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Aromatic Character (aromatic + character)
Selected AbstractsAn ab initio valence bond (VB) calculation of the , delocalization energy in borazine, B3N3H6,HETEROATOM CHEMISTRY, Issue 5 2005Daniel Benker Valence bond (VB) calculations using a double-zeta D95 basis set have been performed for borazine, B3N3H6 and for benzene, C6H6 in order to determine the relative weights of individual standard Lewis structures. In the delocalized resonance scheme of borazine, the structure (I) with no double bonds and three lone pairs of electrons at the three nitrogen atoms is the major contributor with a structural weight of 0.17, followed by six equivalent Lewis structures with one double bond and two lone pairs at two nitrogen atoms (II) with weights of 0.08 each. In the case of benzene, the two Kekulé structures (III) contribute with structural weights of 0.15 each, followed by 12 equivalent ionic structures (IV) with weights of 0.03 each, followed by the three equivalent Dewar-type structures (V) with structural weights of 0.02 each. The values of 54.1 and 45.8 kcal mol,1 for the delocalization energies of borazine and benzene were estimated. Therefore, B3N3H6 is calculated to have substantial aromatic character, similar to benzene, when we assume that the resonance energy can provide a criterion for aromaticity. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:311,315, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20095 [source] Electronic properties of tricoordinated phosphorus in hexagonal phosphininium compounds and molecular aromaticityJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2007Wei-Qi Li Abstract Calculations with B3LYP within quantum chemical density functional theory have been carried out for 1- H -phosphininium cation and a series of 1- R -phosphininium molecules (R = cyclopentadiene, , and , pyrroles, , and , phosphole, C5BH5 and CH2). The negative nuclear-independent chemical shift values and the positive aromatic stabilization energies confirm that they are aromatic compounds. In particular, the 1- H -phosphininium cation even exhibits stronger aromatic character than the well-known aromatic phosphinine. The aromatic substituents have strong capability to attract electrons. It is the conjugation and aromaticity that keeps the stability and conformations of the molecules investigated. Owing to the perturbation of the aromatic substituted groups, the predicted large T values and the enlarged HOMO-LUMO gap of the phosphininium cation indicate that these compounds are expected in experiment. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] The role of tryptophan in the antibacterial activity of a 15-residue bovine lactoferricin peptideJOURNAL OF PEPTIDE SCIENCE, Issue 4 2001Bengt Erik Haug Abstract Bovine lactoferricin is a 25-residue antibacterial peptide isolated after gastric cleavage of the iron transporting protein lactoferrin. A 15-residue fragment, FKCRRWQWRMKKLGA of this peptide sustains most of the antibacterial activity. In this truncated sequence, the two Trp residues are found to be essential for antibacterial activity. The anchoring properties of Trp, as have been observed in membrane proteins, are believed to be important for the interaction of Trp containing antibacterial peptides with bacterial cell membranes. We have investigated the molecular properties which make Trp important for the antibacterial activity of the 15-residue peptide by replacing Trp with natural and unnatural aromatic amino acids. This series of peptides was tested for antibacterial activity against Echerichia coli and Staphylococcus aureus. We found that neither the hydrogen bonding ability nor the amphipathicity of the indole system are essential properties for the effect of Trp on the antibacterial activity of the peptides. Replacement of Trp with residues containing aromatic hydrocarbon side chains gave the most active peptides. We propose that aromatic hydrocarbon residues are able to position themselves deeper into the bacterial cell membrane, making the peptide more efficient in disrupting the bacterial cell membrane. From our results the size, shape and aromatic character of Trp seem to be the most important features for the activity of this class of Trp containing antibacterial peptides. Copyright © 2001 European Peptide Society and John Wiley & Sons, Ltd. [source] Acid,base behavior of triazoleporphyrazines in proton-donating mediaJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2005O. G. Khelevina Abstract The acid,base properties of some peripheral substituted triazoleporphyrazines in proton-donating media with poor-donating character were experimentally studied. The substitution of one pyrrole moiety in the porphyrazine-like compounds by one triazole ring in the triazoleporphyrazines leads to an increase in the basicity. The protonation of the triazoleporphyrazines results in a hypsochromic shift of the Q-bands in the UV,visible spectra. A DFT study of some selected structural models of the unsubstituted triazoleporphyrazine shows that the protonation strongly influences on the molecular electron structure of this compound and that it occurs preferentially through the nitrogen atom located at position 4 of triazole ring. The protonation through the other basic centers (the triazoleporphyrazine is a multicenter conjugated base) leads to different protonated forms which differ notably in their aromatic character. Therefore, this compound could be considered an intramolecular switch of aromaticity. Copyright © 2004 John Wiley & Sons, Ltd. [source] Sandwich Compounds of Transition Metals with Cyclopolyenes and Isolobal Boron AnaloguesCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2010Tatyana Abstract A series of sandwich compounds of transition metals (M=Ni, Fe, Cr) with cyclic hydrocarbon (M(CH)n) and borane (M(BH2)n), ligands (including mixed hydrocarbon/borane sandwiches) has been studied using density functional theory (B3LYP/6-311+G(df,p)). Multicenter bonding between the central metal atom and basal cycloborane rings provides stabilization to planar cycloborane species. Large negative NICS values allude to aromatic character in the cycloboranes similar to the analogous cyclic hydrocarbons. The ability of cycloborane sandwiches to stabilize attached carbocations, radicals and carbanions is also assessed. [source] Is Cyclopropane Really the ,-Aromatic Paradigm?CHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2009Wei Wu Prof. Abstract Dewar proposed the ,-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts,"There is no need to involve ,,-aromaticity',",other analyses, also indirect, resulted in wide-ranging estimates of the ,-aromatic stabilization energy. Moreover, the aromatic character of "in-plane", "double", and cyclically delocalized ,-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A2007, 111, 8163,8169) challenged the existence of an induced ,-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the ,-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the ,-aromatic stabilization energy of cyclopropane is, at most, 3.5,kcal,mol,1 relative to propane, and is close to zero when n -butane is used as reference. Trisilacyclopropane also has very little ,-aromatic stabilization, compared to Si3H8 (6.3,kcal,mol,1) and Si4H10 (4.2,kcal,mol,1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported. [source] Conformational Control of [26]Hexaphyrins(1.1.1.1.1.1) by meso -Thienyl SubstituentsCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2007Masaaki Suzuki Abstract Conformational preference and chemical stability of meso -aryl-substituted [26]hexaphyrins(1.1.1.1.1.1) ([26]ArH) depend upon meso -aryl substituents. Although only a planar and rectangular conformation (type-II conformation) has been identified for [26]ArH so far, we have demonstrated here that a different conformation with all the pyrroles pointing inward (type-I conformation) is preferred for [26]ArH (7 and 11 -I) bearing small 2-thienyl or 3-thineyl substituents at 15- and 30-positions. Both type-I and type-II [26]ArH exhibit diatropic ring currents, reflecting aromatic character. Type-I [26]ArH, such as 7 and 11 -I, have been shown to serve as an effective ligand for PdII ions to provide bis-PdII complexes 12 and 13 with N3C1 coordination through facile CH bond activation. [source] High-Electron-Density C6H6 Units: Stable Ten-,-Electron Benzene ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2005Martin Diefenbach Dr. Abstract The first stable benzene molecule with ten , electrons is predicted. Stability is achieved through barium atoms acting as an electron-donating "matrix" to C6H6 in the inverted sandwich complex [Ba2(C6H6)]. The bis(barium)benzene complex has been computed at the density functional level of theory by using the hybrid functional mPW1PW91. Ab initio calculations were performed by using the coupled-cluster expansion, CCSD(T). Nucleus independent chemical shift (NICS) indices imply distinct aromatic character in the benzene ring of bis(barium)benzene. The D6h -symmetric structure with a 1A1g electronic ground state represents a thermochemically stable, aromatic benzene molecule with four excess , electrons, stabilised by two barium ions. A possible molecular wire, built up from Ba end-capped thorium,benzene "sandwiches", is discussed. [source] Electronic Structure of the Ground and Excited States of ,-CarbolineCHEMPHYSCHEM, Issue 6 2008Vicente Pérez Mondéjar Abstract Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of ,-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects. [source] | ||||||||||