The Remarkable Structure of Lithium Cyanide/Isocyanide

Electron correlation corrections have a considerable influence on the relative stabilities of lithium isocyanide ( 1 ), lithium cyanide ( 2 ), and the bridged form, 3 . While Hartree-Fock theory finds 1 to be most stable and 3 not to be a minimum, MP2/6-31G* optimization indicates 3 to be the global minimum. At higher levels employing full fourth-order Møller-Plesset theory and a quadruply split valence and polarized basis set (MP4STDQ/6-311+G*), 2 is only about 2 kcal/mol less stable than 1 and 3 , which are indicated to have nearly the same energy. LiNC thus is similar to C(Na)N and C(K)N, both of which are known to prefer T-shaped (bridged) structures in the gas phase. However, to an even greater extent than formerly realized, rotation of the lithium cation around the cyanide anion nucleus should be practically free. ΔH (LiCN) = 32.8 kcal/mol is estimated from the calculated lithium cation affinity of 151.2 kcal/mol. In addition, we find at the MP4SDTQ/6-31+G*//MP2/6-31G* level that the bridged form of NaCN is favored by 2–3 kcal/mol over the corresponding linear forms, which have nearly the same energy.     

Toward true DNA base-stacking energies: MP2, CCSD(T), and complete basis set calculations

Stacking energies in low-energy geometries of pyrimidine, uracil, cytosine, and guanine homodimers were determined by the MP2 and CCSD(T) calculations utilizing a wide range of split-valence, correlation-consistent, and bond-functions basis sets. Complete basis set MP2 (CBS MP2) stacking energies extrapolated using aug-cc-pVXZ (X = D, T, and for pyrimidine dimer Q) basis sets equal to -5.3, -12.3, and -11.2 kcal/mol for the first three dimers, respectively. Higher-order correlation corrections estimated as the difference between MP2 and CCSD(T) stacking energies amount to 2.0, 0.7, and 0.9 kcal/mol and lead to final estimates of the genuine stacking energies for the three dimers of -3.4, -11.6, and -10.4 kcal/mol. The CBS MP2 stacking-energy estimate for guanine dimer (-14.8 kcal/mol) was based on the 6-31G(0.25) and aug-cc-pVDZ calculations. This simplified extrapolation can be routinely used with a meaningful accuracy around 1 kcal/mol for large aromatic stacking clusters. The final estimate of the guanine stacking energy after the CCSD(T) correction amounts to -12.9 kcal/mol. The MP2/6-31G(0.25) method previously used as the standard level to calculate aromatic stacking in hundreds of geometries of nucleobase dimers systematically underestimates the base stacking by ca. 1.0-2.5 kcal/mol per stacked dimer, covering 75-90% of the intermolecular correlation stabilization. We suggest that this correction is to be considered in calibration of force fields and other cheaper computational methods. The quality of the MP2/6-31G(0.25) predictions is nevertheless considerably better than suggested on the basis of monomer polarizability calculations. Fast and very accurate estimates of the MP2 aromatic stacking energies can be achieved using the RI-MP2 method. The CBS MP2 calculations and the CCSD(T) correction, when taken together, bring only marginal changes to the relative stability of H-bonded and stacked base pairs, with a slight shift of ca. 1 kcal/mol in favor of H-bonding. We suggest that the present values are very close to ultimate predictions of the strength of aromatic base stacking of DNA and RNA bases.     

Continuous chemiluminescence determination of formaldehyde in air based on Trautz-Schorigin reaction

A new continuous method for the determination of formaldehyde in air is described. A cylindrical wet effluent diffusion denuder is used for the collection of formaldehyde from air into a thin film of absorption liquid (distilled-deionized water). Formaldehyde in the denuder concentrate is on-line detected employing a chemiluminescence flow method based on a reaction of formaldehyde and gallic acid with hydrogen peroxide in an alkaline solution. The collection efficiency of formaldehyde is quantitative at the air flow rate of 0.5 L min⁻¹ (absorption liquid flow rate of 336 μL min⁻¹). The limit of detection (S/N = 3) is 0.60 μg m⁻³ HCHO (0.49 ppb). The calibration graph is linear up to 300 μg m⁻³ HCHO (244 ppb). The relative standard deviations of chemiluminescence method for 1 × 10⁻⁶ and 5 × 10⁻⁶ M HCHO are 2.87% and 1.49%, respectively. Acetaldehyde interferes negligible, other compounds do not interfere. The method was employed for formaldehyde measurement in ambient air. The comparison measurement illustrates the good agreement of results obtained by proposed method with those obtained by reference fluorimetric method.     

Beitrag zur Stickstoffschnellbestimmung nach Merz

Zusammenfassung Einige Änderungen an der Apparatur zur Stickstoffschnellbestimmung nachMerz wurden vorgenommen. Dadurch wird erreicht, daß zwischen den einzelnen Verbrennungen Sauerstoff das Verbrennungsrohr durchspült und vor dem Reduktionsrohr nach außen tritt. Der Verbrennungsraum ist daher bereits zu Beginn jeder Analyse mit Sauerstoff gefüllt, wodurch günstigere Bedingungen für die schlagartige Verbrennung erzielt werden. Der Umbau der von der Firma Heraeus gelieferten Apparatur und die Herstellung eines Oxydationskatalysators (Kobaltoxid-Wolframoxid) werden beschrieben.

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Contribution to the rapid nitrogen determination according to merz

Summary The apparatus for rapid nitrogen determination according toMerz has been modified. Between each two combustions, oxygen is flushed through the combustion tube and is vented before the reduction tube. Therefore, at the beginning of each analysis, the combustion chamber is filled with oxygen, providing a more favorable atmosphere for the rapid ignition. The alterations in the Hereaus apparatus and the preparation of an oxidation catalyst (cobalt oxide and tungsten oxide) are described.

     

Efficient enantiomeric analysis of primary amines and amino alcohols by high-performance liquid chromatography with precolumn derivatization using novel chiral SH-reagent N-(R)-mandelyl-(S)-cysteine

Novel N-acylated-(S)-cysteine derivative-N-(R)-mandelyl-(S)-cysteine (R-NMC), containing additional chiral center, aromatic and polar alpha-substituents in contrast to the traditionally used enantiomerically pure thiols, has been demonstrated to be an efficient SH-reagent for enantiomeric HPLC analysis of primary nonfunctionalized amines and amino alcohols after precolumn derivatization with o-phthalaldehyde. The R-NMC-derived isoindoles as well as adducts formed using traditional SH-reagents had a characteristic absorption maximum at 340 nm with a molar absorbance 6000 M(-1) cm(-1), were stable during the HPLC-analysis and highly fluorescent allowing to detect 1 fmol of amino compound. Using diastereomeric R-NMC all tested amino alcohols were resolved effectively as well as nonfunctionalized amines, some of which were not resolved by a direct method on a chiral phase. Applying traditional enantiomeric N-acetyl-(S)-cysteine (NAC) only some isoindoles formed by aliphatic amino alcohols have been separated satisfactorily. The enhanced selectivity for R-NMC-derived isomers has been achieved, obviously, due to the involvement of the substituents at an extra chiral center into additional intramolecular interactions.     

Purification of waste waters containing 60Co2+, 115mCd2+ and 203Hg2+ radioactive ions by ETS-4 titanosilicate

Summary The sorption of ⁶⁰Co²⁺, ^115mCd²⁺ and ²⁰³Hg²⁺ from diluted solutions (as analogues for radioactive waste waters) on ETS-4 microporous titanosilicate was studied at 277, 293, 313 and 333 K by measuring the sorption kinetics using a batch-method. The sorption of these radiocations was compared by means of the distribution coefficient and of the sorption capacity. The maximum sorption capacities follow the order: ²⁰³Hg²⁺>^115mCd²⁺3⁶⁰Co²⁺. The thermodynamic functions of the sorption processes have been estimated. The increase of the absolute value of DG° with increasing temperatures shows that higher temperatures favor ionic exchange.     

Dichloro[N,N‐dimethyl‐1‐(1‐methyl‐1H‐tetrazol‐5‐yl‐κN4)methanamine‐κN]copper(II)

The title compound, [CuCl₂(C₅H₁₁N₅)], is the first structurally characterized molecular chelate complex involving an α‐­amino­alkyl­tetrazole. There are two complex mol­ecules in the asymmetric unit. The ligand mol­ecules are bidentate. Both Cu atoms reveal rather distorted square‐planar coordinations. The complex mol­ecules are linked together by van der Waals interactions only.     

Coordination compounds of nickel with trithiocyanuric acid. Part IV. Structure of [Ni(pmdien)(ttcH)] (pmdien = N,N,N′,N′,N′′-pentamethyldiethylenetriamine,ttcH3 = trithiocyanuric acid)

Ni^II mixed-ligand complexes of compositions [Ni(pmdien)(ttcH)] (1), [Ni(baphen)₂(ttcH)] · 4H₂O (2), [Ni-(dpa)(ttcH)(H₂O)] (3), [Ni(cyclam)(ttcH)] · 2H₂O (4), [Ni(hexaa)](ttcH) (5) and [Ni(hexab)(ttcH)] · 2H₂O (6), (baphen = 4,7-diphenyl-1,10-phenanthroline, dpa = 2,2-dipyridylamine, cyclam = 1,4,8,11-tetraazacyclotetradecane, hexaa = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9]-octadecane, hexab = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) have been prepared and characterized by means of i.r., u.v.–vis. spectroscopies and magnetochemical measurements. The redox properties of the complexes were studied by cyclic voltammetry. The crystal and molecular structure of [Ni(pmdien)(ttcH)] was determined. The nickel atom is penta-coordinated by three N atoms of pmdien, and by S and N atoms of trithiocyanurate(2–) anion.