Showing papers on "HOMO/LUMO published in 1996"
TL;DR: In this article, the authors measured the currentvoltage and electroluminescence characteristics of single-heterojunction, vacuum-deposited organic light-emitting devices (OLEDs) over a wide range of materials, temperatures, and currents.
Abstract: We measure the current–voltage and electroluminescencecharacteristics of single‐heterojunction, vacuum‐deposited organic light‐emitting devices(OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate‐based devices,electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.
824 citations
TL;DR: In this paper, a synthetic strategy that provides access to oligopyridine-based ditopic ligands bridged by an alkyne spacer comprising one to four ethynyl groups is described.
279 citations
TL;DR: In this paper, the origin of the unusually long UV-vis absorption maximum, λmax = 398 nm, of 3,3′, 4,4,4′-tetraphenyl-2,2′-bisilole was investigated.
Abstract: The purpose of this work is the theoretical elucidation of the origin of the unusually long UV-vis absorption maximum, λmax = 398 nm, of 3,3′,4,4′-tetraphenyl-2,2′-bisilole, which we have observed ...
217 citations
TL;DR: In this paper, the semi-empirical tight-binding method was used to study the electronic structure of spherical PbS nanocrystals, and the calculated band gaps were found to agree well with previously published experimental values.
Abstract: The semiempirical tight-binding method was used to study the electronic structure of spherical PbS nanocrystals. The effects of spin-orbit coupling were included, and the calculated band gaps were found to agree well with previously published experimental values for PbS clusters. The size dependence of the band gaps was studied for clusters containing as many as 912 atoms (35-A diameter). Direct diagonalization was used for small clusters, and Lanczos recursion was used to determine the band gaps and the eigenfunctions for the larger clusters. Analysis of the eigenfunctions revealed that the HOMO and LUMO states were spread throughout the cluster. Densities of states for the PbS nanoclusters converged to the bulk density of states, and the joint densities of states were computed as an approximation to the absorption spectra of the nanoclusters.
204 citations
TL;DR: In this paper, a plausible degradation mechanism of aluminum(III) 8-hydroxyquinoline (Alq3)-based devices is presented, initiated by traces of moisture that facilitate a ligand exchange with the oxyquinoline moiety of Alq3, followed by a condensation polymerization of the freed 8-Hq to produce a deep brown, nonemissive polymeric material and water.
Abstract: A plausible degradation mechanism of aluminum(III) 8-hydroxyquinoline (Alq3)-based devices is presented. This is initiated by traces of moisture that facilitate a ligand exchange with the oxyquinoline moiety of Alq3, followed by a condensation polymerization of the freed 8-hydroxyquinoline (8-Hq) to produce a deep brown, nonemissive polymeric material and water (which can in turn free more 8-Hq). The polymer was found to be an efficient photoluminescence and electroluminescence quencher due to the introduction of nonemissive midgap states within the Alq3 HOMO/LUMO levels.
193 citations
TL;DR: In this article, low-temperature optical absorption, circular dichroism, magnetic circular dichroidism, and sulfur K-edge X-ray absorption spectra have been measured for the green "blue" copper center (type 1) in Achromobacter cycloclastes nitrite reductase.
Abstract: Low-temperature optical absorption, circular dichroism, magnetic circular dichroism, and sulfur K-edge X-ray absorption spectra have been measured for the green “blue” copper center (type 1) in Achromobacter cycloclastes nitrite reductase. Combined with density functional calculations, the results of these spectroscopies have been used to define the extremely “perturbed” electronic structure of this site relative to that of the prototypical “classic” site found in plastocyanin. Experimentally calibrated density functional calculations have been further used to determine the specific geometric distortions which generate the perturbed electronic structure. These studies indicate that the principal electronic structure changes in nitrite reductase, relative to plastocyanin, are a rotation of the Cu dx2-y2 half-filled, highest occupied molecular orbital (HOMO) and an increase in the ligand field strength at the Cu center. The HOMO rotation increases the pseudo-σ interaction and decreases the π interaction of ...
188 citations
TL;DR: In this article, a linear correlation between the logarithm of the decomposition rate constant and the position of the highest occupied molecular orbital (HOMO)-level of the photosensitizers is found.
Abstract: Unsymmetrical zinc(II) complexes of benzonaphthoporphyrazines 5a-12a bearing between one and eight pyridyloxy substituents are synthesized by statistical tetramerization of 6-(1,1-dimethylethyl)-2,3-naphthalenedicarbonitrile (1) with 4-(3-pyridyloxy)- or 4,5-bis-(3-pyridyloxy)-1,2-benzenedicarbonitrile (2, 3). Methylation of 5a-12a leads to the catianic pyridyloxybenzonaphthoporphyrazines 5b-12b having between one to eight positive charges. The Q-band transition in the visible spectra exhibits a bathochromic shift from 680 to 760 nm dependent upon the number of annelated naphthalene rings. The singlet oxygen quantum yields of the benzonaphthoporphyrazines determined by the dye-sensitized photooxidation of 1,3-diphenylisobenzofurane is surprisingly high (in the range of zinc phthalocyanine). The photooxidative stabilities of the photosensitizers described quantitatively by first-order kinetics decrease with the number of annelated naphtho groups. A linear correlation between the logarithm of the decomposition rate constant and the position of the highest occupied molecular orbital (HOMO)-level of the photosensitizers is found. Destabilization of the HOMO leads to a decrease of the photostability. Due to their adjustable long wavelength absorption, their intramolecular polarity axis and their different hydrophilic/hydrophobic character, these novel compounds may be suitable photosensitizers for the photodynamic therapy of cancer.
137 citations
TL;DR: UV/vis spectra recorded during stepwise oxidation of the dication to the +3 and +4 states indicate that the mixed-valence state is valence-trapped (class II); the EPR spectrum of the reduced species is consistent with a largely ligand-based radical.
Abstract: Complex 1 was prepared from [Ru(bipy)2Cl2]·2H2O and K2L [bipy = 2,2‘-bipyridine; H2L = 2,5-dihydroxy-1,4-benzoquinone]. It undergoes two reversible one-electron oxidations separated by 0.34 V (Kc ≈ 6 × 105 for the mixed-valence state), and a one-electron reduction. A molecular orbital analysis showed that the HOMO is delocalized between the metals and the bridging ligand, so the oxidations have both metal and ligand character. The LUMO in contrast is largely based on the bridging ligand, and the EPR spectrum of the reduced species is consistent with a largely ligand-based radical. UV/vis spectra recorded during stepwise oxidation of the dication to the +3 and +4 states indicate that the mixed-valence state is valence-trapped (class II).
123 citations
TL;DR: In this article, the authors used ab initio calculations to systematically design and study polymers derived from squaraine dyes and found that a second class of polymer topology leads to much smaller band gaps.
Abstract: Small band gaps simplify the use of polymers as semiconductors. Dye molecules can be used to construct semiconducting polymers with small band gaps. We use ab initio calculations to systematically design and study polymers derived from squaraine dyes. The calculated band gaps range from 2.3 eV to as low as 0.2 eV. Simple arguments based upon a Huckel analysis of the ab initio results enable us to identify the factors that control the size of the band gap. Squaraine polymers synthesized up till now fall into a class in which the band gap (≥1.3 eV) essentially reflects the HOMO/LUMO energy difference of the squaraine monomer fragment. We predict that a second class of polymer topology leads to much smaller band gaps. The reason for this is that band formation in the polymer shifts the energy of the highest occupied state of the polymer up with respect to the HOMO of the squaraine monomer, whereas the energy of the lowest unoccupied state of the polymer is fixed at the level of the monomer LUMO because of sy...
114 citations
TL;DR: Two classes of synthetically useful bimetallic complexes of the form [(tpy)M(tpp)RuCl(3)](PF(6)) and [tpy]M( tpp) Ru(tipp)]( PF(6)(4) have been prepared and their spectroscopic and electrochemical properties investigated and the synthetic variation of the components within this framework has allowed for understanding the spectroscopy and electro chemical properties.
Abstract: Two classes of synthetically useful bimetallic complexes of the form [(tpy)M(tpp)RuCl3](PF6) and [(tpy)M(tpp)Ru(tpp)](PF6)4 have been prepared and their spectroscopic and electrochemical properties investigated (tpy = 2,2‘:6‘,2‘‘-terpyridine, tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, and M = RuII or OsII). Synthetic methods have been developed for the stepwise construction of tpp-bridged systems using a building block approach. In all four complexes, the tpp that serves as the bridging ligand is the site of localization of the lowest unoccupied molecular orbital (LUMO). The nature of the HOMO (highest occupied molecular orbital) varies depending upon the components present. In the systems of the type [(tpy)M(tpp)RuCl3](PF6), the ruthenium metal coordinated to tpp and three chlorides is the easiest to oxidize and is the site of localization of the HOMO. In contrast, for the [(tpy)M(tpp)Ru(tpp)](PF6)4 systems, the HOMO is based on the metal, M, that is varied, either Ru or Os. This gives rise to systems wh...
109 citations
TL;DR: The first structural characterization of a hexahapto C{sub 60} complex has been reported in this article, where the authors show strong charge transfer from the metal center to coordinated C{ sub 60, consistent with experimental data on stable compounds.
Abstract: We report the synthesis and first structural characterization of a hexahapto C{sub 60} complex, in which C{sub 60} displays arene-like coordination to the open face of a triruthenium cluster. The bonding of C{sub 60} to a single transition metal center has been analyzed in terms of the familiar Dewar-Chatt-Duncanson donation/back-donation model, with a `double bond` donor orbital largely generated from the HOMO (h{sub u}) and the corresponding acceptor orbital formed from the LUMO (t{sub 1u}). Because of the relatively low energies of the C{sub 60} orbitals (high electron affinity), there is strong charge transfer (back-donation) from the metal center to coordinated C{sub 60}, consistent with experimental data on stable compounds. A qualitatively similar bonding model, in terms of donor/acceptor interactions of C{sub 60} with the Ru{sub 3}(CO){sub 9} fragment, is likely for 1; theoretical studies of the bonding in the benzene complexes 2 and 3 provide analogies. However. the relative contribution of back-donation to bonding with the metal triangle should be higher for C{sub 60} than for benzene, which is consistent with both the higher v{sub CO} values and the shorter Ru-C distances seen for 1 compared with those of 2. 21 refs., 1 fig.
TL;DR: In this article, an ab initio model of scanning tunneling microscopy (STM) is presented in the framework of the perturbation theory of Bardeen, where the tip and sample are represented by the cluster model, and the first-order wave functions are calculated by Hartree-Fock (HF), configuration interaction (CI), and symmetry adapted cluster (SAC)/SAC•CI methods.
Abstract: An ab initio model of scanning tunneling microscopy (STM) is presented in the framework of the perturbation theory of Bardeen. The tip and sample are represented by the cluster model, and the first‐order wave functions are calculated by ab initio Hartree–Fock (HF), configuration interaction (CI), and symmetry adapted cluster (SAC)/SAC‐CI methods. This model was applied to simple tip–sample systems such as Pd2–Ag2, Pd2–C6H6, Pd2–(C6H6)2, and Li2–Li2. The calculated STM images were related to the HOMO, LUMO, etc. orbital symmetries of the tip–sample systems. The contribution of the Rydberg orbital was examined at different tip–sample distances. Electron correlations were found to modify the behavior of the tunneling transition probability of the Li2–Li2 system vs that calculated by the HF method.
TL;DR: In this article, the importance of Coulombic energy transfer in intramolecular triplet energy transfer was discussed, and it was considered that the PtII center decreases the magnitude of the electronic coupling matrix element (HDA < 0.2 cm-1) for electron exchange, in part because of the high energy of its LUMO and HOMO states, relative to the butadiynyl bridge.
Abstract: Intramolecular triplet energy transfer occurs from a ruthenium(II) centre to an osmium(II) centre in heteropolynuclear polypyridine complexes at room temperature in acetonitrile solution. When the metal complexes are linked together by way of a butadiynyl bridge (polypyridine = 2,2′: 6′,6″-terpyridine), energy transfer is extremely fast and takes place via Dexter-type (through-bond) electron exchange. This process is facilitated by selective electron donation to the bridging ligand in the MLCT triplet state of the RuII complex, the electron residing in an extended π*-orbital that partially encompasses the bridge. In marked contrast, insertion of a PtII centre into the butadiynyl bridge (polypyridine = 2,2′-bipyridine) serves to inhibit through-bond electron exchange so that triplet energy transfer occurs slowly. It is considered that the PtII centre decreases the magnitude of the electronic coupling matrix element (HDA < 0.2 cm–1) for electron exchange, in part because of the high energy of its LUMO and HOMO states, relative to the butadiynyl bridge (HDA≈ 12 cm–1). The PtII centre, which donates charge to the bridging ligand, also induces localisation of the promoted electron at an unsubstituted 2,2′-bipyridyl ligand in the triplet excited state. The corresponding cis isomer (with respect to the PtII centre) gives a slightly slower rate of intramolecular triplet energy transfer. The importance of Coulombic energy transfer in these systems is discussed.
TL;DR: In this article, the origin of the high nonlinear optical response in tellurite glasses is tentatively elucidated from a detailed structural investigation of typical glasses existing in the TeO2-Al2O3 and TeO 2-Nb2O5 systems joined to a study of their electronic properties throughab initiocalculations.
TL;DR: In this article, a semi-empirical molecular orbital calculation of adsorbents and adsorbates was performed to determine the electronic state of adsorents and adorbates.
TL;DR: In this article, the authors present a measurement of the C Is ionisation potential (IP) for solid C-60, as well as a reliable determination of the highest occupied molecular orbital IP.
TL;DR: In this article, the same authors compared the spectral and redox properties of free base, copper and zinc derivatives of 5,10,15,20-tetraaryl porphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and the corresponding brominated 2,3,7,8,12,13,17,18,18-octabromo-5, 10, 15,20 -tetrasporphrin derivatives by using cyclic voltammetric methods.
Abstract: The free-base, copper(II) and zinc(II) derivatives of 5,10,15,20-tetraarylporphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and the corresponding brominated 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin derivatives have been synthesized and their spectral and redox properties compared by UV/VIS, H-1 NMR, ESR and cyclic voltammetric methods. Substitution with the electron-withdrawing bromine groups at the pyrrole carbons has a profound influence on the UV/VIS and H-1 NMR spectral features and also on the redox potentials of these systems. On the other hand, electron-withdrawing chloro or electron-donating methyl groups at the para positions of the four phenyl rings have only a marginal effect on the spectra and redox potentials of both the brominated and the non-brominated derivatives. The ESR data for the copper(II) derivatives of ail these systems reveal that substitution at either the beta-pyrrole carbons and/or the para positions of the meso-phenyl groups does not significantly affect the spin-Hamiltonian parameters that describe the metal centre in each case. Collectively, these observations suggest that the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of the octabromoporphyrins involve the porphyrin pi-ring system as is the case with the non-brominated derivatives.-Investigations have been carried out to probe the electronic structures of these systems by three different approaches involving spectral and redox potential data as well as AMI calculations. The results obtained suggest that the electron-withdrawing beta-bromine substituents stabilize the LUMOs and, to a lesser degree, the HOMOs and that the extent of these changes can be fine-tuned, in a subtle way, by substituting at the meso-aryl rings of a given porphyrin.
TL;DR: In this article, a new type of intramolecular charge-transfer dye has been prepared, which is based on the LUMO of the acceptor part (1,3-indandione) in these compounds is spiroconjugated with the HOMO (aromatic diamine or amino thiol).
Abstract: A new type of intramolecular charge-transfer dye has been prepared. The LUMO of the acceptor part (1,3-indandione) in these compounds is spiroconjugated with the HOMO of the donor part (aromatic diamine or amino thiol). The interaction between the donor and acceptor is controlled by the energy and symmetry of the frontier orbitals. The ground state dipole moments of these compounds are aligned along the long molecular axes. In the solid state, distortions of structures are observed that are consistent with partial electron shift from the donor to the acceptor. Much more pronounced electron density shifts accompany the electronic transition that is observed in the visible region of the spectrum. These transitions are of the charge-transfer (CT) type, as shown by solvatochromic and electrooptical studies. The excited state dipole moments are in the direction opposite to that of the ground state. These observations are consistent with the excited state having radical ion pair character. The new dyes are mode...
TL;DR: A series of three-ring heteroaromatic compounds consisting of a pyridine central ring and thiophene, selenophene, or N-methylpyrrole outer rings, and their polymers have been investigated as discussed by the authors.
Abstract: A series of three-ring heteroaromatic compounds consisting of a pyridine central ring and thiophene, selenophene, or N-methylpyrrole outer rings, and their polymers have been investigated. Differences in redox potentials and electronic absorbance spectra of the three-ring monomers have been interpreted with the aid of HOMO and LUMO energies from ab initio calculations. Both p- and n-doping of the polymers were investigated electrochemically in acetonitrile. In all cases, the p-doped states were not stable, and measured conductivities were well below intrinsic values. In most cases, the n-doped states were more stable, but conductivities were very low.
TL;DR: Extended Huckel calculations help rationalize the observed regioselectivity of allylic alkylations catalyzed by Pd complexes containing either C2-symmetric or electronically asymmetric bidentate chiral ligands.
TL;DR: In this paper, the Diels-Alder strategy was found suitable for the functionalization of C 60 using 1,3-butadienes substituted with an electron-withdrawing group as well as with electron-donating group, giving cyclohexene-fused C 60 derivatives having ethoxycarbonyl, acetyl, cyano, phenylsulfonyl and nitro substituents.
TL;DR: The first azo-imine chelate system, Pd(N(H)C(R)NNPh)(2) (Pd(RA)(2), has been isolated in the form of diamagnetic solids by the 6e(-)-6H(+) reduction of bis(phenylazooximato)palladium(II), Pd
Abstract: The first azo−imine chelate system, Pd(N(H)C(R)NNPh)2 (Pd(RA)2), has been isolated in the form of diamagnetic solids by the 6e-−6H+ reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)2 (abbreviated Pd(RB)2), with ascorbic acid in a mixed solvent (R = Ph, α-naphthyl) Selected spectral features are described The X-ray structures of Pd(PhA)2 and Pd(PhB)2 have revealed trans-planar geometry consistent with metal oxidation state of +2 Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors In Pd(PhA)2 a total of 10 ligand π electrons are present, each formally monoanionic ligand contributing five Model EHMO studies have revealed that the filled HOMO (au) in Pd(RA)2 is a bonding combination of two ligand π* orbitals with large azo contributions The LUMO (bg) is roughly the corresponding antibonding combination The outer π-electron configuration of Pd(RA)2 is (au)2(bg)0 Four successive voltammetric responses, two oxidative and two reductive, ar
TL;DR: In this article, the relative positioning of the closely situated low-lying unoccupied π* MOs a2(χ) and b1(ψ) in 1,10-phenanthroline (phen) and its 3,4,7,8-tetramethyl (tmphen) and four symmetrical diaza derivatives (n,m-dap) was assessed.
Abstract: Ab initio, semiempirical, and HMO perturbation calculations were employed to assess the relative positioning of the closely situated low-lying unoccupied π* MOs a2(χ) and b1(ψ) in 1,10-phenanthroline (phen) and its 3,4,7,8-tetramethyl (tmphen) and four symmetrical diaza derivatives (n,m-dap). Compared to a2(χ), the b1(ψ) π MO is distinguished by markedly higher MO coefficients at the chelating nitrogen π centers in 1,10-positions; eventually, a higher Coulomb integral value at those positions will thus always favor the lowering of b1 beyond a2. Using the Coulomb integral parameter at the chelating 1,10-nitrogen π centers as the HMO perturbation variable, the crossing of both energy levels in terms of decreasing preference for the a2(χ) over the b1(ψ) orbital as the lowest unoccupied MO follows the sequence 5,6-dap > 2,9-dap > 4,7-dap > phen > 3,8-dap. The calculations reveal a2(χ) as the LUMO in 5,6-dap for all reasonable perturbation parameters, in agreement with previous observations for ruthenium(II) c...
TL;DR: The electronic spectrum of planar 2,2′:5′,2″-terthiophene has been studied using multiconfigurational second-order perturbation theory as mentioned in this paper.
01 May 1996-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors proposed a model for dislocation mobility based on a sinusoidal potential energy, which accounts quantitatively for the hardness of covalent semiconductors (C, Si, Ge, Sn, SiC, and III-V compounds).
Abstract: In simple metals and ionic substances, the bonding is delocalized, and hardness is determined by extrinsic factors such as impurities, precipitates, grain boundaries, dislocation dipoles, and the like. However, in covalent substances, the bonding is localized in electron spin-pairs, and the hardness is intrinsic. This results from low kink mobilities of dislocations. The standard model for dislocation mobility is that of Orowan, Peierls, and Nabarro. In this model, the atomic “roughness” of the glide plane is represented by a sinusoidal potential energy. This model does not agree with observations, and is intrinsically flawed because there are singularities at the cores of dislocations that cannot be represented by periodic potentials. The author has proposed that kink motion on dislocation lines is analogous with chemical exchange reactions. Adjacent atoms lying at the top and bottom of a glide plane exchange their partners for new partners when a kink moves. The reaction is of the disconcerted type, so it is sluggish for covalent bonds, making the kink mobility low. The theory indicates that chemical hardness and mechanical hardness have the same reaction barrier. It is the difference between the energy of the lowest unoccupied electronic orbital (LUMO), and the highest occupied orbital (HOMO). This energy gap determines the strengths of chemical bonds, so it is not surprising that is also determines mechanical strength. The theory accounts quantitatively for the hardnesses of covalent semiconductors (C, Si, Ge, Sn, SiC, and III–V compounds); and with some modification for the hardnesses of “hard metals”, such as TiC and WC.
TL;DR: Density-functional calculations of chemisorption processes and of potential energy surfaces of the dissociation of simple molecules over surfaces, which have become available in the last few years, have greatly improved the fundamental understanding of reactions at solid surfaces, but those extensive computations are limited to a restricted number of model systems.
Abstract: Density-functional calculations of chemisorption processes and of potential energy surfaces of the dissociation of simple molecules over surfaces, which have become available in the last few years, have greatly improved the fundamental understanding of reactions at solid surfaces. However, those extensive computations are limited to a restricted number of model systems. It remains an important task to develop a methodology that allows the prediction and interpretation of reactions at surfaces in terms of the properties of the noninteracting systems. Such concepts, well known in molecular chemistry, are the essence of “reactivity theory.” They are based on low-order perturbation theory and aim at a description of the early stages of chemical interactions. The ensuing response functions are called “reactivity indices” and characterize the changes of the electronic structure of one reactant as stimulated by the presence of the other or vice versa. In an early contribution to this field, Fukui et al. [1,2] established the correlation between the frontier-orbital density, i.e., the density of the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), and the reactivity of a system toward electron donation or acceptance. Pearson [3] introduced the electronic “softness,” the magnitude of the change of the electronic structure due to a change of the number of electrons in the system, as a measure of the reactivity. Species are classified as “soft” if only a small energy is required to change their electronic configuration, i.e., if the valence electrons are easily distorted, polarized, removed or added. A “hard” species has the opposite properties, holding its valence electrons more tightly [3,4]. The utility of the hardnesssoftness concept is based on the so called hard and soft acid and base (HSAB) principle formulated by Pearson [3] which states that hard-hard (soft-soft) interactions are preferred. In the case of polyatomic or extended systems, the HSAB principle is used in a local version: The soft (hard) parts of one reactant prefer to interact with soft (hard) areas of the other [5]. Parr and collaborators [5] gave a foundation in density functional theory to those mostly
TL;DR: It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair-LUMO-mediated pericyclic reaction) in 1,3- and 1,5-chlorine migrations.
Abstract: High-level ab initio molecular orbital calculations, using the G2(MP2,SVP) theory (and semiempirical methods) have been used to examine several 1,3- and 1,5-chlorine migrations. It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair−LUMO-mediated pericyclic reaction). The activation barriers for the 1,3-migration in chloro oxo ketene 1 (Cl(CO)CHCO) and the 1,5-migration in (2-(chlorocarbonyl)vinyl)ketene 2 (Cl(CO)CHCHCHCO) are only 53 and 61 kJ mol-1, respectively, compared to the 216 and 173 kJ mol-1 barriers for the corresponding unassisted 1,3- and 1,5-sigmatropic shifts of Cl in 3-chloro-1-propene and 5-chloro-1,3-pentadiene. The transition structures for 1 and 2 reveal that migration of the chlorine atoms takes place in the molecular planes. The 1,5-chlorine shift in 6-chlorocyclohexa-2,4-dienone (3) has a significantly higher barrier due to a lack of appropriate orbital interaction. The related 1,3-shift in the (chloroc...
TL;DR: Although this study raises several intriguing issues relating mutagenicity to chemical properties, further study will be required to determine the plausibility of the nitrenium ion as the ultimate mutagen for binding to DNA.
Abstract: A set of 16 mutagenic aminoimidazo-azaarenes, including four that have been isolated from cooked foods and identified as bacterial mutagens and rodent carcinogens, was selected from a larger series previously published [Hatch et al. (1991): Environ Mol Mutagen 17:4–19] for an in-depth structure-activity study using computational methods. Structural features believed to affect mutagenic potency were tabulated. Molecular orbital energies and other electronic properties of these compounds were calculated using Huckel, semiempirical AM1, and ab initio quantum mechanical methods. Factor interrelationships were studied by multiple linear regression and canonical correlation analyses. Our goal was an improved understanding of the chemical basis of mutagenicity for this class of heterocyclic amines. The major findings were as follows: 1) mutagenic potency is related to the size of the aromatic ring system; 2) potency is enhanced by the presence and location of an N-methyl group; 3) potency is enhanced by addition of ring nitrogen atoms in pyridine, quinoline, and quinoxaline configurations; 4) potency is inversely related to the energy of the LUMO (lowest unoccupied molecular orbital) of the parent amines; 5) potency is directly, though weakly, related to the LUMO energy of the derived nitrenium ions; and 6) the calculated thermodynamic stability of the nitrenium ions (relative to the parent amine) is directly correlated with nitrenium LUMO energy and with the negative charge on the exocyclic nitrogen atom. Although this study raises several intriguing issues relating mutagenicity to chemical properties, further study will be required to determine the plausibility of the nitrenium ion as the ultimate mutagen for binding to DNA. © 1996 Wiley-Liss, Inc.
TL;DR: In this article, a model including charging and discharging processes of these surface states is presented for the analysis of photocurrent transients as obtained by a chopped light beam, and the dependence of the photocurrent on the reactant concentration could be attributed to an adsorption step.
TL;DR: In this article, a model for estimating the reaction rate constants of unsaturated hydrocarbons with hydroxyl radical has been developed by application of the PLS (partial least squares projections to latent structures) multivariate technique and a large set of 18 empirical variables and calculated quantum chemical or structural descriptors.
Abstract: A reliable QSAR model for estimating the reaction rate constants of unsaturated hydrocarbons with hydroxyl radical has been developed by application of the PLS (partial least squares projections to latent structures) multivariate technique and a large set of 18 empirical variables and calculated quantum chemical or structural descriptors. It was found that five calculated quantum chemical and structural descriptors have a pronounced influence on the reaction rate constants of unsaturated hydrocarbons with hydroxyl radical : energy of the highest occupied molecular orbital (E HOMO ), hardness (HA), average number of alkyl substitutents per unsaturated bond (n Alk ), energy of the lowest unoccupied molecular orbital (E LUMO ) and number of carbon atoms in unsaturated bonds (n c ). The derived QSAR model was rationalized within the framework of the frontier molecular orbital (FMO) theory and by thermodynamic arguments. In addition, a QSAR model has been derived using multiple linear regression (MLR) and a stepwise procedure with the same set of measured and calculated descriptors in order to assess the advantages and limitations of both statistical methods. Finally, a comparison was made between estimates from this study and the estimates from QSAR models published earlier, i.e. Atkinson's fragment contribution method and the non-linear QSAR model of Klamt based on calculated quantum chemical descriptors. This comparison has provided important information on the performance of individual models as well as on their advantages and limitations. It can be safely concluded that the PLS model derived in this study is the method of choice for future reliable estimation of log k OH data for unsaturated chemicals.