Reaction of tributyl phosphite with oxidized iron: surface and tribological chemistry

The surface chemistry of a model lubricant additive, tributyl phosphite (TBPi), is investigated on Fe3O4 in ultrahigh vacuum. A portion of the TBPi desorbs molecularly following adsorption at approximately 200 K, the remainder decomposing either by C-O bond scission to form 1-butyl species or by P-O bond cleavage to form butoxy species. Adsorbed butyl species either undergo beta-hydride elimination to desorb 1-butene or decompose to deposit carbon and hydrogen on the surface. The resulting adsorbed hydrogen reacts with the oxide to desorb water or with the butoxy species to form 1-butanol. Butoxy species are stable up to approximately 600 K at which temperature they also undergo beta-hydride elimination to form butanal and the released hydrogen reacts with other butoxy species to form 1-butanol. Only a small amount of carbon is deposited onto the surface following adsorption at approximately 200 K, which then desorbs as CO above approximately 750 K. Adsorbing TBPi at 300 K results in the deposition of more carbon and an Auger depth profile reveals that the carbon is located predominantly on the surface, while the phosphorus is rather uniformly distributed throughout the oxide film. This result is in accord with previous near-edge X-ray absorption fine structure measurements, which show the formation of phosphates and polyphosphate glasses. The resulting tribological film appears to be composed of a relatively hard polyphosphate glass formed by rapid diffusion of POx species into the oxide, covered by a low shear strength graphitic layer.     

13C NMR spectra of allylic boron “ate” complexes. Relative importance of σ-π conjugation between the double bond and the carbon-boron bond

Carbon-13 NMR spectra of allylic boron “ate” complexes clearly indicate, for the first time, the relative importance of σ-π conjugation between the double bond and the carbon-boron bond.     

碘乙醇在Ni(100)表面的吸附和热分解-碳氢化合物氧化的中间产物：羟乙基和氧金属环

利用热脱附(TPD)实验和X射线光电子能谱(XPS)研究了碘乙醇在Ni(100)表面的吸附和热反应过程. 实验结果表明碘乙醇在100 K时以两种分子的形式吸附在Ni(100)的表面, 即: 以碘原子端吸附在表面或以碘原子端和羟基端同时吸附在表面. 由于两种吸附形式的分子的一致分解和吸附分子的不均匀性, 在140 K引起了较复杂的化学反应, 伴有少量的乙烯和水产生. 碘乙醇在150 K经过C—I键断裂, 有80%碘乙醇生成—O(H)CH2CH2—中间产物, 20%的碘乙醇生成羟乙基中间产物. 羟乙基在160 K的转化过程中包括两个互相竞争的化学反应: 与表面的氢原子进行还原反应生成乙醇, 或失去一个β-H原子生成表面乙烯醇. 另外, 在相同的温度下—O(H)CH2CH2—中间产物经过脱氢反应产生—OCH2CH2—氧金属环. 羟乙基和氧金属环都会发生异构, 分别在210 K和250 K生成乙醛, 这些乙醛一部分从表面脱出, 其余的部分发生分解反应产生氢气、水和一氧化碳. 在实验基础上, 进一步探讨了这种化学过程在催化中的作用和指导意义.     

C-O bond scission of methoxide on Pd nanoparticles: a density functional study

C-O bond scission of methoxide species adsorbed at the surface of Pd nanoparticle was studied by DF calculations for the example of cuboctahedral Pd(79). To investigate different locations of adsorbed intermediates as well as the transition state of C-O bond scission, a substrate model was used, which allows one to consider adsorbates without any local geometry restrictions. In contrast to reaction sites on the flat Pd(111) surface and on extended facets, scission of the C-O bond of methoxide at cluster edges is exothermic by approximately 40 kJ mol(-1) and the decomposition product CH(3) is found to be stabilized there. However, the high calculated activation barrier, approximately 140 mol(-1), implies only a very slow reaction compared to dehydrogenation of CH(3)O.     

Ethanol decomposition: C--C cleavage selectivity on Rh(111).

Ethanol adsorption, desorption and decomposition on Rh(111) have been studied by X-ray photoelectron spectroscopy and tem-perature-programmed desorption experiments. The evolution of the C is and O is core level spectra was monitored as a function of ethanol exposure and surface temperature. Ethanol adsorption at 90 K results in two nonequivalent ethanol-adsorbed species at low surface coverage, while a third species--related to multilayer formation--appears after longer exposures. Upon surface annealing, ethanol undergoes both desorption and dissociation, thus creating intermediate surface species which further decompose to hydrogen, carbon monoxide and atomic carbon. Our results clearly show that C--C bond cleavage is the preferential dissociation channel, while C--O bond scission is not observed. Calculations performed within the framework of the unity bond index-quadratic exponent potential model, have been used to test and compare different competing dissociation channels, providing an estimate of adsorption energies and dissociation barriers.     

An Empirical Proton NMR Shielding Equation for Alkenes Based on Ab Initio Calculations

Nuclei of hydrogen atoms located over a carbon-carbon double bond in the presence of a strong magnetic field experience a perturbed magnetic field caused primarily by the magnetic anisotropy of the bond. However, the commonly used theoretical model for predicting the shielding effect of an alkene double bond on hydrogen nuclei is sometimes inconsistent with the observed proton NMR chemical shifts in structures that have covalently bonded hydrogens located over a carbon-carbon double bond. We have used the ab initio gauge including atomic orbital (GIAO) method to calculate isotropic shielding values and to determine the proton NMR shielding increments for a simple model system: methane held at various positions over ethene. These shielding increments calculated for one proton of methane have been mapped as a function of their position in Cartesian coordinates relative to the center of ethene. A mathematical function has been fit to this three-dimensional shielding increment surface at each of four distances from the face of the ethene molecule. Additionally, a single mathematical equation has been developed for predicting the shielding caused by the carbon-carbon double bond in ethene. In contrast to the traditionally employed shielding model, our results predict deshielding for protons within 3 Å above the center of a carbon-carbon double bond, consistent with experimental observations in several molecular systems. The NMR shielding increments predicted by this equation are compared to observed shielding increments in some test alkenes.     

The beneficial effect of hydrogen on CO oxidation over Au catalysts. A computational study

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O(2) hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O-O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O(2) and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H(2)O and CO(2). CO is selectively oxidized in a H(2) atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.     

Absolute surface coverage measurement using a vibrational overtone

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm(-1) line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm(-1), while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO(2) surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection.     

Hydrogenolysis of benzylic alcohols on rhodium catalysts

Alumina-supported catalysts from various sources and with different rhodium dispersions predominantly yield the hydrogenation product perhydro-1-indanol in the liquid-phase hydrogenation of 1-indanol, while carbonsupported catalysts mainly give the C-O bond scission - hydrogenation product perhydroindane. Addition of organic or inorganic bases to the reaction mixture suppresses C-O bond scission. To distinguish between the two possible pathways for C-O bond scission of direct hydrogenolysis or dehydration followed by hydrogenation, deuteration studies have been carried out with carbon-supported catalysts. Not only 1-indanol but also indane and indene (the two possible mechanistic intermediates in the C-O bond scission routes) were deuterated. Information about the actual pathway has been obtained by determining the degree of deuteration and the positions at which deuterium is incorporated in the resulting perhydroindane product by means of mass spectrometry and 13C NMR spectroscopy. The results prove that C-O bond scission takes place primarily through the direct hydrogenolysis pathway on the carbon-supported catalysts. Direct hydrogenolysis occurs on the carbon support because of the formation of a better leaving group (OH2+) from the benzylic hydroxy group and its subsequent substitution by spillover hydrogen.     

A carbon-13 NMR study of some sulfur-containing organic compounds

Carbon-13 NMR spectra have been obtained and analyzed for a series of saturated linear and cyclic sulfur-containing organic molecules. For the large cyclic polythiaethers whose proton spectra are broad and unresolved, it is found that carbon-13 spectra are sharp and well resolved. The resonances of all carbons fall within the expected chemical shift ranges. Based upon the comparative resolution in the spectra of large and small cyclic species, it is concluded that whatever conformational motion may exist within the rings must be occurring very rapidly on the NMR time scale.     

A spectroscopic study of calcium surface sites and adsorbed iron species at aqueous fluorapatite by means of 1H and 31P MAS NMR

Assignments of the protolytic speciation at the calcium hydroxyl surface sites of synthetic fluorapatite and the chemical interactions between fluorapatite-maghemite and fluorapatite-Fe2+ ions have been studied by means of 1H and 31P single-pulse and 31P CP MAS NMR. Three possible forms of calcium hydroxyl surface sites have been suggested and assigned to [triple bond] CaOH, [triple bond]Ca(OH)2-, and [triple bond]CaOH2+, and their mutual ratios were found to vary as a function of pH. Due to their paramagnetic properties, iron species and Fe2+ ions adsorbed at the fluorapatite surface display a broad spinning sideband manifold in the single-pulse 31P MAS NMR spectra. The resonance lines in the 31P CP MAS NMR spectra originating from the bulk phosphate groups PO4(3-) and phosphorus surface sites [triple bond]POx and [triple bond]POxH decrease with increasing Fe2+ ion adsorption. When iron species originating from maghemite are adsorbed at the fluorapatite surface, no 31P NMR signal is detected, which supports the hypothesis that surface reactions occur between the phosphorus surface sites of fluorapatite and iron species.     

Surface reaction of alkynes and alkenes with H-Si(111): a density functional theory study

Recent experiments on the addition of alkene and alkyne molecules to H-terminated silicon surfaces have provided evidence for a surface chain reaction initiated at isolated Si dangling bonds and involving an intermediate carbon radical state, which, after abstraction of a hydrogen atom from a neighboring Si-H unit, transforms into a stable adsorbed species plus a new Si dangling bond. Using periodic density functional theory (DFT) calculations, together with an efficient method for determining reaction pathways, we have studied the initial steps of this chain reaction for a few different terminal alkynes and alkenes interacting with an isolated Si dangling bond on an otherwise H-saturated Si(111) surface. Calculated minimum energy pathways (MEPs) indicate that the chain mechanism is viable in the case of C(2)H(2), whereas for C(2)H(4) the stabilization of the intermediate state is so small and the barrier for H-abstraction so (relatively) large that the molecule is more likely to desorb than to form a stable adsorbed species. For phenylacetylene and styrene, stabilization of the intermediate state and decrease of the H-abstraction barrier take place. While a stable adsorbed species exists in both cases, the overall heat of adsorption is larger for the alkyne molecules.     

Interaction of glycine with isolated hydroxyl groups at the silica surface: first principles B3LYP periodic simulation

The adsorption of a glycine molecule on a model silica surface terminated by an isolated hydroxyl group has been studied ab initio using a double-zeta polarized Gaussian basis set, the hybrid B3LYP functional, and a full periodic treatment of the silica surface/glycine system. The hydroxylated silica surface has been simulated using either a 2D slab or a single polymer strand cut out from the (001) surface of an all-silica edingtonite. A number of B3LYP-optimized structures have been found by docking glycine on the silica surface exploiting all possible hydrogen bond patterns. Whereas glycine is generally adsorbed in its neutral form, two structures show glycine adsorbed as a zwitterion, the surface playing the role of a "solid solvent" whereas intrastrand hydrogen bond cooperativity stabilizes the zwitterions. The adsorbed zwitterionic structures are no longer formed at a lower glycine coverage as simulated by enlarging the unit cell so as to break intrastrand hydrogen bonds, showing the importance of H-bond cooperativity in stabilizing the zwitterionic forms. Each structure has been characterized by computing its harmonic vibrational spectrum at the Gamma point, which also allowed us to calculate the free energy of adsorption. The experimental infrared features of chemical-vapor-deposited glycine on a silica surface are in agreement with those computed for glycine adsorbed in its neutral form and engaging three hydrogen bonds with the surface silanols, two of them involving the C=O bond and one originating from the glycine OH group. The NH(2) group plays only a minor role as a weak hydrogen bond donor.     

In situ FTIR study of the adsorption geometry of bisulfate accepted ions on a platinum electrode

It is shown that a better understanding of the geometry of adsorbed bisulfate species on the surface is gained by monitoring simultaneously more than one vibrational mode of the molecule. We propose that in the double layer region the bisulfate ions are adsorbed via an oxygen atom of its SO₃ unit with the Pt-O-S linkage along the surface normal. In the oxide region the bisulfate ions are adsorbed on the oxygen covered surface of the platinum electrode via the hydrogen atom by hydrogen bonding with the hydrogen bond showing a significant tilt (approximately 60° ).     

Structural and kinetic effects on a simple catalytic reaction: oxygen reduction on Ni(110)

Oxygen hydrogenation at 100 K by gas phase atomic hydrogen on Ni(110) has been studied under ultrahigh vacuum conditions by temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Formation of adsorbed water and hydroxyl species was observed and characterized. The coverage of the reaction products was monitored as a function of both temperature and initial oxygen precoverage. On the contrary, when high coverage oxygen overlayers were exposed to gas phase molecular hydrogen, no hydrogenation reaction took place. The results are compared to the inverse process, exposing the hydrogen covered surface to molecular oxygen. In this case, at 100 K, simple Langmuir-Hinshelwood modeling yields an initial sticking coefficient for oxygen adsorption equal to 0.26, considerably lower than for the clean surface. Moreover, formation of hydroxyl groups is found to be twice as fast as the final hydrogenation of OH groups to water. Assuming a preexponential factor of 10(13) s(-1), an activation barrier of 6.7 kcal/mol is obtained for OH formation, thus confirming the high hydrogenating activity of nickel with respect to other transition metals, for which higher activation energies are reported. However, oxygen is hardly removed by hydrogen on nickel: this is explained on the basis of the strong Ni-O chemical bond. The hydrogen residual coverage is well described including a contribution from the adsorption-induced H desorption process which takes place during the oxygen uptake and which is clearly visible from the TPD data.     

Interaction of oxygen adsorbed on silver film with hydrogen and ethylene

Kinetic data on hydrogen interaction with adsorbed oxygen testify to the uniformity and constancy of the oxygen bond with the silver surface. The surface reaction of ethylene epoxidation proceeds when the surface coverage by oxygen is higher than a certain threshold value, which is close to 0.4 for silver films at 333 K.     

Rh（100表面乙醇吸附与分解的HREELS研究

利用热脱附谱和高分辨能量损失谱技术研究了乙醇在Ｒｈ（１００）表面的吸会和分解过程，结果表明，１３０时Ｒｈ（１００）面暴露乙醇，表面首先形成化学附层，随乙醇暴露增加，表面出现多层凝聚态，表面升温至１５０Ｋ，吸附乙醇从Ｒｈ（１００）表面脱附，同时部分化学吸附乙醇分子发生羟基断裂，生成表面乙氧基，进一步升谩，表面乙氧基脱氢分解，其分解的主要途径是发生甲基脱氧，β－Ｃ与表面发生作用，生成一种含氧的金属有机     

Structural and conformational properties and intramolecular hydrogen bonding of (methylenecyclopropyl)methanol, as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectra of (methylenecyclopropyl)methanol (H(2)C=C(3)H(3)CH(2)OH) and one deuterated species (H(2)C=C(3)H(3)CH(2)OD) have been investigated in the 20-80 GHz spectral range. Accurate spectral measurements have been performed in the 40-80 GHz spectral interval. The spectra of two rotameric forms, denoted conformer I and conformer IX, have been assigned. Both these rotamers are stabilized by intramolecular hydrogen bonds formed between the hydrogen atom of the hydroxyl group and the pseudo-pi electrons on the outside of the cyclopropyl ring, the so-called "banana bonds". The carbon-carbon bond lengths in the ring are rather different. The bonds adjacent to the methylene group (H(2)C=) are approximately 7 pm shorter that the carbon-carbon bond opposite to this group. It is found from relative intensity measurements of microwave transitions that conformer IX, in which the hydrogen bond is formed with the banana bonds of the long carbon-carbon bond, is 0.4(3) kJ/mol more stable than conformer I, where the hydrogen bond is formed with the pseudo-pi electrons belonging to the shortest carbon-carbon bond of the ring. The microwave study has been augmented by quantum chemical calculations at the MP2/6-311++G, G3 and B3LYP/6-311++G levels of theory.     

Metallic nature and surface diffusion of CO adsorbed on Ru nanoparticles in aqueous media: A 13C NMR study

We report the first observation of the 13C nuclear magnetic resonance spectroscopy (NMR) of 13CO, adsorbed from 13CO saturated 0.5 M sulfuric acid solutions, onto the surfaces of commercial Ru-black nanoparticles. The 13C NMR spectra consist of a symmetrically broadened peak having a large isotropic shift as compared to CO adsorbed onto supported Ru catalysts. The variation of the spin-lattice relaxation rate follows Korringa behavior, indicating the metallic nature of adsorbed CO, in addition to varying across the spectrum in a Korringa-like manner. Motional narrowing of the NMR spectrum at higher temperatures, together with an additional contribution to the spin-lattice relaxation rate, indicate that adsorbed CO undergoes rapid diffusion on the particle surfaces. A two-band model analysis of the NMR results indicates that the CO adsorption bond is weaker on Ru as compared to either Pt or Pd. This is also supported by a reduction in the activation energy for CO diffusion on Ru vs either Pt or Pd nanoparticles.     

Nondissociative chemisorption of short chain alkanethiols on Au(111)

The adsorption of methanethiol and n-propanethiol on the Au(111) surface has been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-temperature scanning tunneling microscopy (LT-STM). Methanethiol desorbs molecularly from the chemisorbed monolayer at temperatures below 220 K in three overlapping desorption processes. No evidence for S-H or C-S bond cleavage has been found on the basis of three types of observations: (1) A mixture of chemisorbed CH3SD and CD3SH does not yield CD3SD, (2) no sulfur remains after desorption, and (3) no residual surface species remain when the adsorbed layer is heated to 300 K as measured by STM. On the other hand, when defects are introduced on the surface by ion bombardment, the desorption temperature of CH3SH is extended to 300 K and a small amount of dimethyl disulfide is observed to desorb at 410 K, indicating that S-H bond scission occurs on defect sites on Au(111) followed by dimerization of CH3S(a) species. Propanethiol also adsorbs nondissociatively on the Au(111) surface and desorbs from the surface below 250 K.