Showing papers on "Chemical bond published in 1986"

Density Functional Calculations of Molecular Bond Energies

Abstract: The calculation of molecular spectroscopic properties is an interesting application of the local density approximation (LDA) for the exchange-correlation energy of many-electron systems. LDA bond lengths and vibrational frequencies agree remarkably well with experiment. Dissociation energies are also reasonably good, but tend to overestimate the experimental results. Therefore, we consider in this work the effect of non-local gradient-type correction terms on LDA bond energies. In particular, we consider the gradient-corrected exchange-correlation functional of Langreth and Mehl, and also a semi-empirical exchange approximation recently developed by the present author.

Fluids with highly directional attractive forces. IV. Equilibrium polymerization

Abstract: We investigate approximation methods for systems of molecules interacting by core repulsion and highly directional attraction due to several attraction sites. The force model chosen imitates a chemical bond by providing for bond saturation when binding occurs. The dense fluid is an equilibrium mixture ofs-mers with mutual repulsion. We use a previously derived reformulation of statistical thermodynamics, in which the particle species are monomeric units with a specified set of attraction sites bonded. Thermodynamic perturbation theory (TPT) and integral equations of two types are derived. The use of TPT is illustrated by explicit calculation for a molecular model with two attraction sites, capable of forming chain and ring polymers. Successes and defects of TPT are discussed. The integral equations for pair correlations between particles of specified bonding include calculation of self-consistent densities of species. Methods of calculating thermodynamic properties from the solutions of integral equations are given.

Natural bond orbital analysis of molecular interactions: Theoretical studies of binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3

Abstract: The binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3 have been studied by ab initio molecular orbital theory and natural bond orbital (NBO) analysis. Most of the complexes involving N2, O2, F2, CO, and CO2 are found to have both hydrogen‐bonded and non‐hydrogen‐bonded structures. The NBO analysis provides a consistent picture of the bonding in this entire family of complexes in terms of charge transfer (CT) interactions, showing the close correlation of these interactions with the van der Waals penetration distance and dissociation energy of the complex. Contrary to previous studies based on the Kitaura–Morokuma analysis, we find a clear theoretical distinction between H‐bonded and non‐H‐bonded complexes based on the strength of CT interactions. Charge transfer is generally stronger in H‐bonded than in non‐H‐bonded complexes. It plays an intermediate role in non‐H‐bonded CO2 complexes which have been studied experimentally. However, the internal rotation barrier (1 kcal mol−...

Symmetric vs. asymmetric linear M-X-M linkages in molecules, polymers, and extended networks.

Abstract: Linear M-X-M linkages in which X is a nitride, oxide, or halide commonly occur in dimers, square tetramers, one-dimensional polymers, and extended three-dimensional solids. For low d electron counts a second-order Jahn- Teller mixing of metal d, and X p, orbitals favors asymmetric M-X-M bridges. M-X u bonding works against the distortion. Going to higher d electron counts also favors the symmetrical bridge by filling M-X x* levels. For the cyclic (MI,,X), tetramer and ;(ML.,X) chain, d electron counts greater than two favor a symmetric bridge; for perovskites, d" metals with n 1 1 are calculated to be symmetric. The extent of M-X bond length alternation can also be decreased by increasing the electronegativity difference between M and X to widen the HOMO-LUMO gap. Vertex-sharing polyhedra abound in solid-state and molecular transition-metal chemistry. Ever since Taube's classic work with Cr(OH2)62+, studies of inner-sphere electron-transfer reactions have focused on an intermediate with two octahedrally coordinated metal atoms sharing a common bridging ligand.' Similarly, some of the oldest, most extensively studied metallic compounds are the tungsten bronzes2 The cubic modification of the tungsten bronzes adopts the simple cubic array of corner linked WO, octahedra shown in 1. The octahedra are arranged to form linear W-0-W bridges and cube centers are randomly occupied by metal 2 3

A simple quantitative model of hydrogen-bonding

Abstract: A simple model for the computation of intermolecular interactions is described. It consists of atom–atom potentials for the representation of repulsion and dispersion energies, and an evaluation of the electrostatic energy in terms of partitioned multipole moments of the monomer electron distributions. Applications are given in detail for hydrogen‐bonded dimers of the molecules HF, HCl, CO, N2, Cl2, HCN, CO2, N2O, OCS, HCCH, NCCN, and HCCCN, and the results compared with ab initio and experimental results. Hydrogen bond energies are obtained to better than 4 kJ mol−1, intermolecular separations to typically better than 0.15 A, and intermolecular angles within 5°, all compared with experiment. Force constants and vibrational frequencies are also well reproduced.

A study of the low‐lying electronic states of Fe2 and Co2 by negative ion photoelectron spectroscopy

Abstract: The anions Fe−2 and Co−2 were prepared and cooled to 300 K in a flowing afterglow ion source, and the low‐lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe2 and Co2 have predicted single 4s–4s bonds, and extremely high densities of low‐lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902±0.008 eV Fe2, 1.110±0.008 eV Co2), the bond elongation on electron attachment (0.08±0.02 A Fe2, Co2), and the vibrational frequencies of the anions (250±20 cm−1 Fe−2, 240±15 cm−1 Co−2). Related studies of the atomic anions yield improved values for the electron affi...

Multiple-quantum NMR study of clustering in hydrogenated amorphous silicon.

Abstract: Multiple-quantum nuclear-magnetic-resonance techniques are used to study the distribution of hydrogen in hydrogenated amorphous silicon. Using the fact that multiple-quantum excitation is limited by the size of the dipolar-coupled spin system, we show that the predominant bonding environment for hydrogen is a cluster of four to seven atoms. For device quality films, the concentration of these cluster defects increases with increasing hydrogen content. At very high hydrogen content, the clusters are replaced with a continuous network of silicon-hydrogen bonds.

The nature of the bonding in XCO for X=Fe, Ni, and Cu

Abstract: The bonding in the 5,3Σ− and 3Δ states of FeCO, the 3,1Σ+, 3Δ, and 3 Π states of NiCO, and the 2Σ+ state of CuCO are analyzed using the constrained space orbital variation (CSOV) technique for both (CASSCF) and SCF wave functions. The bonding is discussed in terms of σ repulsion between the metal 4s and the CO 5σ, CO to metal σ donation when there is an empty or partly occupied dσ orbital and metal to CO 2π * backdonation. The bonding is compared for the different metals and between the different states.

Metal polyimide interface: A titanium reaction mechanism

Abstract: Chemical bonding and reaction of in situ deposited Ti on a series of polyimide surfaces have been investigated. X‐ray photoelectron spectroscopy has been used to probe site specific information for bonding of various functional groups of polyimide molecules, while ultraviolet photoelectron spectroscopy has been applied to monitor the change of the Ti d bands and the polyimide molecular orbitals near the Fermi energy as a function of overlayer coverage. At low coverages, the polyimide surfaces react strongly with Ti and strong charge transfer via the carbonyl group occurs. As the coverage increases, a Ti–C bond formation has been identified. A detailed mechanism for the reaction of Ti with polyimides is described, and this mechanism is confirmed by model pyromellitimide reaction with Ti0 suspension in solution.

π* ← π Shakeup Satellites for the Analysis of Structure and Bonding in Aromatic Polymers by X-Ray Photoelectron Spectroscopy:

Abstract: The applications of ESCA to polymer surface analysis include the use of the secondary final-state effects which lead to satellite structure near the core-level photoemission (PE) lines. Specifically, unsaturated and aromatic functionalities in organic compounds and polymers lead to π* ← π shakeup peaks of less than 10 eV lower kinetic energy (higher binding energy). In the surface analysis of polymers, these features can be utilized for qualitative analysis, identification of the presence and structure of aromatic bonding, and quantitative analysis in determining the amount of a particular block or the aromatic containing function in the near-surface region. Carbon 1s shakeups are most often used, but the present study includes detailed qualitative and quantitative analysis of shakeup structures from PE lines from each type of atom in hydrocarbon-, siloxane-, and sulfur-containing polymers. These results show the importance of including the shakeup intensity in quantitative peak area calculations and in peak fitting of complex PE envelopes. These studies prove in a variety of systems that the effects of third-row atoms on the final state lead to the presence of shakeup features in atoms with orbitals which do not participate in the aromatic orbital initial state, thus complicating interpretation of structure from the presence of these features. Results from the siloxane and sulfone polymers indicate that previously held assumptions about the nature of the initial-state molecular orbital may overlook the contribution of empty 3d orbitals or increased charge density on the Si or S atom which would spread the pi orbitals to the oxygen in the aromatic siloxane or sulfone systems. Finally, analysis of these features can provide quantitative analysis of polymeric surface structure by monitoring the relative intensity of the feature to the main PE line.

Mechanisms of atomic ion emission during sputtering

Abstract: Several major experimental and theoretical findings made in the last few years on the mechanisms of secondary ion emission are summarized. There is a strong indication that the phenomena can be divided into two categories: Ion emission from the surfaces of metals and semiconductors tends to have a strong correlation with the work function and can be described quite well with an electron tunneling model. Ion emission from systems which show large chemical enhancement has only a weak correlation with global surface properties like the work function or the bandgap. Instead it seems to be related more to local chemical bonds and coordination numbers. A localized bond breaking picture may be more appropriate in these circumstances.

Gas-phase photodissociation of organometallic ions. Bond energy and structure determinations

Abstract: The photodissociation of a variety of gas-phase organometallic ions was investigated with Fourier transform mass spectrometry in order to obtain spectroscopic and thermodynamic information on these complexes. The results indicate that these ionic complexes absorb broadly and photodissociate readily in the ultraviolet and visible spectral regions, with cross sections for lambda/sub max/ ranging from 0.02 to 0.30 A/sup 2/. Because of this broad absorption, photodissociation thresholds are attributed to thermodynamic and not spectroscopic factors. Bond energies and heats of formation obtained by monitoring photodissociation onsets show good agreement with those obtained by other techniques. Interestingly, product ions generated by photodissociation are found to differ significantly in a number of instances from those produced by collision-induced dissociation. Finally, differentiation of two FeC/sub 4/H/sub 6//sup +/ and four NiC/sub 4/H/sub 8//sup +/ isomers is demonstrated by observing differences in cross sections, spectral band positions, and neutral losses.

Solvent molecular dynamics in regions of hydrophobic hydration

Abstract: We present a dynamical analysis of the solvent in an aqueous solution of two nonpolar atomic solutes which are constrained to an interatomic distance corresponding to a solvent‐separated free energy minimum. The results are obtained from a molecular dynamics simulation using ST2 model water. Molecular mobility for solvent near the solutes is seen to be retarded, as evidenced in translational diffusion and rotational reorientation. These slower net motions are analogous to pure solvent dynamics at a temperature reduced by 10–15 °C. An analysis of intermolecular hydrogen bonding reveals that solvation shell molecules have correspondingly longer bond half‐lives compared to bulk molecules, by a factor of 1.5–2.0. The spectral densities for intermolecular vibrations are computed from translational and rotational velocity autocorrelation functions for shell and bulk motions. These densities are seen to correlate well with the local binding energy distributions.

Ab initio study of the hydrogen bonding interactions of formamide with water and methanol

Abstract: Ab initio calculations of hydrogen bond energies for a number of water–formamide and methanol–formamide complexes are reported at both the SCF and correlated levels. Full gradient optimizations of these structures have been performed for basis sets of double zeta and double zeta plus polarization quality. For both water and methanol, the most stable 1:1 complex is found to be a cyclic double hydrogen bonded structure. Basis set effects on the calculated hydrogen bond energies were investigated as was the magnitude of the basis set superposition error. In all cases investigated, the addition of polarization functions to the basis set is found to decrease the calculated binding energy by approximately 2–4 kcal/mol, while correlation is found to increase the binding energy by ≂1 kcal/mol. Calculations on dihydrated formamide indicate a small three‐body contribution to the total binding energy.

Carbon-hydrogen bond activation by ruthenium for the catalytic synthesis of indoles

Abstract: The intramolecular cyclometalation of sp/sup 3/ hybridized carbon-hydrogen bonds in ligand alkyl groups is well documented in transition-metal chemistry. Recent studies have demonstrated the feasibility of intermolecular activation of sp/sup 3/ hybridized carbon-hydrogen bonds in free alkane molecules, which has stimulated a search for the functionalization of the C-H bond in a catalytic fashion. The authors report here the activation of benzylic sp/sup 3/ hybridized C-H bonds and their catalytic conversion into an indole product using an isocyanide moiety to trap the activated species.

The generalized resonating valence bond description of cyclobutadiene

Abstract: The low-lying electronic states of square and rectangular cyclobutadiene (CBD) are calculated by using the generalized resonating valence bond (GRVB) method and compared with the results from Hartree-Fock and configuration interaction wavefunctions. We find that simple valence bond concepts correctly predict the sequence of excited states (including ground-state singlet) and the distortion to a rectangular geometry for the ground state. Contrary to common expectation, we find that the singlet ground state of square CBD has 22 kcal of resonance energy (relative to a single valence bond structure). Thus, CBD is not antiresonant, though it is much less stable than normal conjugated systems.

Mechanism of carbonyl insertion reaction of Pd and Pt complexes. An ab initio MO study

Abstract: The carbonyl insertion reaction of Pd(CH/sub 3/)(H)(CO)(PH/sub 3/), 1, and Pt(CH/sub 3/(H)(CO)(PH/sub 3/), 3, has been studied by means of the ab initio MO method with the energy gradient. The transition state has been determined for the reaction of both 1 and 3, which shows unequivocally that the reaction proceeds via methyl group migration. The reaction of 1 has a lower activation energy and is less endothermic than that of 3. These differences can be ascribed to the difference of M-CH/sub 3/, M-CO, and M-COCH/sub 3/ bond strengths between the Pd and the Pt complexes, those in the Pt complexes being stronger than in the Pd complexes. Substitution by an electron-withdrawing fluorine on the alkyl group makes the metal-alkyl bond stronger and the activation barrier higher. An electron-releasing methyl substitution gives a reverse effect. A stronger trans effect makes the metal-carbon bond weaker to give a lower activation barrier.

Sulfur K-edge absorption spectra of Na2SO4, Na2SO3, Na2S2O3, and Na2S2Ox(x=5―8)

Abstract: X-Ray absorption spectra of the sulfur K-edges of SO42−, SO32−, S2O32−, and S2Ox2− (x=5–8) were measured with synchrotron radiation. The spectra show similarly strong absorption bands which originate in bound-state transitions to antibonding 3p-type orbitals and in shape resonances related to 3d-type orbitals. The K-edge spectrum for the terminal sulfur atom in S2O32−, however, shows only a bound-state transition. Differences in chemical environments and bonding around the X-ray absorbing atoms are discussed to interpret the spectra.

Radiationless transitions in gas phase phenol and the effects of hydrogen bonding

Abstract: Quantum yields and rates of intersystem crossing and internal conversion are obtained for the lowest excited singlet state of collisionless phenol. It is found that internal conversion is the dominant decay channel, even at the origin of S1, in contrast to previously studied molecules. Also, triplet lifetimes measured as a function of vibrational energy are an order of magnitude shorter than in other isoelectronic aromatics. Hydrogen bonding by water and in a dimer is found to lengthen the singlet lifetime and decrease the quantum yield of internal conversion. This behavior parallels the effects, in solution, of going from a nonpolar to a polar solvent, indicating that substantial solvation effects are caused by a single hydrogen bond.

A revised set of values of single-bond radii derived from the observed interatomic distances in metals by correction for bond number and resonance energy

Abstract: An earlier discussion [Pauling, L. (1947) J. Am. Chem. Soc. 69, 542] of observed bond lengths in elemental metals with correction for bond number and resonance energy led to a set of single-bond metallic radii with values usually somewhat less than the corresponding values obtained from molecules and complex ions. A theory of resonating covalent bonds has now been developed that permits calculation of the number of resonance structures per atom and of the effective resonance energy per bond. With this refined method of correcting the observed bond lengths for the effect of resonance energy, a new set of single-bond covalent radii, in better agreement with values from molecules and complex ions, has been constructed.

Transition metal-ligand bonding. II

Abstract: The nature of the bonding of CO, H2O, and NH3 to transition metal atoms is analyzed using the constrained‐space‐orbital‐variation (CSOV) technique. The cooperative effects for Ni(CO)2 are found to be different than those for Ni(H2O)2. The bonding between neutral systems and the positive ions is found to be quite different; NiCO+ has little π bonding, while NiCO has strong π bonding. The positive ion of NiH2O is far more strongly bound than the neutral, while for NiCO the positive ion and neutral are bound by about the same energy.

Thermal decomposition of silane.

Abstract: The essential features of the potential energy surface for the thermal decomposition of silane have been calculated with extended basis sets, augmented by correlation corrections. It is predicted that the transition state for the molecular elimination lies 56.9 kcal/mol above silane. For the reverse reaction, the transition state is less than 2 kcal/mol above the separated fragments, silylene and molecular hydrogen, but 4.8 kcal/mol above a long-range potential well. In the latter, the silylene-H/sub 2/ separation is 1.78 A, and the bond in H/sub 2/ has stretched by more than 0.05 A. This indicates a significant electronic interaction between the fragments even at the large fragment separation. The depth of the well is less than 1 kcal/mol at the SCF level of theory, but it increases substantially when correlation is introduced into the wave function. Since the calculated SiH bond energy is 22 kcal/mol larger than the activation energy for the molecular elimination, the homolytic cleavage of silane to form silyl radical is not expected to be an important process in the low-energy pyrolysis of silane.

Intramolecular hydrogen bonding as reflected in the deuterium isotope effects on carbon-13 chemical shifts. Correlation with hydrogen bond energies

Abstract: The carbon-13 resonances of atoms bearing phenolic or enolic hydroxyl groups, that are engaged in intramolecular hydrogen bonds, experience large (/sup 2/..delta.. may exceed 1 ppm) upfield deuterium isotope effects. The magnitude of the two-bond isotope effect, /sup 2/..delta.., correlates with the hydrogen bond energy as obtained from the hydroxyl proton chemical shift. In the conjugated systems investigated in this work, the isotope effects extend over several (up to six) chemical bonds. The signs and magnitudes of the long-range isotope effects are related to molecular structure.

Bond selective photochemistry in CH2BrI through electronic excitation at 210 nm

Abstract: To explore the possibility of bond selective photochemistry in an excited electronic state, we have studied the photolysis of CH2BrI in a molecular beam at 210 nm. Following the direct local excitation of a repulsive transition on the C–Br bond at 210 nm, the fragments were detected by time‐of‐flight mass spectrometry. The dominant channel was found to be C–Br fission (60%) releasing an average of 15 kcal/mol into translation with the remainder reacting to form CH2+IBr and CH2+I+Br. There was no evidence for the primary fission of the C–I bond, making this the first clear example of the selective cleavage of a stronger bond in a molecule over the weakest one.

Annealing of plasma silicon oxynitride films

Abstract: The anneal behavior of plasma‐enhanced chemical vapor deposited silicon oxynitride films has been studied using Fourier transform infrared absorption spectroscopy, nuclear reaction analysis, and electron‐spin resonance. The anneal temperature range was 500–1000 °C. It is observed that the oxynitrides which contain only N–H bonds are thermally stable in the temperature range under study. The layers which also contain Si–H bonds are considerably less thermally stable. Abundant hydrogen effusion from these layers is observed at temperatures as low as 600 °C, accompanied by cracking and shrinkage of the films. It is suggested that the coexistence of both Si–H and N–H bonds offers the possibility for cross linking and that consequently the decomposition temperature of both types of bonds is lowered. Evidence for the occurrence of cross linking is found in the infrared difference spectra. Consistently, the silicon unpaired electron density does not increase upon annealing. The Si–H and N–H bands effectively shift towards higher wave numbers upon annealing at higher temperatures. This is ascribed to the inhomogeneity in bond strength, which in turn is related to a variation in electronegativity of the surrounding groups.

Chemical shifts and bond modification effects for some small first‐row‐atom molecules

Abstract: Chemical shifts and shift changes with bond modification have been calculated for some small first‐row‐atom molecules employing a (6311/311/1)=[4s, 3p, d] heavy atom, (31)=[2s] hydrogen basis in the GIAO approach. Shifts and shift anisotropies using this intermediate size basis agree as well with experiment as other, more extensive basis sets; heavy atom shifts are determined reasonably well on an absolute basis and hydrogen shifts are acceptable on a relative scale. Nearly all shift first derivatives are negative with respect to bond lengthening as are the second derivative terms. These results suggest that in most cases a negative temperature coefficient for the chemical shifts of the heavy atoms will be observed, an effect which in fact does obtain in most cases known experimentally.