A justification for using NMR model-free methods when investigating the solution structures of rhombic paramagnetic lanthanide complexes

The detailed analysis of the 1H NMR hyperfine shifts according to the model-free methods shows that the semi-rigid monometallic complexes [Ln(L)(NO3)3] (Ln = Eu-Yb) are isostructural in solution. The associated separation of contact and pseudo-contact contributions to the hyperfine NMR shifts in each rhombic lanthanide complex at room temperature provides paramagnetic susceptibility tensors whose principal magnetic axes match the expected symmetry requirements. Moreover, both axial (Delta chi(ax)) and rhombic (Delta chi(rh)) paramagnetic anisotropies display satisfactory linear dependence on Bleaney's factors, a correlation predicted by the approximate high-temperature expansion of the magnetic susceptibility limited to T(-2). Consequently, the simple, and chemically attracting NMR model-free methods are not limited to axial systems, and can be safely used for the investigation of the solution structures of any lanthanide complexes. Molecular-based structural criteria for the reliable estimation of paramagnetic susceptibility tensors by NMR are discussed, together with the assignment of the labels of the crystal-field and magnetic axes within Bleaney's approach.     

Electron density distribution of an oxamato bridged Mn(II)-Cu(II) bimetallic chain and correlation to magnetic properties

The electron density distribution of the ferrimagnetic MnCu(pba)(H2O)3.2H2O chain compound, where pba stands for 1,3-propylenebis(oxamato), has been derived from high resolution X-ray diffraction measurements at 114 K using a multipolar model. The analysis of the chemical bonding has been carried out through the "Atoms in Molecules" formalism and thoroughly interpreted with regards to the strong intrachain and weak interchain magnetic couplings. The topological properties of the electron density on the oxamato bridge indicate large electron delocalization and conjugation effects, in addition to high charge transfer from both metals to the bridge. The resulting positive charges on Mn (+1.45 e) and Cu (+1.56 e) induce charge polarization of the bridge, leading to a shift of electron density from the central C atoms to the metal coordinating O and N atoms. The Mn-bridge interactions are mainly closed-shell interactions with low electron density at the corresponding bond critical points, whereas the Cu-bridge interactions exhibit significant covalent character. The Cu-N bonds are moreover stronger than the Cu-O bonds. The 3d Cu and Mn orbital populations are consistent with pyramidal and regular octahedral environments, respectively, in agreement with the loss of degeneracy due to ligand field effects. Interchain interaction pathways are evidenced by the existence of four bond critical points in hydrogen bond regions. Finally, these intrachain and interchain bonding features are correlated to the results of experimental and theoretical spin density distributions, as well as magnetic measurements.     

Detection of volatile organic compounds indicative of human presence in the air

Volatile organic compounds were collected and analyzed from a variety of indoor and outdoor air samples to test whether human‐derived compounds can be readily detected in the air and if they can be associated with human occupancy or presence. Compounds were captured with thermal desorption tubes and then analyzed by gas chromatography with mass spectrometry. Isoprene, a major volatile organic compound in exhaled breath, was shown to be the best indicator of human presence. Acetone, another major breath‐borne compound, was higher in unoccupied or minimally occupied areas than in human‐occupied areas, indicating that its majority may be derived from exogenous sources. The association of endogenous skin‐derived compounds with human occupancy was not significant. In contrast, numerous compounds that are found in foods and consumer products were detected at elevated levels in the occupied areas. Our results revealed that isoprene and many exogenous volatile organic compounds consumed by humans are emitted at levels sufficient for detection in the air, which may be indicative of human presence.     

Studies of benzylisoquinolines. Comparison of the reactivity of the methylene group in 1-benzyl and 4-benzylisoquinolines

While examining the oxidation of 4-benzylisoquinolines to the corresponding 4-benzoylisoquinolines as well as the reactivity of these ketones towards hydroxylamine, reduction reagents and Grignard's reagents, there was found a definite difference in the behavior of the methylene group for these compounds as compared to that of papaverine (1-benzylisoquinolines).     

Inter- and intramolecular [4 + 1]-cycloadditions between electron-poor dienes and electron-rich carbenes

Inter- and intramolecular [4 + 1]-annulations between dialkoxy carbenes and electron-deficient dienes afford mono- or bicyclic products in moderate to good yield.     

DFT studies on the magnetic exchange across the cyanide bridge

Exchange coupling across the cyanide bridge in a series of novel cyanometalate complexes with CuII-NC-MIII (M = Cr and low-spin Mn, Fe) fragments has been studied using the broken-symmetry DFT approach and an empirical model, which allows us to relate the exchange coupling constant with sigma-, pi-, and pi*-type spin densities of the CN- bridging ligand. Ferromagnetic exchange is found to be dominated by pi-delocalization via the CN- pi pathway, whereas spin polarization with participation of sigma orbitals (in examples, where the dz2 orbital of MIII is empty) and pi* orbitals of CN- yields negative spin occupations in these orbitals, and reduces the CuII-MIII exchange coupling constant. When the dz2 orbital of MIII is singly occupied, an additional positive spin density appears in the sigma(CN) orbital and leads to an increase of the ferromagnetic Cu-NC-M exchange constant. For low-spin [MIII(CN)6]3- complexes, the dz2 orbital occupancy results in high-spin metastable excited states, and this offers interesting aspects for applications in the area of molecular photomagnetism. The DFT values of the exchange coupling parameters resulting from different occupations of the t2g orbitals of low-spin (t2g5) FeIII are used to discuss the effect of spin-orbit coupling on the isotropic and anisotropic exchange coupling in linear Cu-NC-Fe pairs.     

Chiral capillary electrophoretic determination of the enantiomeric purity of homocamptothecin derivatives, promising antitumor topoisomerase I inhibitors, using highly sulfated CDs and fluorescence detection

EKC methods for the enantiomeric resolution of homocamptothecin derivatives, potent anticancer agents targeting DNA topoisomerase I selected for clinical trials, were developed using highly sulfated beta-CD as chiral selectors at acidic pH. Optimal electrophoretic conditions, with migration times under 15 min, were as follows: for the neutral homocamptothecin analog 1, a BGE of 75 mM phosphate buffer pH 2.5 (H(3)PO(4) + triethanolamine)/ACN - 95/5 v/v, with 7.5% w/v highly S-beta-CD, an applied field of 0.2 kV/cm and a fused capillary temperature control of 30 +/- 0.1 degrees C (typical current approximately 175 microA); for the cationic homocamptothecin 2, a BGE of 25 mM phosphate buffer pH 2.5 (H(3)PO(4) + TEA)/ACN - 90/10 v/v, with 2.5% w/v highly S-beta-CD, an applied field of 0.15 kV/cm and a fused capillary temperature control of 25 +/- 0.1 degrees C (typical current approximately 45 muA), and both are validated. The best results in terms of LOQ were obtained by EC with fluorescence detection: 10 ng/mL and 20 ng/mL for 1 and 2, respectively (LOQ divided by 150 for 1 and 5 for 2 with respect to UV), thus making this method particularly convenient for enantiomeric purity determination of galenic forms. UV detection appears to be an alternative to fluorescence for the analysis of the main component either for the control of galenic forms or for therapeutic adaptation. Moreover, this method exhibits better performances than HPLC.     

Regioselectivite des substitutions nucleophiles sur des fluoro olefines fonctionnelles

The fluoroolefines CF₃C$\text{F}\text{β}$=C$\text{F}\text{α}$-Y where Y=C₆H₅, C₆H₄CF₃, CONET₂, COOC₃H₇ have been synthesized. Diethylaminolithium or lithium ethoxide attack preferentially these olefines at the α position when Y=C₆H₅ or exclusively at the β position when YCOOC₃H₇. With Y=CONEt₂ this regioselectivity is not so neat, therefore the polarising effect of CF₃ group should be not very much weaker than the amide's. The increasing of the electronic effect in the order

show that vinylic fluorine atoms have no influence on the transmission of these electronic effects.     

Structure and dynamics of t-butyldimethylsilyl amides

A series of N-alkyl- and N-aryl-t-butyldimethylsilyl amides have been prepared by amination and their structures determined by IR and NMR spectroscopy. Like their trimethylsilyl counterparts, the N-alkyl derivatives exist as amides while the N-aryl derivates exist as amide/imidate mixtures. The percentage of imidate and the free energies of activation for the imidate/amide exchange in the aryl derivatives are greater than those in the trimethylsilyl derivatives. The barriers to rotation in the amide form of the aryl derivatives are similar to those of the trimethylsilyl derivatives. The barrier for rotation in t-butyldimethylsilyl-N-methyl formamide, however, is lower than that of the trimethylsilyl derivative. Isomer ratios and free energies of activation are rationalized in terms of the steric effect of the t-butyl group.     

Alcoylation d'acridanone-9. Premiers exemples de O-alcoylation dans cette série de composés

The results of nucleophilic substitution of the 2-methoxy-9-acridone are discussed in terms of competition between both ionic sites N^? and O^?, of the molecule. They are the first examples of O-alkylation of substituted 9-acridone.