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

Structural evidence of anomeric effects in the anesthetic isoflurane

The conformational and structural properties of the inhalational anesthetic isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. Two conformers of the isolated molecule were identified from the rotational spectrum of the parent and several 37Cl and 13C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anticarbon skeleton (τ(C1–C2–O–C3) = 137.8(11)° or 167.4(19)°), differing in the trans (AT) or gauche (AG) orientation of the difluoromethyl group. The conformational abundances in the jet were estimated from relative intensity measurements as (AT)/(AG) ≈ 3 ∶ 1. The structural preferences of the molecule have been rationalized with supporting ab initio calculations and natural-bond-orbital (NBO) analysis, which suggest that the molecule is stabilized by hyperconjugative effects. The NBO analysis of donor–acceptor (LP → σ*) interactions showed that these stereoelectronic effects decrease from the AT to AG conformations, so the conformational preferences can be accounted for in terms of the generalized anomeric effect