SEARCH · Search PubMed
Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
The di-pi-methane rearrangement is firmly established as a mode of synthesizing three-membered-ring compounds. We now report the tri-pi-methane counterpart.
[reaction: see text] An unusual diversion of the di-pi-methane rearrangement has been encountered. The reaction is characteristic of di-pi-methane systems having one vinyl group bearing one or two carbonyl groups and provides a synthesis of dihydrofurans.
[reaction: see text] Oxidized bis(indolyl)methane, 1, a simple chromophore containing an acidic H-bond donor moiety and a basic H-bond acceptor moiety, can act as a selective colorimetric sensor either for F(-) in aprotic solvent or for HSO(4)(-) and weak acidic species in water-containing medium. The deprotonation/protonation of oxidized bis(indolyl)methane 1 is responsible for the dramatic color change.
High resolution pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectroscopy has been used to record the photoelectron spectra of CH4, CDH3, CD2H2 and CD4. The observed extensive progression of rotationally resolved transitions between 100,800 cm-1 and 104,100 cm-1 reveals for the first time the complex energy level structure of the methane cation. The high resolution enabled the determination of accurate values for the adiabatic ionization potentials of the different isotopomers. Based on a simple one-dimensional model for the pseudorotation in the different isotopomers, progress has been made towards the understanding of the Jahn-Teller effect at low energies. The static Jahn-Teller distortion in the ion could be determined directly from the vibrationless photoelectron transition in CD2H2. The analysis of the rotational structure in this spectrum with a rigid rotor model leads to an approximate experimental C2v structure. The dynamics of the other methane isotopomers near the adiabatic ionization potentials is dominated by large amplitude vibrational motions between equivalent structures. The corresponding ground state tunneling motions takes place on a picosecond time scale.
We calculated the cross sections for vibrational predissociation of methane-Ar induced by excitation of the methane nu 3 mode with the aid of an ab initio CH4-Ar potential depending explicitly on the nu 3 and nu 1 normal coordinates of the CH4 monomer. We found that dissociation into CH4 fragments excited in the nu 1 mode, a V-->V' process with very low kinetic energy release, strongly dominates over direct dissociation into Ar and ground state CH4, and is responsible for the line broadening observed experimentally. The (observed and calculated) strong variation of the line widths for the Van der Waals levels excited in combination with the nu 3 mode (giving states of A, F and E symmetry) is related to the opening up of appropriate nu 1 dissociation channels and the occurrence of rotational resonances in the nu 1 continuum in the energy range of the quasi-bound nu 3 levels.
The induction period of dehydroaromatization of methane to benzene over Mo/HZSM-5 had been investigated in real-time by the resonant-enhanced two-photon ionization (RE2PI) technique; it is remarkable that there is a small amount of benzene formed in the early stage of the induction period; we suggest that the trace amount of benzene was caused by the reduction of the original Mo6+ ion during the induction period and the Mo6+ species has a slight catalytic activity for methane-benzene conversion.
We report the synthesis of open-ended MoS2 nanotubes and their application as the catalyst for methanation of carbon monoxide with hydrogen. Because the catalysis of the methanation reaction occurs at relatively low temperatures, such nanotubes may provide new solutions toward harnessing the environmental load caused by CO emission.
Methane is transformed selectively to methanesulfonyl chloride at low temperature by liquid-phase reaction of methane with SO(2)Cl(2) in the presence of a free radical initiator and a promoter using 100% H(2)SO(4) as the solvent.
Particulate methane monooxygenase is a copper-containing, membrane-bound metalloenzyme that converts methane to methanol in Nature. How pMMO accomplishes this difficult reaction under ambient conditions is one of the major unsolved problems in bioinorganic chemistry. Despite considerable research efforts in the past 20 years, the active site of the enzyme remains unknown. We recently solved the first crystal structure of pMMO to 2.8 è resolution, revealing the overall structure, oligomerization state, subunit ratio, and composition and location of the metal centers. Almost none of the key structural features were predicted. In this Perspective, we review the state of knowledge before and after the structure determination, emphasizing elucidation of the pMMO active site.
The trioxo [ReO(3){SO(3)C(pz)(3)}] (1) (pz = pyrazolyl) and oxo [ReOCl{SO(3)C(pz)(3)}(PPh(3))]Cl (2) compounds with tris(pyrazolyl)methanesulfonate were obtained by treatment of Re(2)O(7) or [ReOCl(3)(PPh(3))(2)], respectively, with Li[SO(3)C(pz)(3)], whereas [ReCl(3){HC(pz)(3)}] (3), [ReCl(3){HC(3,5-Me(2)pz)(3)}] (4) and [ReCl(4){eta(2)-HC(pz)(3)}] (5) were prepared by reaction of [ReOCl(3)(PPh(3))(2)] (3,4) or [ReCl(4)(NCMe)(2)] (5) with hydrotris(pyrazolyl)methane HC(pz)(3) (3,5) or hydrotris(3,5-dimethyl-1-pyrazolyl)methane HC(3,5-Me(2)pz)(3) (4). [ReO{SO(3)C(pz)(3)}{OC(CH(3))(2)pz}][ReO(4)] 6, with a chelated pyrazolyl-alkoxide, was derived from an unprecedented ketone-pyrazolyl coupling on reaction of crude 1 with acetone. The compounds have been characterized by elemental analyses, IR and NMR spectroscopies, FAB-MS spectrometry and cyclic voltammetry and, in the case of 5 and 6, also by single-crystal X-ray diffraction. The electrochemical E(L) Lever parameter has been estimated, for the first time, for the SO(3)C(pz)(3)(-) and oxo ligands allowing the measurement of their electron-donor character and comparison with other ligands. Compounds 1, 2 and 6 appear to be the first tris(pyrazolyl)methanesulfonate complexes of rhenium to be reported.
A new type of 1D infinite inorganic [n]catenane framework was self-assembled by silver nitrate and bis(2-methylimidazolyl)methane, while a comparatively one-dimensional zigzag cationic chain was generated by AgNO(3) and bis(imidazol-1-yl)methane.
The alkyne functionalised bidentate N-donor ligand (2-propargyloxyphenyl)bis(pyrazolyl)methane was prepared in high yield from the reaction of (2-hydroxyphenyl)bis(pyrazolyl)methane with propargyl bromide in the presence of base. A series of transition-metal complexes including [MCl2] (M=Cu, Co, Ni, Zn, Pt), [M2](NO3)2 (M=Cu, Co, Ni, Zn), [Ag]NO3 and [Pd(dppe)](OTf)2 were prepared and characterised by spectroscopic techniques. In addition, ligand as well as the Co(II) and Zn(II) complexes [CoCl2]2, [ZnCl2] were structurally characterized by single-crystal X-ray diffraction. The organometallic gold(I) and platinum(II) acetylide complexes [Pz2CH(C6H(4)-2-OCH2C[triple bond, length as m-dash]CAuPPh3)] and trans-[{Pz2CHC6H(4)-2-OCH2C[triple bond, length as m-dash]C}2Pt(PPh3)2] were prepared from and [AuCl(PPh3)] and trans-[PtCl2(PPh3)2], respectively. Treatment of these complexes with [Pd(OTf)2(dppe)] or [Cu(MeCN)4]PF6 results in formation of the cationic, mixed-metal complexes, which were isolated (Pt/Pd, Au/Pt) or detected by electrospray mass spectrometry (Au/Cu, Pt/Cu).
The methanotrophic bacteria Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b convert methane to methanol using the enzyme, methane monooxygenase (MMO). These bacteria are able to express two distinct MMOs: a cytoplasmic or soluble form (sMMO) and a membrane-bound or particulate form (pMMO). Differential expression of sMMO and pMMO is regulated by the amount of copper ions available to the cells; sMMO is expressed at low copper-biomass ratios, whereas pMMO is expressed at high copper-biomass ratios. In both methanotrophs, transcription of the sMMO gene cluster is negatively regulated by copper ions. Data suggest that transcription of the M. trichosporium OB3b sMMO gene cluster is directed from a sigma54-like and a sigma70-like promoter. The pMMO (pmo) genes of M. capsulatus (Bath) are transcribed into a polycistronic mRNA of 3.3 kb. The synthesis of this mRNA was activated by copper ions. Activation of pmo transcription by copper ions was concomitant with repression of sMMO gene transcription in both methanotrophs. This suggests that a common regulatory pathway may be involved in the transcriptional switch between sMMO and pMMO gene expression.
Soluble methane mono-oxygenase (sMMO) of Methylococcus capsulatus (Bath) catalyses the O2-dependent and NAD(P)H-dependent oxygenation of methane and numerous other substrates. During purification, the sMMO enzyme complex, which comprises three components and has a molecular mass in excess of 300 kDa, becomes inactivated because of cleavage of just 12 amino acids from the N-terminus of protein B, which is the smallest component of sMMO and the only one without prosthetic groups. Here we have shown that cleavage of protein B, to form the inactive truncated protein B', continued to occur when intact protein B was repeatedly separated from protein B' and all detectable contaminants, giving compelling evidence that the protein was cleaved autocatalytically. The rate of autocatalytic cleavage decreased when the residues flanking the cleavage site were mutated, but the position of cleavage was unaltered. Analysis of a series of incremental truncates showed that residue(s) essential for the activity of sMMO, and important in determining the stability of protein B, lay in the region Ser4-Tyr7. Protein B was shown to possess intrinsic nucleophilic activity, which we propose initiates the cleavage reaction via a novel mechanism. Proteins B and B' were detected in approximately equal amounts in the cell, showing that truncation of protein B is biologically relevant. Increasing the growth-medium copper concentration, which inactivates sMMO, did not alter the extent of in vivo cleavage, therefore the conditions under which cleavage of protein B may fulfil its proposed role as a regulator of sMMO remain to be identified.
The interactions of Ga(2P:4s(2)4p1, 2S:4s(2)5s1, and 2P:4s(2)5p1) with CH4 is studied by means of Hartree-Fock self-consistent-field (SCF) calculations using relativistic effective core potentials and multiconfigurational-SCF plus multireference variational and perturbational on second-order Möller-Plesset configuration interaction calculations. The Ga atom 2P(4s(2)5p1) state can spontaneously insert into the CH4. In this interaction the 4 2A potential energy surface is initially attractive and becomes repulsive only after meeting with the 3 2A surface, adiabatically linked with the Ga(2S:4s(2)5s1) + CH4 fragments. The Ga atom 2S(4s(2)5s1) excited state inserts in the C-H bond. In this interaction the 3 2A potential energy surface initially attractive, becomes repulsive after meet the 2 2A' surface linked with the Ga(2P:4s(2)4p1) + CH4 fragments. The two 2A curves (2 2A and X 2A) derived from the interaction of Ga(2P:4s(2)4p1) atoms with methane molecules are initially repulsive. The 2 2A curve after an avoided crossing with the 3 2A curve goes smoothly down and reaches a minimum: after this point, it shows an energy barrier. The top of this barrier is located below the energy value of the Ga(2S:4s(2)5s1) + CH4 fragments. After this energy top the 2 2A curve goes down to meet the X 2A curve. The 2 2A curve becomes repulsive after the avoided crossing with the X 2A curve. The X 2A curve becomes attractive only after its avoided crossing with the 2 2A curve. The lowest-lying X 2A potential leads to the HGaCH3 X 2A intermediate molecule. This intermediate molecule, diabatically correlated with the Ga(2S:4s(2)5s1) + CH4 fragments, which lie 6 kcal/mol, above the ground-state reactants, the dissociation channels of this intermediate molecule leading to the GaH + CH3 and H + GaCH3 products. These products are reached from the HGaCH3 intermediate without activation barriers. The work results suggest that Ga atom in the first excited state in gas-phase methane molecules could produce better quality a-C:H thin films through CH3 radicals, as well as gallium carbide materials.
The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl(2), CH(4), and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (nu(3)=3019 cm(-1)) of CH(4) is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (nu(1)=2917 cm(-1)) of CH(4) is prepared by stimulated Raman pumping. Photolysis of Cl(2) at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290+/-175 cm(-1) (0.16+/-0.02 eV). Finally, the nascent HCl or CH(3) products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH(3) products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. [J. Chem. Phys. 119, 9568 (2003)] report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH(3) products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.
Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.
We present a study of the structure of a fully deuterated methane hydrate under the geological conditions found in the world's oceans. In situ high-resolution neutron diffraction experiments have been performed at temperatures of 220, 275, and 280 K and a pressure of 100 bar, corresponding to the conditions at 1000 m water depth. The data were analyzed with a combination of Rietveld refinement and maximum entropy methods. From the Rietveld refinement, precise atomic parameters of the host lattice could be determined, indicating increasing distortions of the structure of the cages at elevated temperatures and pressures. Debye-Waller factors of the encaged CD(4) molecules have been found to exceed the values of the Debye-Waller factors of the D(2)O molecules considerably. In the large cage of structure type I the thermal center-of-mass displacements of the guests are 5-10 times larger than those of the water molecules. From the maximum entropy analysis maps of the scattering length density have been obtained, showing details of the vibrational amplitudes of the atoms in methane hydrate. The Debye-Waller factors of all molecules have been found to deviate considerably from a simple spherical geometry.