Absolute configuration of C2-symmetric spiroselenurane: 3,3,3',3'-tetramethyl-1,1'-spirobi[3 H,2,1]benzoxaselenole

The enantiomers of 3,3,3',3'-tetramethyl-1,1'-spirobi[3 H,2,1]benzoxaselenole have been separated on a chiral preparative chromatographic column. The experimental vibrational circular dichroism (VCD) spectra have been obtained for both enantiomers in CH(2)Cl(2). The theoretical VCD spectra have been obtained by means of density functional theoretical calculations with the B3 LYP density functional. From a comparison of experimental and theoretical VCD spectra, the absolute configuration of an enantiomer with positive specific rotation in CH(2)Cl(2) at 589 nm is determined to be R. This conclusion has been verified by comparing results of experimental optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) to predictions of the same properties using the B3 LYP functional for the title compound.     

Molecular structure retention relationships in comprehensive two-dimensional gas chromatography

Comprehensive two-dimensional gas chromatography (GC x GC) offers new opportunities to develop relationships between molecular structure and retentions in the two dimensional (2D) separation space defined by the GC x GC retention in each dimension. Whereas single dimension GC provides only one retention property for a solute, and hence the specific relationship between retention and chemical property is not readily apparent or derivable, the 2D presentation of compounds in GC x GC provides a subtle and exquisite correlation of chemical property and retention unlike any other GC experiment. The 'orthogonality' of the two separation dimensions is intimately related to the manner in which different separation mechanisms, available through use of two dissimilar phases, are accessible to the different chemical compounds or classes in a sample mixture, and indeed the specific chemical classes present in the sample. The GC x GC experiment now permits various processes such as chemical decompositions, molecular interconversions, various non-linear chromatography effects, and processes such as slow reversible interactions that may arise with stationary phases or in the injector or column couplings, to be identified and further investigated. Here, we briefly review implementation of the GC x GC method, consider the molecular selectivity of GC x GC, and highlight a selection of molecular processes that can be probed by using GC x GC.     

On the formation of protonated acetic acid from ionized n-alkanoic acids in the gas phase

By means of collisional activation mass spectrometry and semi-empirical molecular orbital calculations (MNDO) it is shown that the [C₂H₅O₂]⁺ ions from metastable molecular ions of n-pentanoic and hexanoic acid have the structure of carbonyl protonated acetic acid [CH₃C(OH)₂]⁺. The MNDO calculated geometries of this ion and some other isomeric forms are given together with the ΔH_f⁰ values.     

Determination of aluminum in highly concentrated iron samples: Study of interferences using high-resolution continuum source atomic absorption spectrometry

High-resolution continuum source atomic absorption spectrometry (HR-CS AAS) has been used to investigate spectral and non-spectral interferences found with a conventional line source atomic absorption spectrometer in the determination of aluminum in pharmaceutical products containing elevated iron and sugar concentrations. A transversely heated graphite furnace was used as the atomizer in both spectrometers. The two most sensitive aluminum lines at 309.3 nm and 396.2 nm were investigated and it was found that an iron absorption line at 309.278 nm, in the vicinity of the aluminum line at 309.271 nm, could be responsible for some spectral interference. The simultaneous presence of iron and the organic components of the matrix were responsible for radiation scattering, causing high continuous and also structured background absorption at both wavelengths. The aluminum and iron absorption could not be separated in time, i.e., the iron interference could not be eliminated by optimizing the graphite furnace temperature program. However, an interference-free determination of aluminum was possible carrying out the measurements with HR-CS AAS at 396.152 nm after applying least squares background correction for the elimination of the structured background. Analytical working range and other figures of merit were determined and are presented for both wavelengths using peak volume registration (center pixel ± 1) and the center pixel only. Limits of detection and characteristic masses ranged from 1.1 to 2.5 pg and 13 to 43 pg, respectively. The method was used for the determination of the aluminum contamination in pharmaceutical formulations for iron deficiency treatment, which present iron concentrations from 10 to 50 g l^− 1. Spike recoveries from 89% to 105% show that the proposed method can be satisfactorily used for the quality control of these formulations.     

Highly Soluble and Green Indigo Dyes: 4,4′,7,7′‐Tetraalkoxy‐5,5′‐diaminoindigotins

Two new types of 4,4′,7,7′‐tetraalkoxyindigotins, 1a – f and 2a – f along with the new N‐substituted indigotins 4e – f , were synthesized from dinitrobenzaldehydes 5a – f , which were prepared from 2‐hydroxy‐5‐methoxybenzaldehyde ( 7 ) via dialkoxybenzaldehydes 6a – f (Scheme). The new dialkoxyindigotin 3g was obtained from dialkoxybenzaldehyde 6g via nitrobenzaldehyde 8g . The 1,4‐dialkoxy‐2,3‐dinitrobenzenes 9 were isolated as by‐products. The 4,4′,7,7′‐tetraalkoxy‐5,5′‐diaminoindigotins 1 are soluble in organic solvents, and their solutions are green, which is highly uncommon for indigotins and is primarily caused by electronic effects of substituents, steric effects playing a minor role. The indigotins 1 produce a strong red shift of the longest‐wavelength absorption and negative solvatochromism indicating the predominance of polar resonance structures in the ground state. Tautomeric structures were excluded. These indigotins are valuable compounds for technical applications, for synthetic purposes, and for analytical studies. SANS (Small‐angle neutron scattering) experiments showed that certain 4,4′,7,7′‐tetraalkoxy‐5,5′‐diaminoindigotins 1 form rod‐like aggregates in solution. The similarly substituted 4,4′,7,7′‐tetraalkoxy‐5,5′‐dinitroindigotins 2 are far less soluble. They produce red monoanions (preferably dimers) and bluish‐purple dianions in organic solvents.     

Massenspektrometrische Untersuchungen an Organophosphorverbindungen. II—Elektronenstoßinduzierte Fragmentierungen von Tetraorganodiphosphandisulfiden

The behaviour under electron impact (70 eV) which includes some rearrangement processes of some tetraorganodiphosphanedisulfides R₂P(S)-P(S)R₂ (R ? CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, C₃H₅, C₆H₅) and CH₃RP(S)–P(S)CH₃R (R ? C₂H₅, n-C₃H₇, n-C₄H₉, C₆H₅, C₆H₅, C₆H₅,CH₂) is reported and discussed. Fragmentation patterns which are consistent with direct analysis of daughter ions and defocusing metastable spectra are given. The atomic composition of many of the fragment ions was determined by precise mass measurements. In contrast to compounds R₃P(S) loss of sulphur is not a common process here. The first step in the fragmentation of these compounds is cleavage of one P–C bond and loss of a substituent R?. The second step is elimination of RPS leading to [R₂PS]⁺ from which the base peaks in nearly all the spectra arise. The phenyl substituted compounds give spectra with very abundant [(C₆H₅)₃P]^+. and [(C₆H₅)₂CH₃P]^+. ions respectively, resulting from [M]^+. by migration of C₆H₅. Rearrangement of [M]^+. to a 4-membered P-S ring system prior to fragmentation is suggested.     

Photocatalyzed [2 + 2 + 2]-cycloaddition of nitriles with acetylene: an effective method for the synthesis of 2-pyridines under mild conditions

The photocatalyzed [2 + 2 + 2]-cycloaddition of nitriles with 2 equiv of acetylene to 2-pyridines can be carried out under mild conditions and represents a valuable extension to common synthetical methods. For the ideal wavelength range (350-500 nm), lamps as well as sunlight can be used. Working at room temperature and in organic solvents such as toluene or hexane as well as in water gives satisfying results in many cases. However, it is also possible to vary the solvent and the reaction temperature of the photocatalyzed synthesis and to choose, with respect to the specific substrate, specific requirements for this particular reaction and general requirements of the method. This simple and selective method derives its potential mainly from the large variety of applicable nitriles. Suitable substrates include (functionalized) aliphatic and aromatic nitriles as well as cyanamides derived from secondary amines.     

Acid-base properties of xanthosine 5'-monophosphate (XMP) and of some related nucleobase derivatives in aqueous solution: micro acidity constant evaluations of the (N1)H versus the (N3)H deprotonation ambiguity

The first acidity constant of fully protonated xanthosine 5'-monophosphate, that is, of H3(XMP)+, was estimated by means of a micro acidity constant scheme and the following three deprotonations of the H2(XMP)+/- (pKa=0.97), H(XMP)- (5.30), and XMP2- (6.45) species were determined by potentiometric pH titrations; further deprotonation of (XMP-H)3- is possible only with pKa>12. The most important results are that the xanthine residue is deprotonated before the P(O)2(OH)- group loses its final proton; that is, twofold negatively charged XMP carries one negative charge in the pyrimidine ring and one at the phosphate group. Micro acidity constant evaluations reveal that this latter mentioned species occurs with a formation degree of 88 %, whereas its tautomer with a neutral xanthine moiety and a PO3(2-) group is formed only to 12 %; this distinguishes XMP from its related nucleoside 5'-monophosphates, like guanosine 5'-monophosphate. At the physiological pH of about 7.5 mainly (XMP-H)3- exists. The question, which of the purine sites, (N1)H or (N3)H, is deprotonated in this species cannot be answered unequivocally, though it appears that the (N3)H site is more acidic. By application of several methylated xanthine species intrinsic micro acidity constants are calculated and it is shown that, for example, for 7-methylxanthine the N1-deprotonated tautomer occurs with a formation degree of about 5 %; a small but significant amount that, as is discussed, may possibly be enhanced by metal ion coordination to N7, which is known to occur preferably to this site.     

Bildung oktaedrischer Komplexe durch cis-Addition an planar-quadratisches Bis(oxamidoximato)nickel(II): drei Strukturbeispiele

Formation of Octahedral Complexes via cis-Addition to Square Planar Bis (oxamideoximato)nickel(II): Three Structure Examples In the reaction of orange square planar bis(oxamide oximato)nickel(II) with acids, blue to blue-green octahedral complexes are formed with neutral oxamide oximide ligands and two acid anions in cis-positions. Three compounds are described: cis-dichlorobis(oxamide oxime)nickel(II) ( 1 ), NiCl₂(C₂H₆N₄O₂)₂, M_r = 365.81, monoclinic P2₁/n, a = 6.641(2), b = 14.086(4), c = 13.473(3) Å, β = 96.26(2)°, V = 1 252.8 ų, Z = 4, d_c = 1.94 gcm^?3, final R_w = 0.031 for 4090 reflections. In cis-di(sulfanilato)bis(oxamide oxime)nickel(II) dihydrate ( 2 ) one sulfanilic anion coordinates via the sulfonic acid group, the other one via the amino group; Ni(C₆H₆NO₃S)₂(C₂H₆N₄O₂)₂ · 2 H₂O, M_r = 675.30, monoclinic P2₁, a = 6.879(3), b = 14.305(5), c = 13.930(5) Å, β = 103.62(4)°, V = 1332 Å, Z = 2, d_c = 1.68 gcm^?3, R = 0.067 for 2693 reflections. In catena-μ-(phthalato)bis(oxamide oxime)nickel(II) tetrahydrate ( 3 ) bidentate bridging phthalate anions lead to chain formation; Ni(C₈H₄O₄)(C₂H₆N₄O₂)₂ · 4H₂O, M_r = 531.09, monoclinic P2₁/c, a = 10.633(8), b = 11.324(5), c = 17.680(14) Å, β = 98.25(7)°, V = 2107 ų, Z = 4, d_c = 1.67 gcm^?3. Final R = 0.110 for 3290 reflections.