1H-NMR investigations of the conformation of aryl-(hydroxynaphthyl)-methylpiperidines. Intramolecular interactions,IV

Summary The¹H-NMR spectra of aryl-(hydroxynaphthyl)-methylpiperidines, which are model compounds for intramolecular hydrogen bonding, have been analyzed in order to investigate their conformations in solution. As dynamic phenomena can be assumed from line broadening, low temperature spectra have been measured to evaluate the coalescence temperatures and the energy barriers. The latter have been discussed with respect to the size and position of selected substituents. It can be shown that the molecules exist in one energetically favorable conformation with the aryl ring perpendicular to the plane of the naphthol ring system. The interaction between the naphthol ring and the aryl ring influences the conformation at the piperidine ring moiety. This effect leads to an increase of the inversion barrier of the piperidine residue.On Sabbatical Leave from Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan     

Crystal structure and conformational analysis of 1-[N-(2-bromophenyl)]naphthaldimine

1-[N-(2-bromophenyl)]naphthaldimine (C₁₇H₁₂NOBr) (1) was synthesised and its crystal structure was determined. The compound 1 is orthorhombic, space group P2₁2₁2₁ with a=12.653(2), b=13.7311(14), Z=4, R=0.032 for 499 reflections I>2σ(I)]. There is an intramolecular hydrogen bond of distance 2.473(3) Å between the hydroxyl oxygen atom and imine nitrogen atom, the hydrogen atom essentially being bonded to the oxygen atom. Minimum energy conformation was calculated as a function of torsion angle θ (C10-C11-N1-C12) varied every 5 degrees. The optimized geometry of the crystal structure corresponding to the non-planar conformation is the most stable conformation in all calculations. The results strongly indicate that the minimum energy conformation is primarily determined by hydrogen-hydrogen repulsions between the ortho-hydrogen atoms on the aldehyde rings. Complementary IR, ¹H NMR and UV measurements in solution and in the solid state were carried out.     

Statistical theory for hydrogen bonding fluid system of A_aD_d type (I): The geometrical phase transition

The influence of hydrogen bonds on the physical and chemical properties of hydrogen bonding fluid system of AaDd type is investigated from two viewpoints by the principle of statistical mechanics. In detail, we proposed two new ways that can be used to obtain the equilibrium size distribution of the hydrogen bonding clusters, and derived the analytical expression of a relationship between the hydrogen bonding free energy and hydrogen bonding degree. For the nonlinear hydrogen bonding systems, it is shown that the sol-gel phase transition can take place under proper conditions, which is further proven to be a kind of geometrical phase transition rather than a thermodynamic one. Moreover, several problems associated with the geometrical phase transition and liquid-solid phase transition in nonlinear hydrogen bonding systems are discussed.     

Conformational study of (Z)‐5‐(4‐chlorobenzylidene)‐2‐[4‐(2‐hydroxyethyl)piperazin‐1‐yl]‐3H‐imidazol‐4(5H)‐one in different environments: insight into the structural properties of bacterial efflux pump inhibitors

The 2‐amine derivatives of 5‐arylidene‐3H‐imidazol‐4(5H )‐one are a new class of bacterial efflux pump inhibitors, the chemical compounds that are able to restore antibiotic efficacy against multidrug resistant bacteria. 5‐Arylidene‐3H‐imidazol‐4(5H )‐ones with a piperazine ring at position 2 reverse the mechanisms of multidrug resistance (MDR) of the particularly dangerous Gram‐negative bacteria E. coli by inhibition of the efflux pump AcrA/AcrB/TolC (a main multidrug resistance mechanism in Gram‐negative bacteria, consisting of a membrane fusion protein, AcrA, a Resistant‐Nodulation‐Division protein, AcrB, and an outer membrane factor, TolC). In order to study the influence of the environment on the conformation of (Z )‐5‐(4‐chlorobenzylidene)‐2‐[4‐(2‐hydroxyethyl)piperazin‐1‐yl]‐3H‐imidazol‐4(5H )‐one, ( 3 ), two different salts were prepared, namely with picolinic acid {systematic name: 4‐[(Z )‐4‐(4‐chlorobenzylidene)‐5‐oxo‐3,4‐dihydro‐1H‐imidazol‐2‐yl]‐1‐(2‐hydroxyethyl)piperazin‐1‐ium pyridine‐2‐carboxylate, C₁₆H₂₀ClN₄O₂⁺·C₆H₄NO₂⁻, ( 3 a )} and 4‐nitrophenylacetic acid {systematic name: 4‐[(Z )‐4‐(4‐chlorobenzylidene)‐5‐oxo‐3,4‐dihydro‐1H‐imidazol‐2‐yl]‐1‐(2‐hydroxyethyl)piperazin‐1‐ium 2‐(4‐nitrophenyl)acetate, C₁₆H₂₀ClN₄O₂⁺·C₈H₆NO₄⁻, ( 3 b )}. The crystal structures of the new salts were determined by X‐ray diffraction. In both crystal structures, the molecule of ( 3 ) is protonated at an N atom of the piperazine ring by proton transfer from the corresponding acid. The carboxylate group of picolinate engages in hydrogen bonds with three molecules of the cation of ( 3 ), whereas the carboxylate group of 4‐nitrophenylacetate engages in hydrogen bonds with only two molecules of ( 3 ). As a consequence of these interactions, different orientations of the hydroxyethyl group of ( 3 ) are observed. The crystal structures are additionally stabilized by both C—H…N [in ( 3 a )] and C—H…O [in ( 3 a ) and ( 3 b )] intermolecular interactions. The geometry of the imidazolone fragment was compared with other crystal structures possessing this moiety. The tautomer observed in the crystal structures presented here, namely 3H‐imidazol‐4(5H )‐one [systematic name: 1H‐imidazol‐5(4H )‐one], is also that most frequently observed in other structures containing this heterocycle.     

Structure of the 2-isopropylaminoethanol isolated molecule: Conformational analysis and intramolecular interactions

In this paper, a systematic exploration of all the possible conformers of 2-isopropylaminoethanol (2-IPAE) was carried out using the Density Functional Theory (B3LYP) and the 6-311++G(d,p) basis set. At this level, 66 unique conformers within a Gibbs energy range of ca. 31 kJ mol⁻¹ were found in the potential energy surface and their geometrical and thermodynamic properties were determined and discussed. A significant molecular strain was evidenced by the dihedrals and distances between non-bonded hydrogen atoms. According to the geometrical parameters, a O–H···N hydrogen bond was found to be present in the three most stable conformers, representing 68% of the conformational composition at 298.15 K. The energetic and geometrical data derived from the DFT calculations were further complemented by a NBO analysis of the most stable conformers.     

Statistical theory for hydrogen bonding fluid system of AaDd type (I): The geometrical phase transition

The influence of hydrogen bonds on the physical and chemical properties of hydrogen bonding fluid system of A _a D _d type is investigated from two viewpoints by the principle of statistical mechanics. In detail, we proposed two new ways that can be used to obtain the equilibrium size distribution of the hydrogen bonding clusters, and derived the analytical expression of a relationship between the hydrogen bonding free energy and hydrogen bonding degree. For the nonlinear hydrogen bonding systems, it is shown that the sol-gel phase transition can take place under proper conditions, which is further proven to be a kind of geometrical phase transition rather than a thermodynamic one. Moreover, several problems associated with the geometrical phase transition and liquid-solid phase transition in nonlinear hydrogen bonding systems are discussed.     

Conformations of alpha-aminobutyric acid in the gas phase.

A laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectrometer has been successfully applied to the structural study of alpha-aminobutyric acid. Three neutral conformers have been identified in the gas phase by comparing their experimental rotational and 14N nuclear quadrupole coupling parameters with those predicted by ab initio calculations at the MP2/6-311++G(d,p) level. The most stable conformer is stabilized by a bifurcated amine-to-carbonyl hydrogen bond (N--HO=C) and a cis-COOH group, and the side-chain adopts a configuration with a torsion angle tau(C(gamma)-C(beta)-C(alpha)-C') of about 180 degrees. The second most stable conformer exhibits the same configuration for the amino acid skeleton but adopts a different orientation for the side chain with tau(C(gamma)-C(beta)-C(alpha)-C') approximately -60 degrees. In the third conformer an intramolecular hydrogen bond is established between the hydroxyl group and the nitrogen atom (NH--O), with a side-chain orientation similar to that of the most stable conformer.     

Equilibrium constants and the prediction of miscibility windows for polymer blends containing poly(tetrafluoroethylene-alt-vinyl alcohol)

Theoretical calculations of miscibility windows for binary polymer blends in which one component is an essentially alternating copolymer of tetrafluoroethylene and vinyl alcohol (FVOH) are reported. FVOH has an inherently low solubility parameter [≈ 6.2 (cal. cm^?3)^0.5] that is outside the range commonly encountered in miscible polymer blends and thus represents a stringent test of the predictive capabilities of an association model we have used in previous work. The application of this model requires that we determine dimensionless equilibrium constants describing the self-association of a model compound 3,4-pentafluorobutan-2-ol (PFB) at 25°C from infrared spectroscopic data. Analogous equilibrium constants for FVOH were scaled from those of PFB by taking into account differences in the molar volume of the model and the specific repeat of the copolymer (see M. M. Coleman, J. F. Graf, and P. C. Painter: Specific Interactions and the Miscibility of Polymer Blends, Technomic, Lancaster, PA, 1991). Equilibrium constants describing the inter-association of FVOH with ester type carbonyl groups were obtained from spectroscopic studies of miscible blends with poly(ethyl methacrylate). These equilibrium constant values were then used to calculate theoretical miscibility windows for the complete range of blends of FVOH with polymethacrylates, ethylene-co-methyl acrylate, styrene-co-methyl acrylate, and ethylene-co-vinyl acetate copolymers. Experimental results performed in our laboratories confirm the general validity of the predicted miscibility windows. © 1993 John Wiley & Sons, Inc.     

Evidence for a Bulky Unit of a Pillar[5]arene Flipping in the Solid State

It is well known that pillar[5]arenes have two most stable conformations (pS and pR) in their crystal structures. Because of the intramolecular H‐bonding interactions, substituents, temperature, solvent and so on, the rotational behaviors of the phenolic units on pillararenes are also common. This paper showed some other kinds of conformations in the functionalized pillar[5]arenes and gave evidence for a bulky unit (1,4‐methoxycarbonylmethoxybenzene unit) flipping in the solid state. The presence of hydrogen bonding facilitated the intermolecular self‐assembly in terms of energy‐minimized packing in the crystals. Thus, the main driving force for the flipping of this bulky unit might be both the intramolecular hydrogen bonding between the phenolic units on pillararenes and quadrupolar hydrogen bonding between the host and water. This paper helps us to have a better understanding on the conformations of pillar[5]arenes.     

Hydrogen bonding in diols and binary diol-water systems investigated using DFT methods. II. Calculated infrared OH-stretch frequencies,force constants,and NMR chemical shifts correlate with hydrogen bond geometry and electron density topology. A reevaluation of geometrical criteria for hydrogen bonding

Although the two hydroxyl groups in 1,2-diols interact as evidenced by NMR and IR spectroscopic shifts, electron density topological analysis has shown a bond critical point (BCP) and atomic bond path to be absent (Klein, R. A.; J Comp Chem 2002, 23, 585-599; J Am Chem Soc 2002, 124, 13931-13937), indicating that no intramolecular hydrogen bond is formed. Here, we demonstrate that small NMR or IR shifts are neither necessarily diagnostic nor sufficient as indicators of hydrogen bond formation; moreover, modified van der Waals atomic radii are needed for estimating maximum nuclear interaction distances and nuclear interpenetration.     

Single‐Crystal X‐ray Diffraction Structure of the Stable Enol Tautomer Polymorph of Barbituric Acid at 224 and 95 K

The thermodynamically stable enol crystal form of barbituric acid, previously prepared as powder by grinding or slurry methods, has been obtained as single crystals by slow cooling from methanol solution. The selection of the enol crystal was facilitated by a density‐gradient method. The structure at 224 and 95 K confirms the enol inferred on the basis of powder data. The enol has bond lengths that are consistent with the expected bond order and with DFT calculations that include treatment of hydrogen bonding. In isolation, the enol is higher in energy than the tri‐keto form by 50 kJ mol^?1 which must be more than compensated by enhanced hydrogen bonding. Both crystal forms have four normal H‐bonds; the enol has two additional H‐bonds with O–O distances of 2.49 Å. Conversion into the enol form occurs spontaneously in the solid state upon prolonged storage of the commercial tri‐keto material. Slurry conversion of tri‐one to enol in ethanol is reversed in direction in ethanol‐D₁.     

2-羟基吡啶与水氢键作用的理论研究

本文采用量子化学的Hatree-Fock方法和密度泛函理论(DFT)的B3LYP方法,在6-31G(d)水平上,研究了2-羟基吡啶分子(Hy)及其酮式互变异构体2(1H)-吡啶酮(Py)与水的相互作用。考察它们之间在形成Hy…H2O,Py…H2O,Hy…Hy,Py…Py和Hy…Py等复合物前后的能量变化和分子结构参数变化特点。计算结果表明,在这些复合物中都形成了较强的氢键作用,在水合物中,Py与水形成复合物时能量降低较多,与实验结果一致。经过零点振动能(ZPVE)和基组叠加误差(BSSE)校正后的复合物离解能分别为38.3,40.8,73.0,82.7和71.1 kJ/mol(B3LYP/6-31G(d)),水合物的离解能远小于二聚体复合物,而酮式结构的二聚体的离解能最大。     

Direct Evidence for the Effect of Intermolecular Hydrogen Bonding on Organogels

In order to get direct evidence for the effect of intermolecular hydrogen bonding on the organogels, one arnide group in N-（3, 4, 5-octyloxybenzoyl）-N＇-（4＇-aminobenzoyl）hydrazine（D8） was replaced by a Schiff base group, forming N-（3,4,5-octyloxybenzoyl）-N＇-（4＇-amidobenzoyl） acylhydrazone（T8SchA）. D8 and T8SchA organogels in cyclohexane show the same hexagonal columnar structure. And the hydrogen bonding was demonstrated to be still interacting in the organogels. However, although the molecular geometry of D8 was well retained in T8SchA, the molecular dipole moment of T8SchA is bigger than that of D8 due to the reduction of the number of hydrogen bonds. Thus, the decreased gelling stability of T8SchA compared to that of D8 can only be attributed to the reduction of the number of intermolecular hydrogen bonds, which provides direct evidence that intermolecular hydrogen bonding plays an important role in stabilising organogels.     

Spectroscopic Evidence for Clusters of Like‐Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding

Direct spectroscopic evidence for hydrogen‐bonded clusters of like‐charged ions is reported for ionic liquids. The measured infrared O?H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion‐corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like‐charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT‐D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.     

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy-Functionalized Pyridinium Cations

The paradigm of supramolecular chemistry relies on the delicate balance of noncovalent forces. Here we present a systematic approach for controlling the structural versatility of halide salts by the nature of hydrogen bonding interactions. We synthesized halide salts with hydroxy-functionalized pyridinium cations [HOC_nPy]⁺ (n=2, 3, 4) and chloride, bromide and iodide anions, which are typically used as precursor material for synthesizing ionic liquids by anion metathesis reaction. The X-ray structures of these omnium halides show two types of hydrogen bonding: ‘intra-ionic’ H-bonds, wherein the anion interacts with the hydroxy group and the positively charged ring at the same cation, and ‘inter-ionic’ H-bonds, wherein the anion also interacts with the hydroxy group and the ring system but of different cations. We show that hydrogen bonding is controllable by the length of the hydroxyalkyl chain and the interaction strength of the anion. Some molten halide salts exhibit a third type of hydrogen bonding. IR spectra reveal elusive H-bonds between the OH groups of cations, showing interaction between ions of like charge. They are formed despite the repulsive interaction between the like-charged ions and compete with the favored cation-anion H-bonds. All types of H-bonding are analyzed by quantum chemical methods and the natural bond orbital approach, emphasizing the importance of charge transfer in these interactions. For simple omnium salts, we evidenced three distinct types of hydrogen bonds: Three in one!     

Inclusion Compounds of Thiourea and Peralkylated Ammonium Salts. Part IV. Hydrogen-Bonded Host Lattices Built of Thiourea and Formate Ions

New inclusion complexes(C2H5)4N+HCO2-·(NH2)2CS·H2O (1),[(C2H5)4N]2+[(HCO2)2H]-(HCO2-)·2(NH2)2CS(2), (n-C3H7)4N+HCO2-·3(NH2)2CS·H2O (3)and (n-C4H9)4N+[(HCO2)2H]-·2(NH2)2CS(4) have been prepared and characterized by X-ray crystallography. Crystal data, MoK radiation: 1, space group P21/c, a = 7.199(2), b = 16.851(2),c = 13.044(2) Å, = 100.13(2)°, Z = 4, and RF = 0.065 for 1011 observed data; 2, space group Pca21, a = 25.803(5), b = 7.190(2), c = 17.394(2) Å, Z = 4, and RF= 0.073 for 1515 observed data; 3, space group P21/n, a = 8.533(2), b = 9.423(5), c = 33.517(7) Å, = 90.44(2)°, Z = 4, and RF = 0.052 for 2521 observed data; 4, space group Pbca, a = 17.389(3), b = 16.622(2),c = 20.199(3) Å, Z = 8, and RF = 0.056for 1910 observed data. In both 1 and 2 the tetraethylammonium ions are sandwiched between puckered layers, which are constructed by the cross-linkage of a parallel arrangement of infinite chains. In 1 each chain is composed of twisted(thiourea–formate)2 tetramers bridged by water molecules, whereas in 2 it comprises an alternate arrangement of thiourea dimers and protonated formate trimers each formed by the linkage of a hydrogen diformate ion, [(HCO2)2H]-, to a formate ion via} a C–-H·sO hydrogen bond. In compound 3 two independent thiourea molecules are used to construct a hydrogen-bonded puckered layer normal to thec axis, whereas the remaining thiourea molecule, together with the formate ion and water molecule, generate another puckered layer that is parallel to the first one. Hydrogen bonding between these two types of layers gives rise to a network containing channels running parallel to the [100] direction, and the cations are stacked regularly within each column. In the crystal structure of 4, the thiourea molecules form hydrogen-bonded zigzag ribbons running parallel to the b axis, which are cross-linked by the dimeric formate moieties [(HCO2)2H]- to form a puckered layer, and the(n-C4H9)N+ cations occupy the space between adjacent layers.