Essential methodological improvements in the oxygen isotope ratio analysis of N‐containing organic compounds

The quantitative conversion of organically bound oxygen into CO, a prerequisite for the ¹⁸O/¹⁶O analysis of organic compounds, is generally performed by high‐temperature conversion in the presence of carbon at ～1450°C. Since this high‐temperature procedure demands complicated and expensive equipment, a lower temperature method that could be utilized on standard elemental analyzers was evaluated. By substituting glassy carbon with carbon black, the conversion temperature could be reduced to 1170°C. However, regardless of the temperature, N‐containing compounds yielded incorrect results, despite quantitative conversion of the bound oxygen into CO. We believe that the problems were partially caused by interfering gases produced by a secondary decomposition of N‐ and C‐containing polymers formed during the decomposition of the analyte. In order to overcome the interference, we replaced the gas chromatographic (GC) separation of CO and N₂ by reversible CO adsorption, yielding the possibility of collecting and purifying the CO more efficiently. After CO collection, the interfering gases were vented by means of a specific stream diverter, thus preventing them from entering the trap and the mass spectrometer. Simultaneously, a make‐up He flow was used to purge the gas‐specific trap before the desorption of the CO and its subsequent mass spectrometric analysis. Furthermore, the formation of interfering gases was reduced by the use of polyethylene as an additive for analytes with a N:O ratio greater than 1. These methodological modifications to the thermal conversion of N‐containing analytes, depending on their structure or O:N ratio, led to satisfactory results and showed that it was possible to optimize the conditions for their individual oxygen isotope ratio analysis, even at 1170°C. With these methodological modifications, correct and precise δ¹⁸O results were obtained on N‐containing analytes even at 1170°C. Differences from the expected standard values were below ±1‰ with standard deviations of the analysis <0.2‰. Copyright © 2010 John Wiley & Sons, Ltd.     

On a group of two-parameter intramolecular distributions and their properties

A group of two-parameter intramolecular distributions for degenerate vibrations has been investigated. The distributions are of physical significance in calculating electronic transitions and have certain important properties which are formulated in terms of addition theorems.     

Unusual Regioselectivity in the Reactions of Chlorocyclophosphazene-Containing Crown Ethers with Dinucleophiles

Abstract

New macrocyclic ligands with cyclophosphazene sub-units, representing geminally diamino-bridged derivatives of BINO-crown type (III), and bisansa-cyclosubstituted ones of chiral PNP cryptand type (IV) have been obtained by the regioselective substitution reactions of monoansa(oxytetraethylenoxy) reactive PNP-crown, N₃P3CI₄[O(CH₂CH20)₄] (II) with the dinucleophiles, like alkylenediamines or sodium diarylates. The supramolecular control of these processes is discussed.     

Prediction of water's isotropic nuclear shieldings and indirect nuclear spin–spin coupling constants (SSCCs) using correlation‐consistent and polarization‐consistent basis sets in the Kohn–Sham basis set limit

Density functional theory (DFT) was used to estimate water's isotropic nuclear shieldings and indirect nuclear spin–spin coupling constants (SSCCs) in the Kohn–Sham (KS) complete basis set (CBS) limit. Correlation‐consistent cc‐pVxZ and cc‐pCVxZ (x = D, T, Q, 5, and 6), and their modified versions (ccJ‐pVxZ, unc‐ccJ‐pVxZ, and aug‐cc‐pVTZ‐J) and polarization‐consistent pc‐n and pcJ‐n (n = 0, 1, 2, 3, and 4) basis sets were used, and the results fitted with a simple mathematical formula. The performance of over 20 studied density functionals was assessed from comparison with the experiment. The agreement between the CBS DFT‐predicted isotropic shieldings, spin–spin values, and the experimental values was good and similar for the modified correlation‐consistent and polarization‐consistent basis sets. The BHandH method predicted the most accurate ¹H, ¹⁷O isotropic shieldings and ¹J(OH) coupling constant (deviations from experiment of about ? 0.2 and ? 1 ppm and 0.6 Hz, respectively). The performance of BHandH for predicting water isotropic shieldings and ¹J(OH) is similar to the more advanced methods, second‐order polarization propagator approximation (SOPPA) and SOPPA(CCSD), in the basis set limit. Copyright © 2008 John Wiley & Sons, Ltd.     

On the convergence of zero-point vibrational corrections to nuclear shieldings and shielding anisotropies towards the complete basis set limit in water

ABSTRACT

The method and basis set dependence of zero-point vibrational corrections (ZPVCs) to nuclear magnetic resonance shielding constants and anisotropies has been investigated using water as a test system. A systematic comparison has been made using the Hartree–Fock, second-order Møller–Plesset perturbation theory (MP2), coupled cluster singles and doubles (CCSD), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) and Kohn–Sham density functional theory with the B3LYP exchange-correlation functional methods in combination with the second-order vibrational perturbation theory (VPT2) approach for the vibrational corrections. As basis sets, the correlation consistent basis sets cc-pVXZ, aug-cc-pVXZ, cc-pCVXZ and aug-cc-pCVXZ with X = D, T, Q, 5, 6 and the polarisation consistent basis sets aug-pc-n and aug-pcS-n with n = 1, 2, 3, 4 were employed. Our results show that basis set convergence of the vibrational corrections is not monotonic and that very large basis sets are needed before a reasonable extrapolation to the basis set limit can be performed. Furthermore, our results suggest that coupled cluster methods and a decent basis set are required before the error of the electronic structure approach is lower than the inherent error of the VPT2 approximation.     

Performance of revised STO(1M)-3G basis set for prediction of 5-fluorocytosine chemical shifts

Nuclear shieldings and chemical shifts of 5-fluorocytosine (5FC) were predicted in the gas phase and DMSO solution modeled by polarizable continuum model using B3LYP density functional and revised STO(1M)-3G basis set. For comparison, eight arbitrary selected basis sets including STO-3G and medium-size Pople-type and larger dedicated Jensen-type ones were applied. The former basis sets were significantly smaller, but the calculated structural parameters, harmonic vibrational frequencies, were very accurate and close to those obtained with larger, polarization-consistent ones. The predicted ¹³C and ¹H chemical shieldings of 5FC and cytosine, selected as parent molecule, were acceptable (root mean square for ¹³C chemical shifts in DMSO of about 5 ppm and less) though less accurate than those calculated with large basis sets, dedicated for prediction of nuclear magnetic resonance parameters.     

Die vibronische Kopplung in Koordinationsverbindungen am Beispiel von Hexahalogenokomplexen

Ohne Zusammenfassung

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The vibronic coupling in coordination compounds. hexahalogeno complexes as example