Showing papers on "Chemical bond published in 1994"
TL;DR: In this paper, a topological analysis of local quantum-mechanical functions related to the Pauli exclusion principle is presented, where the local maxima of these functions define "localization attractors", of which there are only three basic types: bonding, non-bonding and core.
Abstract: THE definitions currently used to classify chemical bonds (in terms of bond order, covalency versus ionicity and so forth) are derived from approximate theories1–3 and are often imprecise. Here we outline a first step towards a more rigorous means of classification based on topological analysis of local quantum-mechanical functions related to the Pauli exclusion principle. The local maxima of these functions define 'localization attractors', of which there are only three basic types: bonding, non-bonding and core. Bonding attractors lie between the core attractors (which themselves surround the atomic nuclei) and characterize the shared-electron interactions. The number of bond attractors is related to the bond multiplicity. The spatial organization of localization attractors provides a basis for a well-defined classification of bonds, allowing an absolute characterization of covalency versus ionicity to be obtained from observable properties such as electron densities.
3,146 citations
TL;DR: A critical review of three different experimental methods used to measure bond energies: radical kinetics, gas-phase acidity cycles, and photoionization mass spectrometry is presented in this article.
Abstract: We present a critical review of three different experimental methods used to measure bond energies: radical kinetics, gas-phase acidity cycles, and photoionization mass spectrometry. These experimental techniques are currently in use to measure the bond energies of a large number of molecules. We discuss the basic premises of each of these experiments and explicitly try to point out the strengths and weaknesses of each method. We directly compare each of these procedures using a set of about 30 important molecules. These three different techniques have only a few disagreements among them, the case of ethylene being the most serious. For the overwhelming number of studies, all measurements lead to bond energies within each other's error bars. We include tables of ionization potentials, electron affinities, and heats of formation for about 40 important organic and inorganic radicals. We also list bond energies for the parent molecules at 0 and 298 K. 273 refs., 10 figs., 5 tabs.
1,016 citations
TL;DR: Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water—surfactant-coated quartz, water-hexane, and water-air, and a partly wettable water-quartz interface.
Abstract: Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water molecules. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.
707 citations
TL;DR: Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C60 molecules are linked, in accord with the reduced solubility of the pressure-induced phases.
Abstract: The fullerene C60 can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C60 on reheating to 300°C at ambient pressure. For synthesis temperatures between 300° and 400°C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter ao = 13.6 angstroms. When treated at 500° to 800°C at the same pressure, C60 transforms into a rhombohedral structure with hexagonal lattice parameters of ao = 9.22 angstroms and co = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C60 molecules are linked.
572 citations
TL;DR: Optical sum-frequency generation was used to study OH stretch vibrations of water molecules at fused quartz-water interfaces and indicates that orientations and bond ordering of interfacial water molecules are strongly affected by electrostatic interaction and hydrogen bonding.
Abstract: Optical sum-frequency generation was used to study OH stretch vibrations of water molecules at fused quartz-water interfaces. The results indicate that orientations and bond ordering of interfacial water molecules are strongly affected by electrostatic interaction and hydrogen bonding of the molecules with the quartz surface.
491 citations
385 citations
TL;DR: In this paper, the authors present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li[sup+], Na[sup +], K[sup+, Rb[sup], and Cs[sup+) at the restricted Hartree-Fock (RHF) level.
Abstract: We present an ab inito, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li[sup +], Na[sup +], K[sup +], Rb[sup +], and Cs[sup +]. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31 + G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NBO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol[sup [minus]1], depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K[sup +]/18c6 (-71.5 kcal mol[sup [minus]1]) is about 30 kcal mol[sup [minus]1] stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. 48 refs., 12 figs., 5 tabs.
349 citations
TL;DR: Direct studies of diradicals, the molecular species hypothesized to be archetypal of chemical bond transformations in many classes of reactions, have been made using femtosecond laser techniques with mass spectrometry in a molecular beam to establish the nature of these intermediates and define their existence during reactions.
Abstract: Direct studies of diradicals, the molecular species hypothesized to be archetypal of chemical bond transformations in many classes of reactions, have been made using femtosecond laser techniques with mass spectrometry in a molecular beam. These studies are aimed at "freezing" the diradicals in time and in the course of the reaction. The passage of these species through the transition-state region was observed and the effect of total energy and alkyl substitution on the rates of bond closure and cleavage was examined. The results establish the nature of these intermediates and define their existence during reactions.
245 citations
221 citations
TL;DR: The modified ionic and covalent valence indices as discussed by the authors are defined in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL).
Abstract: The modified ionic and covalent valence indices are introduced, defined in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL). They include only quadratic contributions in changes of the molecular charge-and-bond order matrix elements, relative to the SAL. General properties of the modified valence indices are examined and illustrative qualitative results for model systems are presented. Numerical UHF SCF MO valence data for selected diatomic and triatomic molecules are reported and interpreted in terms of the valence saturation effect and the ionic vs. covalent valence competition. A three-orbital valence model of a symmetric transition state of the bond-forming–bond-breaking reaction supports the BEBO model postulate of preservation of the total “bond order.” The model predictions are compared with the UHF numerical values. © 1994 John Wiley & Sons, Inc.
218 citations
TL;DR: In this article, the authors re-examined the relationship between the covalent O-H bond length and the H...O and O...O separations in a low-temperature neutron diffraction study of organic molecules.
Abstract: The lengthening of the covalent O-H bond in O-H..O hydrogen bonds is re-examined from high-precision low-temperature neutron diffraction studies of organic molecules (32 crystal structures, 136 hydrogen bonds, T<130 K, R<0.06, H atoms on symmetry elements excluded). Accuracies are around or better than 0.002 A. The dependencies of the covalent O-H bond length on the H...O and O...O separations are smooth with no sign of discontinuities. For all types of O-H and O-D hydrogen-bond donors and in combination with all types of O acceptors, the O-H bond length follows the same function of the H...O separation (within the experimental accuracy)
TL;DR: In this paper, a new interlayer dielectric film using fluorine-doped silicon oxide (SiOF) for multilevel interconnection of very large scale integration (VLSI) has been fabricated.
Abstract: A new interlayer dielectric film using fluorine-doped silicon oxide (SiOF) for multilevel interconnection of very large scale integration (VLSI) has been fabricated The film is deposited by a simple technique, which is hexafluoroethane ( C2F6) addition to conventional tetraethoxysilane (TEOS)-based plasma-enhanced chemical vapor deposition (PE-CVD) Si–F bond formation in the film is detected by chemical bonding structural studies using Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) Low dielectric constants caused by Si–F bond formation and good gap-filling ability due to in situ etching by C2F6 plasma are obtained Therefore, SiOF film has very high applicability as an interlayer dielectric film for advanced VLSI devices
TL;DR: In this article, the first complete LEED-IV analysis of an ordered water layer adsorbed on a metal surface, the structure of p(√3 × √3)R30°-2D 2 O/Ru(001), commonly assumed to be a bilayer was presented.
TL;DR: Two-dimensional magic angle spinning nuclear magnetic resonance spectra collected just above the glass transition of K2Si4O9 at temperatures as high as 583�C are presented and rates of exchange for silicon among silicate species, which involves Si—O bond breaking, are measured and shown to be closely related in time scale to those defined by viscosity.
Abstract: The atomic-scale dynamics of the glass-to-liquid transition are, in general, poorly understood in inorganic materials. Here, two-dimensional magic angle spinning nuclear magnetic resonance spectra collected just above the glass transition of K2Si4O9 at temperatures as high as 583°C are presented. Rates of exchange for silicon among silicate species, which involves Si—O bond breaking, have been measured and are shown to be closely related in time scale to those defined by viscosity. Thus, even at viscosities as high as 1010 pascal seconds, local bond breaking (in contrast to the cooperative motion of large clusters) is of major importance in the control of macroscopic flow and diffusion.
TL;DR: In this paper, a simple valence-bond charge transfer model is proposed to describe the nonlinear optical properties of charge transfer organic materials, including the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters related to the band gap, bandwidth, and dipole moment of the charge transfer state.
Abstract: The nonlinear optical properties of charge-transfer organic materials are discussed in the framework of a simple valence-bond charge-transfer model. This model leads to analytic formulas for the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters, V, t, and Q related to the band gap, bandwidth, and dipole moment of the charge-transfer state. These parameters are derivable from experiment or from theory. The valence-bond charge-transfer model provides a clear physical picture for the dependence of the hyperpolarizabilities on the structure of charge-transfer molecules and leads to good agreement with the trends predicted by the AM1 calculations.
TL;DR: In this article, the density functional calculations using the Perdew nonlocal corrections to exchange and correlation have been carried out for a sequence of hydrated proton clusters, and the optimized structures were obtained up to H13O+6.
Abstract: The density functional calculations using the Perdew nonlocal corrections to exchange and correlation have been carried out for a sequence of hydrated proton clusters. The optimized structures were obtained up to H13O+6. It is found that H3O+ is indeed the central unit in all the lowest energy structures we found. Our results support the argument that the structure with a four‐coordinate first solvation shell is very unlikely in small hydrated proton clusters. The density functional calculations with the Perdew nonlocal corrections to exchange and correlation give somewhat shorter hydrogen bond lengths, but slightly longer chemical bond lengths as compared with the post‐Hartree–Fock calculations. The harmonic vibrational frequencies and IR intensities of various vibrational modes have been generated for all the structures optimized. Results for small clusters are compared with the high resolution experimental spectroscopy studies of Yeh et al. and Begemann et al. Results for larger clusters are used to in...
TL;DR: In this paper, the I2 caging in solid Kr is followed in real-time following its dissociative excitation on the A(3Π1u) surface by pump-probe measurements with a time resolution of ≥150 fs.
Abstract: The caging of I2 in solid Kr is followed in real‐time following its dissociative excitation on the A(3Π1u) surface. The experiments involve pump–probe measurements with a time resolution of ≥150 fs. The experimental signals are reproduced using classical molecular dynamics simulations, and the classical Franck approximation. The comparison between experiment and simulation allows an unambiguous interpretation of features in the observed signal as being due to the initial impulsive stretch of the I–I bond, collision of the atoms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I2 molecule. These detailed observations are possible due to retention of coherence along the I–I coordinate throughout the caging process. The extent of coherence is dictated mainly by the initial impact parameters of the molecule‐cage collision, which in turn is controlled by the thermal and zero‐point amplitudes of lattice vibrations. The caging is well‐described as a sudden pro...
TL;DR: In this article, the authors extended the meaning of chemical threshold to such chemical compositions and bonding arrangement (even for the case R < 1) where the energy of formation reaches the most negative value.
TL;DR: The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core-and valence-level photoemission, and CO chemisorption as mentioned in this paper.
Abstract: The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of [approximately] +0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and Pd[sub n]Zn[sub n] (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) [yields] Zn(4p) charge transfer and Pd(4d) [yields] Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2[pi]) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. 57 refs., 12 figs., 1 tab.
TL;DR: In this article, the abstraction kinetics for atomic hydrogen removal of chemisorbed D and atomic deuterium (Dat) removal of adsorbed H are studied on single crystal Si surfaces, and the surface H and D coverages are measured in real time by mass analyzing the recoiled H+ and D+ ion signals.
Abstract: The abstraction kinetics for atomic hydrogen (Hat) removal of chemisorbed D and atomic deuterium (Dat) removal of chemisorbed H are studied on single crystal Si surfaces. The surface H and D coverages are measured in real time by mass analyzing the recoiled H+ and D+ ion signals. On both Si(100) and Si(111) surfaces, the abstraction reactions are efficient, and have very low activation energies ≂0.5–1 kcal/mol. For abstraction from surfaces containing only monohydride species, the abstraction reaction probability is ≂0.36 times the adsorption rate of Hat or Dat. For the same Hat and Dat exposures, the reaction rates for Hat removal of adsorbed D and Dat removal of adsorbed H are nearly identical. All observations are consistent with a generalized Eley–Rideal abstraction mechanism, and a two‐dimensional quantum‐mechanical model is used to calculate reaction probabilities for these reactions. According to the model, the activation energies are due to enhanced abstraction rates from excited vibrational states of the adsorbed Si–H or Si–D bond. With SiH2 and SiH3 species present on the surface, the removal rate of H using Dat is decelerated, suggesting that the higher hydrides have a lower cross section for abstraction.
TL;DR: In this article, it was shown that molecular building blocks are actually orbital (or energetic) building blocks, showing that there is an energetic basis for the use of fundamental building blocks in the representation.
Abstract: The goals of theoretical crystallography may be summarized as follows: (1) predict the stoichiometry of the stable compounds; (2) predict the bond topology (i.e. the approximate atomic arrangement) of the stable compounds; (3) given the bond topology, calculate accurate bond lengths and angles (i.e. accurate atomic coordinates and cell dimensions); (4) given accurate atomic coordinates, calculate accurate static and dynamic properties of a crystal. For oxides and oxysalts, we are now quite successful at (3) and (4), but fail miserably at (1) and (2). The current situation in the first two areas is briefly reviewed, prior to discussing in some detail an approach to topological aspects of structure in oxide and oxysalt crystals. The structure of a molecule or crystal may be represented by a graph, in which the vertices represent orbitals, atoms or groups of atoms, and the edges represent orbital interactions or chemical bonds. The topological characteristics of the bond network are contained in the (weighted) adjacency matrix of the graph and the corresponding eigenvalues constitute the spectrum of the graph. Simple graph theory arguments show that molecular (fundamental) building blocks are actually orbital (or energetic) building blocks, showing that there is an energetic basis for the use of fundamental building blocks in the representation
TL;DR: In this paper, the authors focus on the relation between the propensity of a molecular crystal to undergo chem reactions after impact or shock and energy transfer rates and derive a simple formula for the total energy transfer rate into a given vibron band in terms of the density vibrational states and the vibron-phonon coupling.
Abstract: In this paper we focus on the relation between the propensity of a molecular crystal to undergo chem reactions after impact or shock and energy transfer rates. When a crystal receives a shock, low-frequency lattice vibrations (called phonons) are excited. Typical phonon frequencies are 0-200 cm[sup [minus]1]. This end must then be converted to bond stretch frequencies (1000-200 cm[sup [minus]1]) before bond breaking can occur, derive a simple formula for the total energy transfer rate into a given vibron band in terms of the density vibrational states and the vibron-phonon coupling. We are able to estimate the phonon upconversion in widely varying energetic materials such as TATB. HMX, and Pb styphnate by examining existing inelastic neutron scattering data. We find that the estimated energy transfer rates in pure unreacted material are several times greater for the sensitive explosives studied than the insensitive explosives. 35 refs., 5 figs., 2 tabs.
TL;DR: In this paper, a rhodium system was used to provide a thermodynamically favored C-C cleaved complex by making strong metal-aryl bonds, and the results showed that C-H activation and [eta][sup 2] coordination are probably involved in the process leading to c-C bond cleavage.
Abstract: Recent publications have brought renewed interest to the quest for homogeneous catalytic activation of carbon-carbon bonds. However, except for a few reports of biphenylene cleavage, the mechanism and scope of aryl-aryl C-C bond activation remains relatively uninvestigated. In the hope of overcoming the obstacle of weak M-C bonds, we used a rhodium system that should provide a thermodynamically favored C-C cleaved complex by making strong metal-aryl bonds. Reaction of (C[sub 5]Me[sub 5])Rh(PMe[sub 3])(Ph)(H) (1) with 1.5 equiv of biphenylene in cyclohexane-d[sub 12] at 65[degree]C resulted in the quantitative formation of (CC[sub 5]Me[sub 5])Rh(PMe[sub 3]) (biphenylenyl(H)) (2), along with a small amount of a red complex. The cleavage of the well-hidden carbon-carbon bond of biphenylene described relies both on a strained four-membered ring and on the formation of a stable pentametallacycle. The results show that C-H activation and [eta][sup 2] coordination are probably involved in the process leading to C-C bond cleavage, and valuable information can be obtained from studies of this system. 21 refs., 1 fig.
TL;DR: The theoretically predicted differences in bond strengths for different protonated forms clearly indicate that in peptide fragmentation schemes one should consider even those protonate forms whose formation is not preferred energetically.
TL;DR: In this article, the authors reported related chemistry of the Tp[double prime]Co-moiety (Tp[ double prime] = hydridotris (3-tert-butyl-5-methylpyrazolyl) borate), including the observation of an intermediate and evidence for a tunneling contribution to hydrogen atom abstraction from the ligand.
Abstract: The activation of dioxygen by coordination to metals plays an important role in living organisms as well as artificial oxidation catalysts. We have previously described the dioxygen complex Tp[prime]Co[sup II](O[sub 2]) (Tp[prime] = hydridotris (3-tert-butyl-5-methylpyrazolyl)borate) and its reaction with Tp[prime]Co[sup I](N[sub 2]) to yield Tp[prime]Co[sup II]OH, the apparent product of hydrogen atom abstraction by a highly reactive cobalt - oxo complex. Herein we report related chemistry of the Tp[double prime]Co-moiety (Tp[double prime] = hydridotris (3-isopropyl-5-methylpyrazolyl) borate), including the observation of an intermediate and evidence for a tunneling contribution to hydrogen atom abstraction from the ligand. 16 refs., 1 fig.
TL;DR: In this paper, a general concept of negative ion radical reactive intermediates formation for these components has been proposed, based upon the ionisation mechanism of these compounds caused by the action of electrons of law energy (1-4 eV).
Abstract: To understand the chemical behaviour of lubricant components during boundary lubrication, a general concept of negative ionradical reactive intermediates formation for these components has been proposed. The concept is based upon the ionisation mechanism of these compounds caused by the action of electrons of law energy (1–4 eV). Electrons of such energy (exoelectrons) are spontaneously emitted from most fresh surfaces formed during friction. The principal thesis of the model is that lubricant components form anions which are then chemisorbed on the positively charged areas of rubbing surfaces.
The formation of negative ions and their decomposition process are simulated by electron attachment mass spectrography. This type of mass spectrography uses electrons of an energy range similar to those of exoelectrons. The proposed model encompasses the following major stages: (a) low-energy electron emission process and creation of positively charged spots, (b) action of emitted electrons with lubricant components (anion and radical formation), (c) reactions of anions with metal surfaces forming a film protecting the surface from wear, (d) cracking of chemical bonds producing other radicals. The model explains many lubrication phenomena of hydrocarbons, oxygen-containing compounds, and many types of chemicals used as antiwear and extreme-pressure additives.
TL;DR: Experimental observations of the effects of solvation on the dehydrogenation reaction of Mg+(H2O)n to produce MgOH+1 are presented for n ≥ 6 as mentioned in this paper.
Abstract: Experimental observations of the effects of solvation on the dehydrogenation reaction of Mg+(H2O)n to produce MgOH+(H2O)n−1 are presented for n≤6. The reaction is seen to occur spontaneously at room temperature for n≳4. Ligand switching reactions are used to show the Mg+–OH bonds are stronger than Mg+H2O bonds. The results show the energy required to lose an H atom decreases with the number of water molecules attached because the magnesium changes oxidation state and this results in stronger interactions with the water ligands. Ab initio calculations are used to explain these observations.