Amine/anhydride reaction versus amide/anhydride reaction in polyamide/anhydride carriers

Polyamide 6 (PA6) and poly(metaxylene adipamide) (PAmXD6) were blended in a batch mixer with anhydrides such as phthalic anhydride, n-octadecyl succinic anhydride, and anhydride-grafted ethylene propylene rubber. The melt viscosity, the solution viscosity, and chain end concentration were studied during the mixing. PA was first mixed 5 min to get an homogeneous melt prior to the anhydride addition. The introduction of the anhydride to the molten polyamide resulted in large decreases of melt and solution viscosities and of amine chain end concentrations. The anhydride units react with amine chain ends to form imide groups. The resulting low amine chain end concentration causes hydrolysis reaction to maintain the condensation equilibrium. As a consequence an increased carboxylic chain end concentration is observed. The imide concentration was studied by IR. It was shown that when most of the amine chain ends are consumed, the remaining anhydride reacts with amino groups formed by polyamide hydrolysis. © 1995 John Wiley & Sons, Inc.     

Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO2) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO2 groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO2) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue.     

Theoretical analysis of the triplet excited state of the [Pt2(H2P2O5)4]4- ion and comparison with time-resolved X-ray and spectroscopic results

A full understanding of the nature of excited states of transition metal complexes is important for understanding their chemical reactivity and role as intermediates in photochemically induced reactions. The ground and excited states of the [Pt(2)(pop)(4)](4-) ion are investigated using density functional theory (DFT). Calculations with different functionals employing quasi-relativistic Pauli and ZORA formalisms all predict a Pt-Pt bond shortening and a slight Pt-P lengthening upon excitation to the lowest triplet state, the latter in apparent contradiction to experimental EXAFS results. The PW86LYP functional with the ZORA relativistic treatment is found to produce good agreement with time-resolved crystallographic and spectroscopic results. A topological bond path between the Pt atoms is found in both the ground and the excited states, though the electron localization function (ELF) indicates weak Pt-Pt covalent bonding for the excited state only. The spin density is mainly localized on the Pt atoms, giving insight into the ability of the triplet excited state to abstract hydrogen and halogen atoms from organic substrates.     

X-ray Diffraction Analysis of Geometry Changes upon Excitation: The Ground-State and Metastable-State Structures of K(2)[Ru(NO(2))(4)(OH)(NO)

The structure of the laser-light-induced metastable state MS(1) of the [Ru(NO(2))(4)(OH)(NO)](2)(-) anion in K(2)[Ru(NO(2))(4)(OH)(NO)] was determined by X-ray analysis at 50 K of a crystal with a 16% excited-state population. Results of an independent determination of the ground-state structure were used in the analysis. The most pronounced geometrical change upon excitation was an increase of the Ru-(NO) distance by 0.097(11) ?, significantly larger than the change of the corresponding distance in sodium nitroprusside (Pressprich, M. R.; White, M. A.; Vekhter, Y.; Coppens, P. J. Am. Chem. Soc. 1994, 116, 5233-5238). A decrease in the angleRu-(N-O) angle from 174.0(2) to 169(1) degrees was observed. The diffraction results provide evidence that the photoinduced state MS(1) of the transition metal nitrosyl complexes is a linkage isomer in which the NO group is attached to the metal atom through the oxygen, instead of through the nitrogen atom, rather than an electronic excited state as reported previously.     

The structure of a TNA-TNA complex in solution: NMR study of the octamer duplex derived from alpha-(L)-threofuranosyl-(3'-2')-CGAATTCG

TNA (alpha-( l)-threofuranosyl-(3'-2') nucleic acid) is a nucleic acid in which the ribofuranose building block of the natural nucleic acid RNA is replaced by the tetrofuranose alpha-( l)-threose. This shortens the repetitive unit of the backbone by one bond as compared to the natural systems. Among the alternative nucleic acid structures studied so far in our laboratories in the etiological context, TNA is the only one that exhibits Watson-Crick pairing not only with itself but also with DNA and, even more strongly, with RNA. Using NMR spectroscopy, we have determined the structure of a duplex consisting entirely of TNA nucleotides. The TNA octamer (3'-2')-CGAATTCG forms a right-handed double helix with antiparallel strands paired according to the Watson-Crick mode. The dominant conformation of the sugar units has the 2'- and 3'-phosphodiester substituents in quasi-diaxial position and corresponds to a 4'-exo puckering. With 5.85 A, the average sequential P i -P i+1 distances of TNA are shorter than for A-type DNA (6.2 A). The helix parameters, in particular the slide and x-displacement, as well as the shallow and wide minor groove, place the TNA duplex in the structural vicinity of A-type DNA and RNA.     

The injectivity of the extended Gauss map of general projections of smooth projective varieties

Let X be a smooth n-dimensional projective variety embedded in some projective space ℙ ^N over the field ℂ of the complex numbers. Associated with the general projection of X to a space ℙ ^N-m (N-m>n+1) one defines an extended Gauss map (in case N-m>2n-1 this is the Gauss map of the image of X under the projection). We prove that is smooth. In case any two different points of X do have disjoint tangent spaces then we prove that is injective.     

Supramolecular solids as a medium for single-crystal-to-single-crystal E/Z photoisomerization: kinetic study of the photoreactions of two Zn-coordinated tiglic acid molecules

[Zn(TA)2(H2O)2] (H-TA=tiglic acid) has been embedded in a framework composed of CECR (CECR=C-ethylcalix[4]resorcinarene) molecules to examine its E-->Z photoisomerization in a periodic framework. The photoisomerization of tiglic acid in CECR-[Zn(TA)2(H2O)2]4 H2O proceeds without the [2+2]-dimerization reaction that often occurs in crystals of uncomplexed analogues, and without breakdown of the crystal lattice that frequently occurs in neat crystals. The two Zn-coordinated TA molecules are located in different size cavities. The rate constants of the isomerization reaction are strongly affected by the size of the reaction cavity. Analysis of the temperature dependence of the reaction rates and the occupancies in the final photostationary state shows that the activation energies and the standard enthalpies of activation are dependent on the difference between the reaction cavities. This is the first quantitative diffraction study of solid-state E/Z isomerization of a metal-coordinated ligand in a periodic host environment.     

Measuring picosecond excited‐state lifetimes at synchrotron sources

A new analysis method for the short excited‐state lifetime measurement of photosensitive species in crystals is described. Based on photocrystallographic techniques, this method is an alternative to spectroscopic methods and is also valid for non‐luminescent excited species. Two different approaches are described depending on the magnitude of the lifetime τ. For very short lifetimes below the width of the synchrotron pulse, an estimated τ can be obtained from the occurrence of the maximal system response as a function of the pump–probe delay time Δt. More precise estimates for both short and longer lifetimes can be achieved by a refinement of a model of the response as a function of the pump–probe delay time. The method also offers the possibility of the structure determination of excited species with lifetimes in the 40–100 ps range.