Ab initio models for multiple-hydrogen exchange: Comparison of cyclic four- and six-center systems

High-level ab initio calculations {QCISD(T)/6-311 +G**//MP2(fu)/6-31 +G**, with corrections for higher polarization [evaluated at MP2/6-311 +G(3df,2p)] and ΔZPE//MP2(fu)/6-31 +G**, i.e., comparable to Gaussian-2 theory} indicate concerted mechanisms for double- and triple-hydrogen exchange reactions in HF and HCl dimers and trimers, in mixed dimers and trimers containing one NH₃, and in mixed dimers of HF, HCl, and NH₃ with formic acid. All these reactions proceed via cyclic four- or six-center transition structures, the latter being generally more favorable. Calculated activation barriers (ΔH^d? at 0 K, kcal/mol) are 42.3 for (HF)₂, 20.3 for (HF)₃, 41.2 for (HCl)₂, 25.6 for (HCl)₃, 36.0 for NH₃-HF, 10.6 for NH₃(HF)₂, 19.9 for NH₃-HCl, 2.3 for NH₃(HCl)₂, 9.7 for HCO₂H-HF, 7.0 for HCO₂H-HCl, and 11.3, for HCO₂H-NH₃. The barriers are lower for the more ionic systems and when more ion pair character is present. © John Wiley & Sons, Inc.     

Can bridged 1,6-X-[10]annulenes (X = SiH(2), SiMe(2), PH,and S) exist?

[structure: see text] In contrast to the 1,6-X-[10]annulenes (X = CH(2), O, NH) with delocalized forms (c), their X = PH and S counterparts favor the bisnorcaradiene structures (b). Forms b and c are close in energy with X = SiH(2) and SiMe(2). The computed nucleus independent chemical shifts (NICS), show both annulenes (c) and cyclic polyenes (a) to be aromatic. Strain-introduced structural localization, e.g., due to four bulky SiMe(3) substituents, reduces but does not eliminate aromaticity.     

Investigation of a putative mobius aromatic hydrocarbon. The effect of benzannelation on mobius [4n]annulene aromaticity

The first experimental example of a [4n]annulene derivative with one Mobius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the Mobius [16]annulene core of 1 shows large bond alternation (Deltar up to 0.157 A). Delocalization in this core is inhibited by large dihedral angles, which hinders effective pi overlap. This conclusion is supported by computational results (B3LYP/6-311+G) on 1 and several less benzannelated derivatives, based on geometric (Deltar, Deltar(m), Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Mobius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE(II) values of just the polyene bridge portion of 1 and its Huckel counterpart suggests that stabilization of 1 relative to its Huckel isomer is confined to the polyene bridge and is not due to a delocalized pi circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE(II) value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic.     

Ligand-exchange processes on solvated lithium cations: DMSO and water/DMSO mixtures.

Solutions of LiClO(4) in solvent mixtures consisting of dimethylsulfoxide (DMSO) and water, or DMSO and gamma-butyrolactone, were studied by (7)Li NMR spectroscopy (for complexation by cryptands in gamma-butyrolactone as a solvent, see: E. Pasgreta, R. Puchta, M. Galle, N. J. R. van Eikema Hommes, A. Zahl, R. van Eldik, J. Incl. Phen., 2007, 58, 81-88). Chemical shifts indicate that the Li(+) ion is coordinated by four DMSO molecules. In the binary solvent mixture of water and DMSO, no selective solvation is detected, thus indicating that on increasing the water content of the solvent mixture, DMSO is gradually displaced by water in the coordination sphere of Li(+). The ligand-exchange mechanism of Li(+) ions solvated by DMSO and water/DMSO mixtures was studied using DFT calculations. Ligand exchange on [Li(DMSO)(4)](+) was found to follow a limiting associative (A) mechanism. The displacement of coordinated H(2)O by DMSO in [Li(H(2)O)(4)](+) follows an associative interchange mechanism. The suggested mechanisms are discussed in reference to available experimental and theoretical data.     

Mechanistic studies on the formation of Pt(II) hydroformylation catalysts in imidazolium-based ionic liquids

The kinetics of the formation of the active species cis-[Pt^II(PPh₃)₂Cl(SnCl₃)] and cis-[Pt^II(PPh₃)₂(SnCl₃)₂] from the hydroformylation catalyst precursor cis-[Pt^II(PPh₃)₂Cl₂] in the presence of SnCl₂, was studied in two different imidazolium-based ionic liquids. A large range of different chlorostannate melts consisting of 1-butyl-3-methyl-imidazolium cations and [Sn_xCl_y]^{(−y + 2x)} anions with varying molar fraction of SnCl₂, were prepared and characterized by ¹H and ¹¹⁹Sn NMR. The observed chemical shifts point to major changes in the composition of the anionic species within the melt. The second ionic liquid employed, viz., 1-butyl-3-methyl-imidazolium-bis(trifluormethylsulfonyl)amide was prepared in a colorless quality that enabled its application in kinetic studies. The concentration and temperature dependence of the substitution of Cl⁻ by [SnCl₃]⁻ to yield cis-[Pt^II(PPh₃)₂Cl(SnCl₃)], could be studied in detail. Theoretical (DFT) calculations were employed to model the reaction progress and to resolve the role of the ionic liquid in the activation of the catalyst. The available results are presented and a plausible mechanism for the formation of the catalytically active species is suggested.     

Bicyclic diironcryptates with mobile guest cations: A new class of solid ionic conductors

Polycrystalline bicyclic diironcryptates with different mobile guest cations were synthesized and characterized by XRD analysis, IR spectroscopy, FAB-mass spectroscopy, elemental analysis, electrical conductivity spectroscopy and ion exchange experiments. We find strong indication that the guest cations exhibit a long-range mobility and that their mobility is governed by their size relative to the size of the cryptand molecules. On the other hand, the valence of the guest cations seems to play a minor role for the mobility. These results are discussed in comparison to the properties of conventional solid cationic conductors.     

A rapid assay for on-site monitoring of infliximab trough levels: a feasibility study

Monitoring levels of biologicals against tumor necrosis factor (TNF) has been suggested to improve therapeutic outcomes in inflammatory bowel diseases (IBDs). This pilot study describes a rapid lateral flow (LF)-based assay for on-site monitoring of serum trough levels of humanized monoclonal antibody infliximab (IFX). The applied chromatographic method utilizes sequential flows of diluted serum, wash buffer, and an immunoglobulin generic label on LF strips with a Test line comprised of TNF-α. The successive flows permitted enrichment of IFX at the Test line before the label was applied. The label, luminescent upconverting phosphor (UCP) particles coated with protein-A, emits a 550-nm visible light upon excitation with 980-nm infrared light. IFX concentrations were determined through measurement of UCP fluorescence at the Test line. The assay was optimized to detect IFX levels as low as 0.17 μg/mL in serum. For patients with IBD, this limit is appropriate to detect levels associated with loss of response (0.5 μg IFX/mL). The assay was evaluated with clinical samples from patients with Crohn’s disease and correlated well within the physiologically relevant range from 0.17 to 10 μg/mL with an IFX-specific ELISA. Performance of the assay was further successfully validated with samples from blood donors, IFX negative IBD patients, and rheumatoid arthritis patients that had developed anti-IFX antibodies. Because of its generic nature, the assay is suited for detecting most therapeutic anti-TNF-α monoclonal antibodies.

Is tetrahedral H42+ a minimum? Anomalous behavior of popular basis sets with the standard p exponents on hydrogen

The nature of the tetrahedral H₄²⁺ stationary point (minimum or triply degenerate saddle) depends remarkably upon the theoretical level employed. Harmonic vibrational analyses with, e.g., the 6-31G** (and 6-31 + +G**) and Dunning's [4s2p1d;2s1p] [D95(d,p)] basis sets using the standard p exponent suggest (erroneously) that the T_d geometry is a minimum at both the HF and MP2 levels. This is not the case at definitive higher levels. The C₃H₄²⁺ structure with an apical H is another example of the failure of the calculations with the 6-31G**, 6-311G**, and D95(d,p) basis sets. Even at MP2/6-31G** and MP2/ cc-pVDZ levels, the C_3v structure has no negative eigenvalues of the Hessian. Actually, this form is a second-order saddle point as shown by the MP2/6-31G** calculation with the optimized exponent. The D_4h methane dication structure is also an example of the misleading performance of the 6-31G** basis set. In all these cases, energy-optimized hydrogen p exponents give the correct results, i.e., those found with more extended treatments. Optimized values of the hydrogen polarization function exponents eliminate these defects in 6-31G** calculations. Species with higher coordinate hydrogens may also be calculated reliably by using more than one set of p functions on hydrogen [e.g., the 6-31G(d,2p) basis set]. Not all cases are critical. A survey of examples, also including some boron compounds, provides calibration. © 1993 John Wiley & Sons, Inc.