Strong Orbital Interactions Research Articles

Charge Delocalization in a Cyclometalated Bisruthenium Complex Bridged by a Noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene Ligand

Two ruthenium atoms are covalently connected to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)benzene (tpb) to form a linear Ru-tpb-Ru arrangement. This unique structure leads to appealing electronic properties for the biscyclometalated complex [(tpy)Ru(tpb)Ru(tpy)](2+), where tpy is 2,2';6',2″-terpyridine. It could be stepwise oxidized at substantially low potential (+0.12 and +0.55 V vs Ag/AgCl) and with a noticeably large comproportionation constant (1.94 × 10(7)). In addition to the routinely observed metal-to-ligand charge-transfer transitions, [(tpy)Ru(tpb)Ru(tpy)](2+) displays a separate and distinct absorption band at 805 nm with appreciable absorptivity (ε = 9000 M(-1) cm(-1)). This band is assigned to the charge transition from the Ru-tpb-Ru motif to the pyridine rings of tpb with the aide of density functional theory (DFT) and time-dependent DFT calculations. Complex [(tpy)Ru(tpb)Ru(tpy)](2+) was precisely titrated with 1 equiv of cerium ammonium nitrate to produce [(tpy)Ru(tpb)Ru(tpy)](3+), which shows intense multiple NIR transitions. The electronic coupling parameters H(ab) of individual NIR components are determined to be 5812, 4942, 4358, and 3560 cm(-1). DFT and TDDFT calculation were performed on [(tpy)Ru(tpb)Ru(tpy)](3+) to elucidate its electronic structure and spin density population and the nature of the observed NIR transitions. Electron paramagnetic resonance studies of [(tpy)Ru(tpb)Ru(tpy)](3+) exhibit a discernible rhombic signal with the isotropic g factor of ⟨g⟩ = 2.144. These results point to the strong orbital interaction of tpb with metal centers and that tpb behaves as a redox noninnocent bridging ligand in [(tpy)Ru(tpb)Ru(tpy)](2+). Complex [(tpy)Ru(tpb)Ru(tpy)](3+) is determined to be a Robin-Day class III system with full charge delocalization across the Ru-tpb-Ru motif.

Spectroscopic and theoretical studies of some 2-ethylsulfinyl-(4′-substituted)-phenylthioacetates

The analysis of the IR v CO bands of the 2-ethylsulfinyl-(4′-substituted)-phenylthioacetates 4′-Y-C 6H 4SC(O)CH 2S(O)Et (Y = NO 2 1, Cl 2, Br 3, H 4, Me 5, OMe 6) supported by B3LYP/6-31G(d,p) calculations along with the NBO analysis for 1, 4 and 6 and X-ray analysis for 3, indicated the existence of four gauche ( q-g-syn, g 3-syn , g 1-anti and q-g 2-syn ) conformers for 1– 6. The calculations reproduce quite well the experimental results, i.e. the computed q-g-syn and g 3-syn conformers correspond in the IR spectrum (in solution), to the v CO doublet higher frequency component of larger intensity, while the computed g 1-anti conformer correspond to the v CO doublet lower frequency component (in solution). NBO analysis showed that the n s → π C 1 O 2 ∗ , n O(CO) → σ C 1 – S 3 ∗ , n O(CO) → σ C 1 – C 4 ∗ orbital interactions are the main factors which stabilize the q-g-syn, g 3-syn , g 1-anti and q-g 2-syn conformers for 1, 4 and 6. The n O(CO) → σ C 1 – S 3 ∗ interaction which stabilizes the q-g-syn, g 3-syn and q-g 2-syn conformers into a larger extent than the g 1-anti conformer, is responsible for the larger v CO frequencies of the former conformers relative to the latter one. The q-g-syn, g 3-syn and q-g 2-syn conformers are further stabilized by σ C 4 – S 5 → π CO ∗ (strong), π CO ∗ / σ C 4 – S 5 ∗ , n O(CO) → σ C 6 – H 17 [ Et ] ∗ (weak) and π CO ∗ / σ C 4 – S 5 ∗ (strong) orbital interactions. The g 1-anti conformer is also stabilized by σ C4–S5 → π CO ∗ (strong), π CO/ σ C 4 – S 5 ∗ , n O(CO) → σ C 6 – H 17 [ Et ] ∗ , π C9 C11[Ph] → σ C 4 – H 16 [ α - CH 2 ] ∗ (weak), n O(SO) → σ C 11 – H 23 [ o-Ph ] ∗ (medium) and π CO ∗ / σ C 4 – S 5 ∗ (strong) orbital interactions. The q-g-syn conformer is further stabilized by O ( CO ) δ - ⋯ S ( SO ) δ + attractive Coulombic interaction while the q-g 2-syn conformer is destabilized by the n S5⋯ O ( CO ) δ - repulsive Coulombic interaction. This analysis indicates the following conformer stabilization order: q-g-syn; g 3-syn > g 1-anti ≫ q-g 2-syn . X-ray single crystal analysis of 3 indicates that it assumes in the solid a distorted q-g 2-syn geometry which is stabilized through almost the same orbital and Coulombic interaction which takes place for the q-g 2-syn conformer, in the gas, along with dipole moment coupling and a series intermolecular C H⋯O interactions.

Radiation and Quantum Chemical Studies of Chalcone Derivatives

The reactions of oxidizing radicals ((*)OH, Br(2)(*-), and SO(4)(*-)) with -OH-, -CH(3)-, or -NH(2)-substituted indole chalcones and hydroxy benzenoid chalcones were studied by radiation and quantum chemical methods. The (*)OH radical was found to react by addition at diffusion-controlled rates (k = 1.1-1.7 x 10(10) dm(3) mol(-1) s(-1)), but Br(2)(*-) radical reacted by 2 orders of magnitude lower. Quantum chemical calculations at the B3LYP/6-31+G(d,p) level of theory have shown that the (C2-OH)(*), (C11-OH)(*), and (C10-OH)(*) adducts of the indole chalcones and the (C7-OH)(*) and (C8-OH)(*) adducts of the hydroxy benzenoid chalcones are more stable with DeltaH = -39 to -28 kcal mol(-1) and DeltaG = -32 to -19 kcal mol(-1). This suggests that (*)OH addition to the alpha,beta-unsaturated bond is a major reaction channel in both types of chalcones and is barrierless. The stability and lack of dehydration of the (*)OH adducts arise from two factors: strong frontier orbital interaction due to the low energy gap between interacting orbitals and the negligible Coulombic repulsion due to small absolute values of Mulliken charges. The transient absorption spectrum measured in the (*)OH radical reaction with all the indole chalcone derivatives exhibited a maximum at 390 nm, which is in excellent agreement with the computed value (394 nm). The formation of three phenolic products under steady-state radiolysis is in line with the three stable (*)OH adducts predicted by theory. Independent of the substituent, identical spectra (lambda(max) = 330-360 and approximately 580 nm) were obtained on one-electron oxidation of the three indole chalcones. MO calculations predict the deprotonation from the -NH group is more efficient than from the substituent due to the larger electron density on the N1 atom forming the chalcone indolyl radical. Its reduction potential was determined to be 0.56 V from the ABTS(*-)/ABTS(2-) couple. In benzenoid chalcones, the (*)OH adduct spectrum is characterized by a peak at 270 nm and a broad maximum centered in the range 430-450 nm with an intense bleaching at 340 nm. The spectrum formed by electron transfer in these derivatives with lambda(max) = 280 and 380 nm (epsilon(280) = 5000 dm(3) mol(-1) cm(-1) and epsilon(380) = 700 dm(3) mol(-1) cm(-1)) was assigned to its phenoxyl radical. Our pulse radiolysis experiments in combination with quantum chemical calculations demonstrate that chalcones are efficient scavengers of damaging oxyl radicals.

The tri–μ–hydrido–bis[(η5–C5Me5)aluminum(III)] theoretical study, the assets of sandwiched M2H3 (M of 13th group elements) stability

The stability of the tri-mu-hydrido-bis[(eta(5)-C(5)Me(5))aluminum], Cp*(2)Al(2)H(3), 1 is studied at B3LYP/6-311+G(d,p), CCSD(T)//B3LYP/6-311+G(d,p) and MP4//B3LYP/6-311+G(d,p) levels. The coordination between Al(2)H(3) entity and both C(5)(CH(3))(5) groups is ensured by strong electrostatic and orbital interactions. The orbital analysis of the interacting fragments shows that Al(2)H(3) acceptor, which keeps its tribridged structure, implies the vacant [Formula: see text] and five antibonding ([Formula: see text], e' and e'') molecular orbitals to interact with two orbitals mixtures, b(1) and e" of the donors (C(5)Me(5)). When we take into account the solvent effect, the computation shows that 1 seems to be stable in condensed phase with a tribridged bond between the Al atoms [Cp*Al(micro-H)(3)AlCp*], whereas in the gas phase, the monobridged Cp*AlH(micro-H)AlHCp* 4 is slightly favored (4 kcal mol(-1)). We propose that 1 could be prepared thanks to Cp*Al (2) and Cp*AlH(2) (3) reaction in acidic medium. The experimental treatment of this type of metallocenes would contribute to the development of the organometallic chemistry of 13th group elements.

Unusual Interchromophoric Interactions in β,β′ Directly and Doubly Linked Corrole Dimers: Prohibited Electronic Communication and Abnormal Singlet Ground States

Directly and doubly beta,beta'-linked corrole dimers (DH(3)CD, DH(2)CD, and DZnCD) are excellent platforms for the investigation of intercorrole interactions because of their enforced coplanar geometries and short edge-to-edge distances. Through the use of these molecules along with the reference monomer H(3)CM and the singly beta,beta'-linked corrole dimer SH(3)CD, the intercorrole interactions have been systematically studied by density functional theory calculations, ultrafast photophysical measurements, and two-photon absorption measurements. A particular focus was placed on revealing factors that are important for the induced photophysical properties of the doubly linked corrole dimers compared with corrole monomer. In the doubly linked corrole dimers, strong molecular orbital interactions caused by the coplanar geometry and the short interchromophoric distance give rise to perturbations of the electronic states that are responsible for the red-shifted and intensified Q-like band in DH(3)CD and the broad NIR absorption bands and fast excitation-energy relaxation processes in DH(2)CD and DZnCD. On the other hand, electronic communication between corrole units is prohibited by the structurally constrained octagonal core in the center, so each constituent corrole unit in the doubly linked corrole dimers maintains an intrinsic pi-conjugation system. Consequently, the overall aromaticity of the directly linked corrole dimers can be explained in terms of a linear sum of two constituent corrole monomers, and the singlet biradical character of DH(2)CD and DZnCD can be understood in terms of two unpaired electrons (one from each constituent oxidized corrole monomer) and their appropriate interaction. In addition, the nonlinear optical properties of DH(2)CD and DZnCD with singlet biradical character have been confirmed to be significantly enhanced compared with those of closed-shell DH(3)CD. Collectively, double beta,beta'-linkages of corroles provide the coplanar geometry with a short interchromophoric distance and the strained octagonal core that play key roles in allowing the strong molecular orbital interactions and restricting the electronic communication between the two corroles, respectively.

Donor Acceptor Complexes of Noble Gases

Donor-acceptor (DA) complexes of noble gases (Ng) of the general type A <-- Ng <-- D (A = Lewis acid, D = Lewis base) have been theoretically studied using ab initio and DFT methods. Chemical bonding in these compounds is realized via a 3-center 4-electron bond, which is formed by a lone pair of the noble gas, a lone pair of the donor molecule and a vacant orbital of the acceptor molecule. Detailed bonding analysis of the model compounds F(3)Al-Ng-NH(3) reveals that Ng-ammonia interaction is repulsive due to Pauli repulsion. Bonding interaction between Ng and N is mostly electrostatic. In contrast, strong orbital interactions are responsible for the attractive interactions between Ng and AlF(3). Due to the repulsive interactions with the donor molecule and a sizable reorganization energy of the acceptor molecule, optimization attempts of the A <-- Ng <-- D compounds, which feature individual donor and acceptor molecules, always lead to the dissociation of the complex and eventual formation of free Ng. To overcome this obstacle, the concept of a rigid C(3v) symmetric cryptand-type ligand, which features spacially separated pyramidalized donor and acceptor fragments, is introduced. Such "push-pull" ligands are predicted to exothermically form complexes with noble gases. These are the first examples of the thermodynamically stable Ar and Kr compounds. Application of the push-pull cryptand ligands featuring multiple (two and three) donor-acceptor induced chemical bonds is expected to yield stable complexes with virtually any electron-rich element in the periodic table.

THEORETICAL STUDY OF NITROGEN MONOXIDE ADSORPTION ON RHODIUM CLUSTERS AT DIFFERENT SITES

The adsorption of nitrogen monoxide NO with charged and neutral [Formula: see text] clusters at atop, bridge, and threefold hollow sites had been investigated by density functional theory calculations. The results showed that rhodium clusters had strong orbital interactions with NO and formed the complex [ Rh n NO ]-/0/+. The stretching vibrational frequencies of the N–O bonds changed with the different adsorption sites and clusters sizes. The interactions between rhodium clusters and NO molecular could be described through the donation and back-donation of their frontier orbitals. The more back donation from Rh to NO , the weaker the N–O bonds, exhibiting that the lengthening of the N–O bond length and the lowering of its vibrational frequency. In general, the donation and back-donation interactions followed the tendencies: anionic &gt; neutral &gt; cationic, big size &gt; small size, threefold hollow site &gt; bridge site &gt; atop site.

Electron−Phonon Interactions and Jahn−Teller Effects in the Monocation of Corannulene

Electron-phonon interactions in the monocation of corannulene are studied by using the hybrid Hartree-Fock (HF)/density-functional-theory (DFT) method in the Gaussian 98 program package. The C-C stretching mode of 1498 cm(-1) most strongly couples to the e1 highest occupied molecular orbitals (HOMO) in corannulene. The total electron-phonon coupling constant for the monocation (l(HOMO)) of corannulene is estimated to be 0.165 eV. The l(HOMO) value for corannulene is much larger than those for coronene and acenes with similar numbers of carbon atoms. The delocalized electronic structures and the intermediate characteristics between the strong sigma-orbital interactions and weak pi-orbital interactions originating from a bowl-shaped C(5v) geometry are the main reason that the l(HOMO) value for corannulene is much larger than those for planar D(6h) symmetric pi-conjugated coronene and D(2h) symmetric pi-conjugated acenes with similar numbers of carbon atoms. The electron transfer in the positively charged corannulene is also discussed. Intramolecular electron mobility (sigma(intra,monocation)) in the positively charged corannulene is estimated to be smaller than those for the positively charged pi-conjugated acenes and coronene. The reorganization energy for the positively charged corannulene (0.060 eV) is estimated to be larger than those for the positively charged acenes and coronene. The strong orbital interactions between two neighboring carbon atoms in the HOMO of corannulene with the bowl-shaped structure are the main reasons for the calculated results. Thus, the larger overlap integral between two neighboring molecules is needed for the positively charged corannulene to become a better conductor than those for positively charged coronene and acenes. The smaller density of states at the Fermi level n(0) values are enough for the conditions of the attractive electron-electron interactions to be realized in the monocation of corannulene than in the monocations of coronene and acenes with similar numbers of carbon atoms. The multimode problem is also treated in order to investigate how consideration of the multimode problem is closely related to the characteristics of the electron-phonon interactions.

Computer-assisted design of asymmetric 1,3-dipolar cycloadditions between dimethylvinylborane and chiral nitrones

This report describes a theoretical study of the 1,3-dipolar cycloadditions between dimethylvinylborane and different chiral nitrones, aimed at identifying the requisite structural features to achieve high levels of selectivity. Excellent regioselectivities towards the formation of the 5-borylisoxazolidines are computed in all cases due to the presence of a strong secondary orbital interaction between the boron of the vinylborane and the oxygen of the nitrones. While these transition structures show [3+3] character, 4-boryl regioisomeric structures have classical [3+2] character with weak carbon–boron secondary orbital interactions. Endo transition structures leading to the favoured products adopt chair-like conformations only, unlike their exo counterparts, which exhibit either boat or twist-boat structures. Placing a stereocentre adjacent to the nitrone nitrogen appears to be an effective strategy to stereodiscriminate the anti and syn approaches of the dipolarophile.

Vibronic interactions and possible electron pairing in positively charged cyanodienes

The conditions under which the attractive electron-electron interactions are realized in the monocations of sigma-conjugated cyanodienes such as C(6)N(4)H(4), C(8)N(6)H(4), and C(10)N(8)H(4) and of pi-conjugated acenes are discussed. The total electron-phonon coupling constants for the monocations l(HOMO) of cyanodienes are much larger than those for the monocations of acenes. The strong sigma orbital interactions between two neighboring atoms in the highest occupied molecular orbitals (HOMO) of sigma-conjugated cyanodienes are the main reason for the calculated results. Furthermore, we discuss how the conditions under which the monocation crystals become good conductor are related to the molecular size. Both the l(HOMO) values and the reorganization energies between the neutral molecules and the monocations decrease with an increase in molecular size in cyanodienes. The calculated results for the sigma-conjugated cyanodienes are compared with those for the pi-conjugated acenes in order to investigate how the CH-N substitutions in cyanodienes are closely related to the l(HOMO) values and the reorganization energies. Both the l(HOMO) and the reorganization energies in the positively charged sigma-conjugated cyanodienes are much larger than those in the positively charged pi-conjugated acenes. This means that in order to become good conductors, the positively charged sigma-conjugated cyanodienes need larger overlap integral between two adjacent molecules than the positively charged pi-conjugated acenes. On the other hand, since the l(HOMO) values for cyanodienes are much larger than those for acenes, the condition of attractive electron-electron interactions is more easily to be realized in the monocations of cyanodienes than in the monocations of acenes. It is suggested that the positively charged sigma-conjugated cyanodienes cannot easily become good conductors, but the conditions under which the electron-electron interactions become attractive are realized more easily in the positively charged sigma-conjugated cyanodienes than in the positively charged pi-conjugated acenes.

The essential role of the electronegativity perturbation in vibronic interactions in positively charged B, N-substituted acenes

Total electron–phonon coupling constants for the monocations ( l HOMO) of B, N-substituted acene-series such as B 3N 3H 6, B 5N 5H 8, and B 7N 7H 10 are estimated. The estimated l HOMO values of 0.357, 0.209, and 0.182 eV for B 3N 3H 6, B 5N 5H 8, and B 7N 7H 10, respectively, are larger than those of 0.244, 0.173, and 0.130 eV for benzene, naphthalene, and anthracene, respectively. Furthermore, the l HOMO value for B, N-substituted polyacene (0.096 eV) is estimated to be larger than that for polyacene (0.036 eV). Stronger orbital interactions between B and N atoms in the highest occupied molecular orbitals (HOMO) in B, N-substituted acene-series than those between two neighboring carbon atoms in the HOMO in acene-series as a consequence of electronegativity perturbation on acene-series, are the main reason why the l HOMO values for B, N-substituted acene-series are larger than those for acene-series.

Orbital and spin physics in LiNiO2 and NaNiO2

We derive a spin–orbital Hamiltonian for a triangular lattice of eg orbital degenerate (Ni3+) transition-metal ions interacting via 90° superexchange involving (O2−) anions, taking into account the onsite Coulomb interactions on both the anions and the transition metal ions. The derived interactions in the spin–orbital model are strongly frustrated, with the strongest orbital interactions selecting different orbitals for pairs of Ni ions along the three different lattice directions. In the orbital-ordered phase, favoured in mean field theory, the spin–orbital interaction can play an important role by breaking the U(1) symmetry generated by the much stronger orbital interaction and restoring the three-fold symmetry of the lattice. As a result, the effective magnetic exchange is non-uniform and includes both ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic interactions still dominate, this offers yet insufficient explanation for the absence of magnetic order and the low-temperature behaviour of the magnetic susceptibility of stoichiometric LiNiO2. The scenario proposed to explain the observed difference in the physical properties of LiNiO2 and NaNiO2 includes small covalency of Ni–O–Li–O–Ni bonds inducing weaker interplane superexchange in LiNiO2, insufficient to stabilize orbital long-range order in the presence of stronger intraplane competition between superexchange and Jahn–Teller coupling.

Structures and its dipole moments of half-sandwich type metal–benzene (1:1) complexes determined by 2-m long electrostatic hexapole

The transition metal–benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti–C6H6 and V–C6H6 are larger than the ones of Co–C6H6 and Ni–C6H6. Such large dipole moments of Ti–C6H6 and V–C6H6 can be ascribed to the strong orbital interaction due to charge transfer between the metal atom and benzene molecule.

Investigation of Interporphyrin Charge Resonance of Dihedral Angle Controlled Porphyrin Dimers by Resonance Raman Spectroscopy and MO Approaches

Strongly interacting porphyrin dimers as a form of cofacial or side-to-side linkage have been ideally suited for investigation on the structure−electronic property relationship in porphyrin dimers. The relative orientation and interchromophore distance determined by the dihedral angle between the two porphyrin units are key factors in controlling the interchromophoric interaction. In the present work, directly linked porphyrin dimers with a systematic change in the dihedral angle between the two porphyrin planes have been investigated by resonance Raman (RR) and emission spectroscopies. Extremely strong molecular orbital interactions between the porphyrin units by decreasing the dihedral angle give rise to strongly enhanced interporphyrin charge-resonance transitions. With a decrease of the dihedral angle, unique RR spectra of the porphyrin dimers were also observed depending on the excitation wavelengths of 406.7, 457.9, and 488.0 nm. With the aid of supermolecular MO approaches it was found that the cha...

A study on the correlation between electronic structures of RENi 5 and their hydrides and hydrogen absorption properties

The electronic structures of RENi 5 (RE=La, Ce, Pr, Nd) and their hydrides were investigated by the SCC-DV-X α (Self-Consistent-Charge Discrete Variational X α ) method. The results show that the stabilities of RENi 5 hydrides are related to the charge transferred to the hydrogen atom, and decrease with increase of the transferred charge. There is a strong orbital interaction between the 3d orbit in the Ni atom and the 4f orbit in the RE atom, which is weakened when the hydride is formed. The equilibrium plateau pressure of RENi 5 rises with lowering of the Fermi energy of the hydride.

Theoretical investigation of hydrogen bonded water molecules in Ca(IO 3) 2·6H 2O

Routine quantum chemical calculations, possibly together with routine vibrational spectroscopic experiments, seem to yield more reliable OH bond lengths of crystal water molecules than routine neutron diffraction experiments. With OH being the different OH bonds in the crystal water molecules and H⋯O′ the nearly linear hydrogen bonds to adjacent iodate oxygens, the R OH- ω OH- R H⋯O′ variations have correlation coefficients better than 0.95. In contrast to infrared intensities, the Raman intensities are only moderately influenced by the crystal field. This supports the ‘oriented gas molecule model’ for estimating Raman intensities. The agreement between theoretical and measured intensities, however, is disappointing. This may indicate strong orbital interactions between the water molecules and the iodate and calcium ions.

Theoretical Studies on the Gas-Phase Nucleophilic Ring Opening of 3,4-Epimino-, 3,4-Epoxy-, and 3,4-Epithio-1-butene

The ring opening reaction of 3,4-epimino-1-butene (EIP), 3,4-epoxy-1-butene (EOB) and 3,4-epithio-1-butene (ETB) initiated by the attack of hydroxide ion at three carbons (Cα, Cγ, and Cδ) have been investigated using MP2/6-31+G*//MP2/6-31+G* ab initio molecular orbital theory. The CαCβCγCδX process by the α-attack is favored by the strong frontier orbital interactions due to the low π* orbital, but is disfavored by a lower degree of progress in the developing π orbital as well as in the ring opening. In contrast, the processes by the γ-and δ-attacks are disfavored by the high σ* orbitals, but are favored by a greater strain energy release in a greater degree of ring opening in the transition state. The barriers to the ring opening processes are found to depend on the strain energy of the three-membered heterocycles, bond strength of the C−X bonds, frontier orbital (π*, σ*(1), and σ*(2)) levels, exothermicity of the reaction, and steric and entropy effects.

Semiempirical electronic-structure calculations of hydrogen-phosphorus-vacancy complexes in crystalline silicon

The author reports on self-consistent calculations for the electronic structure of three hydrogen-phosphorus-vacancy complexes in crystalline silicon, based on a semiempirical tight-binding theory. These complexes, denoted by V-1H-3P, V-2H-2P, and V-3H-1P, consist of a vacancy and substitutional phosphorus impurities on the nearest-neighbour sites of the vacancy. The remaining silicon dangling bonds in the complexes are all saturated by hydrogen atoms. Each of the complexes is described by a large repeated supercell and the authors use the recursion method for computing local densities of states and orbital electron occupancies. They find that the three complexes do not introduce energy levels into the fundamental band gap, revealing that the electrical activity of the silicon dangling bonds can be well passivated by hydrogen atoms at bonding positions through strong orbital interactions and by phosphorus atoms at substitutional positions through Coulomb attractions. However, the complexes can have defect resonant states outside of the band gap. The authors show that these resonant states can be well described in terms of the symmetric combinations of the phosphorus orbitals and the symmetric combinations of the hydrogen and the hydrogen-saturated silicon orbitals.

Cyclic Acetals. Structural Analysis of 1,3‐Dioxepine and Related Compounds

AbstractThe He(I) PE spectra of 1,3‐dioxepine (1) and the related bicyclic acetals 1,3,7,7‐tetramethyl‐2,11‐dioxabicyclo[4.41]undeca‐3,5‐dien‐10‐one (2) and 1,3,7,7‐tetramethyl‐10‐methylidene‐2,11‐dioxabicyclo[4.4.1]undeca‐3,5‐diene (3) have been recorded and analyzed. Interpretation of the PE data indicate strong orbital interactions between the diene and acetal moieties in 1. Different conformations have been evaluated by a joint MMP2 molecular mechanics and ab initio molecular orbital analysis, which shows 1 to prefer a slightly boat‐shaped conformation with the diene part being planar and the CH2 group placed considerably out‐of‐plane. The bicyclic acetals 2 and 3 show a frontier orbital picture similar to 1 superimposed with the characteristics of the CO and CC groups, respectively. The crystal structure of the keto‐acetal 2 was determined by an X‐ray analysis.

Excited states of mixed-ligand chelate complexes of ruthenium(II). A re-examination of the evidence for strong interligand coupling

An investigation of the effects of the coupling between dissimilar ligands in the excited states of the complexes [Ru(bipy)n(phen)3–n]2+, [Ru(bipy)n(4,7-Ph2phen)3–n]2+ and [Ru(bipy)n(5-NO2phen)3–n]2+(n= 0, 1, 2 or 3)(bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline) is presented. Proton magnetic resonance measurements upon the ground states of the complexes [Ru(bipy)n(phen)3–n]2+ show that the chemical shifts of the 3,3′-protons of bipy and the 4,7-protons of phen do not depend in a significant manner upon the ligand composition. Excited-state lifetime determinations for all three mixed-ligand series at 298 K in aqueous solution suggest that the coupling between dissimilar ligands is strong enough to produce kinetic effects. Simulations of the emission spectra of {[Ru(bipy)n(phen)3–n]2+}*(n= 1 or 2) show that the shifts in experimental band maxima in this series can be reproduced by a simple weighted addition of the emission profiles of {[Ru(bipy)3]2+}* and {[Ru(phen)3]2+}*, demonstrating that the shifts are not necessarily symptomatic of strong interligand orbital interactions. Finally, the variation of the absorption spectra in the wavelength region 240–300 nm across the series [Ru(bipy)n(phen)3–n]2+(n= 0, 1, 2 or 3) suggests that the π→π* transitions characteristic of [Ru(bipy)3]2+ and [Ru(phen)3]2+ maintain their discrete identity in the mixed-ligand complexes, an observation from which the presence of ligand-localised excited electronic states may be inferred. These apparently conflicting observations are reconciled through a kinetic model which demonstrates that coupling may occur on a time-scale which is comparable with or significantly shorter than the excited-state lifetimes and still be too weak to cause detectable spectroscopic shifts.