Spectroscopic study of hydrogen bonding in the enol form of β-diketones—II. Symmetry of the hydrogen bond

The effects of proton potential on the i.r. and Raman spectra of β-diketones are discussed. The spectroscopic behaviour of the asymmetric hydrogen bond stretching mode and of the out-of-plane hydrogen bond bending modes indicates a relatively high barrier double minimum potential.From the observed O…O distances and these predicted from the spectroscopic results for intermolecular H-bonds, a bent hydrogen bond was concluded. The spectroscopic data show a great change in the internal modes of β-diketones upon deuteration. This was attributed to the lengthening of the O…O distance which is a known characteristic of a double minimum potential.     

Spectroscopic study of hydrogen bonding in the enol form of β-diketones—I. Vibrational assignment and strength of the bond

The i.r. and Raman spectra of the trans-enol form of some β-diketones and their deuterated analogues in α position are examined in the 4000-50cm⁻¹ range and an approximate assignment is given for the fundamental vibrations. The ν_as(OH) and ν(CO) values suggest that the intramolecular H-bond strength increases in the order: hexafluoroacetylacetone < trifluoroacetylacetone < acetylacetone < α - Cl - acetylacetone < benzoylacetone < dibenzoylmethane < tetraacetylethane.The observed relative Raman intensities of the ν(CC) and ν(CC) bands also support this feature. The spectroscopic results show a great change in the internal modes of the β-diketones upon deuteration; this behaviour indicates a greater localization of the π-electron system of the chelate ring and a weakening of the H-bond on deuteration. The ν_s(OH) vibration seems to be involved in one of the in-plane ring deformational modes.     

Specific features of coupling reactions of polystyrene in the presence α-dinitrones based on glyoxal

Russian Chemical Bulletin - Bromine-containing polystyrene synthesized by atom transfer radical polymerization was used as a model to study coupling reactions in the presence of such free radical...     

Mechanistic study on the gold-catalyzed C-S bond formation of α-thioallenes to form 2,5-dihydrothiophenes

The reaction mechanism of the AuCl-catalyzed reaction of the α-thioallenes to give 2,5-dihydrothiophenes has been computationally studied using DFT (B3LYP/6-31G*, SDD for Au). Calculations indicate the complexation of α-thioallene with AuCl occurs preferentially at the distal double bond, followed by the C-S bond formation, the proton transfer from the sulfur to the carbon "b", and the [1,2]-hydride shift to give the 2,5-dihydrothiophene gold complex. The proton transfer is the rate-limiting step with very high activation energy in the gas phase. In the presence of one water molecule, the activation free energy of the proton transfer was lowered by as much as 19.9 kcal/mol. Furthermore, one dichloromethane molecule stabilized all of the transition structures by its hydrogen bonds.     

Neutron diffraction study of the HF adducts containing a hydrogen bond F-H?O

Single-crystal structures of HF adducts of acid salts CsH₂PO₃·HF, KH₂PO₄·HF, and CsH₂PO₄·HF were determined by neutron diffraction using the Laue method. In the crystals, HF molecules are connected to anions by means of new type of hydrogen bonds, F-H?O, which are significantly shorter (F?O distances 2.356-2.386(3) Å) than strong O-H?O or O-H?F hydrogen bonds. The H-F distances in the structures of these adducts, 1.020-1.027(5) Å, are compared with those in crystalline HF and hydrofluoride anions.     

Molecular structure and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine based on X-ray diffraction data at 120 and 150 K. “Bent” bonds in the six-membered heterocycle

Characteristic features of the molecular geometry and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine have been studied by X-ray structural analysis and quantum-chemical (RHF and MP2) and molecular mechanics (MM3) calculations. An unusual shift of the maxima of the deformation electron density from the N-N bonds toward the center of the heterocycles was found, which may be interpreted as a bending of the corresponding bonds. This bending was confirmed by calculations of characteristics of the electron density distribution within the multipole model.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1995.The work was supported by the International Science Foundation (Grant No. M1F 000) and the Russian Foundation for Basic Research (Project No. 94-03-08338). We thank Dr. P. Mallinson (Chemistry Department, University of Glasgow, Scotland) for help in performance of multipole calculations.     

Structure of the X-ray emission CuLα-line in YBa2Cu3O7−δ and CuO based on the SCFXα-SW calculation

Using the SCF X-SW method we have calculated the final states of the X-ray L-transition of the CuO ₄ ^–6 cluster modeling the nearest environment of the copper atom (one of two types) in YBa₂Cu₃O_7- and the nearest environment of copper in CuO. The energies of transitions forming the L-spectrum (with or without many-electron excitations) have been calculated. It has been shown that for interpretation of the main singularities of the X-ray emission L-spectrum, not only should the Coster-Kronig transitions be considered but also the shake-up excitation 2b_1g3b_1g processes and the multiple ionization satellites 3p^–1. An experimental schame has been suggested that allows separation of the effects of different many-electron processes on the L-spectrum.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 3–9, July–August, 1993.Translated by L. Smolina     

Relationship between calculated NMR data and intermolecular hydrogen bond properties in X-pyridine⋯HF

The effect of different substituents in para and metapositions on the NMR data of X-pyridine?HF complex has been studied at B3LYP/6-311++G(d,p) level of theory. The relationship between NMR data and electron donation of substituents has been investigated. The results of topological properties of electron charge density calculated using atoms in molecules (AIM) analysis can be used to predict some NMR data. The magnetism-based indices, nucleus independent chemical shift NICS(1) and its z component NICS(1)_ZZ, were used to investigate the ring aromaticity changes on complexation. A linear correlation between Hammett coefficients and some NMR data could be found with a good correlation coefficient.     

X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose Iβ. The lateral thermal expansion behavior of the prepared cellulose Iβ films was investigated using X-ray diffraction at temperatures from 20 to 300 °C. Cellulose Iβ was transformed into the high-temperature phase when the temperature was above 230 °C, allowing the lateral thermal expansion coefficient of cellulose Iβ and its high-temperature phase to be measured. For cellulose Iβ, the thermal expansion coefficients (TECs) of the a- and b-axes were α_a = 9.8 × 10⁻⁵ °C⁻¹ and α_b = 1.2 × 10⁻⁵ °C⁻¹, respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose Iβ. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were α_a = 19.8 × 10⁻⁵ °C⁻¹ and α_b = −1.6 × 10⁻⁵ °C⁻¹, respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 °C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and γ = 96.4°, with space group = P2₁. The volume of this cell is 4.6% larger than that of cellulose Iβ at 30 °C.     

A novel formula for location of double bond in alkenyl acetates and alcohols based on mass spectral data of dimethyl disulfide derivatives

A new formula was presented for locating double bond position in dodecenols, tetrade-cenols, hexadecenols and their acetates, based on mass spectral data of dimethyl disulfide derivatives. In this procedure, molecular ion and base peak ion were utilized as characteristic parameters to identify the positional isomers. The method was tested with mass spectra of 25 dimethyl disulflde derivatives of mono-unsaturated acetates and alcohols, and the original double-bond position in each isomer was located unambiguously.     

The nature of the P–P bond in carbene-stabilized diphosphorus complex

The nature of the P–P bond in the recently synthesized N-heterocyclic carbene-stabilized diphosphorus complex has been investigated by the built-in fragment-oriented approach using DFT calculations. The result leads us to investigate the stabilization of diphosphorus. It is concluded that a weakening factor, which can be engendered through such methods as orbital mixing, antibonding interaction, or ligand–receptor interactions, is the key to stabilize the highly reactive diphosphorus molecule.     

Evolution of bond orders in the reversible reactions of alkylene oxides with R-β-hydroxyalkyl sulfides and the role of the intermolecular hydrogen bond in initiation and at intermediate reaction stages

Semiempirical (PM3) calculations of the changes in the structures and energies of the reactants, intermediates, transition states, and final products have been carried out for the reversible reaction of β-hydroxyethyl methyl sulfide with propylene oxide. The evolution of the electron density distribution during the reaction is analyzed. It is demonstrated that the transformation proceeds via two intermediate products and the O⋯H⋯O bridge persists throughout the reaction pathway.     

Quantitative convergent-beam electron diffraction and quantum crystallography—the metallic bond in aluminium

The technique of quantitative convergent-beam electron diffraction (QCBED) is reviewed as a method for making very accurate and precise measurements of low-order bonding-sensitive structure factors. As such, it is a technique with the potential to make significant contributions to the field of quantum crystallography. To demonstrate this, the application of QCBED in determining the nature of the metallic bond in aluminium is examined in detail. The importance of precision in structure factor measurement when it comes to determining bonding electron distributions, especially in metals, becomes obvious from this examination. Uncertainties as low as ±0.1% are routinely attainable by QCBED and are shown to be important in reliably locating bonding charge. In the case of aluminium, a nearly free electron gas metal where the bonding electron distribution is subtler than in most other materials, the bonds are entirely tetrahedrally centred. This is a conclusion that could not have been reached experimentally without the levels of precision offered by QCBED.     

A new insight on hydrogen bond donor property of bridged B–H–B protons in the interaction of diborane with some hydrogen bond acceptor molecules

Structural Chemistry - Ab initio calculations at the MP2/6-311++G(d,p) computational level were used to analyze the interactions between a molecule of B2H6 with some small molecules which often...     

Importance of the nature of α-substituents in pyrrolidine organocatalysts in asymmetric Michael additions

The fundamental factors contributing toward the stereoselectivity in organocatalyzed asymmetric Michael reaction between aldehydes (propanal and 3-phenyl propanal) and methyl vinyl ketone (MVK) are established by using density functional theory methods. Three of the most commonly employed α-substituted pyrrolidine organocatalysts are examined. Several key stereochemical modes of addition between (i) a model enamine or (ii) pyrrolidine enamines derived from aldehydes and secondary amine to MVK are examined. Among these possibilities, the addition of (E)-enamine to cis-MVK is found to have a lower activation barrier. The stereochemical outcome of the reaction is reported on the basis of the relative energies between pertinent diastereomeric transition states. Moderate selectivity is predicted for the reaction involving pyrrolidine catalysts I and II, which carry relatively less bulky α-substituents dimethylmethoxymethyl and diphenylmethyl, respectively. On the other hand, high selectivity is computed in the case of catalyst III having a sufficiently large α-substituent (diarylmethoxymethyl or diphenylprolinol methyl ether). The enantiomeric excess in the case of 3-phenyl propanal is found to be much higher as compared to that with unsubstituted propanal, suggesting potential for improvement in stereoselectivity by substrate modifications. The computed enantiomeric excess is found to be in reasonable agreement with the reported experimental stereoselectivities. A detailed investigation on the geometries of the crucial transition states reveals that apart from steric interactions between the α-substituent and MVK, various other factors such as orbital interactions and weak stabilizing hydrogen-bonding interactions play a vital role in stereoselectivity. The results serve to establish the importance of cumulative effects of various stabilizing and destabilizing interactions at the transition state as responsible for the stereochemical outcome of the reaction. The limitations of commonly employed qualitative propositions, relying on the steric protection of one of the prochiral faces of enamines offered by the bulky α-substituent, are presented.     

Effect of pressure on the structure and dynamics of hydrogen bonds in ethylene glycol–water mixtures: Numerical simulation data

Water?ethylene glycol mixtures containing from 0.002 to 0.998 mole fractions of ethylene glycol at T = 298.15 K and P = 0.1 and 100 MPa are simulated by means of classical molecular dynamics. Such structural and dynamic characteristics of hydrogen bonds as the average number and lifetime, along with the distribution of molecules over the number of hydrogen bonds, are calculated; their changes are analyzed, depending on the mixture’s composition and pressure. It is shown that the components are characterized by a high degree of interpenetration and form a uniform infinite hydrogen-bonded cluster over the range of concentrations. It is found that the higher the concentration of ethylene glycol, the greater the stability of all hydrogen bonds. It is concluded that an increase in pressure lowers the number of hydrogen bonds, while the average lifetime of the remaining hydrogen bonds grows.