Pressure-stiffened Raman phonons in group III nitrides: a local bond average approach

It has long been puzzling regarding the atomistic origin of the pressure-induced Raman optical phonon stiffening that generally follows a polynomial expression with coefficients needing physical indication. Here, we show that an extension of the bond-order-length-strength correlation mechanism and a local bond average approach to the pressure domain have led to an analytical solution to connect the pressure-induced Raman optical phonon stiffening directly to the bonding identities of the specimen and the response of the bonding identities to the applied stimulus. It is found that the pressure-induced blue-shift of Raman optical phonons arises from the bond compression and energy storage exerted by the compressive stress. Agreement between predictions and experimental measurements lead to the clarification of the detailed form for the polynomial coefficients, which provide an atomistic understanding of the physical mechanism of the external pressure induced energy gain, thermally induced bond expansion, as well as means of determining the mode atomic cohesive energy in a specimen.     

Electronic structure and chemical bonding in nonstoichiometric zirconium nitrides

X-ray emission spectra were taken and band calculations using the Green function LMTO method and cluster calculations using the discrete variational X method were carried out for the electronic structure and chemical bonding parameters for nonstoichiometric zirconium nitrides containing metallic and metalloid vacancies. The existence of structural defects leads to a redistribution of the occupancies of the major sub-bands of the nitride valence spectrum and the formation of a new group of states between the p-d- and d-like bands of ZrN_1.0.Institute of Chemistry, Urals Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 82–89, September–October, 1989.     

Chemical bonding in phosphane and amine complexes of main group elements and transition metals

The geometries and bond dissociation energies of the main group complexes X3B-NX3, X3B-PX3, X3Al-NX3, and X3Al-PX3 (X = H, Me, Cl) and the transition metal complexes (CO)5M-NX3 and (CO)5M-PX3 (M = Cr, Mo, W) have been calculated using gradient-corrected density functional theory at the BP86/TZ2P level. The nature of the donor-acceptor bonds was investigated with an energy decomposition analysis. It is found that the bond dissociation energy is not a good measure for the intrinsic strength of Lewis acidity and basicity because the preparation energies of the fragments may significantly change the trend of the bond strength. The interaction energies between the frozen fragments of the borane complexes are in most cases larger than the interaction energies of the alane complexes. The bond dissociation energy of the alane complexes is sometimes higher than that of the borane analogues because the energy for distorting the planar equilibrium geometry of BX3 to the pyramidal from in the complexes is higher than for AlX3. Inspection of the three energy terms, DeltaE(Pauli), DeltaE(orb), and DeltaE(elstat), shows that all three of them must be considered to understand the trends of the Lewis acid and base strength. The orbital term of the donor-acceptor bonds with the Lewis bases NCl3 and PCl3 have a higher pi character than the bonds of EH3 and EMe3, but NCl3 and PCl3 are weaker Lewis bases because the lone-pair orbital at the donor atoms N and P has a high percent s character. The calculated DeltaE(int) values suggest that the trends of the intrinsic Lewis bases' strengths in the main-group complexes with BX3 and AlX3 are NMe3 > NH3 > NCl3 and PMe3 > PH3 > PCl3. The transition metal complexes exhibit a somewhat different order with NH3 > NMe3 > NCl3 and PMe3 > PH3 > PCl3. The slightly weaker bonding of NMe3 than that of NH3 comes from stronger Pauli repulsion. The bond length does not always correlate with the bond dissociation energy, nor does it always correlate with the intrinsic interaction energy.     

Chemical bonding in ternary magnesium hydrides

The electronic structure of various alkali and alkaline–earth magnesium‐based hydrides was investigated in detail. These types of crystalline compounds show MgH₄ or MgH₆ units ordered within a light‐metal framework. We investigated the nature of the chemical bonding in these units by means of quantum chemical calculations of several related clusters. The properties of the charge density of the clusters, within the framework of the theory of atoms in molecules, was analyzed. A further set of computations of the band structure of the solid hydrides was conducted using a state‐of the‐art density functional‐based method and the mechanism of stabilization of the Mg? H units is discussed. It was found that the properties obtained at the molecular level correlate well with those of the solid crystals, indicating the molecular nature of the extended systems in which the units MgH_x, x = 4, 6, are stabilized by means of Mg? H closed‐shell interactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 150–164, 2003     

Chemical bonding in the hydrogen molecule

The chemical bond in the hydrogen molecule is examined using the electron density and the generalized overlap amplitudes. Logarithmic derivatives of the electron density provide a clear picture of its behavior in the bonding region as well as in the outer region. The GOA expansion of the density is used to examine the dependence of the rate of decay of the density on the GOA ionization potentials. The increase in the electron density at the nuclei and in the bonding region coincides with the higher ionization potential of H₂ over the H atom. The density in the bonding region along the internuclear axis does not decay exponentially, but its shape is very nearly an inverted Gaussian. © 1996 John Wiley & Sons, Inc.     

Chemical applications of topology and group theory. 34. Structure and bonding in titanocarbohedrene cages

Chemical bonding models are developed for the titanocarbohedrenes Ti14C13 and Ti8C12 by assuming that the Ti atoms use a six-orbital sd5 manifold and there is no direct Ti...Ti bonding. In the 3 x 3 x 3 cubic structure of Ti14C13, the 8 Ti atoms at the vertices of the cube are divided into two tetrahedral sets, one Ti(III) set and one Ti(IV) set, and the 6 Ti atoms at the midpoints of the cube faces exhibit square planar TiC4 coordination with two perpendicular three-center four-electron bonds. The energetically unfavorable Th dodecahedral structure for Ti8C12 has 8 equivalent Ti(III) atoms and C2(4-) units derived from the complete deprotonation of ethylene. In the more energetically favorable Td tetracapped tetrahedral structure for Ti8C12, the C2 units are formally dianions and the 8 Ti atoms are partitioned into inner tetrahedra (Ti(i)) bonded to the C2 units through three-center Ti-C2 bonds and outer tetrahedra (Ti degrees) bonded to the C2 units through two-center Ti-C bonds. The Ti atoms in one of the Ti4 tetrahedra are Ti(0) and those in the other Ti4 tetrahedron are Ti(III). Among the two such possibilities, the lower energy form has the (Ti0)o4(Ti(III))i4 configuration, corresponding to dicarbene C2 ligands with two unpaired electrons in the carbon-carbon pi-bonding similar to the multiple bond in triplet O2. This contrasts with the opposite (Ti(III)o4(Ti0)i4 configuration in the higher energy form of Th-Ti8C12, corresponding to ethynediyl ligands with full C...C triple bonds and unpaired electrons in the C sp hybrid orbitals for sigma-bonding to Ti.     

Nature of bonding in the sulfuryl group

The SO sulfuryl bond in a number of representative sulfoxides and sulfones has been studied at the B3LYP/6-311+G(d,p) level in the atoms-in-molecules (AIM) approach involving the AIM delocalization index and the Cioslowski-Mixon localized orbitals and associated covalent bond order. The sulfur-oxygen covalent bond is strongly polarized toward oxygen and the oxygen lone pairs provide significant backbonding to create short and strong SO bonds, similar in nature to those found in the analogous phosphoryl (PO) bond. Although the sulfoxides in general have larger delocalization indices than the sulfones, there is no correlation between these quantities and the bond dissociation energies.     

Chemical action and chemical bonding

New chemical bonding paradigm in terms of chemical action functional and of its reformulations by means of electronegativity, linear response and density softness kernels is advanced; it makes no use of traditional molecular orbital bonding analysis while providing reliability in identifying the bonding regions through appropriate specialization of the chemical action variational (conservation) principle along the bond length.     

Chemical bonding in transition metal carbene complexes

In this work, we summarize recent theoretical studies of our groups in which modern quantum chemical methods are used to gain insight into the nature of metal–ligand interactions in Fischer- and Schrock-type carbene complexes. It is shown that with the help of charge- and energy-partitioning techniques it is possible to build a bridge between heuristic bonding models and the physical mechanism which leads to a chemical bond. Questions about the bonding situation which in the past were often controversially discussed because of vaguely defined concepts may be addressed in terms of well defined quantum chemical expressions. The results of the partitioning analyses show that Fischer and Schrock carbenes exhibit different bonding situations, which are clearly revealed by the calculated data. The contributions of the electrostatic and the orbital interaction are estimated and the strength of the σ donor and π acceptor bonding in Fischer complexes are discussed. We also discuss the bonding situation in complexes with N,N-heterocyclic carbene ligands.     

Chemical bonding in isostructural Li-containing ternary chalcogenides

First principle calculations were performed for the first time to study the electronic structure of LiGaTe₂, LiInTe₂, and LiInSe₂ chalcogenides with a chalcopyrite structure. Peculiarities of chemical bonding are discussed and electron density and difference density maps are constructed for crystals and sublattices. Major information about chemical bonding in crystals is conveyed by the difference density. The chemical bond in chalcogenides is a donor-acceptor bond.     

Chemical bonding variations and electron-phonon interactions

A new functional, Psib(Phi), of an electronic state in solids based on the bonding indicator B(tau,tau') in terms of Mulliken's electron partitioning approach has been introduced. Using Psib(Phi), the bonding variations of an electronic state caused by electron-phonon coupling can be studied. With this proposed approach, the differences between the "flat band" states for Hg in coupling to the phonons and the peaklike structure of electron-phonon coupling constants in the q space are well explained.     

Comparative study on the nonadditivity of methyl group in lithium bonding and hydrogen bonding

Quantum chemical calculations at the second‐order Moeller–Plesset (MP2) level with 6‐311++G(d,p) basis set have been performed on the lithium‐bonded and hydrogen‐bonded systems. The interaction energy, binding distance, bond length, and stretch frequency in these systems have been analyzed to study the nonadditivity of methyl group in the lithium bonding and hydrogen bonding. In the complexes involving with NH₃, the introduction of one methyl group into NH₃ molecule results in an increase of the strength of lithium bonding and hydrogen bonding. The insertion of two methyl groups into NH₃ molecule also leads to an increase of the hydrogen bonding strength but a decrease of the lithium bonding strength relative to that of the first methyl group. The addition of three methyl groups into NH₃ molecule causes the strongest hydrogen bonding and the weakest lithium bonding. Although the presence of methyl group has a different influence on the lithium bonding and hydrogen bonding, a negative nonadditivity of methyl group is found in both interactions. The effect of methyl group on the lithium bonding and hydrogen bonding has also been investigated with the natural bond orbital and atoms in molecule analyses. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Chemical bonding evolution on compression of crystals

The electronegativity concept has been used to develop a method of determining effective atomic charges in pressurized crystalline binary compounds from the experimental values of volume, compression modulus and thermochemical characteristics. The data obtained are in accord with available spectroscopic data on the chemical bonding.     

Molecular nitrides containing group 4 and 5 metals: single and double azatitanocubanes

Treatment of [[Ti(eta(5)-C(5)Me(5))(micro-NH)](3)(micro(3)-N)] (1) with the imido complexes [Ti(NAr)Cl(2)(py)(3)] (Ar=2,4,6-C(6)H(2)Me(3)) and [Ti(NtBu)Cl(2)(py)(3)] in toluene affords the single azatitanocubanes [[Cl(2)(ArN)Ti]( micro(3)-NH)(3)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]].(C(7)H(8)) (2.C(7)H(8)) and [[Cl(2)Ti](micro(3)-N)(2)(micro(3)-NH)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]] (3), respectively. Similar reactions of complex 1 with the niobium and tantalum imido derivatives [[M(NtBu)(NHtBu)Cl(2)(NH(2)tBu)](2)] (M=Nb, Ta) in toluene give the single azaheterometallocubanes [[Cl(2)(tBuN)M](micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]] (M=Nb (4), Ta (5)), both complexes react with 2,4,6-trimethylaniline to yield the analogous species [[Cl(2)(ArN)M](micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]].(C(7)H(8)) (Ar=2,4,6-C(6)H(2)Me(3), M=Nb (6.C(7)H(8)), Ta (7.C(7)H(8))). Also the azaheterodicubanes [M[micro(3)-N)(2)(micro(3)-NH)](2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)](2)].2C(7)H(8) [M=Ti (8.2C(7)H(8)), Zr (9.2C(7)H(8))], and [M[(micro(3)-N)(5)(micro(3)-NH)][Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)](2)].2 C(7)H(8) (Nb (10.2C(7)H(8)), Ta (11.2C(7)H(8))) were prepared from 1 and the homoleptic dimethylamido complex [M(NMe(2))(x)] (x=4, M=Ti, Zr; x=5, M=Nb, Ta) in toluene at 150 degrees C. X-ray crystal structure determinations were performed for 6 and 10, which revealed a cube- and double-cube-type core, respectively. For complexes 2 and 4-7 we observed and studied by DNMR a rotation or trigonal-twist of the organometallic ligands [[Ti(eta(5)-C(5)Me(5))(micro-NH)](3)(micro(3)-N)] (1) and [(micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]](1-). Density functional theory calculations were carried out on model complexes of 2, 3, and 8 to establish and understand their structures.     

Chemical bonding effect in electron scattering by gaseous molecules

The experimental intensity of 30 keV electron small angle scattering by a gaseous molecule is much different from the calculation using usual independent atom model. This is due to the rearrangement of electron distribution in a molecule by the formation of chemical bonds, and is called chemical bonding effect (CBE). The molecules studied are mainly hydrocarbons such as methane, acetylene, ethane, etc. and some non-hydrocarbons. The measurement was carried out on both elastic and total scattering and the effect was found for not only elastic but also inelastic scattering. The effect is relatively large for hydrogen rich molecules as H₂O, NH₃ and hydrocarbons, but is essentially related to the number of atoms contained in molecules. The origin of CBE will attribute mainly to the concentration of inner atomic electrons resulting from chemical bonding.     

Chemical bonding in the crystal structure of 1-hydrosilatrane

A study has been made of the crystal and molecular structure of 1-hydrosilatrane HSi(OCH_2CH₂)₃N. The quantum chemical calculations of its crystal structure have been carried out. According to an estimate of the energy, the coordination bond N→Si is by 5 kcal mol^?1 stronger than that in the crystal of 1-methylsilatrane. The charge values calculated within the framework of the topological analysis of the electron density demonstrate that the electron density of the coordination bond N→Si is primarily transferred to the region of the equatorial bonds Si—O and, to a lesser extent, to the bond Si—H. On going from the isolated molecule of 1-hydrosilatrane to its crystal, the interatomic distance N—Si decreases, mainly owing to the weak intermolecular interaction C—H...O.