Prototropic and metallotropic migration of isolobal fragments on indol rings. Theoretical study and NBO analysis

Molecular structures, energies, NBO analysis and sigmatropic behaviour of 1-Indenyl(dihydro)borane (1) and 1-Indenyl-threecarbonylcobalt(I) (2) were investigated using DFT and ab initio molecular orbital methods. In these compounds BH2 and Co(CO)3 fragments areisolobal. The Results of calculations using B3LYP, HF and MP2methods [Basis set 6-311+G**] showed that -BH2 and -Co(CO)3 had similar behaviour in sigmatropic shifts. Prototropic shifts in compounds 1 and 2 have similar mechanisms too. Results showed that metallotrotropic shift is faster than Prototrpic shift in compounds 1 and 2. The activation energies (Ea) of Prototropic shift in compounds 1 and 2 are 18.83 and 17.38 kcal.mol-1. These energies are higher than -BH2 shifts in compound 1 (10.11 kcal.mol-1) or migration of -Co(CO)3 fragment in compound 2 (12.39 kcal.mol-1). Lower amount of activation energy in borotropic shift and cobalt`s fragment shift show that rotation of boron and cobalt on the indol ring can happen in the ambient temperature. Calculation results revealed that migration of proton and Co(CO)3 was carried out via suprafacial[1,2]-sigmatropic mechanism while -BH2 shift took place via antrafacial [1,3]-rearangment.Â

Complexes of thallium(I) and cadmium(II) with dipeptides of L-phenylalanylglycine and Glycyl-L-phenylalanine

The stability constants of the complexes of thallium(I) and cadmium(II) ions with dipeptides of glycyl-L-phenylalanine and L-phenylalanylglycine were determined in aqueous solution at 25 ºC and 0.1 mol dm-3 ionic medium using a combination of potentiometric and spectrophotometric techniques. Sodium perchlorate was used to maintain the ionic strength. The composition of the formed complexes was determined and it was shown that thallium(I) and cadmium(II) forms two mononuclear 1:1 species with the ligands, of the type [Tl(HL)]+, TlL, [Cd(HL)]2+ and [CdL]+ in the pH range of study (1.5-10.5), where L represents a fully dissociated ligand. The logarithms of the cumulative stability constants, betaxyz, of the complexes, [(Metal ion)x(H+)y(ligand) z], are log beta111 and log beta101: 12.15, 3.39 (for Tl+ with L-phenylalanylglycine), 11.36, 2.13 (for Tl+ with glycyl-L-phenylalanine), 12.06, 2.82 (for Cd2+ with L-phenylalanylglycine), 10.70 and 1.70 (for Cd2+ with glycyl-L-phenylalanine), respectively

Natural Bond Orbital Theory of Pseudo-Jahn–Teller Effects

We describe a unified picture of symmetry-breaking electronic interactions that are usually described as “pseudo-Jahn–Teller (PJT) effects” and attributed to vibronic coupling but can also be associated with hyperconjugative donor–acceptor interactions in the framework of natural bond orbital (NBO) and natural resonance theory (NRT) analysis. We show how NBO/NRT descriptors offer a simplified alternative to the vibronic coupling picture of PJT effects that yields both improved cause–effect specificity and chemically enriched understanding of symmetry-breaking phenomena but with no necessary input from ground-state vibrational or excited-state electronic properties. Comparative NBO/NRT vs vibronic coupling analyses of PJT effects are illustrated for two well-known cases: trans-bending in Si₂H₄ and higher Group-14 homologues of ethylene and chain-kinking in cyclopentadienylideneketene (C₅H₄CCO) and related cumulene ketones. The conceptual and practical advantages of the NBO-based hyperconjugative approach may be expected to extend to numerous PJT-type symmetry-breaking phenomena throughout the chemical sciences

On the Covalent Character of Rare Gas Bonding Interactions: A New Kind of Weak Interaction

At the averaged quadratic coupled-cluster (AQCC) level, a number of selected rare gas (Rg) containing systems have been studied using the quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and several other analysis methods. According to the criteria for a covalent bond, most of the Rg–M (Rg = He, Ne, Ar, Kr, Xe; M = Be, Cu, Ag, Au, Pt) bonds in this study are assigned to weak interactions instead of van der Walls or covalent ones. Our results indicate that the rare gas bond is a new kind of weak interaction, like the hydrogen bond for example

Exploring the Origin of the Generalized Anomeric Effects in the Acyclic Nonplanar Systems

Contrary to the published conclusions in the literature concerning the origin of the generalized <i>anomeric</i> relationships in open-chain nonplanar systems, its origin has remained an open question. In order to explore the origin of the generalized <i>anomeric</i> relationships in open-chain nonplanar systems, we assessed the roles and contributions of the effective factors on the conformational properties of methyl propargyl ether (<b>1</b>), methyl propargyl sulfide (<b>2</b>), and methyl propargyl selenide (<b>3</b>) by means of the G3MP2, CCSD­(T), MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. We examined the contributions of the hyperconjugative interactions on the conformational preferences of compounds <b>1</b>–<b>3</b> by the deletions of the orbitals overlapping from the Fock matrices of the <i>gauche</i>- and <i>anti</i>-conformations. The trend observed for energy changes in the Fock matrices justify the variations of the <i>gauche</i>-conformations preferences going from compound <b>1</b> to compound <b>3</b>, revealing that the hyperconjugative interactions are solely responsible for the generalized <i>anomeric</i> relationships in compounds <b>1</b>–<b>3</b>. Accordingly, the conclusions published in the literature concerning the origin of the generalized <i>anomeric</i> effect in the acyclic nonplanar compounds should be revised by these findings. The Pauli exchange type repulsions (PETR) are in favors of the <i>gauche</i>-conformations and the variations of the PETR differences between the <i>gauche</i>- and <i>anti</i>-conformations of compounds <b>1</b>–<b>3</b> correlate well with their <i>gauche</i>-conformations preferences, revealing that the generalized <i>anomeric</i> relationships in compounds <b>1</b>–<b>3</b> have also the Pauli exchange-type repulsions origin. The resemblance between the preorthogonal natural bond orbitals (that are involved in the hyperconjugative interactions) and their corresponding molecular orbitals have been investigated