Molecular Structure of Complexes with Bifurcated Hydrogen Bond: III. Solvate H-Complexes Formed by Trifluoromethanesulfonamide and Its Cyclic Dimer

According to the data of IR spectroscopy, dielcometry, and B3LYP/6-31G* quantum-chemical calculations, trifluoromethanesulfonamide homoassociates in weakly basic protophilic media are converted into 2 : 1 solvate H-complexes. Cyclic trifluoromethanesulfonamide dimer decomposes in heteroassociate with dioxane with a composition of 1 : 3. Cyclic trifluoromethanesulfonamide dimer with dioxane forms a 1 : 4 H-complex with bifurcated (three-center) hydrogen bond involving the bridging hydrogen atom of the NH group (NH?O).     

Solvatochromism of Heteroaromatic Compounds: XXVI. Solvent Effect on the Solvatochromic Parameters of IR Bands of <Emphasis Type="Italic">N</Emphasis>-Methylanilines and Related Hydrogen Bond Donors

Formation of a hydrogen bond between molecules of a proton donor (phenol, pyrrole, N-methylanilines) and a solvent decreases the sensitivity of the XH stretching frequency to the polarity/polarizability of solvents. A change in the bond configuration in the amine moiety of N-methylaniline and related compounds upon formation of a solvation H complex is manifested in that the absolute terms of the solvatochromic equations for inert and protophilic media are different. The spectroscopic effect from the geometric reorganization of molecules is determined by their structure and the capability to act as hydrogen bond donors. Multicentered hydrogen bond with π bases affects the geometry of the amine fragment of N-substituted anilines to a lesser extent than does two-centered H bond with onium bases.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 4, 2005, pp. 642–649.Original Russian Text Copyright © 2005 by Vokin, Oznobikhina, Shulunova, Ustinov, Bozhenkov, Levkovskaya, Turchaninov.For communication XXV, see [1].     

Molecular Conformation of 2,2'-Dinitrodiphenylamine According to Dielectric Studies and Ab Initio Quantum Chemical Calculations

According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer.     

Molecular Structure of Complexes with Bifurcated Hydrogen Bond: I. N-(4-Methyl-2-nitrophenyl)acetamide

According to the results of HF/6-31G* and B3LYP/6-31G* nonempirical calculations, N-(4-methyl- 2-nitrophenyl)acetamide in the synperiplanar and antiperiplanar conformations gives stable complexes with protophilic solvents, which are stabilized by bifurcated (three-center) hydrogen bond. The complexes are characterized by (1) opposite variations of the interatomic distances and angles corresponding to intra- and intermolecular components of the bifurcated hydrogen bond, (2) extension of the intramolecular component and a more pronounced shortening of the intermolecular component with increase in the strength of the three-center H-complex, and (3) nonlinear and nonmonotonic relation between the NH stretching vibration frequency and the energy of complex formation.     

Molecular structure of complexes with bifurcated hydrogen bond: IV. Solvate H-complexes of <Emphasis Type="Italic">N</Emphasis>-methyltrifluoromethanesulfonamide in aprotic protophilic media

Analysis of the IR spectra of N-methyltrifluoromethanesulfonamide, as well as quantum-chemical calculations of solvate complexes of its monomer and open-chain dimer have demonstrated inadequacy of the model assuming the presence in protophilic solvents of a 1:1 monomer H-complex only. A model is suggested, which is based on an equilibrium between the 1:1 monomer H-complexes and open-chain dimer complexes of variable composition. The proposed model is much better consistent with experimental data.     

Solvatochromism of Heteroaromatic Compounds: XXIII. Reversible Specific Solvatochromic Effect in the IR Spectra of 2-Nitrophenol and 2,4,6-Trinitrophenol

2-Nitrophenol and 2,4,6-trinitrophenol form with protophilic solvents complexes with a bifurcate (three-center) hydrogen bond. Their O-H stretching vibration frequencies nonlinearly and unsteadily vary with the H-bond acceptor ability of the medium, which is a spectroscopic evidence for three-center H-complex formation.     

Orientation of hydrogen bond in H-complexes of sulfones and sulfonamides

According to the data of quantum chemical calculations using the DFT method (B3LYP/6-311G*), dimethylsulfone (I), N,N-dimethylmethanesulfonamide (II) and N,N-dimethyltrifluoromethanesulfonamide (III) form with methanol and phenol H-complexes of two types: (1) having nonlinear structure and (2) bidentate complexes with participation of both oxygen atom lone pairs in the formation of the H-bond. The angle determining the directionality of the hydrogen bond with respect to the axis of the nearest lone electron pair of the SO₂ group oxygen atom in the conformational isomers of these complexes falls within the range 3°–100°. Variation of the calculated values of Δν(OH) in the H-complexes is parallel to that observed in their IR spectra and reflects the decrease of basicity of the sulfonyl group in the order I > II > III.     

Cyclic Heptapeptides Axinastatin 2, 3, and 4: Conformational analysis and evaluation of the biological potential

The conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two-dimensional NMR spectroscopy in combination with distance-geometry (DG) and molecular-dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D₆)DMSO. Axinastatin 2 was also investigated in CD₃OH. In all structures, Pro² is in the i + 1 position of a βI turn and Pro⁶ occupies the i + 2 position of a βVIa turn about the cis amide bond between residue 5 and Pro^6. In all peptides, a bifurcated H-bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn I_g turn was found about Asn¹ and Pro^2. We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A β-bulge motif with two β turns and one bifurcated H-bond is found as the dominating backbone conformation of cyclic all-L-heptapeptides. Axinastatin 2, 3, and 4 can be characterized by six trans and one cis amide bond resulting in a β/βVI(a)-turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be inactive or of low activity.     

NH3BH3的双接触蓝移双氢键的理论研究

运用量子化学从头算方法研究了NH3BH3与含大的分子内超共轭的质子给体CHF3, H2CO, HCOOH, HCOCl和HNO形成的双接触弯曲双氢键B—H2…H—X(X＝C, N)的分子结构、电子密度拓扑性质与频率位移特征. 计算结果表明, 在所有体系中, 由于双氢键的形成, B—H键拉长且伸缩振动频率红移, 而X—H键长减小且伸缩频率蓝移. 弯曲的双接触构型导致分子间超共轭减小、X—H键的大的正重极化与正重杂化以及分子内超共轭减小三个因素导致了X—H键蓝移; B—H键红移的主要原因是B—H键的负的重极化与负的重杂化.     

Characteristics of antiaromatic ring pi multi-hydrogen bonds in (H2O)n-C4H4 (n = 1, 2) complexes

By counterpoise-corrected optimization method, the six antiaromatic ring pi multi-hydrogen bond structures with diversiform shapes for (H2O)n-C4H4 (n = 1,2) have been obtained at the MP2/aug-cc-pVDZ level. At the CCSD(T)/aug-cc-pVDZ level, the interaction energy obtained mainly depends on the numbers of H2O and fold numbers of the pi multi-hydrogen bond. The interaction energy order is -2.342 (1a with pi mono-hydrogen) < -2.777 (1b with pi bi-hydrogen) < -4.683 (2a with pi bi-hydrogen) < -4.734 (2b with pi tri-hydrogen) < -4.782 (2c with pi tri-hydrogen) < -5.009 kcal/mol (2d with pi tetra-hydrogen bond). Strangely, why is the interaction energy of the pi bi-hydrogen bond in 1b close to that of the pi mono-hydrogen bond in 1a (their difference is only 15.7%)? The reason is that a pi-type H-bond (as an accompanying interaction) between two lone pairs of the O-atom and a near pair of H-atoms of C4H4 exists shoulder by shoulder in structures 1a, 2a, 2b, and 2c and contributes to the interaction energy. Another accompanying interaction, a repulsive interaction between the pi H-bond (using the H-atom(s) of H2O) and the near pair of H-atoms of C4H4, is also found. For the structures and interaction energies, the pi-type H-bond produces four effects: bending the strong pi H-bond, attracting the pair of H-atoms of C4H4 so that they deviate from the C4 ring plane, showing the interaction energy contribution, and bringing the larger electron correlation contribution. The repulsive interaction also produces four effects: pushing the pair of H-atoms of C4H4 so that they deviate from its ring plane, elongating the distance of the pi H-bond, promoting the formation of pi-type H-bond, and slightly influencing the interaction energy. In the present paper, one C=C bond with two H2O (over and below the ring plane) forms a pi H-bond link in two ways: a strong-weak pi H-bond link and a strong-strong pi H-bond link. The stability contribution of the former is more favorable than the latter. One H2O forms a pi H-bond with C4H4 in two ways. One strong pi H-bond part (over or below the ring plane) always is accompanied by another H-bond part. The accompanying part is either a weak pi H-bond or pi-type H-bond.     

Hydrogen‐bonded structures of the isomeric 2‐, 3‐ and 4‐carbamoylpyridinium hydrogen chloranilates

In the three isomeric salts, all C₆H₇N₂O⁺·C₆HCl₂O₄⁻, of chloranilic acid (2,5‐dichloro‐3,6‐dihydroxy‐1,4‐benzoquinone) with 2‐, 3‐ and 4‐carbamoylpyridine, namely, 2‐carbamoylpyridinium hydrogen chloranilate (systematic name: 2‐carbamoylpyridinium 2,5‐dichloro‐4‐hydroxy‐3,6‐dioxocyclohexa‐1,4‐dienolate), (I), 3‐carbamoylpyridinium hydrogen chloranilate, (II), and 4‐carbamoylpyridinium hydrogen chloranilate, (III), acid–base interactions involving H‐atom transfer are observed. The shortest interactions between the cation and the anion in (I) and (II) are pyridinium N—H...(O,O) bifurcated hydrogen bonds, which act as the primary intermolecular interaction in each crystal structure. In (III), an amide N—H...(O,O) bifurcated hydrogen bond, which is much weaker than the bifurcated hydrogen bonds in (I) and (II), connects the cation and the anion.     

Covalent nature of the hydrogen bond

Conclusions The main conclusion derived from this work is that the H bond in all three FHF^–, FHFH, and HFHFH⁺ systems is a three-centered, two-electron, covalent chemical bond formed at the expense of 2a _1g (2a ₁) MO bonding. The 1a _1g MO bonding has little effect on H-bond stabilization. Thus the H bond is a one-orbital chemical bond with its formation corresponding, to that of a three-centered MO, as distinguished from molecules bonded by a two-centered MO (e.g., F₂ or HOOH [41]; hence the H bond is much weaker. The uniqueness of the H bond lies in its being the weakest covalent bond. It is precisely the covalent nature of the H bond that gives it its characteristic properties, i.e., saturability and strict compliance to structural requirements. In addition, the low dissociation barrier makes it easy to control the H bond under mild conditions, which is very important in biological systems.Translated from Zhurnal Strukturnoi Khimii, Vol. 26, No. 2, pp. 13–21, March–April, 1985.     

The Crystal Structure of Methyl β-orcinol-carboxylate (= Methyl 2,4-dihydroxy-3,6-dimethylbenzoate)

The mono-arylic compound, methyl β-orcinol-carboxylate ( I ) is one of the principal components of lichens which grow on the oak tree. In the crystal structure of I the molecules are H-bonded to form sheets separated by 3.56 (5) Å. The carbonyl O-atom of the ester group and the ortho-hydroxyl group form a strong intramolecular H-bond. The IR. spectrum of the solid contains two (O--H)-stretching frequencies which, by comparison with the spectra of the 4-methoxy and the 2-methoxy analogues, can be assigned to one intra- and one intermolecular H-bond.     

Correlation analysis of quantum chemical data and the polarizability effect in H-complexes

The study is concerned with analysis of the energies of formation (E), frequency shifts (Δν) in IR spectra, ionization potentials (IP) of H-complexes, hydrogen bond lengths (r), and spin densities (sd) in H-complexes involving radical cations, obtained from quantum chemical calculations for 20 series of H-complexes. It was for the first time established that the E, IP, r, and sd values and the changes in enthalpy (δH) depend not only on the inductive and resonance effects but also on the polarizability effect of the substituents bound to the donor and acceptor centers in the H-complexes. Interrelations between the polarizability effect and the molecular structure of H-complexes are considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 602–608, April, 2006.     

Supramolecular structures containing ‘isolated’ pentaborate anions and non-metal cations: Crystal structures of [Me3NCH2CH2OH][B5O6(OH)4] and [4-MepyH, 4-Mepy][B5O6(OH)4]

The solid-state structures of two non-metal pentaborates [Me₃NCH₂CH₂OH][B₅O₆(OH)₄] (1) and [4-MepyH, 4-Mepy][B₅O₆(OH)₄] (2) have been determined by single-crystal X-ray diffraction methods. Structures 1 and 2 both contain supramolecular pentaborate frameworks held together by extensive H-bond interactions. The framework of 1 exists essentially as planes of pentaborate anions linked via three pairwise ‘planar’ β → α interactions, with a fourth β → β interaction crosslinking the planes. The framework of 2 is very similar except that one of the three pairwise linkages within the plane is replaced by pairwise ‘step-like’ bifurcated H-bonds to both α sites of a neighboring anion. The cations in 1 and the cations and neutral 4-Mepy ligands in 2 are present in the framework cavities and channels, with additional H-bond interactions existing between cations and anions.     

An X‐ray crystallographic and density functional theory study of (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one

The Schiff base enaminones (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₃H₁₇NO₄S, (I), and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₅H₂₁NO₂, (II), were studied by X‐ray crystallography and density functional theory (DFT). Although the keto tautomer of these compounds is dominant, the O=C—C=C—N bond lengths are consistent with some electron delocalization and partial enol character. Both (I) and (II) are nonplanar, with the amino–phenol group canted relative to the rest of the molecule; the twist about the N(enamine)—C(aryl) bond leads to dihedral angles of 40.5 (2) and −116.7 (1)° for (I) and (II), respectively. Compound (I) has a bifurcated intramolecular hydrogen bond between the N—H group and the flanking carbonyl and hydroxy O atoms, as well as an intermolecular hydrogen bond, leading to an infinite one‐dimensional hydrogen‐bonded chain. Compound (II) has one intramolecular hydrogen bond and one intermolecular C=O...H—O hydrogen bond, and consequently also forms a one‐dimensional hydrogen‐bonded chain. The DFT‐calculated structures [in vacuo, B3LYP/6‐311G(d,p) level] for the keto tautomers compare favourably with the X‐ray crystal structures of (I) and (II), confirming the dominance of the keto tautomer. The simulations indicate that the keto tautomers are 20.55 and 18.86 kJ mol⁻¹ lower in energy than the enol tautomers for (I) and (II), respectively.     

Solvatochromism of Heteroaromatic Compounds: VII. Bifurcate Hydrogen Bond in Solvatochromic Complexes of 2-(1,2,2-Tritricyanovinyl)pyrrole

IR and UV spectrosocopy and quantum chemistry were used to reveal a bifurcate (three-center) hydrogen bond in the complexes of 2-(1,2,2-tricyanovinyl)pyrrole with hydrogen-bond acceptors. In the gas phase and aprotic inert solvents this compound exists predominantly as the sp conformer stabilized by intra- molecular hydrogen bond involving an orthogonal system of the nitrile group and the NH hydrogen. The formation of the three-center hydrogen bond with an aprotic protophilic solvent shifts the conformational equilibrium to the ap conformer. As the strength of the intermolecular hydrogen bond increases in the series nitromethane < acetonitrile < dioxane, the fraction of the ap conformer increases and becomes prevailing already in the moderately protophilic THF. Solvatochromism of the long-wave absorption band in the elec- tronic spectra of the sp and ap conformers was studied in detail.     

Analysis of multiple H-bond interactions in self-assembly between polyurethane with pendent carboxyl and poly(4-vinylpyridine)

As an extension study, FTIR and molecular simulation methods were combined in the present paper to analyze the H-bond interactions resulting from multiple donors and acceptors that have led to self-assembly based on segmented polyurethane with carboxyl (PUc) and poly(4-vinylpyridine) (P4VP) in our previous work. Of them, FTIR was used to analyze the H-bonding types and interactions as well as their changes before and after self-assembly; molecular mechanics (MM/COMPASS) was used to study the effect of possible conformations on the H-bonds involved and analyze the most probable H-bond patterns; quantum mechanics (QM/B3LYP) was used to help confirm the experimental FTIR band assignments and calculate the H-bond energy. It was found that two types of H-bonds exist, namely, COOH...P4VP (type I) and (OCO)NH...P4VP (type II), based on OH and NH as the strong donors in the interaction between PUc and P4VP. Strong evidence has been obtained for a type II H-bond, which is the specialty in PUc/P4VP assembly. The type I and type II H-bonding energies are -11.293 and -7.150 kcal/mol, respectively. The forming probability of the type I H-bond accounts for 95.87%, while that of the type II H-bond is 4.13%, showing the primary driving force for the assembly based on PUc and P4VP is still the H-bond between COOH and P4VP, yet the H-bonds based on NH and pyridyl in P4VP cannot be ignored.     

<Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR Study of Bifurcated Intramolecular Hydrogen Bonds in 2,6-Bis(2-pyrrolyl)pyridine and 2,6-Bis(1-vinyl-2-pyrrolyl)pyridine

According to the ¹H and ¹³C NMR data, bifurcated intramolecular hydrogen bond NH?N?HN in 2,6-bis(2-pyrrolyl)pyridine fixes its molecule in a conformation with syn orientation of the pyrrole rings. An analogous bifurcated hydrogen bond CH?N?HC is formed in 2,6-bis(1-vinyl-2-pyrrolyl)pyridine. 2-(1-Vinyl-2-pyrrolyl)-6-(2-pyrrolyl)pyridine is characterized by unsymmetrical bifurcated hydrogen bond NH? N?HC.     

Investigation of H-bonds in 1,1-diphenylethanol by IR spectra measurements

The parameters of stretching vibrations nu(OH) (frequencies and integral intensities) have been calculated from IR spectra of a large number of H-complexes of 1,1-diphenylethanol [(C(6)H(5))(2)C(CH(3))OH] with solvents of various proton accepting strength which were not regularly reported. The data has been used to estimate the formation enthalpies of H-complexes and the proton-donor action of the mentioned alcohol. Also, correlations between those spectral characteristics have been investigated. The H-bonds in 1,1-diphenylethanol (DPE) have been investigated depending on concentrations in CCl(4) and temperature by means of infrared (IR) absorption spectra measurements. It has been found that the hydrogen bonds of the crystalline DPE are formed in the manner of cyclic tetramer, while in solutions with the concentration 0.5 mol/l, cyclic dimers are formed, which in higher concentrations change into cyclic tetramers.