CH Fragments Research Articles

State-resolved dynamics of 248 nm methyl-iodide fragmentation on GaAs(110)

The 248 nm initiated dissociation dynamics of methyl iodide physisorbed on GaAs (110) is investigated using 2+1 REMPI detection of the CH3 umbrella mode. The velocity and vibrational distributions of the photofragment are dependent on the adsorbate coverage and surface termination. Two translational energy distributions are measured for the CH3 fragment at high coverage, i.e., 25 ML, with maxima at 1 and 1.9 eV. These maxima are within 0.5–0.7 eV of those obtained in direct dissociation of gas-phase CH3I. At 1 ML one distribution is measured with its maxima at 0.3 eV. A modification of the surface with chemisorbed iodine reduces the yield of the CH3 at 1 ML but not at 25 ML. The dissociation dynamics observed at 1 ML is attributed to bond scission following electron attachment while that at 25 ML to direct photodissociation. At 3 ML coverage both the 1 and 25 ML components are observed. The measured fragment vibrational distribution at 1 ML has a 4 times lower v″=1/v″=0 ratio compared with that at 25 ML. Differences in the translational and vibrational distributions for the two dissociation mechanisms are attributed to the available energy and molecular geometry of photoexcited CH3I.

Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)

The broadband UV (230−420 nm) photoinduced chemistry of CH3Br adsorbed on Cu(2ML)/Ru(001) in the coverage range of 1−50 ML was studied by monitoring the desorption products (Δp-TPD mode) in combination with post irradiation work function change measurements before and during surface heating (Δφ-ΤPD mode). Δφ measurements enabled us to follow multilayer restructure and desorption of parent molecules and photochemical reaction products in the temperature range of 80−700 K. Methyl radicals accumulated on the surface are the precursor for the thermal formation of methane and ethylene at 450 K. Dehydrogenation of the methyl group is the rate-limiting step of the surface reaction resulting in the formation of these molecules. Based on work function change measurements, an estimate of the adsorbed methyl dipole moment is μ0 = 0.48 D. Dissociative electron attachment (DEA) driven CH3Br dissociation produced CH3 and CH2 fragments within the parent molecules multilayer matrix. At the multilayer coverage range, Δφ increases by up to 1.1 eV after 10 min UV irradiation. Model calculations qualitatively describe the post irradiation work function changes induced by the embedded photofragments (mostly Br- ions) inside the CH3Br dielectric film. Comparison of the Δφ-TPD spectra on Cu(2ML)/Ru(001) to those on clean Ru(001) indicates that the nature of the molecule−surface interaction and the structure of the first few layers strongly influence the resulting photochemistry at layer thickness up to at least 20 ML.

Production of electronically excited CH via the vacuum ultraviolet photodissociation of ethylene and the possible role of the ethylidene isomer

The visible fluorescence of CH fragments (A 2Δ→X 2Π and B 2Σ−→X 2Π transitions) formed in the vacuum ultraviolet photodissociation of ethylene in the 11.7–21.4 eV energy region, was recorded. Two formation thresholds for each excited fragment, CH* (A) or CH* (B), were identified and associated with two dissociation channels namely CH*+CH3 and CH*+CH+H2. Unlike previous studies of the dissociation dynamics on the ground-state potential energy surface, neither of these channels exhibit an energy barrier within the experimental uncertainty, even in the latter case of molecular H2 elimination. It is proposed that both channels pass via an ethylidene intermediate (H3CCH:), an isomer never previously experimentally detected and whose existence has been debated in theoretical publications. The observed behavior, at the excitation energies used in the present work, also suggests that fast isomerization and internal conversion to excited states of ethylene precede fragmentation. Above 18.5 eV, that is around the ionization limit C2H4+ (D 2B1u), dissociative ionization starts to compete with neutral dissociation into excited CH fragments giving rise to a substantial decrease in the neutral fragment signal.

Adsorption of methylchloride on Si(100) from first principles

The chemisorption of methylchloride (CH3Cl) on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is one in which the methylchloride molecule is dissociated into CH3 and Cl fragments which are bound to the two Si atoms of the same surface dimer. Our calculations show that dissociative chemisorption of methylchloride on Si(100) may proceed along different reaction paths characterized by different energy barriers that the system must overcome: some dissociation processes are mediated by a molecular precursor state and, at least in one case, we find that the dissociation process is nonactivated, in agreement with recent experimental findings. We have also generated, for many possible adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements.

Adhesive Strength Of DLC Films Prepared By Ionization Deposition

Diamond-like carbon (DLC) films are considered to be promising to develop extensively for practical applications owing to their good properties. Improvement of adhesive strength of DLC films to substrates is required to enhance their suitability and reliability. The purpose of this study is to investigate the influence of the preparation conditions of DLC films on the adhesion. DLC films were prepared from benzene gas by the ionization deposition method. Three different anode voltages of 50, 75 and 100 V were applied during deposition. The substrates used were WC-CO, austenitic 18Cr8Ni stainless steel and AI-Mg alloy. The adhesive strength was measured by a tensile pull-off tester. We examined and discussed the adhesion of DLC films from the viewpoints of the internal stress in the films and the species in plasma during deposition. The main results obtained from this study are as follows: (1) The adhesive strength of DLC films to substrates can be enhanced by controlling the anode voltage. (2) For thicker films, the adhesive strength of DLC films increases with increasing internal stress. (3) At the anode voltage of 50 V, higher adhesive strength of DLC films can be attained than that prepared at other anode voltages, but the internal stress is much higher. It is presumed that DLC films prepared at an anode voltage of 50 V have large amounts of CZ, CH and C5HS fragments against hydrogen in deposited films.

Catalytic decomposition of acetylene on Fe(001): A first-principles study

The catalytic decomposition process in the initial stage of carbon nanotube (CNT) growth is investigated for the system of acetylene $({\mathrm{C}}_{2}{\mathrm{H}}_{2})$ on the Fe(001) surface through first-principles electronic structure calculation. In its most stable configuration, the C-C bond of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ is lying on the hollow site along the [110] direction of the Fe(001) surface. The formation of the CH fragments accompanied by C-C bond breaking is found to be an intermediate step toward the full dissociation of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ molecules on Fe(001) and the whole process is exothermic.

Triple coincidence experiments to study the fragmentation of core ionized molecules by electron impact

An apparatus to study the fragmentation of molecular dications following inner shell ionization by electron impact has been built at CNR-IMAI. The fragmentation of the dications is studied via Auger electron–ion–ion coincidence experiments. This coincidence technique has been used to identify N 1s −1v −1→v −3 transitions in the N 2 Auger spectrum and to study the fragmentation of CH 4 2+ after C 1s ionization.

Approximate transferability in alkanols

The approximate transferability of OH, CH2, and CH3 groups in unbranched primary alkanols has been studied by comparing several atomic and bond properties of the 12 smallest members of this series. These properties were obtained by employing the Atoms in Molecules theory on HF/6-31++G∗∗//HF/6-31G∗ and QCISD/6-31++G∗∗//QCISD/6-31G∗ wave functions. The properties computed at both levels follow parallel evolutions along the series, which allow to conclude that the OH group can be considered approximately transferable along the set of 1-alkanols larger than ethanol, whereas ethanol and methanol present specific hydroxyls. The electron population of the oxygen atom is lower than in ethers, aldehydes, and ketones. The hydroxyl affects significantly the CH3 and CH2 groups that are in positions α, β, γ, or δ, whereas those groups separated from the oxygen by more than 4 bonds can be considered similar to those included in a n-alkane. CH2 groups in the series can be classified into 6 quasi-transferable fragments taking into account their position with regard to the OH (α, β, γ, δ, or beyond (ν)), and respect to the terminal CH3 (attached to it or not). The simultaneous occurrence of both facts gives rise to four specific CH2 fragments: α-CH2 in ethanol, β-CH2 in 1-propanol, γ-CH2 in 1-butanol, and δ-CH2 in 1-pentanol. It has been found that all the CH2 and CH3 fragments that are γ or δ to the OH group do not differ significantly from the corresponding fragments of a dialkyl ether. The energy of oxygen, CH2 and CH3 fragments depend on the molecular size. The effect of the basis set size error on this quantity has been investigated, concluding that the molecular-size dependence is not an artifact due to it. The destabilization experienced by the oxygen atom for a common increase in the molecular size in alcohols is equivalent to that of ethers and smaller than the one displayed by aldehydes and ketones. It was also concluded that the effect due to the variation of the molecular size is independent on the number of alkyl chains that are increased.

Characterization of diesel particulate matter with excimer laser fragmentation fluorescence spectroscopy

Excimer laser fragmentation fluorescence spectroscopy (ELFFS) is an effective technique for monitoring diesel particulate matter in engine exhaust. Measurements were made in the exhaust of a single-cylinder, 4 kW diesel generator with several different air to exhaust dilution ratios. Light from a 193 nm ArF laser photofragments the particles and produces fluorescence from the atomic carbon fragments at 248 nm and from CH (431 nm) and C2 (468 nm) fragments. Interferences from carbon monoxide and carbon dioxide are negligible. The interference from unburned gas-phase hydrocarbons is significant when the engine is not under load, but insignificant when the engine is loaded. The gas-phase hydrocarbons can be removed by a diffusion denuder to eliminate this interference under no-load conditions. The 100-shot (1 s) detection limit for particles in the exhaust is approximately 0.2 ppb volume fraction. The single-shot detection limit of ELFFS is low enough to allow for real-time monitoring of particulate matter from diesel engines, as well as from other combustion systems. ELFFS is sensitive to both the volatile and non-volatile fraction of the particulate matter, and monitoring fluorescence from multiple fragments provides information on the chemical composition of the particles.

A novel method for film thickness measurement of perfluoropolyether lubricant by secondary ion mass spectroscopy

A novel method for measuring the thickness of lubricant coated on magnetic recording media using time-of-flight secondary ion mass spectroscopy (TOF-SIMS) is proposed. This method employs a pulsed primary ion beam to uniformly etch the lubricant layer down to the underlying carbon overcoat layer. The lubricant thickness is determined by detecting the etching time taken for the lubricant backbone fragments CF2O and C2F4O to converge or alternatively by monitoring the maximum position of the depth profile of CH fragment. The film thickness determined by this new method showed good linearity across a wide range and correlated well with current available techniques such as Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.

Real-Time Measurement of Combustion Generated Particles with Photofragmentation-Fluorescence

Excimer laser fragmentation-fluorescence spectroscopy (ELFFS) is a viable technique for real-time monitoring of carbonaceous particles in combustion exhausts. The exhaust from a single-cylinder two-stroke engine is diluted and diverted into the laser interrogation region, resulting in a particle concentration of approximately 1 × 107/cm3. Light from a 193 nm ArF laser photofragments the particles and then produces fluorescence from the atomic carbon fragments at 248 nm, CH fragments at 431 nm, and C2 fragments at 468 nm. The atomic carbon fluorescence signal is proportional to the number concentration of particles in the laser interrogation region. The 100-shot (1 s) detection limit for particles in the exhaust is 1 mg/m3, expressed as a mass concentration of particulate matter. Interferences from carbon monoxide and carbon dioxide are negligible. The relative fluorescence yield at 248 nm is four times greater from particles than from the gas phase hydrocarbons present in the exhaust. This high yield suggests that the gas phase hydrocarbon interference would not be problematic for measurements of diesel exhaust, where the ratio of particulate carbon to gas phase hydrocarbon is high.

Time-of-flight study of the ionic and neutral particles produced by pulsed-laser ablation of frozen glycerol

The emitted particles from pulsed-laser ablation (PLA), λ=193 nm and fluence=88–400 mJ/cm2, of frozen glycerol was examined using time-of-flight mass spectrometry. The data are analyzed using supersonic molecular-beam theory and the result is interpreted using a thermal/fluid-dynamic model. Both intact and fragmented glycerol are emitted in the PLA process at all fluences and their concentration ratio is fluence dependent. Fragmentation occurs predominantly at one of the C–C bonds forming CH2–OH (31 amu) and HO–CH2–CH–OH (61 amu). CH3 is produced at the target which requires the protonation of a CH2 fragment. At fluences higher than 250 mJ/cm2, ions are detected. These ions have very high velocity, >2000 m/s, and their intensity increases with fluences. PLA is thus not suitable for glycerol transfer under these conditions due to fragmentation. The data show that particle emission proceeds as a simple thermal vaporization process at fluences <200 mJ/cm2. Higher fluences will yield a Knudsen layer (KL), which is formed in front of the target surface. For fluences >300 mJ/cm2, particles from the KL go through unsteady adiabatic expansion prior to free flight. Models of particle and ion formation and interaction are proposed and discussed. © 2001 American Institute of Physics.

An ab initio molecular dynamics study of S ketene fragmentation

The dynamical origins of product state distributions in the unimolecular dissociation of S0 ketene, CH2CO (X̃ 1A1)→CH2(ã 1A1)+CO, are studied with ab initio molecular dynamics. We focus on rotational distributions associated with ground vibrational state fragments. Trajectories are integrated between an inner, variational transition state (TS) and separated fragments in both the dissociative and associative directions. The average rotational energy in both CO and CH2 fragments decreases during the motion from the TS to separated fragments. However, the CO distribution remains slightly hotter than phase space theory (PST) predictions, whereas that for CH2 ends up significantly colder than PST, in good agreement with experiment. Our calculations do not, however, reproduce the experimentally observed correlations between CH2 and CO rotational states, in which the simultaneous formation of low rotational levels of each fragment is suppressed relative to PST. A limited search for nonstatistical behavior in the strong interaction region also fails to explain this discrepancy.

Transferability in alkyl monoethers. II. Methyl and methylene fragments

The transferability of the atomic and bond properties of the methyl and methylene fragments in linear unbranched alkyl monoethers was studied using the Theory of Atoms in Molecules (AIM). This theory has been applied to the analysis of the HF/6-31++G**//HF/6-31G* electron charge distributions of a series of 33 dialkyl ethers, CH3(CH2)mO(CH2)nCH3, [n=0,1(n⩽m⩽9), n=2,3(n⩽m⩽8), n=4(n⩽m⩽5)]. The results obtained indicate that the methyl and methylene fragments situated in α, β, γ, or δ positions with respect to the oxygen atom are different to those of an n-alkane. Nevertheless, CH3 and CH2 at more distant positions can be considered as standard units, whose nonenergetic properties coincide with those of the corresponding fragment in an n-alkane. On the contrary, the energetic properties of the fragments maintain a differential value with respect to the n-alkane in all of the positions studied in the series. The properties of the methyl or methylene fragments in α to the oxygen depend on the size (methyl or larger) of the other alkyl chain bonded to the oxygen. The properties of methylenes are also different when they are α to the terminal CH3. Thus, the CH2 and CH3 fragments of dialkyl ethers can be classified into 9 CH3 and 12 CH2 groups. All of the groups proposed verify the transferability of bond properties, charge, and volume throughout all the fragments that it includes. Though the energy of the fragments depend on the size of the molecule, fragments included in the same group display a common dependence. This dependence does not impede the appearance of excellent linear relationships between the total molecular energy and the number of CH2 groups. Nevertheless, the AIM computed energies for the oxygen atom are always more negative than those obtained from the fittings of total electron energies to the number of CH2 groups in the molecule. This stabilization is produced at the cost of destabilizing the CH2 or CH3 groups in the α position. Whereas, if the CH2 groups bonded to a methyl group are excluded, the remaining CH2 and CH3 groups are slightly stabilized (in a magnitude that depends on the size of the molecule and which oscillates slightly in its position with respect to the oxygen atom).

Atomic charges in conformers of gaseous glycine

The potential energy surface of gaseous glycine determined years ago in ab initio MP2/6-311++G ∗∗ calculations (A.Császár, J. Am. Chem. Soc., 114 (1992) 9568) is composed of 13 stable conformers with relative energies within 11 kcal/mol. Glycine being the simplest amino acid and having a large number of conformers with close energies, it offers a stringent benchmark to explore the conformational dependences of properties of interest for further studies on peptides or proteins. The conformational behavior of atomic charges computed with the MP2/6-311++G ∗∗ electron density using population analyses and partition approaches of different nature are analyzed. The behavior of every atom as well as that of molecular fragments COOH, CH 2 and NH 2 show that atoms in molecules (AIM) and natural population analysis (NPA) charges exhibit not only a good conformational independence but also provide a meaningful picture of the electron charge distribution in this system.

Hot-atom chemiluminescence: a beam study of the [formula omitted] system

Beams of fast N( 4 S) atoms (10–1000 eV lab) were generated by charge transfer of N + ions with Kr atoms. NH(A–X) chemiluminescence (CL) resulting from N+CH 4 collisions was spectrally resolved. The CL cross-section was determined relative to the N ++CH 4 CL reaction. It is only half as large, but has a very similar energy dependence. NH(A) formation up to unusually high collision energies (>200 eV CM) is explained by a `billiard ball' model. Spectra of CH ( A 2Δ , B 2Σ − , C 2Σ + ) fragments were also observed at high energies. Computer simulations of the emission spectra from both the neutral and the ionic reaction yielded rovibrational populations. The differences are discussed.

A new focal adhesion protein that interacts with integrin-linked kinase and regulates cell adhesion and spreading.

Integrin-linked kinase (ILK) is a multidomain focal adhesion (FA) protein that functions as an important regulator of integrin-mediated processes. We report here the identification and characterization of a new calponin homology (CH) domain-containing ILK-binding protein (CH-ILKBP). CH-ILKBP is widely expressed and highly conserved among different organisms from nematodes to human. CH-ILKBP interacts with ILK in vitro and in vivo, and the ILK COOH-terminal domain and the CH-ILKBP CH2 domain mediate the interaction. CH-ILKBP, ILK, and PINCH, a FA protein that binds the NH(2)-terminal domain of ILK, form a complex in cells. Using multiple approaches (epitope-tagged CH-ILKBP, monoclonal anti-CH-ILKBP antibodies, and green fluorescent protein-CH-ILKBP), we demonstrate that CH-ILKBP localizes to FAs and associates with the cytoskeleton. Deletion of the ILK-binding CH2 domain abolished the ability of CH-ILKBP to localize to FAs. Furthermore, the CH2 domain alone is sufficient for FA targeting, and a point mutation that inhibits the ILK-binding impaired the FA localization of CH-ILKBP. Thus, the CH2 domain, through its interaction with ILK, mediates the FA localization of CH-ILKBP. Finally, we show that overexpression of the ILK-binding CH2 fragment or the ILK-binding defective point mutant inhibited cell adhesion and spreading. These findings reveal a novel CH-ILKBP-ILK-PINCH complex and provide important evidence for a crucial role of this complex in the regulation of cell adhesion and cytoskeleton organization.

The photodissociation of CH3SCH3 and CD3SCD3 at 220–231 nm investigated by velocity map ion imaging

The photodissociation of the two isotopomers of dimethyl sulfide, CH3SCH3 and CD3SCD3, through the first electronic absorption band at wavelengths 220–231 nm has been studied employing velocity map ion imaging to detect the methyl products. Translational energy and recoil angle distributions have been determined for the CH3 and CD3 fragments either in the vibrational ground state or without product state selection. The measurements indicate that the S–C bond photolysis yields strongly anisotropic product scattering distributions characterized by β parameters within the range (−0.4, −1.0). The β values closest to the limiting value of −1.0 (corresponding to a dipole perpendicular transition) are observed for the methyl fragments produced in the vibrational ground state. The product recoil energy distributions are centered at ≈1.5–1.7 eV (i.e., 65%–75% of the available energy) and are quite narrow (full width at half maximum, FWHM≈0.3–0.5 eV), which indicates that methyl and methylthio fragments are born internally cold but with substantial translational excitation. The recoil energy distribution is practically independent of the photolysis wavelength within the interval studied, indicating that changes in the photon energy correlate primarily with the rovibrational excitation of the CH3S (or CD3S) fragment whose internal dynamics appear to be more active in the dissociation process than that of the CH3 (or CD3) partner.

Modulation of the electronic communication between two equivalent ferrocene centers by proton transfer, solvent effects and structural modifications

A series of six new dinuclear ferrocene compounds with the bridging fragment CH 2–N(R)–CH 2 were prepared and characterized. Bis(ferrocenylmethyl)- t-butylamine ( 1), bis(ferrocenylmethyl)-isopropylamine ( 2), bis(ferrocenylmethyl)-hexylamine ( 3), bis(ferrocenylmethyl)- p-methoxyaniline ( 4), bis(ferrocenylmethyl)aniline ( 5) and bis(ferrocenylmethyl)- p-nitroaniline ( 6), as well as the quaternized amine bis(ferrocenylmethyl)hexylmethylammonium hexaflurophosphate ( 3 +·PF 6 −), were investigated using electrochemical techniques, 1H NMR spectroscopy and single crystal X-ray diffraction analysis. The voltammetric data indicate that the extent of electronic communication between the equivalent ferrocene centers increases in low polarity solvents and when bulky aliphatic groups are covalently attached to the central tertiary nitrogen. The observed experimental data are consistent with through-space communication effects, essentially electrostatic in nature. The extent of electronic communication can be modulated by reversible and irreversible reactions in the solution phase. Protonation or methylation of the tertiary nitrogen in the middle of the bridge disrupts the electronic communication between the ferrocene residues.

Vibrational Spectroscopy and Normal-Mode Analysis of Tungsten−Methylidyne Complexes. Insight into the Nature of the M⋮CH Bonds

Raman and infrared spectra are reported for the tungsten−methylidyne complex trans-W(⋮CH)(PMe3)4Cl (1) and its deuterated isotopomers W(⋮CD)(PMe3)4Cl (1-d1) and W(⋮CH)(PMe3-d9)4Cl (1-d36). The bands attributable to the vibrational modes of the W⋮CH fragment are clearly identified on the basis of their frequency shifts upon isotopic substitution: ν1[ν(CH)] ≅ 2980 cm-1 (1, 1-d36), 2240 cm-1 (1-d1); ν2[ν(W⋮C)] ≅ 911 cm-1 (1, 1-d36), 871 cm-1 (1-d1); ν3[λ(W⋮CH)] = 788/755 cm-1 (1), 642/611 cm-1 (1-d1). These data and normal-coordinate calculations on both the W⋮CH and W(⋮CH)P4Cl fragments of these compounds reveal that the W⋮C stretch is negligibly coupled either to the CH stretch or to motions involving the ancillary ligands; thus, for 1, the W⋮C force constant given by the pseudodiatomic-oscillator approximation (5.945 mdyne Å-1) is within 2% of the best value from the normal-coordinate calculations (6.021 mdyne Å-1). The W⋮C stretching frequency is much smaller than those previously assigned for the M⋮CR stretches of metal−alkylidyne complexes with larger R groups (R = Me, Ph; ν ≅ 1270−1600 cm-1); normal-coordinate calculations on the general W⋮CB oscillator, where B is a dummy atom of variable mass, reveal that this disparity is the result of large mixing between ν(W⋮C) and ν(CB) for cases where B is heavier than H and, in some cases, of mixing among ν(W⋮C) and internal vibrational modes of B. The W⋮C force constant is consistent with the W⋮C bond length, based on an empirical bond-distance/force-constant correlation for selected third-transition-series metal complexes. The relatively low CH stretching frequency and force constant (5.207 mdyne Å-1) are consistent with a CH bond length that is longer than typical for an sp-hybridized carbon atom, suggesting that this bond is correspondingly rich in carbon p-orbital character.