NMR studies of strong hydrogen bond to fluoride ion

Systems such as MF/diol (M = alkali metal) and }1F/carboxylic acid were subjected to IH, I9F and 13C nmr study to investigate the nature of the very strong H-bonding of fluoride ions with these systems. Evidence indicates a strong H-bond in diol-fluoride systems (~H ~ -(56) kJ mol-I) which is stronger than most 'typical' H-bonds (~H = -(12-40) kJ mol-I), but weaker than that reported for carboxylic acid-fluoride systems (~H ~ -(120) kJ mol-I). Approximate fluoride H-bonded shifts (o(OH)OHF) were evaluated for MF/diol systems from IH chemical shift measurements. No direct correlation was observed between I9F chemical shift and H-bond strength. Thermodynamic parameters were calculated from temperature dependent IH and 19F shifts. Preliminary studies of BUn 4NF-acetylacetone by I9F nmr were conducted at low temperatures and a possible Jmax (ca. 400 Hz) is reported for the fluoride ion H-bonded to acetylacetone. Highfield shift for non-protonated carbons and downfield shift for protonated carbons were observed in carboxylic acid/KF systems. Significant decreas$in I3C TI due to strong H-bonding to fluoride ions were also detected in both diol and carboxylic acid systems. Anomalous results were obtained, such as increasing NOE with increasing temperature in neat 1,2-ethanediol (values above the theoretical maximum of 1.988) and in 1,2-ethanediol/KF. The large 13C NOE's for carboxy carbons in neat carboxylic acids which are. further enhanced by the addition of KF are also unusual.

Ligações hidrogênio fortes em ácidos dicarboxílicos e diaminas aromáticas

A very short-strong hydrogen bond (<2 Å, >20kcal/mol) is found in the monoanion of certain dicarboxylic acids derived from maleic and dialkylmalonic acids. Certain aromatic diamines that are known as proton sponge have exceptionally high basicity (pKa) and are only monoprotonated with strong acids like percloric acid. The closed proximity between the two basic centers provokes a strong steric interaction that is relieved upon protonation. Similar effects are found in dicarboxylic acids (hydrogen maleate and hydrogen dialkylmalonates) that present a very short distance between the two oxygens and a short-strong hydrogen bond.

Electronic spectra, ESR and crystal structure of tetrahedral halocuprates(II). The existence of cations (2tbpo·H+) with a very short hydrogen bond

Two compounds [2tbpo·H+)2[CuCl4]= (yellow) and (2tbpo·H+)2[CuBr4]= (dark purple) (tbpo = tribenzylphosphine oxide) have been prepared and investigated by means of crystal structure, electronic, vibrational and ESR spectra. The crystal structure of the (2tbpo·H+)2[CuCl4]= complex was determined by three-dimensional X-ray diffraction. The compound crystallizes in the space group P42/n with unit-cell dimensions a = 19.585(2), c = 9.883(1)Å, V = 3790 (1)Å3, Z = 2, Dm = 1.303 (flotation) Dx = 1.302 Mg m-3. The structure was solved by direct methods and refined by blocked full-matrix least-squares to R = 0.053 for 2583 observed reflections. Cu(II) is coordinated to four chlorides in a tetrahedral arrangement. Tribenzylphosphine oxide molecules, related by a centre of inversion, are connected by a short hydrogen bridge. Chemical analysis, electronic and vibrational spectra showed that the bromide compound is similar to the chloride one and can be formulated as (2tbpo·H+)2[CuBr4]=. The position of the dd transition bands, the charge transfer bands, the ESR and the vibrational spectra of both complexes are discussed. The results are compared with analogous complexes cited in the literature. © 1983.

NMR evidence for the participation of a low-barrier hydrogen bond in the mechanism of delta 5-3-ketosteroid isomerase.

Delta 5-3-Ketosteroid isomerase (EC 5.3.3.1) promotes an allylic rearrangement involving intramolecular proton transfer via a dienolic intermediate. This enzyme enhances the catalytic rate by a factor of 10(10). Two residues, Tyr-14, the general acid that polarizes the steroid 3-carbonyl group and facilitates enolization, and Asp-38 the general base that abstracts and transfers the 4 beta-proton to the 6 beta-position, contribute 10(4.7) and 10(5.6) to the rate increase, respectively. A major mechanistic enigma is the huge disparity between the pKa values of the catalytic groups and their targets. Upon binding of an analog of the dienolate intermediate to isomerase, proton NMR detects a highly deshielded resonance at 18.15 ppm in proximity to aromatic protons, and with a 3-fold preference for protium over deuterium (fractionation factor, phi = 0.34), consistent with formation of a short, strong (low-barrier) hydrogen bond to Tyr-14. The strength of this hydrogen bond is estimated to be at least 7.1 kcal/mol. This bond is relatively inaccessible to bulk solvent and is pH insensitive. Low-barrier hydrogen bonding of Tyr-14 to the intermediate, in conjunction with the previously demonstrated tunneling contribution to the proton transfer by Asp-38, provide a plausible and quantitative explanation for the high catalytic power of this isomerase.

Different supramolecular hydrogen bond structures and significant changes in magnetic properties in dinuclear mu(2)-1,1-N-3 copper(II) complexes with very similar tridentate Schiff base blocking ligands

Three new basal-apical, mu(2)-1,1-azide bridged complexes, [CuL1(N-3)](2) (1), [CuL2(N-3)](2) (2) and [CuL3(N-3)]2 (3) with very similar tridentate Schiff base blocking ligands [L-1=N-(3-aminopropyl) salicylaldimine, L-2=7-amino-4-methyl-5-azahept-3-en-2-one and L-3=8-amino-4-methyl-5-azaoct-3-en-2-one) have been synthesised and their molecular structures determined by X-ray crystallography. In complex 1, there is no inter-dimer H-bonding. However, complexes 2 and 3 form two different supramolecular structures in which the dinuclear entities are linked by strong H-bonds giving one-dimensional systems. Variable-temperature (300-2 K) magnetic susceptibility measurements and magnetization measurements at 2 K reveal that complexes 1 and 2 have antiferromagnetic coupling while 3 has ferromagnetic coupling which is also confirmed by EPR spectra at 4-300 K. Magnetostructural correlations have been made taking into consideration both the azido bridging ligands and the existence of intermolecular hydrogen bonds in complexes 2 and 3.

Hydrogen bond-induced pair formation of glycine on the chiral Cu{531} surface

Enantio-specific interactions on intrinsically chiral or chirally modified surfaces can be identified experimentally via comparison of the adsorption geometries of similar nonchiral and chiral molecules. Information about the effects of substrate-related and in interactions on the adsorption geometry of glycine, the only natural nonchiral amino acid, is therefore important for identifying enantio-specific interactions of larger chiral amino acids. We have studied the long- and short-range adsorption geometry and bonding properties of glycine on the intrinsically chiral Cu{531} surface with low-energy electron diffraction, near-edge X-ray absorption One structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. For coverages between 0.15 and 0.33 ML (saturated chemisorbed layer) and temperatures between 300 and 430 K, glycine molecules adsorb in two different azimuthal orientations, which are associated with adsorption sites on the {110} and {311} microfacets of Cu{531}. Both types of adsorption sites allow a triangular footprint with surface bonds through the two oxygen atoms and the nitrogen atom. The occupation of the two adsorption sites is equal for all coverages, which can be explained by pair formation due to similar site-specific adsorption energies and the possibility of forming hydrogen bonds between molecules on adjacent {110} and {311} sites. This is not the ease for alanine and points toward higher site specificity in the case of alanine, which is eventually responsible for the enantiomeric differences observed for the alanine system.

Modeling the Histidine–Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ~80% of the T-shaped histidine···aromatic contacts are CH···π, and only ~20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C₆ axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by −73 cm⁻¹ relative to that of bare imidazole at 3518 cm⁻¹, indicating a moderately strong NH···π interaction. While the Sₒ(A1g) → S₁(B₂u) origin of benzene at 38 086 cm⁻¹ is forbidden in the gas phase, Im·Bz exhibits a moderately intense Sₒ → S₁ origin, which appears via the D₆h → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm⁻¹). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)₂ cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz₂ are cleanly separable by IR hole burning. The UV spectrum of Im·Bz₂ exhibits two 000 bands corresponding to the Sₒ → S₁ excitations of the two inequivalent benzenes, which are symmetrically shifted by −86/+88 cm⁻¹ relative to the 000 band of benzene.

On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions

The electronic nature of low-barrier hydrogen bonds (LBHBs) in enzymatic reactions is discussed based on combined low temperature neutron and x-ray diffraction experiments and on high level ab initio calculations by using the model substrate benzoylacetone. This molecule has a LBHB, as the intramolecular hydrogen bond is described by a double-well potential with a small barrier for hydrogen transfer. From an “atoms in molecules” analysis of the electron density, it is found that the hydrogen atom is stabilized by covalent bonds to both oxygens. Large atomic partial charges on the hydrogen-bonded atoms are found experimentally and theoretically. Therefore, the hydrogen bond gains stabilization from both covalency and from the normal electrostatic interactions found for long, weak hydrogen bonds. Based on comparisons with other systems having short-strong hydrogen bonds or LBHBs, it is proposed that all short-strong and LBHB systems possess similar electronic features of the hydrogen-bonded region, namely polar covalent bonds between the hydrogen atom and both heteroatoms in question.

Hydrogen bond interactions between acetone and supercritical water

Hydrogen bond interactions between acetone and supercritical water are investigated using a combined and sequential Monte Carlo/quantum mechanics (S-MC/QM) approach. Simulation results show a dominant presence of con. gurations with one hydrogen bond for different supercritical states, indicating that this specific interaction plays an important role on the solvation properties of acetone in supercritical water. Using QM MP2/aug-cc-pVDZ the calculated average interaction energy reveals that the hydrogen-bonded acetone-water complex is energetically more stable under supercritical conditions than ambient conditions and its stability is little affected by variations of temperature and/or pressure. All average results reported here are statistically converged.

Comparative electron impact and fast atom bombardment mass spectrometric studies of some HMPA adducts of phenyltin and phenyllead halides and studies of strong hydrogen bonding by FAB-MS

The fragmentation patterns and mass spectra of some phenyl tin and -lead halide adducts with hexamethylphosphoramide are compared by subjecting them t~ electron impact and fast atom bombardment ionization in a mass spectrometer. This comparison is restricted to the metal-containing ions. Ligand-exchange mechanisms of some of the metal-containing species are explored by FAB-MS. Several moisturesensitive organo-metallics and H-bonded systems have been examined by FAB for attempted characterization, but without any success. Scavenging and trapping of water molecules by complex aggregates in solutions of quaternary ammonium fluorides and hydroxides are investigated by FAB to complement previous NMR-studies.

Strong hydrogen bonding in organic synthesis

The preparation of phenacyl and para-phenylphenacyl esters, the reactions of carboxylic acids, phenols, 2-nitropropane and alcohols with alkyl halides in the presence of fluoride anion are described. The reactions are thought to be accelerated by the formation of hydrogen bonds between the fluoride anion and the organic electron acceptor. The fluoride ,carboxylic acids, fluoride-phenols and fluoride-2-nitropropane are better reaction systems than the fluoride-alcohol. The source of the fluoride anion and the choice of solvents are also discussed.

Supramolecular hydrogen-bond structures and magnetic interactions in basal-apical, dinuclear, azide-bridged copper(II) complexes

The synthesis, characterisation, X-ray single crystal structures and magnetic properties of three new basal-apical mu(2)-1,1-azide-bridged complexes [(CuLN3)-N-1](2) (1), [(CuLN3)-N-2](2) (2) and [(CuLN3)-N-3](2) (3) with very similar tridentate Schiff-base blocking ligands {HL1 = N-[2-(ethylamino) ethyl] salicylaldimine; HL2 = 7-(ethylamino)-4-methyl-5-azahept-3-en-2-one; HL3 = 7-amino-4-methyl-5-azaoct-3-en-2-one} have been reported [complex 1: monoclinic, P2(1)/c, a = 8.390(2), b = 7.512(2), c = 19.822(6) Angstrom, beta = 91.45(5)degrees; complex 2: monoclinic, P2(1)/c, a = 8.070(9), b = 9.787(12), c = 15.743(17) A, beta = 98.467(10)degrees; complex 3: monoclinic, P2(1)/n, a = 5.884(7), b = 16.147(18), c = 11.901(12) Angstrom, beta = 90.050(10)degrees]. The structures consist of neutral dinuclear entities resulting from the pairing of two mononuclear units through end-on azide bridges connecting an equatorial position of one copper centre to an axial position of the other, The copper ions adopt a (4+1) square-based geometry in all the complexes. In complex 2, there is no inter-dimer hydrogen-bonding. However, complexes 1 and 3 form two different supramolecular structures in which the dinuclear entities are linked by H-bonds giving one-dimensional systems. Variable temperature (300-2 K) magnetic-susceptibility measurements and magnetisation measurements at 2 K reveal that all three complexes have antiferromagnetic coupling. Magneto-structural correlations have been made taking into consideration both the azido bridging ligands and the existence of intermolecular hydrogen bonds. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

Hydrogen-bond assisted stabilization of the less favored conformation of a tridentate Schiff base ligand in dinuclear nickel(II) complex: an experimental and theoretical study

The Schiff base ligand, HL (2-[1-(3-methylamino-propylimino)-ethyl]-phenol), the 1:1 condensation product of 2-hydroxy acetophenone and N-methyl-1,3-diaminopropane, has been synthesized and characterized by X-ray crystallography as the perchlorate salt [H2L]ClO4 (1). The structure consists of discrete [H2L](+) cations and perchlorate anions. Two dinuclear Ni-II complexes, [Ni2L2(NO2)(2)] (2), [Ni2L2(NO3)(2)] (3) have been synthesized using this ligand and characterized by single crystal X-ray analyses. Complexes 2 and 3 are centrosymmetric dimers in which the Ni-II ions are in distorted fac- and mer-octahedral environments, respectively, bridged by two mu(2)-phenolate ions of deprotonated ligand, L. The plane of the phenyl rings and the Ni2O2 basal plane are nearly coplanar in 2 but almost perpendicular in 3. We have studied and explained this different behavior using high level DFT calculations (RI-BP86/def2-TZVP level of theory). The conformation observed in 3, which is energetically less favorable, is stabilized via intermolecular non-covalent interactions. Under the excitation of ultraviolet light, characteristic fluorescence of compound 1 was observed; by comparison fluorescence intensity decreases in case of compound 3 and completely quenched in compound 2.

Mono-aqua-bridged dinuclear complexes of Cu(II) containing NNO donor Schiff base ligand: Hydrogen-bond-mediated exchange coupling

Two new mono-aqua-bridged dinuclear Cu(II) complexes of tridentate NNO Schiff bases, [Cu-2(mu-H2O)L-2(1)(H2O)(2)](BF4)(2)center dot 2H(2)O (1) and [Cu-2(mu-H2O)L-2(2)(H2O)(2)](BF4)(2)center dot 2H(2)O (2) where HL1 = 2-[1-(2-dimethylamino-ethylimino)-ethyl]-phenol and HL2 =2-[(2-dimethylamino-ethylimino)-methyl]-phenol were synthesized. Both the complexes were characterized by single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. For both the complexes each Cu(II) ion is in a square-pyramidal environment being bonded to three atoms from the tridentate NNO Schiff base and a terminal H2O molecule in the equatorial plane; a second H2O ligand acts as a bridge between the two Cu(II) centres through the axial positions. Hydrogen bonds between the terminal H2O ligand and the Schiff base of the adjacent centre complete the intra-dimer linkages. Variable-temperature (4-300 K) magnetic susceptibility measurement shows the presence of significant antiferromagnetic coupling for both the complexes (J = -12.2 and -12.5 cm(-1), respectively, for 1 and 2), mediated mainly through the intra-dimer H-bonds.

Application of hydrogen-bond propensity calculations to an indomethacin–nicotinamide (1:1) co-crystal

The crystal structure of an indomethacin–nicotinamide (1 : 1) cocrystal produced by milling has been determined from laboratory powder X-ray diffraction (PXRD) data. The hydrogen bonding motifs observed in the structure represent one of the most probable of all the possible combinations of donors and acceptors in the constituent molecules.