Diatomic Fragments Research Articles

On the determination of effective one‐electron Hamiltonians to be used for studying large molecules

AbstractWe studied the possibility of defining a one‐electron effective Hamiltonian in the form of a Hückel‐type matrix, taking into account the effect of the environment of the atoms in different molecules (the E2HT). It is found that the appropriate basis set in which this Hamiltonian is defined can be determined by using diatomic fragments. Concerning the matrix elements themselves, the appropriate information can be taken in fragments with a size similar to that of ethane. © 1994 John Wiley & Sons, Inc.

Activity decline and diffusional changes in a magnesium-promoted cobalt-silica catalyst

A Co-Mg/diatomaceous earth catalyst was prepared by slow precipitation. Transmission electron microscopy (TEM) indicated two separate phases: small needles of a Co-Mg phase and the large particles of the diatom fragments. Nitrogen sorption measurements indicated much higherpore volume (0.32 cm 3 g ) for the catalyst than for the control catalyst made by fast precipitation (0.10 cm 3 g ). The catalyst activity in the Fischer-Tropsch synthesis at 155 kPa declined at an unusually fast rate. The decline was accompanied by an increase of the methane selectivity and a decrease of the Schulz-Flory growth factor. Hydrogen treatments temporarily restored the catalyst activity but did not arrest the declines. Increasing the H 2:CO feed ratio for the deactivated catalyst not only restored the high activity but also stabilized the catalyst. An increase of the methane selectivity and a small decrease of the growth factor also accompanied these changes. The changes in catalyst activity and product selectivity correlate well with the anticipated diffusional changes from gas phase diffusion to diffusion through liquids which have been deposited in the pores during the course of the reaction. It is inferred that the diffusional changes cause significant changes in the steady-state concentrations of the reaction intermediates and these, in turn, affect the reaction rate (conversion) and the chain termination (growth factor).

A Coulomb expolsion model applied to charge separation in nitrosyl halide and related dications

Photoelectron—photoion—photoion coincidence spectra of NOF, NOCl, NOBr and NOCF 3 from ionisation of the parent molecules by 40.8 and 48.4 eV light are reported. The peak shapes arisng from atomisation of the triatomic dications cannot be well fitted by reasonable models of intermediate diatomic fragment formation, and a Coulomb explosion model incorporating charge exchange is proposed to explain them.

Constituents of a remote pacific marine aerosol: A tem study

Marine aerosol particles from the Equatorial Pacific were studied using transmission electron microscopy. Sea-salt aggregates consist largely of NaCl and Na-Ca sulfate crystals. Variations in the composition of sea-salt aggregates occur, and these may reflect the effects of relative humidity. There are several species of submicron, S-bearing particles, presumably including ammonium sulfate and strong acids. Diatom fragments are common, and sparse kaolinite and rutile crystals are the mineral component of this marine aerosol. The samples we studied appeared to be free of obvious anthropogenic effects.

High resolution near infrared spectroscopy of HCl–DCl and DCl–HCl: Relative binding energies, isomer interconversion rates, and mode specific vibrational predissociation

Both D- and H-bonded isomers of the mixed dimers formed between HCl and DCl are investigated via high resolution infrared difference frequency and diode laser spectroscopy in the 2885 and 2064 cm−1 regions. From an analysis of the relative integrated absorption intensities, the D-bonded complex (i.e., HCl–DCl) is determined to be more stable by 16±4 cm−1 than the H-bonded (i.e., DCl–HCl) species. All four chlorine isotopic combinations of the lower energy (HCl–DCl) complex are probed via excitation of both HCl (vHClacc=1←0) and DCl (vDCldon=1←0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl–HCl) complex are investigated through HCl excitation. Compared to the facile tunneling observed in both (HCl)2 or (DCl)2 complexes, these mixed dimers exhibit more rigid behavior characteristic of two distinct isomeric species. However, the relatively small energy difference (16±4 cm−1) between the two isomers still allows the wave functions for both species to sample both the HCl–DCl and DCl–HCl local minima on the potential surface. This intermediate level of angular localization of the wave function is modeled via 3D quantum mechanical calculations including all three internal rotor angular degrees of freedom. Additionally, a 1D treatment along the minimum energy tunneling path is investigated, which quantifies the asymmetry in the tunneling coordinate due to isotopic dependence of the H- and D-bonded zero point bending and torsion energies. Vibrational predissociation lifetimes in excess of the slit jet instrument line shape are determined from homogeneous broadening of the spectral line widths. The HCl stretch excited lifetime of H-bonded DCl–HCl [ΔνHCldon=44(6) MHz, τHCldon=3.6(5) ns] is threefold shorter than the corresponding lifetime of D-bonded HCl–DCl [ΔνHClacc=16(3) MHz, τHClacc=9.6(16) ns]. This ratio is quite comparable to the results obtained in investigations of (HCl)2 and consistent with a stronger, mode specific coupling to the dissociation coordinate for excitation of the bonded-HX vs free-HX moiety. However, the absolute lifetimes of both vHClacc=1 HCl–DCl and vHCldon=1 DCl–HCl complexes are tenfold shorter than the corresponding excited vibrational state lifetimes in (HCl)2. This suggests a near resonant channel for predissociation into HCl(v=0)+DCl(v=1) which minimizes the energy deposited into rotation and relative translation of the diatomic fragments.

Large-Amplitude Vibrations in Triatomic Molecules: A New Approach

Large-amplitude vibrations of the symmetric triatomic molecule are considered as the coupled rotations of two diatomic fragments. This procedure does not require a molecule-fixed coordinate system to be defined. Thus Eckart conditions, Coriolis coupling, and similar concerns are avoided. An algebraic approach is used to generate a Hamiltonian which describes the stretching vibrations and a second Hamiltonian which describes the bend. A model potential is used which treats the stretches as coupled Morse oscillators and the bend as a bond-length-dependent anharmonic oscillator. This paper gives detailed results only for states of zero angular momentum. However it can be used for all rotational states

Femtosecond real-time probing of reactions. 13. Multiphoton dynamics of mercury iodide (IHgI)

Real-time studies of the dynamics were performed on the reaction of HgI_2 in a molecular beam. Excitation was by either one or multi pump photons (311 nm), leading to two separate sets of dynamics, each of which could be investigated by a time-delayed probe laser (622 nm) that ionized the parent molecule and the fragments by REMPI processes. These dynamics were distinguished by combining the information from transients taken at each mass (HgI_2, HgI, Hgl, I_2, and I) with the results of pump (and probe) power dependence studies on each mass. A method of plotting the slope of the intensity dependence against the pump-probe time delay proved essential. In the preceding publication, we detailed the dynamics of the reaction initiated by a one photon excitation to the A-continuum. Here, we present studies of higher-energy states. Multiphoton excitation accesses predissociative states of HgI_2, for which there are crossings into the symmetric and asymmetric stretch coordinates. The dynamics of these channels, which lead to atomic (I or Hg) and diatomic (Hgl) fragments, are discussed and related to the nature of the intermediates along the reaction pathway.

Hydrothermal palygorskite and ferromanganese mineralization at a central California margin fracture zone

Ferromanganese oxyhydroxide crusts and nodules associated with palygorskite were recovered from the Santa Lucia Escarpment where the Morro Fracture Zone intersects the central California continental margin. Palygorskite was found in pure, high-Mg, low-Al, boxwork-textured veins, and disseminated in poorly consolidated palygorskite-rich mudstone. The purity of the palygorskite boxwork blades and the boxwork structure suggest formation by direct precipitation rather than by diagenetic or detrital processes. Interaction of hydrothermal fluids with oceanic basalt and/or deeper ultramafic rocks produced a Mg-Si enriched fluid supersaturated with respect to palygorskite that precipitated directly from the fluid at or near the seafloor. The close association of Fe-Mn crusts and nodules with both the palygorskite-rich mudstone and boxwork-vein palygorskite suggests a genetic link between the three types of mineralization. Mixed origin hydrothermal-hydrogenetic Fe-Mn crusts, with up to 50% hydrothermal input, formed contemporaneously with and subsequent to palygorskite formation. Fe-Mn nodules collected in the same dredge are of combined hydrogenetic and diagenetic origin and appear to be unrelated to hydrothermal mineralization that produced the crusts and palygorskite. The thickness of the Fe-Mn crusts and rare diatom fragments within the mudstone suggest an age of formation between 13 and 5 Ma.

Semiempirical molecular orbital theory: Parameterization of the MINDO/SR method for the first transition metal series

A suitable parameterization of the MINDO/SR method is presented for homonuclear diatomic fragments and clusters for some of the first transition metals. The central idea of this work is to find a set of parameters which gives both good bond energies for diatomic molecules and good cohesive energies for the solid represented by a cluster of few atoms. From the set of parameters of the original MINDO equations, the one ones varied here are the Slater-Condon parameters F 0(s,s) and the diatomic α and β parameters. Satisfactory agreement is found between the calculated and experimental binding energies for the elements Sc, Ti, V, Fe, Co, and Ni.

Electron-Spin Electron-Spin, Electron-Spin Rotation Interactions, and Quartic Centrifugal Distortion Terms for an Open-Shell Diatomic Molecule van der Waals Bonded to a Closed-Shell Partner

Effective Hamiltonians H sr, H ss, H λd, H cd, and their matrix elements are derived for calculating electron-spin rotation, electron-spin electron-spin splittings, and centrifugal distortion correction terms to electron-spin electron-spin interaction and rotational energy levels, respectively. This formalism is valid for a near rigid, nonlinear planar open-shell complex consisting of an open-shell diatomic unit in an electronic state described by 2 S+1 Λ, where Λ = Σ, Π, Δ, Φ, etc,; S ≥ 1 2 , and a closed-shell partner (a rare gas atom or a closed shell diatomic molecule). Electron-spin electron-spin interaction and its centrifugal distortion correction terms (described by H ss and H λd, respectively) are needed only for complexes containing an open-shell diatomic unit with S > 1 2 , whereas electron-spin rotation and centrifugal distortion correction terms to rotational energy levels (described by H sr and H cd respectively) are required for complexes containing an open-shell diatomic unit with S ≥ 1 2 . For calculating effects of H sr, H ss, H λd, and H cd on rotational energy levels in the mentioned types of complexes, the total Hamiltonian is written as H = H rot + H so + H q + H sr + H ss + Hλ d + H cd. Rotational energy levels are then obtained by numerically diagonalizing this Hamiltonian matrix for each given J. Matrix elements of H rot, H so, H q, and calculations of relative intensities in spin and orbitally allowed transitions in the open-shell diatomic fragment and in rotationally or vibrationally allowed transitions in the closed-shell partner (when the closed-shell partner is a dipolar diatom) are the same as described in our previous work (W. M. Fawzy and J. T. Hougen, J. Mol. Spectrosc. 137, 154-165 (1989)). A brief discussion of the matrix elements of H sr, H ss, H Λd, H cd, and their inclusion in our previous computer program for calculating rotational energy levels and relative intensities of allowed transitions is given.

Electron impact dissociation of H2O: emission cross sections for OH*, OH+*, H*, and H2O+* fragments

The optical emission spectrum in the near ultraviolet and visible following electron impact on H2O was studied in a crossed-beam and a static gas-target experiment. Emissions of H*, OH*, OH+*, and H2O+* fragments were detected and absolute emission cross sections for the different fragments were determined. A nonthermal rotational population was observed for the diatomic fragments which gives insight into the dissociation process. Further conclusions on the dissociation mechanism are possible based on appearance potentials and the shape of the emission cross sections as a function of impact energy.

An adiabatic-bend Franck-Condon model for final rotational distributions in the H + H 2O and H + D 2O reactions

We report H 2 and OH rotational distributions for the H + H 2O reaction, and HD and OD rotational distributions for the H + D 2O reaction, using a model based on the adiabatic-bend theory of the reaction dynamics together with a Franck-Condon model to obtain the rotational distributions. A realistic, six-degree-of-freedom potential is used to describe the four-atom reaction, and the bending eigenstates are accurately calculated using a direct-product basis of spherical harmonics for the two diatomic fragments.

Vibrational modification of the spatial anisotropy parameter in photodissociation. A classical trajectory study

Classical trajectory calculations have been performed to explore the vibrational influence on the spatial anisotropy parameter β in triatomic molecular photodissociation. Our calculations indicate that in a fast dissociation, accompanied by a large angular momentum excitation of the diatomic fragment, the recoil direction can change as much as 20° thereby resulting in a measurable modification of β.

Fragmentation dynamics of the photo-dissociated triatomic molecule

A theory is presented of the angular momentum distribution of diatomic fragments ejected in the photodissociation of triatomic molecules. The theory is based on semi-classical dynamics and treats dissociation from an ensemble of nuclear configurations reflecting the zero-point oscillations of the parent molecule. The mean angular momentum of the diatomic is determined by the range parameter of the repulsive potential, while the width of its distribution is determined by the gradient parameters of the potential surface as expressed in the Franck-Condon spatial and angular coordinates. The odd moments of the asymmetrical distribution are related to the nonlinear part of the potential energy surface. Using available experimental data, the model is applied to the photo-dissociation of NOCl at 440 nm, leading to an estimated value of 0·2 eV rad-1 for the local angular gradient of the repulsive potential, and a consequent lower limit of 20 fs for the fragmentation time.

Isotope specific photodissociation of SO 2 at 193.3 nm

The 193 nm photodissociation of the isotopomers SO 2, SO * 2 and SOO * (with O *= 18O) was studied by high-resolution photo-fragment translational spectroscopy. The translational energy distributions P( E T) of the fragment pairs SO+O, SO *+O *, SO+O * and SO *+O were determined from systematic analysis of the time-of-flight distributions obtained with various degrees of O * substitution and were found to be significantly different due to a different ro-vibrational distribution of the diatomic fragments. In the case of the symmetric isotopomers SO 2 and SO * 2 the difference of the P( E T) distributions is attributed to a slightly different state excitation in the C̃ electronic band of the parent molecule. The SO and SO * bond fission of the asymmetric SO0 * have equal probabilities but lead to different energy distributions in the SO and SO * fragments which reflect subtle differences in the dissociation dynamics of the two reaction modes.

Isotopic studies of atomic site selectivity in molecular multiphoton ionization of N2O.

Isotopic studies of multiphoton ionization of ${\mathrm{N}}_{2}$O give clear evidence for a significant atomic site selectivity in the production of atomic fragments. The observation of the diatomic fragments ${\mathrm{N}}_{2}^{+}$ and ${\mathrm{NO}}^{+}$ suggests a possible role for curve-crossing mechanisms that could lead to electronically excited fragments.

An AEM-TEM study of weathering and diagenesis, Abert Lake, Oregon: II. Diagenetic modification of the sedimentary assemblage

This paper compares the mineralogy and chemistry of clay minerals in sediments from various depths and positions in Abert Lake and surrounding playa with those of the weathered materials entering the lake in order to reveal the nature and extent of post-depositional mineralogical modification. Analytical electron microscope (AEM) data from individual clay particles reveal that each sample is comprised of a highly inhomogeneous smectite assemblage. The thin clay flakes (commonly less than 10 nm wide) display a complete range in octahedral sheet compositions from nearly dioctahedral to nearly trioctahedral. The very abundant Mg-rich lake smectites with an estimated composition K 0.29(Al 0.23-Mg 2.16Fe 0.30)Si 3.80Al 0.20O 10(OH) 2 are not formed by weathering. This confirms the importance of diagenetic Mg uptake. Lattice-fringe imaging failed to reveal distinct brucite-like or vermiculite-like layers, suggesting that interstratifications of this type are rare or absent. Siliceous coatings on clay particles (identified by silica excess in smectite analyses) seem to favor topotactic overgrowth of stevensite rather than addition of brucite-like layers to the dioctahedral nuclei. The growth of K-stevensite dilutes the Al content of the crystal, and thus the increasing diagenetic modification reduces rather than supplements its illite component. Smectite compositions within individual samples were highly variable, yet source-related characteristics such as the abundance of Fe-rich smectite were apparent. Little evidence for systematic K or Mg enrichment with depth was identified in samples from depths of down to 16 feet below the sediment-water interface. The most magnesian assemblages are associated both with weathering sources of Mg-rich smectite and playa environments subjected to repeated wetting and drying cycles. Thus, the observations suggest that clay compositions primarily reflect changes in lake levels, brine composition, and source characteristics, rather than time and depth/compaction effects. Other diagenetic reactions in the sediment include recrystallization of Na-rich silica gel and diatom fragments. Abundant, submicron-sized, untwinned, euhedral crystals of K-feldspar are interpreted to be authigenic in origin.

Effect of interference inside surroundings on XANES. O K-shell photoabsorption in N2O

In this work an interference picture occurring as a result of inner-shell ionization of an atom in a substance is examined. We focus our attention on the interference of photoelectron scattered waves inside the surroundings of an excited atom. Such interference leads to the interdependence of amplitudes of the photoelectron waves reflected (or back scattered) by different coordination spheres and thus the surroundings act as an indivisible system. The influence of this interference is analysed here using as an example, the O K-shell XANES in the N2O molecule (the case of XANES of a terminal atom in a linear triatomic molecule). To reveal the role of the interferencial effects inside the diatomic fragment ⟨N1-N2⟩ we obtain an approximate analytical expression for XANES description. Such a description becomes possible due to (i) the localized orbitals approach, (ii) the variable phase approach and (iii) the concept of electron-optical properties of surroundings. The performed calculations show that for the correct O K-shell XANES interpretation the interference photoelectron waves inside the fragment must necessarily be taken into account. It is as well stated that the interference effects inside the surroundings explain: (1) the disturbance of the linear correlation between σ-resonance position and bond length, (2) the transformation of the EXAFS-like band to the resonance-like line in the spectrum and (3) the appearance of the extra maximum in the σ-channel of the photoabsorption cross section.

Theoretical aspects of metal cluster chemistry

Abstract

B(2P)+ H2O (X1A1): a quasi-classical 3D trajectory calculation

A quasi-classical 3D trajectory study of the B(2P)+ H2O (X1A1) reaction, which has several possible exothermic reaction channels, has been carried out using an analytical single-valued Sorbie–Murrell-like function for the H2BO doublet ground electronic state, where the reaction is supposed to take place adiabatically. In order to construct the analytical function, ab initio calculations have been carried out at the MP3/6-31G**//HF/6-31G** level for all diatomic and triatomic fragments, as well as for the tetra-atomic system. Several stationary points have been characterized on the ground tetra-atomic potential-energy surface, in good agreement with our previous semiempirical calculations. The dynamical study shows the formation of HOB, HBO and BO oxidation products. Total reaction cross-sections (SR) for the different channels have been calculated at several initial conditions, showing that SR increases with collision energy and that the HOB + H arrangement is more reactive than any other in the energy range studied. The influence of initial vibrational and rotational energies has been analysed at a collision energy of 25.0 kcal mol–1.