p-terphenyl could also be synthesized.
TL;DR: A profound study of halogen complexes in the framework of the AIM theory was performed, and a good correlation between the density at the intermolecular bond critical point and the energy interaction was found.
Abstract: Density functional theory (DFT) and atoms in molecules theory (AIM) were used to study the characteristic of the noncovalent interactions in complexes formed between Lewis bases (NH3, H2O, and H2S) and Lewis acids (ClF, BrF, IF, BrCl, ICl, and IBr). In order to compare halogen and hydrogen bonds interactions, this study included hydrogen complexes formed by some Lewis bases and HF, HCl, and HBr Lewis acids. Ab initio, wave functions were generated at B3LYP/6-311++G(d,p) level with optimized structures at the same level. Criteria based on a topological analysis of the electron density were used in order to characterize the nature of halogen interactions in Lewis complexes. The main purpose of the present work is to provide an answer to the following questions: (a) why can electronegative atoms such as halogens act as bridges between two other electronegative atoms? Can a study based on the electron charge density answer this question? Considering this, we had performed a profound study of halogen complexes...
TL;DR: Hydrophobic association does indeed favor a crystal-like structure over solution, as might be expected, and hydrogen bonding also favored the crystal packing, possibly in part because of the high entropic cost for hydrating glucose hydroxyl groups, which significantly restricts the configurational freedom of the hydrogen-bonded waters.
TL;DR: The protein dynamical transition and its connection with the liquid-glass transition (GT) of hydration water and aqueous solvents are reviewed in this paper, where the protein solvation shell exhibits a regular glass transition, characterized by steps in the specific heat and the thermal expansion coefficient at the calorimetric glass temperature T(G) approximately 170 K.
TL;DR: This Account briefly review and update the use of the method of isotopic perturbation to search for a symmetric, centered, or single-well-potential H-bond in solution, and shows previous evidence for "short, strong, low-barrier" H- bonds to be based on ambiguous comparisons.
Abstract: In a symmetric hydrogen bond (H-bond), the hydrogen atom is perfectly centered between the two donor atoms. The energy diagram for hydrogen motion is thus a single-well potential, rather than the double-well potential of a more typical H-bond, in which the hydrogen is covalently bonded to one atom and H-bonded to the other. Examples of symmetric H-bonds are often found in crystal structures, and they exhibit the distinctive feature of unusually short length: for example, the O−O distance in symmetric OHO H-bonds is found to be less than 2.5 A. In comparison, the O−O distance in a typical asymmetric H-bond, such as ROH···OR2, ranges from about 2.7 to 3.0 A. In this Account, we briefly review and update our use of the method of isotopic perturbation to search for a symmetric, centered, or single-well-potential H-bond in solution. Such low-barrier H-bonds are thought to be unusually strong, owing perhaps to the resonance stabilization of two identical resonance forms [A−H···B ↔ A···H−B]. This presumptive bon...
TL;DR: This reaction is remarkable, as it does not involve any redox-active catalyst or reagent commonly used for C H bond activation and thus holds great potential for the design of sustainable syntheses with simple and cheap reagents and low levels of waste.
Abstract: Activated benzylic CH2-groups in xanthene (I) or dihydroacridines (IV) undergo acid-catalyzed oxidative coupling towards carbonyl compounds.
TL;DR: The strikingly different behavior of the ylide-like, N-heterocyclic silylene LSi versus its LSi-->Ni(CO)(3) complex 13 to activate E-H bonds (E = S, N) of small molecules is reported.
Abstract: The strikingly different behavior of the ylide-like, N-heterocyclic silylene LSi: (5: L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-iPrC6H3) versus its LSi→Ni(CO)3 complex 13 to activate E−H bonds (E = S, N) of small molecules is reported. Remarkably, conversion of 5 with hydrogen sulfide leads exclusively to the first isolable silathioformamide, L′Si(═S)H (16: L′ = CH[C(Me)NAr]2; Ar = 2,6-iPrC6H3) with a donor-supported Si═S double bond and four-coordinate silicon. The latter result demonstrates the unusual ambivalent reactivity of 5 by combining two modes of reactivity involving S−H bond activation and subsequent 1,4- and 1,1-addition, respectively. In addition, 5 can serve as a ligand with well-balanced σ-donor and π-acceptor capabilities toward transition metals. This has been demonstrated by the isolable [Ni0(arene)] complexes 12a−e (arene = MenC6H6−n, n = 0−3), which are ideal precursors for the formation of the corresponding Ni(CO)3 complex 13. The latter activates a S−H bond in hydrogen sulfide, too, but the...
TL;DR: The disruption of antiaromaticity upon reaction of the borole with H(2) provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.
Abstract: Facile metal-free splitting of molecular hydrogen (H2) is crucial for the utilization of H2 without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen−hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H2 in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons α to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H2 provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.