Home About us Contact
|
Home About us Contact | ||
|
|
||
Frontier Orbitals (frontier + orbital)
Selected AbstractsBonding and Bending in Zirconium(IV) and Hafnium(IV) HydrazidesCHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2008Heike Herrmann Dr. Abstract Reaction of the dichloro complexes [M(N2TBSNpy)Cl2] (M=Zr: 1, Hf: 2; TBS: tBuMe2Si; py: pyridine) with one molar equivalent of LiNHNPh2 gave mixtures of the two diastereomeric chlorohydrazido(1,) complexes [M(N2TBSNpy)(NHNPh2)Cl] (M=Zr: 3,a,b, Hf: 4,a,b) in which the diphenylhydrazido(1,) ligand adopts a bent ,1 coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2,) complexes [Zr(N2TBSNpy)(NNPh2)(py)] (5) and [Hf(N2TBSNpy)(NNPh2)(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one , and two , orbitals. A high polarity of the MN bond was found, in accordance with the description of hydrazinediide(2,) acting as a six-electron donor ligand. The pyridine ligand in [M(N2TBSNpy)(NNPh2)(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (,G,=19,kcal,mol,1). A change in denticity of the hydrazido unit from ,1 to ,2 was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid ,1 to ,2 interconversion. This process is believed to happen early on in the NN scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N, is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C6F5)3 to give [Zr(N2TBSNpy)(NNPh2){B(C6F5)3}] (7) and [Hf(N2TBSNpy)(NNPh2){B(C6F5)3}] (8). In these products, B(C6F5)3 becomes attached to the N, atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho -F atom of a C6F5 ring with the metal centre. [source] The role of the Frontier orbitals in acid,base chemistry of organic amines probed by ab initio and chemometric techniquesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2010Felipe A. La Porta Abstract The Frontier effective-for-reaction molecular orbital (FERMO) concept emerges as a powerful and innovative implement to investigate the role of molecular orbitals (MOs) applied in the description of breakage and formation of chemical bonds. In this work, theoretical calculations were carried out for conjugated acids of 18 amines and their acid,base behavior was analyzed using MO energies. We observed that highest occupied MO (HOMO) energies are inadequate to describe the acid,base behavior of these compounds. By using the FERMO concept, the reactions that are driven by HOMO, and those that are not, can be better explained, independent of the calculation method used, as independent of the calculation method used, both HF and Kohn,Sham methodologies lead to the same FERMO. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Synthetic popularity reflects chemical reactivityJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2009Bartlomiej Kowalczyk Abstract Large and diverse databases of chemical reactions contain statistically significant information about the propensities of molecules to undergo specific chemical transformations. It is shown that this information can be quantified to reflect reaction thermodynamics/kinetics and can be used to construct primitive (yet accurate) reactivity indices from the counts of reported reactions involving molecules/molecular positions of interest. These indices correlate with frontier orbital (FO) populations or Hammett , and , parameters for a range of reactions involving aromatic substrates. These findings suggest that large chemical databases are not only a historical repository of chemical knowledge but also tools with which one can make useful chemical predictions. Copyright © 2009 John Wiley & Sons, Ltd. [source] Dispersion-Oriented Soft Interaction in a Frustrated Lewis Pair and the Entropic Encouragement Effect in its FormationCHEMISTRY - A EUROPEAN JOURNAL, Issue 48 2009Woo Kim Abstract The origin of the stability of a frustrated Lewis pair (FLP) tBu3P:B(C6F5)3 is investigated computationally to demonstrate the importance of the dispersion interaction. To this end, the interaction between alkyl-substituted phosphines (Me3P and tBu3P) and hexafluorobenzene (C6F6) is first investigated. Driven by the lone-pair to ,-orbital interaction, the binding energy is found to be even larger than usual ,,, interaction energies between small aromatic compounds. This character, which is inherited to fluorophenyl-substituted B(C6F5)3 in the FLP, induces large flexibility in the FLP over the molecular surface of B(C6F5)3. This soft interaction, in turn, causes an entropic stabilization of the FLP formation in comparison with classical Lewis pairs based on close and tight PB dative bonds. It also suggests a diverse nature of the FLP when it is involved in chemical reactions. Even with the cooperative participation of the perfluorophenyl groups, a detailed inspection of the FLP interaction potential energy surface indicates that the boron atom is still the major interaction site for the pair formation. This non-negligible direct PB interaction, which is related also to the soft nature of the borane frontier orbital, is further supported by substantial spatial overlap between the frontier orbitals on the phosphine/borane fragments and their interaction energy estimations. [source] Theoretical and Experimental Study of the Regioselectivity of Michael AdditionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2004David C. Chatfield Abstract Nucleophilic attack at an ,,,-unsaturated carbonyl moiety usually results in conjugate addition at the ,-carbon atom (1,4 or Michael addition) or, occasionally, in addition at the carbonyl carbon atom (1,2 addition). Recently, however, addition at the ,-carbon atom has been observed when strongly electron-withdrawing groups are positioned at the carbon atom , relative to the carbonyl group [e.g., methyl 3,3-bis(trifluoromethyl)propenoate (8) and ethyl 3-(2,4-dinitrophenyl)propenoate (24)]. We have performed theoretical calculations [HF/6,31+G(d) and B3LYP//HF/6,31+G(d)] for the addition of cyanide anion to model ,,,-unsaturated carbonyl compounds to determine trends in the regioselectivity with respect to properties of the substituents. The difference between the reaction barriers for ,- vs. ,-addition decreases as the strength of electron-withdrawing groups increases until, for sufficiently strong electron-withdrawing groups, ,-addition becomes favored. The calculations are in agreement with the experimental results. We show that the regioselectivity can be predicted from partial atomic charges and properties of the frontier orbitals of the reactants. We also report new experimental evidence of ,-addition to polysubstituted cinnamates and cinnamaldehydes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] New Ruthenium Complexes Containing Oligoalkylthiophene-Substituted 1,10-Phenanthroline for Nanocrystalline Dye-Sensitized Solar Cells,ADVANCED FUNCTIONAL MATERIALS, Issue 1 2007C.-Y. Chen Abstract Two new ruthenium complexes [Ru(dcbpy)(L)(NCS)2], where dcbpy is 4,4,-dicarboxylic acid-2,2,-bipyridine and L is 3,8-bis(4-octylthiophen-2-yl)-1,10-phenanthroline (CYC-P1) or 3,8-bis(4-octyl-5-(4-octylthiophen-2-yl)thiophen-2-yl)-1,10-phenanthroline (CYC-P2), are synthesized, characterized by physicochemical and semiempirical computational methods, and used as photosensitizers in nanocrystalline dye-sensitized solar cells. It was found that the difference in light-harvesting ability between CYC-P1 and CYC-P2 is associated mainly with the location of the frontier orbitals, in particular the highest occupied molecular orbital (HOMO). Increasing the conjugation length of the ancillary ligand decreases the energy of the metal-to-ligand charge transfer (MLCT) transition, but at the same time reduces the molar absorption coefficient, owing to the HOMO located partially on the ancillary ligand of the ruthenium complex. The incident photon-to-current conversion efficiency curves of the devices are consistent with the MLCT band of the complexes. Therefore, the overall efficiencies of CYC-P1 and CYC-P2 sensitized cells are 6.01 and 3.42,%, respectively, compared to a cis- di(thiocyanato)-bis(2,2,-bipyridyl)-4,4,-dicarboxylate ruthenium(II)-sensitized device, which is 7.70,% using the same device-fabrication process and measuring parameters. [source] V-Shaped Thiophene-Based Oligomers with Improved Electroluminescence Properties,ADVANCED FUNCTIONAL MATERIALS, Issue 4 2005G. Barbarella Abstract The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film-forming properties than their linear counterparts. Spin-coated films show photoluminescence efficiencies up to 50,%. Light-emitting diodes with spin-coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10,000,cd,m,2. Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl- S,S -dioxide leads an increase in electron affinity by 0.5,0.7,V, analogous to that of the corresponding linear oligomers. [source] Intermolecular , -dimer of oxoverdazyl radicals with long-distance multicenter (2e/8c) bonding via nitrogen atomsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2010Sergiy V. Rosokha Abstract The 1,5-dimethyl-6-oxoverdazyl radical's solid-state structure shows distinct , -dimeric units with close interplanar separations (3.10,Å) between the head-over-tail cofacial moieties and several interatomic contacts shorter than the sums of the van der Waals radii. Evaluations of the frontier orbitals of monomeric oxoverdazyl and its , -dimer reveal that interaction of the radical SOMOs (concentrated on the nitrogen atoms) leads to the formation of the supramolecular orbital involving four equivalent bonding (N,N) segments between two oxoverdazyl moieties. As such, this , -dimer represents a rare example of nitrogen-based multicenter (2e/8c) long-distance bonding and emphasizes the universal character of this phenomenon in organic (ion-) radical systems. Copyright © 2009 John Wiley & Sons, Ltd. [source] Synthesis and electroluminescent properties of fluorene-based copolymers containing electron-withdrawing thiazole derivativesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2008In Hwan Jung Abstract We synthesized two fluorene-based copolymers poly[(2,5-bis(4-hexylthiophen-2-yl)thiazolo[5,4-day]thiazole-5,5,-diyl)-alt-(9,9,-dioctylfluorene-2,7-diyl)] (PF-TTZT), and poly[(5,5,-bis(4-hexylthiophen-2-yl)-2,2,-bithiazole-5,5,-diyl)-alt-(9,9,-dioctylfluorene-2,7-diyl)] (PF-TBTT), which contain the electron-withdrawing moieties, thiazolothiazole, and bithiazole, respectively. Through electrochemical studies, we found that these two polymers exhibit stable reversible oxidation and reduction behaviors. Moreover, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PF-TBTT are lower than those of PF-TTZT, and the bandgap of PF-TBTT is smaller than that of PF-TTZT. Thus the bithiazole moiety in PF-TBTT is more electron-withdrawing than the thiazolothiazole moiety in PF-TTZT. Light-emitting devices with indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT)/polymer/bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum (BAlq)/LiF/Al configurations were fabricated. The performance of the PF-TBTT device was found to be almost three times better than that of the PF-TTZT device, which is because electron injection from the cathode to PF-TBTT is much easier than for PF-TTZT. We also investigated the planarity and frontier orbitals of the electron donor-acceptor (D-A) moieties with computational calculations using ab initio Hartree,Fock with the split-valence 6-31G* basis set. These calculations show that TBTT has a more nonplanar structure than TTZT and that the bithiazole moiety is more electron-withdrawing than thiazolothiazole. These calculations are in good agreement with the experimental results. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7148,7161, 2008 [source] Dispersion-Oriented Soft Interaction in a Frustrated Lewis Pair and the Entropic Encouragement Effect in its FormationCHEMISTRY - A EUROPEAN JOURNAL, Issue 48 2009Woo Kim Abstract The origin of the stability of a frustrated Lewis pair (FLP) tBu3P:B(C6F5)3 is investigated computationally to demonstrate the importance of the dispersion interaction. To this end, the interaction between alkyl-substituted phosphines (Me3P and tBu3P) and hexafluorobenzene (C6F6) is first investigated. Driven by the lone-pair to ,-orbital interaction, the binding energy is found to be even larger than usual ,,, interaction energies between small aromatic compounds. This character, which is inherited to fluorophenyl-substituted B(C6F5)3 in the FLP, induces large flexibility in the FLP over the molecular surface of B(C6F5)3. This soft interaction, in turn, causes an entropic stabilization of the FLP formation in comparison with classical Lewis pairs based on close and tight PB dative bonds. It also suggests a diverse nature of the FLP when it is involved in chemical reactions. Even with the cooperative participation of the perfluorophenyl groups, a detailed inspection of the FLP interaction potential energy surface indicates that the boron atom is still the major interaction site for the pair formation. This non-negligible direct PB interaction, which is related also to the soft nature of the borane frontier orbital, is further supported by substantial spatial overlap between the frontier orbitals on the phosphine/borane fragments and their interaction energy estimations. [source] Optical, Redox, and NLO Properties of Tricyanovinyl Oligothiophenes: Comparisons between Symmetric and Asymmetric Substitution PatternsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006Juan Casado Dr. Abstract A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits ,-stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs). [source] Bond Orders between Molecular FragmentsCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2006Adam J. Bridgeman Dr. Abstract An extension of the Mayer bond order for the interaction between molecular fragments is presented. This approach allows the classical chemical concepts of bond order and valence to be utilised for fragments and the interactions between the fragments and symmetry-adapted linear combinations to be analysed. For high-symmetry systems, the approach allows the contribution from each irreducible representation to be assessed and provides a semiquantitative measure of the role of each bonding mode to interfragment interactions. The utility of this tool has been examined by a study of the bonding in symmetrical sandwich complexes. The validity of the frontier-orbital approach and the contributions from each frontier-orbital interaction can also be assessed within this model. As demonstrated by a study of a number of mixed-sandwich complexes, the model proves to be especially useful for low-symmetry systems in which separation of the ,, , and , roles in bonding of the ligand is difficult to assess. The fragment bond order describes the interaction between preoptimized fragment orbitals and is independent of the charges that are placed on these fragments. Although the method allows the chemist to define fragments in any way they choose, most insight is gained by using the same frontier orbitals employed so successfully in perturbational molecular-orbital approaches. The results are free from the influence of the electron-counting method used to describe fragments, such as the rings and metals in sandwich complexes. [source] | |||||||||||||