M. Helmle

Bio: M. Helmle is an academic researcher from University of Stuttgart. The author has contributed to research in topics: Knight shift & Hyperfine structure. The author has an hindex of 3, co-authored 3 publications receiving 20 citations.

Knight Shift in the Organic Conductor (Fluoranthenyl)2AsF6

Abstract: Proton and 13C NMR Knight shift investigations performed in the organic conductor Fa2AsF6 provide locally resolved information on the electronic states of the conducting band. From single crystal measurements the principal axis values of the proton hyperfine tensor for two positions are determined.

Spin Density Distribution of α-Nitronyl Aminoxyl Radicals from Experimental and ab Initio Calculated ESR Isotropic Hyperfine Coupling Constants

Abstract: Isotropic ESR spectra have been determined for the following α-nitronyl aminoxyl radicals showing different substituents at the 2-position of the imidazolyl ring: 1H (1); 2H (2); 3‘,5‘-(t-C4H9)2-4‘-(HO)C6H2 (3); 4‘-HOC6H4 (4); 3‘,5‘-(HO)2C6H3 (5); 3‘-HOC6H4 (6); 3‘,4‘-(HO)2C6H3 (7); C6H5 (8); 4‘-NO2C6H4 (9); 2‘-HOC6H4 (10); 2‘,4‘-(HO)2C6H3 (11); and 2‘-ClC6H4 (12). Solvent dependence in a large variety of solvents of the isotropic ESR hyperfine coupling constants (hfcc's) for 1, 4, 6, 8, 9, 10, and 12 has been studied for the first time by the linear solvation energy relationship (LSER) methodology. From this study, the most important solvent−solute interactions governing the spin density distribution in these radicals as well as the estimates of their hyperfine coupling constants in the absence of any significant solvent−solute interaction have been determined. Such solvent-independent hyperfine coupling constants are the expected values in gas phase and, therefore, they are used to evaluate the theoret...

Determination of the spin distribution in nitronylnitroxides by solid-state 1h, 2h, and 13c nmr spectroscopy

Abstract: Nitronylnitroxide radicals substituted by 2‘-hydroxy-, 3‘-hydroxy-, 4‘-hydroxy-, and 4‘-methoxyphenyl groups and by a methyl group (2, 3, 4, 5, and 6, respectively) have been investigated in the solid state with 1H, 2H, and 13C NMR spectroscopy under magic angle spinning (MAS) and in solution with 1H and 2H NMR spectroscopy. Well-resolved 13C MAS NMR spectra have been recorded which show signals in ranges up to almost 1900 ppm. The 1H NMR signal spread does not exceed 60 ppm (except for a unique methyl signal of 6 near −230 ppm), and the resolution is worse than that of 13C NMR spectra. Narrower signals have been obtained with 2H NMR spectroscopy. The signals have been assigned with the help of various criteria including the results of ab initio calculations. From the NMR data it has been concluded that the nitronylnitroxide five-ring is puckered, thus rendering all substituents inequivalent. Signal coalescence at elevated temperatures has shown that these inequivalencies are partly averaged out while the...

Magnetic Properties of Conjugated Polymers

Abstract: Magnetic resonance spectroscopy thrives upon the abundance of nuclear and electronic spins in matter. Conjugated polymers in particular usually contain the following nuclear spins: 1H (100% natural abundance), 13C (1.1%), 14N (100%), 15N(0.37%) and 2H (0.01%). In special cases it is useful to isotopically enrich 13C, 2H and 15N. The nuclear spins carry magnetic moments that can be detected in NMR (nuclear magnetic resonance) experiments, and which provide information on local fields at the nuclear site. These local fields depend on the electronic structure and dynamics of the polymer. Similar arguments hold for electronic spins, which are usually detected in an ESR (electron spin resonance) experiment. It is therefore not surprising that a wealth of important information can be obtained from such magnetic resonance (MR) or spin resonance experiments.

Magnetic and Structural Properties of Mixed-Valence Molecular Conductors (DMeDCNQI) 2 Cu and (DMeODCNQI) 2 Cu

Abstract: Temperature dependence of magnetic susceptibilities and lattice parameters is measured at ambient pressure in the title compounds, which are metallic down to the lowest temperature measured and have a mixed valence state of Cu. In (DMeDCNQI) 2 Cu an anomalously large susceptibility having a thermal hysteresis is found in the range below 100 K. This result combined with the change in the lattice parameter tells that (DMeDCNQI) 2 Cu is a marginal metal in which the number of stable Cu 2+ ions changes with temperature. It is pointed out that the reentrant behavior already found in the title compounds under respective pressure range has a close relation with the lattice-parameter change.