Structural Changes, P−P Bond Energies, and Homolytic Dissociation Enthalpies of Substituted Diphosphines from Quantum Mechanical Calculations

The molecular structures of the diphosphines P2[CH(SiH3)2]4, P2[C(SiH3)3]4, P2[SiH(CH3)2]4, and P2[Si(CH3)3]4 and the corresponding radicals P[CH(SiH3)2]2, P[C(SiH3)3]2, P[SiH(CH3)2]2, and P[Si(CH3)3]2 were predicted by theoretical quantum chemical calculations at the HF/3-21G*, B3LYP/3-21G*, and MP2/6-31+G* levels. The conformational analyses of all structures found the gauche conformers of the diphosphines with C2 symmetry to be the most stable. The most stable conformers of the phosphido radicals were also found to possess C2 symmetry. The structural changes upon dissociation allow the release of some of the energy stored in the substituents and therefore contribute to the decrease of the P−P bond dissociation energy. The P−P bond dissociation enthalpies at 298 K in the compounds studied were calculated to vary from −11.4 kJ mol-1 (P2[C(SiH3)3]4) to 179.0 kJ mol-1 (P2[SiH(CH3)2]4) at the B3LYP/3-21G* level. The MP2/6-31+G* calculations predict them to be in the range of 52.8−207.9 kJ mol-1. All the values are corrected for basis set superposition error. The P−P bond energy defined by applying a mechanical analogy of the flexible substituents connected by a spring shows less variation, between 191.3 and 222.6 kJ mol-1 at the B3LYP/3-21G* level and between 225.6 and 290.4 kJ mol-1 at the MP2/6-31+G* level. Its average value can be used to estimate bond dissociation energies from the energetics of structural relaxation

Molecular Geometry of Benzaldehyde and Salicylaldehyde: A Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Investigation

The molecular geometries of benzaldehyde and salicylaldehyde have been determined by gas-phase electron diffraction and ab initio molecular orbital calculations at the MP2(FC)/6-31G* level. Several parameter differences from the molecular orbital calculations were incorporated as constraints in the electron diffraction analysis of salicylaldehyde. Some selected bond lengths (rg) and angles obtained in the electron diffraction analyses are as follows:  benzaldehyde (C−H)mean 1.095 ± 0.005 Å; (C−C)mean (benzene) 1.397 ± 0.003 Å; C2−C7 1.479 ± 0.004 Å; CO 1.212 ± 0.003 Å; C2−C7O 123.6 ± 0.4°; the benzene ring is undistorted within experimental error; salicylaldehyde (C−H)mean 1.090 ± 0.011 Å; (C−C)mean (benzene) 1.404 ± 0.003 Å; C1−C2 1.418 ± 0.014 Å; C−O 1.362 ± 0.010 Å; O−H 0.985 ± 0.014 Å; C2−C13 1.462 ± 0.011 Å; CO 1.225 ± 0.004 Å; C2−C13O 123.8 ± 1.2°; C2−C1−O 120.9 ± 1.1°. All the data are consistent with planar equilibrium structures for both molecules. The barrier to formyl group torsion is estimated to be appreciably higher for salicylaldehyde (at least 30 kJ/mol) than for benzaldehyde (at least 20 kJ/mol). There is intramolecular hydrogen bonding in the salicylaldehyde molecule of comparable strength with that in o-nitrophenol. The hydrogen bond is characterized by the following observed/calculated distances:  O···H(−O) 1.74(2)/1.80 Å and O···O 2.65(1)/2.68 Å. The structural changes in the rest of the molecule, as compared with the parent benzaldehyde and phenol molecules, are consistent with resonance-assisted hydrogen bonding similar to the o-nitrophenols. These changes include a lengthening of the CO bond (0.013 Å), a shortening of the exocyclic C−C bond (0.020 Å), a lengthening of the ring C−C bond between the substituents (0.017 Å), and a shortening of the hydroxy C−O bond (0.022 Å)

Intramolecular Hydrogen Bonding and Molecular Structure of 2,5-Dihydroxyterephthalaldehyde and 4,6-Dihydroxyisophthalaldehyde: A Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Study

The molecular structure of 2,5-dihydroxyterephthalaldehyde has been determined from a joint electron diffraction/ab initio investigation, and the molecular structure of 4,6-dihydroxyisophthalaldehyde has been obtained from ab initio calculations at the MP2/6-31G* level. There is considerable intramolecular hydrogen bonding in these structures manifested by the O···H and O···O distances as well as by the structural changes in the rest of the molecule. These changes are consistent with the notion of resonance-assisted hydrogen bonding. The hydrogen bonding is somewhat stronger in 4,6-dihydroxyisophthalaldehyde than in 2,5-dihydroxyterephthalaldehyde, and this difference may be linked to the difference in the mutual positioning of the interacting formyl and hydroxy groups in these molecules

Controlled Radiation Damage and Edge Structures in Boron Nitride Membranes

We show that hexagonal boron nitride membranes synthesized by chemical exfoliation are more resistant to electron beam irradiation at 80 kV than is graphene, consistent with quantum chemical calculations describing the radiation damage processes. Monolayer hexagonal boron nitride does not form vacancy defects or amorphize during extended electron beam irradiation. Zigzag edge structures are predominant in thin membranes for both a freestanding boron nitride monolayer and for a supported multilayer step edge. We have also determined that the elemental termination species in the zigzag edges is predominantly N

Controlled Radiation Damage and Edge Structures in Boron Nitride Membranes

We show that hexagonal boron nitride membranes synthesized by chemical exfoliation are more resistant to electron beam irradiation at 80 kV than is graphene, consistent with quantum chemical calculations describing the radiation damage processes. Monolayer hexagonal boron nitride does not form vacancy defects or amorphize during extended electron beam irradiation. Zigzag edge structures are predominant in thin membranes for both a freestanding boron nitride monolayer and for a supported multilayer step edge. We have also determined that the elemental termination species in the zigzag edges is predominantly N

Three-Membered Ring or Open Chain Molecule − (F₃C)F₂SiONMe₂ a Model for the α-Effect in Silicon Chemistry

(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1)°, gas anti 84.4(32)° and gauche 87.8(20)°] and short Si···N distances [solid 1.904(2) Å]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si−N dative bond, as upon formation of the Si···N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si−NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si−N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si···N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an α-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms

Corrosion of Gold by a Nanoscale Gold and Copper Beltlike Structure

We report the observation of corrosion of gold at the nanoscale, in which nanometer-sized gold can be corroded in the air in the presence of copper at room temperature during ageing to form a compound of gold with residual copper. The compound is a nanoscale beltlike two-dimensional structure that grows from gold nanoparticles. Using high-resolution electron microscopy and chemical analyses, we show that the structure consists of alternating rows of gold and copper atoms. The atomic columns of gold are arranged in rows separated by 0.5 nm, and the structure extends in a perpendicular direction to these. Density functional theory calculations of an atomic model of this two-dimensional material consisting of gold, copper, and oxygen suggest that it is a narrow bandgap semiconductor. This beltlike structure, growing around Au fine wire bonding in nanodevices, may cause their failure in the electric contact in nanodevices. Thus, this study has direct relevance to the use of gold as a contact material in semiconductor devices. In addition, possible future applications of the observed structures as additives in organic solar cells are discussed

Three-Membered Ring or Open Chain Molecule − (F₃C)F₂SiONMe₂ a Model for the α-Effect in Silicon Chemistry

(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1)°, gas anti 84.4(32)° and gauche 87.8(20)°] and short Si···N distances [solid 1.904(2) Å]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si−N dative bond, as upon formation of the Si···N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si−NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si−N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si···N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an α-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms

Molecular Structures of <i>a</i><i>rachno</i>-Decaborane Derivatives 6,9-X₂B₈H₁₀ (X = CH₂, NH, Se) Including a Gas-Phase Electron-Diffraction Study of 6,9-C₂B₈H₁₄

The molecular structures of the three heterodecaboranes arachno-6,9-C2B8H14, arachno-6,9-N2B8H12, and arachno-6,9-Se2B8H10 have been determined by ab initio MO theory. In addition, the structure of arachno-6,9-C2B8H14 was experimentally determined using gas-phase electron diffraction (GED). The accuracy of all four of these structures has been confirmed by the good agreement of the 11B chemical shifts calculated at the GIAO-MP2 level with the experimental values. A comparison of the GIAO-HF and GIAO-MP2 methods shows that for these heteroborane clusters, electron correlation effects on the computed δ(11B) values are quite substantial and that it is necessary to go beyond the HF level in the NMR computation