HOMO LUMO Energy Separation Research Articles

Structural design and characterization of a chalcone derivative crystal DAMO with strong SHG efficiency for NLO applications

A new promising NLO material 1-(4-methoxyphenyl)-3-(4-N, N dimethyl amino phenyl)-2-propen-1-one (DAMO) was synthesized and high-quality crystal was grown by slow evaporation technique. The crystal structure was revealed by single crystal XRD scan, and all the functional groups of titled crystal were confirmed by FT-IR and 1 H NMR spectroscopy. The thermal properties, dielectric properties, optical properties were carried out from the sample, and the experimental data were analyzed systematically. The surface morphology of grown crystal was studied through SEM and AFM images. The SHG efficiency was confirmed by Kurtz-Perry powder technique, which shows that DAMO was 42.71 times that of KDP crystal. In addition, the HOMO–LUMO energy separation value was also theoretically calculated by Gaussian 09 package. • DAMO crystal with the sizes of 2 × 2 × 0.5 mm 3 is obtained by slow evaporation technology for the first time. • DAMO crystal has high transmittance (~94%) in the range of 550–2000 nm. • SHG efficiency of DAMO crystal is 42.71 times greater than KDP. • HOMO–LUMO orbital of the DAMO crystal is studied for the first time.

Crystal structure and photoluminescent properties of bis-(4'-chloro-2,2':6',2''-terpyrid-yl)cobalt(II) dichloride tetra-hydrate.

In the title hydrated complex, [Co(C15H10ClN3)2]Cl2·4H2O, the complete dication is generated by symmetry. The CoN6 moiety shows distortion from regular octa-hedral geometry with the trans bond angles of two N-Co-N units being 160.62 (9)°. In the crystal, O-H⋯Cl and C-H⋯O inter-actions link the components into (001) sheets. The title compound exhibits blue-light emission, as indicated by photoluminescence data, and a HOMO-LUMO energy separation of 2.23 eV was obtained from its diffuse reflectance spectrum.

Experimental and Computational Study on the Stability of Nitrogen, Boron, and Sulfur Doped Carbon Catalysts for Oxygen Reduction Reaction

A new approach in electrode catalysis bearing immense potential for electrochemical technologies is the prospect of carbon based electrodes. Pristine carbon nanostructures are relatively inert and modications including nitrogen, boron, sulfur doping and their combinations are known for their beneficial effects on the electrochemical activity of carbon nanomaterials in both alkaline and acidic media. However, long-term stability of these materials, especially in acidic environment, is rarely mentioned. Here, we evaluate the stability and longterm degradation of nitrogen (N), sulfur-nitrogen (SN), and boron-nitrogen (BN) co-doped graphene akes as oxygen reduction electrocatalyst with theoretical and experimental techniques. We assume that nitrogen dopants in the graphene sheet interact with electrons and protons at the electrode-electrolyte interface leading to ammonia scission and continuous catalyst deactivation. We analyse and compare the ammonia scission pathways of N, BN and SN containing graphene structures by the use of Density Functional Theory (DFT) B3LYP method. The stability of the model systems with regard to the investigated reaction pathway are correlated with the HOMO-LUMO energy separation, spin and charge densities showing that degradation of doped graphene and oxygen reduction reaction is driven by similar material parameters. The computational results are corroborated with electrochemical measurements at SAFC operating conditions. Figure 1

Silicon impacts on structure, stability and aromaticity of C20-nSin heterofullerenes (n = 1–10): A density functional perspective

Density functional theory (DFT) calculations are applied to compare and contrast silicon atom substitution doped C20-nSin heterofullerene analogous with n = 1–10, at B3LYP/AUG-cc-pVTZ. Vibrational frequency analysis confirms that all studied systems are true minima. Isolating the dopants is an applicable strategy for obtaining highly doped stable heterofullerenes, since it avoids weak silicon―silicon single bonds. Comparing and contrasting the optimized geometries shows that except C11Si9 and C10Si10 species (with deformed cages of segregated analogous), all eight fullerenic cages are the complete isolated-pentagon analogous. Hence, the dopants must be completely isolated from each other by means of strong CC double bonds. Isolable or extractable fullerene isomers must be not only thermodynamically but also stable against electronic excitations. We then predicted that these isomers must have not only relatively large heats of atomization per carbon as a criterion of thermodynamic stability but also relatively large HOMO–LUMO energy separation against electronic excitations. The calculated the highest binding energy (6.52 eV/atom), heat of atomization per carbon (3193.2 kcal mol−1), band gap (2.86 eV) and nucleus independent chemical shift at the cage center (−50.00 ppm) for C18Si2 reveals it as the most stable heterofullerene. It has Ci symmetry and contains two silicon atoms in equatorial. High charge transfer on the surfaces of our scrutinized heterofullerenes provokes further investigations on their possible application for hydrogen storage. We hope that the present study will stimulate new experiments.

18-Crown-6 ether templated transition-metal dicyanamido complexes: Synthesis, structural characterization and DFT studies

Reaction of ethanolic solution of hydrated metal salts Mn(NO3)2/Fe(BF4)2 with sodium dicyanamide (dca) and 18-crown-6 ether (18C6) in water results in the isolation of 1:1 associates [Mn(dca)2(H2O)4]·18C6 (1) and [Fe(dca)2(H2O)4]·18C6 (2). Single crystal X-ray structure analysis revealed a high degree of isostructurality between the two 18C6 templated metal complexes (1) and (2) exhibiting second sphere coordination via intermolecular O (water)H⋯O (crown) hydrogen bonds. The octahedral Mn(II)/Fe(II) complexes are sandwiched between adjacent 18C6 molecules into one-dimensional polymeric chains along the [100] directions, which are further linked through intermolecular O (water)H⋯N (dca) hydrogen bonds, thus forming a two-dimensional supramolecular framework. The electronic structures of the complexes calculated at the DFT level showed a HOMO–LUMO energy separation of 2.45eV in (1) and 0.83eV in (2), which implied a higher kinetic stability of complex (1) than that of (2).

Structure and Aromaticity of B6H5+Cation: A Novel Borhydride System Containing Planar Pentacoordinated Boron

A novel structural pattern: two-dimensional, five-membered ring-like boron hydride system B(6)H(5)(+), which contains both planar tetra- and pentacoordinated boron (ptB and ppB) was designed and investigated at [B3LYP, MP2(Full) and CCSD(T,FC)]/6-311+G(d,p) theoretical levels. The results indicate that both degenerate ppB B(6)H(5)(+) configurations A (D(5H),(1)A(1)') and B (C(2nu),(1)A(1)) are energetically favorable, sharing the lowest energy comparison with other isomers considered in this work. The computed considerably large smallest vibrational frequencies [197.5 and 197.4 cm(-1), respectively at MP2 (full) level], HOMO-LUMO energy separations [10.99-11.03 eV computed at MP2 (full) level], and fairly large endothermic dissociation reaction energies suggest they are promising candidate molecules for experimental detection. The detailed molecular orbital analysis, natural bond orbital analysis, magnetic susceptibility anisotropy, and NMR chemical shift analyses confirmed that the two low-energy ppB molecules exhibit strong aromaticities.

Controlling the electron transfer mechanism in metal–molecules–metal junctions

We show how the mechanism of electron transfer through molecules can be switched between different regimes by using Hg-based metal–molecules–metal junctions. The junctions are easy to assemble and allow for hosting of self-assembled monolayers (SAMs) of a large variety of molecular structures. In this paper, we compare results from studies of charge transport across two different types of junctions. The first approach has been reported previously and involves the use of a two-electrode junction, Hg–SAM//SAM–Ag, to measure I– V curves through SAMs formed from different organic molecules [alkanethiols HS(CH 2) n − 1 CH 3 ( n = 8, 10, 12, 14, 16), oligophenylene thiols HS(C 6H 4) k H ( k = 1, 2, 3), or benzylic homologs of the oligophenylene thiols HSCH 2(C 6H 4) m H ( m = 1, 2, 3)]. The molecules incorporated have a very large HOMO–LUMO energy separation and their orbitals cannot align with the Fermi levels of the electrodes under an applied voltage. The molecules therefore behave as insulators, and the electron transport mechanism is characterized by a non-resonant tunneling process. The second approach is new and involves the use of an electrochemical junction, Hg–SAM//SAM–Hg, with SAM formed by electroactive thiol molecules [HS(CH 2) 10CONHCH 2pyRu(NH 3) 5](PF 6) 2. Charge transport across the junction is measured using a conventional electrochemical technique, which allows the potentials of the Hg electrodes relative to that of a reference electrode to be controlled with respect to the redox potential of the incorporated redox-active molecules. The incorporated redox sites have energetically low molecular orbitals which can align with the Fermi levels of the electrodes. We show that under this control of potential the electron transport mechanism can be switched to different regimes and the current flowing through the junction can be modulated. The current changes of order of magnitude (NDR effect) as a result of a change of the mechanism of electron transport from non-resonant to resonant tunneling (or hopping).

Thermodynamically and kinetically stable isomers of the C 88 and C 90 fullerenes

Isolable or extractable fullerene isomers must be not only thermodynamically but also kinetically stable. Heats of atomization and anion localization energies (anion LEs) for all isolated-pentagon isomers of the C 88 and C 90 fullerenes were calculated using the PM3 method. The heat of atomization is a direct index of thermodynamic stability, whereas the minimum anion LE (min anion LE) in a molecule serves as a measure of kinetic stability for electron-deficient fullerenes. On this theoretical basis, we successfully predicted the fullerene isomers that might appear in experimental samples of C 88 and C 90. For the fullerene isomers studied, HOMO–LUMO energy separation is not highly correlative with min anion LE. It seems likely that, with a notable exception of C 72, the C 6 n fullerenes are generated more abundantly than their neighboring members.

Aromaticity and stability of furofurans and thienothiophenes

AbstractThe topological resonance energy (TRE) calculated for a polycyclic π‐system cannot always be used to predict the thermodynamic or kinetic stability of the π‐system. For example, furo[3,4‐c]furan (1d) and thieno[3,4‐c]thiophene (2d) are aromatic with large positive TREs and large negative nucleus independent chemical shift (NICS) values at the ring centers. However, these heterobicycles are predicted to be very high in energy with very small HOMO–LUMO energy separations. These unusual properties of the heterobicycles are closely associated with the fact that classical resonance structures cannot be drawn for them. In general, the kinetic stability of a polycyclic π‐system reflects not only the aromaticity of the π‐system but also the kinetic stability of the polyene reference. Copyright © 2004 John Wiley & Sons, Ltd.

PM3 localization energies for the isolated-pentagon isomers of the C84 fullerene

Isolable or extractable fullerene isomers must be not only thermodynamically but also kinetically stable. However, thermodynamically stable isomers are not always kinetically stable. Cation and anion localization energies (cation and anion LEs) for carbon atoms in 24 isolated-pentagon isomers of the C84 fullerene were calculated using the semiempirical PM3 method. The smallest or minimum anion LE (min anion LE) in a molecule was found to be very useful as a measure of kinetic stability for fullerenes. We then predicted that isolable or extractable isomers must have not only relatively large heats of atomization but also relatively large min anion LEs. For the C84 fullerene isomers, the min anion LE is highly correlated with the HOMO–LUMO energy separation.

Aromatic Character of Large Deltahedral Borane Dianions As Estimated Using the Kettle–Tomlinson Bonding Model

Abstract Lipscomb et al. designed the low-energy deltahedral isomers of borane dianions BnHn2− for n ranging from 13 to 24. We evaluated the topological resonance energies (TREs) of these dianions by applying the Kettle–Tomlinson (KT) bonding model to them. It was then found that except for B18H182− (D6d) the TRE per BH unit is highly correlative with the PRDDO binding energy per BH unit. This indicates that the thermodynamic stability of deltahedral borane dianions is determined primarily by aromaticity. B18H182− (D6d) has a very small TRE per BH unit, strongly reflecting its small HOMO–LUMO energy separation. Thus, the KT model is useful for describing electronic characteristics of large deltahedral borane dianions.

Weighted HOMO-LUMO energy separations of properly closed-shell fullerene isomers

The HOMO–LUMO energy separation has been used as an index of kinetic stability for fullerenes. However, its utility is rather limited, in that a larger fullerene molecule in general has a smaller HOMO–LUMO energy separation even if it is kinetically stable. The T value, i.e., the HOMO–LUMO energy separation weighted with the number of carbon atoms, is free from this kind of difficulty. Properly closed-shell isomers of fullerenes with antibonding LUMOs were found to have an exceptionally large T value irrespective of the number of constituent carbon atoms and are hence predicted to be kinetically very stable. Thus, the antibonding character of the LUMOs is one of the determinants for kinetically stabilizing fullerene molecules. © 2001 by Elsevier Science Inc.

Excited states of four hemes in a c-type cytochrome subunit of the photosynthetic reaction center ofRhodopseudomonas viridis: SAC-CI calculations

The excited states of four hemes, c-552, c-554, c-556, and c-559, in a c-type cytochrome subunit of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis were studied using the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. c-552 has two imidazole ligands, while the other three hemes have imidazole and methionine ligands. The electronic states of these four hemes are affected significantly by both the ligands and protein and water environments. The ligand effect classifies the four hemes into two groups, while the environmental effect is unique in each heme. The HOMO–LUMO energy separation of c-552 is much smaller than those of the other hemes, and therefore, the low-lying excited states of c-552 are more stable than those of the other hemes. The low-lying excited states and their oscillator strengths of c-559 are significantly modified by the environment when compared with c-556 and c-554. Some observed UV peaks of the PSRC of Rps. viridis were found to have been assigned reasonably to the excited states of these four hemes in a previous report. The calculated linear dichroism angles also supported our assignment.

FMO treatment of the reactivity and stereochemistry of the cycloaddition processes using ASED-MO method

Frontier molecular orbitals treatment of the reactivity of the 1-substituted pyridinium-3-oxide betaines as 4π-1,3-dipole as well as the stereochemical criteria accompanied the cycloaddition reactions is studied using ASED-MO theory. The calculations show that the effect of the pyridazinyl substituent on increasing the reactivity of the betaines is due to small HOMO–LUMO energy separation. The betaines with small energy gap are able to dimerize. Also, it is shown from the calculations that the reaction of betaines with π-electron deficient dipolarophiles is dipole HOMO-controlled. The interaction between the frontier molecular orbitals is important to rationalize the stereoselectivity of the cycloaddition processes.

Correlation found between the HOMO–LUMO energy separation and the chemical reactivity at the most reactive site for isolated-pentagon isomers of fullerenes

For isolated-pentagon isomers of fullerenes with up to 90 carbon atoms, the HOMO–LUMO energy separation is closely associated with the chemical reactivity at the most reactive site of the molecule. To be exact, the T value correlates very well with the minimum bond resonance energy (min BRE). Here, the T value is the HOMO–LUMO energy separation multiplied by the number of carbon atoms, and the min BRE is the minimum bond resonance energy in the molecule. This correlation explicitly shows that an isolated-pentagon fullerene isomer with chemically reactive substructures in general has a small HOMO–LUMO gap energy.

Electronic Structure of Metallacyclophosphazene and Metallacyclothiazene Complexes.

The electronic structure of metallacyclotriphosphazene complexes with several substituents at the phosphorus atoms and metallacyclothiazene complexes is explored for a variety of transition metal elements using density functional theory methods. Accordingly the metallacyclophosphazenes possess a large HOMO-LUMO energy separation while the metallacyclothiazenes bear stronger open-shell character. In addition our calculations predict the existence of experimentally so far unknown dimetallacyclophosphazenes. All structures show to be highly dynamical. The double bond character of the transition metal nitrogen bond is much less pronounced than in nitrido or imido complexes. For the ring compounds vibrational spectra are reported and compared with experimental data.

Weighted Homo-Lumo Energy Separations for the Polyene References of Fullerenes with Up to 100 Carbon Atoms

The T value represents the HOMO-LUMO energy separation of a ε-electronic system weighted with the number of conjugated atoms. We found that this value is kept almost constant for the polyene references defined graph-theoretically for isolated-pentagon isomers of fullerenes with up to 100 carbon atoms. on this numerical basis, the T value can safely be considered as a realistic index of kinetic stability or chemical inertness for all fullerenes without abutting pentagons.

Weighted HOMO-LUMO energy separation as an index of kinetic stability for fullerenes

A very simple but general index of kinetic stability, T, is proposed for π-electron systems, which is defined as a highest occupied molecular orbital (HOMO)- lowest unoccupied molecular orbital (LUMO) energy separation multiplied by the number of conjugated atoms. This new index can be justified by relating it to the approximate form of the superdelocalizability, an index of chemical reactivity defined by Fukui et al. It is best suited for predicting the kinetic stability of fullerenes. All isolable fullerene isomers have T values larger than 13.

Validity of the weighted HOMO–LUMO energy separation as an index of kinetic stability for fullerenes with up to 120 carbon atoms

The T value, a general index of kinetic stability defined as a HOMO–LUMO energy separation multiplied by the number of conjugated atoms, can be evaluated meaningfully for isolated-pentagon isomers of fullerenes with up to 120 carbon atoms. Fullerenes with 60+6k carbon atoms have one or more isolated-pentagon isomers with very large T values. Here, k is zero or any integer larger than unity.

Why are some polycyclic aromatic hydrocarbons extremely reactive?

Higher members of the polyacene series are extremely reactive although they are aromatic in nature. This aspect of aromatic chemistry can be explained using conventional indices of kinetic stability, such as a localization energy and a HOMO–LUMO energy separation. Large polyacene molecules have smaller localization energies and smaller HOMO–LUMO energy separations than their respective polyene references defined using matching polynomials. These molecules are presumed to be thermodynamically more stable but kinetically less stable than non-aromatic polyolefins. The same situation is sometimes observed in other groups of polycyclic aromatic hydrocarbons (PAHs). Such highly reactive PAH molecules were found to be substructures of metallic one-dimensional benzenoid polymers.