Search results for "MR"
showing 10 items of 3001 documents
Ultrafast Twisting of the Indoline Donor Unit Utilized in Solar Cell Dyes: Experimental and Theoretical Studies
2015
Previous time-resolved measurements on D149, the most-studied dye of the indoline family, had shown a fast time-component of 20–40 ps that had tentatively been attributed to structural relaxation. Using femtosecond transient absorption, we have investigated the isolated indoline donor unit (i.e., without acceptor group) and found an ultrafast decay characterized by two lifetimes of 3.5 and 23 ps. Density functional theory calculations show π-bonding and π*-antibonding character of the central ethylene group for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The LUMO is localized on the flexible vinyl-diphenyl region of the donor unit and a twist…
Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR
2002
Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude.
ChemInform Abstract: 1,6-π-Electrocyclization of 3-Vinyl-Functionalized N,N′-Dimethyl-2,2′-bisindolyl: Some Analysis of the Educt Transformation Proc…
2010
The thermally and photochemically induced 1,6-π-electrocyclization reaction of 3-vinyl-functionalized N, N'-dimethyl-2,2′-bisindolyls 4 has been analysed and some reactivity aspects are presented. On the basis of time dependent 1H nmr and uv spectroscopic measurements the educt transformations can be observed in detail. Some results of the isomerization process of the educts are also given.
Mechanisms of Organic Coating on the Surface of a Poplar Biochar
2017
Background: Recent studies highlighted that biochar efficiency to improve soil fertility is enhanced after it is blended with fresh organic materials. It was suggested that organic coating of inner-porous biochar surfaces acts as a kind of “glue” for plant-nutrients, thereby allowing their slow release towards plant-roots and/or microorganisms. Objective: The aim of the present study is to improve the understanding of the nature of the interactions between fresh organic matter and a poplar biochar. Method: Two fluorinated organic models were used as target molecules in order to apply heteronuclear (i.e. 19F) fast field cycling (FFC) NMR relaxometry. Results: The results suggest that organic…
Predominance of inductive over resonance substituent effect on33S NMR chemical shifts of 4-substituted phenyl-4′-methylphenacyl sulfones
1999
33S NMR chemical shifts have been determined for the first time for a series of 10 substituted phenacyl sulfones. Electron-withdrawing and electron-releasing substituents in 4-substituted phenyl-4′-methylphenacyl sulfones, p-MeC6H4COCH2SO2C6H4R-p, cause a ‘reverse’ substituent effect on the 33S NMR resonance. Dual-substituent parameter (DSP) analysis of δ(33S) values revealed that the inductive effect of the substituent predominates over its resonance effect. This finding shows that the 33S NMR chemical shifts are of importance in estimating the electronic properties of sulfur-containing compounds. The 13C and 17O NMR chemical shifts of the title compounds are also discussed. Copyright © 19…
A statistical study of the additivities of substituent effects in the13C NMR chemical shifts of hydroxy- and chloro-substituted benzenes
1980
The 13C NMR spectra of six hydroxybenzenes, all chlorobenzenes, all chlorophenols and eight chlorocatechols are measured and assigned. The additivity of the substituent effects and the usefulness of some corrective parameters are studied with regression analysis. The order of the chemical shifts is most efficiently predicted by the simplest substituent effect model, containing only the direct effects of the substituents, although the 95% confidence limits of the calculated shifts are as high as 5.6 ppm. If the chemical shifts need to be predicted within the measuring errors (approximately 0.05–0.10 ppm, in the present data), the number of necessary corrections is very impractical. The corre…
Determination of the Relative Stereochemistry of Flexible Organic Compounds by Ab Initio Methods: Conformational Analysis and Boltzmann-Averaged GIAO…
2002
Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) 1 3 C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an openchain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental 1 3 C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations …
Metal complexes of sulfanilamide derivatives. Crystal structure of [Zn(sulfathiazole)2]·H2O
1993
Abstract The crystal structure of the Zn(sulfathiazole) 2 ·H 2 O complex is reported. The complex crystallizes in the monoclinic space group C 2/ c . The cell dimensions are a = 9.098(3) A, b = 17.871(5) A, c = 14.61(1) A, β = 99.44(6)°, V = 2343(2) A 3 , Z = 4, and D x = 1.68 g/cm 3 . The final conventional R-factor = 0.027 for 2669 “observed” reflections and 191 variables. The Zn ion is surrounded in a regular tetrahedral arrangement by two N amino and two N thiazole atoms pertaining to four sulfathiazole (Hstz) ligands. Each stz anion, acting as a bidentate ligand, chelates to two Zn ions as a bridge through the N thiazole and the N amino atoms. The IR, 13 C NMR in solid state, 1 H NMR, …
1H,13C and17O NMR study of substituted nitropyridines
1991
1H, 13C and 17O NMR spectra for 22 substituted nitropyridines were measured and their 1H NMR spectra were analysed. The most significant variations in the NMR parameters are found for isomeric hydroxy derivatives, owing to the possibility of keto–enol tautomerism. The prevalence of the keto form is observed in 2- and 4-hydroxy derivatives, while the 3-hydroxy derivative exists in its enol form. Among the three nuclei studied, 17O seems to be the best nucleus for probing the keto–enol tautomerism. No correlation is observed between the torsion angle of the nitro group and its 17O NMR chemical shift. Molecular mechanics calculations were performed to clarify the torsional energetics of the ni…
Über polygermane
1980
Abstract A complete series of 5-membered heterocycles (Ph 2 Ge) 4 X with X = O, S, Se and Te has been prepared and investigated by mass spectra, 13 C NMR spectra and vibrational spectra. The 13 C signals of the phenyl groups in the compounds (Ph 2 Ge) 4 X were compared with the related values in the homocycle (Ph 2 Ge) 5 . Frequency values for the symmetrical and asymmetrical stretching vibrations of the units GeGeGe and GeXGe are given. The crystal structure of the compound (Ph 2 Ge) 4 S has been determined and refined to a R of 0.051. The 5-membered ring Ge 4 S exhibits envelope conformation.