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.

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 37 Cl and 13 C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anti carbon skeleton (t(C 1 -C 2 -O-C 3 ) = 137.8(11)1 or 167.4(19)1), 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) E 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 -s*) 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.

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The conformational behaviors of trans‐2,3‐dihalo‐1,4‐dithiane [halo = F (1), Cl (2), Br (3)] and trans‐2,5‐dihalo‐1,4‐dithiane [halo = F (4), Cl (5), Br (6)] have been analyzed by means of complete basis set CBS‐4, hybrid‐density functional theory (B3LYP/6‐311 + G**//B3LYP/6‐311 + G**) based methods, and natural bond orbital (NBO) interpretation. Both methods showed that the axial conformations of compounds 1–5 are more stable than their equatorial conformations but CBS‐4 resulted in an equatorial preference for compound 6. The Gibbs free energy difference (Geq−Gax) values (i.e., ΔGeq–ax) at 298.15 K and 1 atm between the axial and equatorial conformations decrease from compound 1 to compound 2 but increase from compound 2 to compound 3. Also, the calculated ΔGeq–ax values decrease from compound 4 to compound 6. The NBO analysis of donor–acceptor (LP → σ*) interactions showed that the anomeric effect (AE) increase from compound 1 to compound 3 and also from compound 4 to compound 6. On the other hand, the calculated dipole moment values between the axial and equatorial conformations [Δ(µeq−µax)] decrease from compound 1 to compound 3. The conflict between the increase of AE and the decrease of Δ(µeq−µax) values could explain the variation of the calculated ΔGeq–ax for compounds 1–3. The Gibbs free energy difference values between the axial and equatorial conformations (i.e., ΔGax–ax and ΔGeq–eq) of compounds 1 and 4, 2 and 5 and also 3 and 6 have been calculated. The correlations between the AE, bond orders, pairwise steric exchange energies (PSEE), ΔGeq–ax, ΔGax–ax, ΔGeq–eq, dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–6 have been investigated. Copyright © 2010 John Wiley & Sons, Ltd.

Natural bond orbital (NBO), nuclear magnetic resonance (NMR) analysis and hybrid-density functional theory based method (B3LYP/Def2-TZVPP) were used to investigate the correlation between the nucleus-independent chemical shifts [NICS, as an aromaticity criterion], σ Al(1)-X2(b) → σ*Al(3)-X₄(b) electron delocalizations and the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ to 2AlX₃ (X = F, Cl, Br, I). The results obtained showed that the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ decrease from Al₂F₆ to Al₂I₆. Like aromatic molecules, these compounds have relatively significant negative NICSiso(0) values. Clearly, based on magnetic criteria, they exhibit aromatic character and make it possible to consider them as σ-delocalized aromatic species, such as Möbius σ-aromatic species. The σ-aromatic character which is demonstrated by their NICSiso(0) values decreases from Al₂F₆ to Al₂I₆. The NICSiso values are dominated by the in-plane σ₂₂ (i.e., σyy, the plane containing halogen atoms bridged) chemical shift components. The increase of the NICSiso values explains significantly the decrease of the corresponding dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆. Importantly, the NBO results suggest that in these compounds the dissociation energies are controlled by the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations. The decrease of the stabilization energies associated with σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations is in accordance with the variation of the calculated NICSiso values. The correlations between the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆, σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic orbitals (NAOs) and NICSiso values have been investigated.

As constantes de estabilidade dos complexos de íons tálio(I) e cádmio(II) com dipeptídeos de glicil-L-fenilalanina e L-fenilalanilglicina foram determinadas em solução aquosa a 25 °C e meio iônico 0,1 mol dm -3 usando uma combinação de técnicas potenciométricas e espectrofotométricas. Perclorato de sódio foi usado para manter a força iônica. A composição dos complexos formados foi determinada e observou-se que tálio (I) 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 °C 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 (

The B3LYP/6-31G à method was used to investigate the configurational properties of allene (,9-decanonaene (9). The calculations at the B3LYP/6-31G à level of theory showed that the mutual interconversion energy barrier in compounds 1-8 are: 209.73, 131.77, 120.34, 85.00, 80.91, 62.19, 55.56, and 46.83 kJ mol À1 , respectively. The results showed that the difference between the average C --C double bond lengths (d) values in cumulene compounds 1 and 2, is larger than those between 7 and 8, which suggest that with large n (number of carbon atoms in cumulene chain), the d values approach a limiting value. Accordingly, based on the plotted data, the extrapolation to n ¼ 1, gives nearly the same limiting d(i. e., d lim ). Also, NBO results revealed that the sum of p-bond occupancies, p occupancy , decrease from 1 to 8, and inversely, the sum of p-antibonding orbital occupancies, p à occupancy , increase from compound 1 to compound 8. The decrease of Dðp occupancy À p à occupancy Þ values for compounds 1-8, is found to follow the same trend as the barrier heights of mutual interconversion in compounds 1-8, while the decrease of the barrier height of mutual interconversion in compounds 1-8 is found to follow the opposite trend as the increase in the number of carbon atom. Accordingly, besides the previously reported allylic resonant stabilization effect in the transition state structures, the results reveal that the d values, Dðp occupancy À p à occupancy Þ, D(E HOMO À E LUMO ), and the C atom number could be considered as significant criteria for the mutual interconversion in cumulene compounds 1-8. This work reports also useful predictive linear relationships between mutual interconversion energy barriers (DE a 0 ) in cumulene compounds and the following four parameters: d, Dðp occupancy À p Ã