CH Hydrogen Research Articles

Catalytic hydrogenation of ethylene on organoboron nanoparticles formed by pyrolysis of C2B10H12 carborane

The ability of organoboron nanoparticles (C2B10H4) n to multiply enhance the yield of C2H6 in the hydrogenation of C2H4 at temperatures 770–970 K is experimentally demonstrated. The observed acceleration is of catalytic nature, since the electronic structure of the nanoparticles remains unchanged in the hydrogenation reaction.

Conformation and interactions of dopamine hydrochloride in solution.

The aqueous solution of dopamine hydrochloride has been investigated using neutron and X-ray total scattering data together with Monte-Carlo based modelling using Empirical Potential Structure Refinement. The conformation of the protonated dopamine molecule is presented and the results compared to the conformations found in crystal structures, dopamine-complexed protein crystal structures and predicted from theoretical calculations and pharmacophoric models. It is found that protonated dopamine adopts a range of conformations in solution, highlighting the low rotational energy barrier between different conformations, with the preferred conformation being trans-perpendicular. The interactions between each of the species present (protonated dopamine molecules, water molecules, and chloride anions) have been determined and are discussed with reference to interactions observed in similar systems both in the liquid and crystalline state, and predicted from theoretical calculations. The expected strong hydrogen bonds between the strong hydrogen bond donors and acceptors are observed, together with evidence of weaker CH hydrogen bonds and π interactions also playing a significant role in determining the arrangement of adjacent molecules.

Improving Far UV Circular Dichroism Calculations of Peptides and Proteins with the Dipole Interaction Model

The dipole interaction model (assembled into the computer package DInaMo) uses a classical electromagnetic theory for calculating the far UV circular dichroism (CD) of peptides and proteins. DInaMo reduces all amide chromophores to a single point with anisotropic polarizability and all nonchomophoric aliphatic atoms to points with isotropic polarizability. By determining interactions among the chomophoric and nonchromoporic parts of the molecule using empirically derived polarizabilities, the rotational and dipole strengths are determined leading to the calculation of CD. Polarizabilities are largest for the chromophoric points and smaller for the nonchromophoric points with hydrogens having the smallest polarizabilities. It is possible to collapse hydrogen polarizability onto the atom to which it is bound or ignore it in the CD calculation creating a united atom approach. Ignoring CH3 group hydrogens and treating only the π-π∗ transition reproduces experiment in the region of 180 to 210 nm well, showing bands with similar morphology and absorption maxima for the π-π∗ transition. Recent calculations on small peptides and proteins in which both CH3 and CH2 group hydrogens have been ignored seem to produce better CD results than ignoring only the CH3 group hydrogens. Also, preliminary calculations using the mean polarizability values of the CH3 and CH2 groups show some good prospects of further improving the CD calculations with the DinaMo package. In addition, initial calculations indicate that DInaMo has the potential of including the n-π∗ transition in the CD calculations.

Molecular and ionic diffusion in aqueous - deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR.

Pulsed field gradient (PFG) NMR has been used to probe self-diffusion of molecular and ionic species in aqueous mixtures of choline chloride (ChCl) based deep eutectic solvents (DESs), in order to elucidate the effect of water on motion and inter-molecular interactions between the different species in the mixtures, namely the Ch(+) cation and hydrogen bond donor (HBD). The results reveal an interesting and complex behaviour of such mixtures at a molecular level. In general, it is observed that the hydroxyl protons ((1)H) of Ch(+) and the hydrogen bond donor have diffusion coefficients significantly different from those measured for their parent molecules when water is added. This indicates a clear and significant change in inter-molecular interactions. In aqueous Ethaline, the hydroxyl species of Ch(+) and HBD show a stronger interaction with water as water is added to the system. In the case of Glyceline, water has little effect on both hydroxyl proton diffusion of Ch(+) and HBD. In Reline, it is likely that water allows the formation of small amounts of ammonium hydroxide. The most surprising observation is from the self-diffusion of water, which is considerably higher that expected from a homogeneous liquid. This leads to the conclusion that Reline and Glyceline form mixtures that are inhomogeneous at a microscopic level despite the hydrophilicity of the salt and HBD. This work shows that PFG NMR is a powerful tool to elucidate both molecular dynamics and inter-molecular interactions in complex liquid mixtures, such as the aqueous DES mixtures.

How many water molecules does it take to ionise HCN/HNC? An NCI exploration.

How many water molecules does it take to ionise HCN/HNC? An NCI exploration.

Catalytic total oxidation of 1,2-dichloroethane over highly dispersed vanadia supported on CeO2 nanobelts

CeO2 nanobelts were synthesized via a facile aqueous-phase precipitation route under mild conditions (template-free and non-hydrothermal), and then the highly dispersed vanadia catalysts with a wide range of VOx loadings were prepared by a conventional incipient-wetness impregnation method. The target VOx/CeO2 catalysts were characterized in detail and used in catalytic combustion of 1,2-dichloroethane (DCE). The results revealed that the monolayer dispersed VOx (6.0%VOx/CeO2) exhibited the most outstanding initial and stable activities, however, the main product containing carbon was CO, not desired CO2. A reaction mechanism of DCE total oxidation was proposed based on the study of TPSR and in situ FTIR. The first step of this mechanism was considered to be a dissociative adsorption of CCl bonds on Lewis acid sites (such as Ce4+/3+, V5+/4+ or Ce3+–O2−–V5+) via Cl abstraction, and then the dissociated DCE can be oxidized directly to CO2 by surface active oxygen species over pure CeO2. Whereas, over VOx/CeO2 catalysts, the formation of intermediate acetaldehyde was a key step via CH bond activation and hydrogen transfer on VOx species, subsequently, partially oxidized to CO by the lattice oxygen of VOx.

Ethanol Synthesis from Syngas on the Stepped Rh(211) Surface: Effect of Surface Structure and Composition

The mechanisms of ethanol synthesis from syngas including CHx (x = 1–3) species and C2 oxygenates on the stepped Rh(211) surface have been systematically investigated by using density functional theory together with a periodic slab model. Our results show that among all CHx (x = 1–3) species, CH3 is the most favorable monomer, which is more favorable than CH3OH formation; this result is quite different from that on the flat Rh(111). Beginning with CH3 species, CH3 + CHO → CH3CHO dominantly contributes to the formation of C2 oxygenates; subsequently, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH4 formation by CH3 hydrogenation is energetically compatible with CH3CHO formation; that is, the productivity and selectivity of ethanol is determined by the formations of CH3CHO and CH4. As a result, the strategy of improving the Rh-based catalyst with the help from the promoter Mn is employed to tune the relative activity of CH3CHO and CH4 formations that control the productivity and selectivity of ethanol from syngas. Our results show that the productivity and selectivity of ethanol from syngas can be improved by introducing promoter Mn into the Rh catalyst.

Opposite face sensitivity of CeO₂ in hydrogenation and oxidation catalysis.

The determination of structure-performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO2 particles, dominates in C2H2 hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO2 are favored over low-vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.

Opposite Face Sensitivity of CeO2 in Hydrogenation and Oxidation Catalysis

AbstractThe determination of structure–performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO2 particles, dominates in C2H2 hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO2 are favored over low‐vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.

Formation of 1D-Chain via C–H⋯Cl Interaction Utilizing [(L3)ZnIICl2] (L3 = 2-[3-(2′-Pyridyl)pyrazol-1-ylmethyl]-(1-methylimidazole)) Tecton

Reaction of 2-[3-(2′-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole) (L3) with ZnCl2 affords [(L3)ZnIICl2] 1. Crystal structure analysis of 1 reveals that the ZnII ion assumes a slightly distorted square-pyramidal geometry with coordination by the tridentate nonplanar ligand L3, providing a pyridine, a pyrazole and an imidazole N donors and two chloride ions. Notably, secondary interactions [C–H⋯Cl (–CH2– hydrogen of the spacer and –CH3 hydrogen of 1-methylimidazole)] triggered by ZnII-coordinated chloride ions acting as hydrogen-bonding acceptors generates self-complementary dimeric tectons, which lead to 1D supramolecular chain.

Insight into the Effect of Promoter Mn on Ethanol Formation from Syngas on a Mn-Promoted MnCu(211) Surface: A Comparison with a Cu(211) Surface

Density functional theory calculations have been employed to investigate the effect of promoter Mn on ethanol formation from syngas on a Mn-promoted MnCu(211) surface. Our results show that CO + 3H → CHO + 2H → CH2O + H → CH3O is an optimal pathway for the overall CO conversion. Starting with CH3O, CH3 is formed via CH3O → CH3 + O. Then, CHO insertion into CH3 can form CH3CHO, and further, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH3OH is formed via CH3O + H → CH3OH. Compared to the pure Cu(211) surface, CH3 formation is found to be energetically compatible with CH3OH formation on the MnCu(211) surface, which can lead to more CH3 sources and less CH3OH; thus, the productivity and selectivity of ethanol can be improved. On the other hand, starting from CH3, the MnCu(211) surface is more favorable for CHO insertion into CH3 to CH3CHO in comparison with CH3 hydrogenation, dissociation and coupling to CH4, CH2, and C2H6 due to their high activation barriers; namely, the MnCu(211) surface exhibits a better selectivity toward C2 oxygenates rather than hydrocarbons. As a result, we can show that, by introducing promoter Mn into Cu catalyst, the productivity and selectivity to ethanol from syngas can be effectively improved.

Can Green Dimethyl Carbonate Synthesis be More Effective? A Catalyst Recycling Study Benefiting from Experimental Kinetics and DFT Modeling

Dibutyldimethoxystannanes are known to catalyze the reaction between carbon dioxide and methanol leading to dimethyl carbonate. Despite similarities between din-butyl- and ditert-butyldimethoxystannane, the recycled complexes have different structural features. In the din-butyl series, a decatin(IV) complex has been characterized and is less active than the stannane precursor. Kinetic experiments likely indicate that all the tin centers are not active, which is confirmed in comparing with the related dinuclear 1,3-dimethoxytetran-butyldistannoxane complex. In the ditert-butyl series, the tritin(IV) complex isolated upon recycling features the steric effect of bulky tBu ancillary ligands. Interestingly enough, the SnOH. . .O(H)CH3 hydrogen bonding found in the structure prefigures Sn-OH/Sn-OCH3 interchange, a crucial step for closing the catalytic cycle. Density functional calculations highlight that the Sn-OH/Sn-OCH3 exchange is endothermic. Taken together, the results cast a clear light on the significant role of complexes of low nuclearity for dimethyl carbonate synthesis.

Neutral CH and cationic CH donor groups as anion receptors.

The design and synthesis of anion selective receptors and chemosensors continues to attract considerable interest within the supramolecular community. In recent years, increasing attention has focused on the use of neutral and cationic CH hydrogen bond donors as anion recognition elements. Over the last five years, motifs that support CHX (X = anion) hydrogen bonds have been actively used in various shape persistent macrocycles, foldamers and "molecular machines". This tutorial review highlights recent developments in host-guest chemistry based on the use of neutral and cationic CH hydrogen bond donors. Also discussed are various structural classifications, including alkyl CH, phenyl CH, triazole-based CH, imidazolium (CH)(+) and triazolium (CH)(+) hydrogen bond donor systems.

Effects of Support on Sulfur Tolerance and Regeneration of Pt Catalysts Measured by Ethylene Hydrogenation and EXAFS

The effect of support on sulfur tolerance and regenerability under reducing environments was investigated by rate measurements for ethylene hydrogenation, hydrogen chemisorption, and extended X-ray absorption fine structure (EXAFS). Catalysts, 1 % Pt/Al2O3 and 1 % Pt/P25 (TiO2), were tested after sulfidation in H2S/H2 at 250 °C followed by regeneration treatments in H2 at 250, 350 and 450 °C. Our combined results showed a 20–27 times decrease in the rate of ethylene hydrogenation on both sulfided catalysts, accompanied by a 4–6 times drop in the Pt surface area. Regenerations up to 450 °C were unable to remove all the sulfur, as evidenced by the presence of Pt–S bonds by EXAFS at about 2.25–2.33 A, characteristic lengths for chemisorbed sulfur and bulk-type PtS. However, a partial recovery of the hydrogenation rate per mole of Pt was observed on sulfided Pt/Al2O3 after reduction at 450 °C, while the induction of strong metal support interactions (SMSI) at reduction temperature above 350 °C was observed on Pt/P25, regardless of the presence of sulfur. For Pt/P25, the reversal of the SMSI state together with sulfur removal by mild oxidation suggests that sequential reduction/oxidation treatments may be more effective in restoring the S-free state of TiO2-supported catalysts. Pt/Al2O3 and Pt/TiO2 (P25) sulfur tolerance and regenerability were evaluated after reduction treatments in H2. Both catalysts were equally poisoned by sulfur based on C2H4 hydrogenation. Reduction treatments up to 450 °C were not able to remove sulfur on either catalyst. Sulfur on Pt may inhibit the formation of the SMSI state on Pt/TiO2, especially below 350 °C.

Substituent Effects on Noncovalent Bonds: Complexes of Ionized Benzene Derivatives with Hydrogen Cyanide

Here, we report the first experimental and computational study of the noncovalent binding energies and structures of ionized benzenes containing electron-withdrawing substituents solvated by one to four HCN molecules. Measured by ion mobility mass spectrometric equilibrium studies, the bond dissociation enthalpies of the first HCN molecule to the fluorobenzene (C6H5F(•+)), 1,4-difluorobenzene (C6H4F2(•+)), and benzonitrile (C6H5CN(•+)) ions (11.2, 11.2, and 9.2 kcal/mol, respectively) are similar to those of HCN with the benzene (C6H6(•+)) and phenyacetylene (C6H5CCH(•+)) radical cations (9.2 and 10.5 kcal/mol, respectively). DFT calculations at the B3LYP/6-311++G(d,p) level show that HCN can form in-plane hydrogen bonds to ring hydrogens, or bind electrostatically to positively charged carbon centers in the ring. The electron-withdrawing substituents increase the bond energy by increasing the partial positive charge on the ring hydrogens that form CH(δ+)---NCH bonds, and by creating a π hole, as evidenced by positive charge centers on the fluorinated ring carbons for electrostatically bonded isomers. In the complexes of benzonitrile(•+), similar to benzene(•+), hydrogen bonded planar isomers have the lowest energy. In the complexes of (fluorinated benzene)(•+), the lowest energy isomers are electrostatically bonded where HCN is perpendicular to the ring and its dipole points to a positively charged ring carbon. However, in all cases the planar hydrogen-bonded and vertical electrostatic isomers have similar binding energies within 1 kcal/mol, although HCN interacts with different sites of the ionized benzenes in these isomers, suggesting that the observed cluster populations are mixtures of the planar and vertical isomers. Further HCN molecules can bind directly to unoccupied ring CH hydrogens or bind to the first-shell HCN molecules to form linear HCN---HCN--- hydrogen bonded chains. The binding energies decrease stepwise to about 6-7 kcal/mol by 4 HCN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal/mol).

On the role of noncovalent interactions in electrocatalysis. Two cases of mediated reductive dehalogenation

Two cases of mediated electron transfer are presented: chloroform reduction catalysed by MoII/I alkoxy scorpionates and debromination of hexabromocyclododecane (HBCD) in the presence of free-base tetraphenylporphyrin (H2TPP). Although H2TPP should act as a typical outer-sphere mediator, it is not active towards analogous dehalogenation of 1,2-dibromocyclododecane. The observed phenomena can be rationalised by considering the catalytically relevant transient adducts formed owing to noncovalent interactions (CH hydrogen bonds and dispersive Chalogen⋯π interactions or directional halogen bonding), which warrants the close and prolonged contact between the catalyst and its substrate, thus increases the probability of electron transfer, and decisively accelerates the reaction. Crucial for this action is thermodynamic stability of the adducts, which can only be explained if dispersive van der Waals interactions are properly accounted for, e.g., as by dispersion-corrected density functional theory (DFT-D) calculations. The structures involving strong and anisotropic interactions, like the surprisingly short CH⋯Oalkoxide H-bonding in the MoI–chloroform adduct, may be reasonably well described by standard DFT calculations and the energy needs only be corrected for dispersion without the need for structure re-optimisation at the DFT-D level. The latter is, however, a method of choice for the prediction of supramolecular structures chiefly controlled by weak non-directional van der Waals forces.

Synthesis, spectral and single crystal X-ray diffraction studies on Co(II), Ni(II), Cu(II) and Zn(II) complexes with o-amino acetophenone benzoyl hydrazone

A series of metal complexes of Co(II), Ni(II), Cu(II) and Zn(II) with o-amino acetophenone benzoylhydrazone (HL) have been synthesized. The complexes were characterized by elemental analyses, magnetic susceptibility measurements, electronic, IR, NMR and ESR spectral techniques. The molecular structures of the Co(II) and Ni(II) complexes were determined by single crystal X-ray diffraction studies. The delocalized charge on the five-membered chelate rings of the Co(II) and Ni(II) complexes forms an unusual intra-molecular CH⋯π interaction with the –CH3 hydrogen. CH⋯π interactions were also observed in the Ni(II) complex between the phenyl ring and phenyl hydrogen. HL acts as a tridentate ligand in most of the complexes, bonding through >CO, >CN and –NH2 groups. Electronic spectral studies indicate an octahedral geometry for the Co(II) and Ni(II) complexes, but a square planar geometry for the Cu(II) complex. The ESR spectra of the Cu(II) complex also suggest a square planar geometry around the metal ion in the solid state, but a tetragonal distorted octahedral geometry in DMSO solution at LNT due to solvent interactions at the axial positions. The electrochemical study of the Cu(II) complex exhibits reversible redox behaviour. The thermal analyses (TGA and DTA) of a few complexes show an exothermic multi-step decomposition pattern of the bonded ligands.

Steric effects in the interaction between transmembrane proteins and polyunsaturated phospholipids

The methylene-interrupted, all-cis configuration is overwhelmingly favoured in polyunsaturated fatty acids (PUFA) of vertebrate membranes, particularly the highly unsaturated membranes of electrically active neural tissue. The hypothesis that hexaene and pentaene homoallylic groups are sterically allowed to follow the groove of a transmembrane α-helix from bovine rhodopsin was investigated using energy minimization (EM) and molecular dynamics (MD) simulations with the GROMOS96 force field. Docosahexaenoic acid (22:6n-3) and docosapentaenoic acid (22:5n-6) were arranged along arbitrary midand C-terminal paths of a model of α-helix 2 of bovine rhodopsin, with the doubly allylic CH 2 hydrogens directed inward and the ethylenic CH hydrogens directed out of the helix. By performing EM and MD simulations for 22:6n-3 and 22:5n-6, it was shown that the rotationally constrained homoallylic regions are more stably contained within the groove than are the saturated regions. The analyses of the initial conformations of 22:6n-3 in phosphatidylcholine-22:6n-3;34:5n-3, and of 22:5n-6 in phosphatidylcholine-22:5n-6;34:5n-3, and of conformations of 22:6n-3 in phosphatidylcholine-22:6n-3;34:5n-3 obtained after EM, showed that the homoallylic regions fit loosely within the groove while the saturated regions are much closer to the outer groove boundary. EM calculations of the phosphatidylcholine22:6n-3;34:5n-3 showed that the 22:6n-3 chain can follow the groove nearest a membrane– water interface, while the homoallylic region of the very long chain 34:5n-3 can follow the groove nearer the membrane centre, tethered by its extended saturated region. These results illustrate that the homoallylic polyunsaturated fatty acid motif is not sterically prevented from occupying the groove of a transmembrane α-helix. Speculations are provided about the biophysical stability and properties conferred by this configuration.

INSIGHTS INTO SURFACE HYDROGENATION IN THE INTERSTELLAR MEDIUM: OBSERVATIONS OF METHANIMINE AND METHYL AMINE IN Sgr B2(N)

Multiple observations of methanimine (CH{sub 2}NH) and methyl amine (CH{sub 3}NH{sub 2}) have been performed toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. In the frequency range 68-280 GHz, 23 transitions of CH{sub 2}NH and 170 lines of CH{sub 3}NH{sub 2} have been observed as individual, distinguishable features, although some are partially blended with other lines. For CH{sub 2}NH, the line profiles indicate V{sub LSR} = 64.2 {+-} 1.4 km s{sup -1} and {Delta}V{sub 1/2} = 13.8 {+-} 2.8 km s{sup -1}, while V{sub LSR} = 63.7 {+-} 1.6 km s{sup -1} and {Delta}V{sub 1/2} = 15.1 {+-} 3.0 km s{sup -1} for CH{sub 3}NH{sub 2}, parameters that are very similar to those of other organic species in Sgr B2(N). From these data, rotational diagrams were constructed for both species. In the case of CH{sub 2}NH, a rotational temperature of T{sub rot} = 44 {+-} 13 K and a column density of N{sub tot} = (9.1 {+-} 4.4) Multiplication-Sign 10{sup 14} cm{sup -2} were determined from the analysis. For CH{sub 3}NH{sub 2}, T{sub rot} = 159 {+-} 30 K and N{sub tot} = (5.0 {+-}more » 0.9) Multiplication-Sign 10{sup 15} cm{sup -2}, indicating that this species is present in much warmer gas than CH{sub 2}NH. The fractional abundances for CH{sub 2}NH and CH{sub 3}NH{sub 2} were established to be f (H{sub 2}) Almost-Equal-To 3.0 Multiplication-Sign 10{sup -10} and f (H{sub 2}) Almost-Equal-To 1.7 Multiplication-Sign 10{sup -9}, respectively. It has been proposed that CH{sub 2}NH is formed on grains via hydrogenation of HCN; further hydrogenation of CH{sub 2}NH on surfaces leads to CH{sub 3}NH{sub 2}. However, given the dissimilarity between the rotational temperatures and distributions of CH{sub 2}NH and CH{sub 3}NH{sub 2} in Sgr B2, it is improbable that these species are closely related synthetically, at least in this source. Both CH{sub 2}NH and CH{sub 3}NH{sub 2} are more likely created by neutral-neutral processes in the gas phase.« less

A new conformational polymorph of N-acetyl-l-cysteine. The role of S–H⋯O and C–H⋯O interactions

A novel polymorph of N-acetyl-L-cysteine (NAC) is discovered three decades after the first report on the X-ray crystal structure of this bioactive compound. The crystal structure of the new orthorhombic polymorph (form II in the P212121 space group) is characterized by X-ray diffraction and compared to the triclinic structure of form I (NALCYS02 and NALCYS10 in the P1 space group). Both polymorphs contain a C(7) chain of COOH⋯OC–CH3 hydrogen bonds, except that the COOH group is rotated by 180° in form II to make an auxiliary C–H⋯O interaction with the methyl group in a R22(8) ring motif. The known form I contains weak S–H⋯OC–OH and N–H⋯S hydrogen bonds whereas the new form II has only N–H⋯S interactions. The conformational polymorphs of NAC were compared by Hirshfeld surface analysis (dnorm) and XPac methods, and were spectroscopically characterized by FT-IR and Raman spectroscopy. The bulk phases were distinguished by their 13C ss-NMR and powder X-ray diffraction line patterns. The two polymorphs are enantiotropically related (form I has a higher melting temperature and lower enthalpy of fusion). However, no phase transition was observed in the DSC analysis. The metastable form II converted to stable form I in solid-state grinding, a slurry medium and storage in ambient conditions for 3 months.