Search results for "MR"

Conformational analysis and DFT calculations of 8α-hydroxy-germacradiene-6,12-olide derivatives

2005

The Systematic Conformational Search Analysis (SCSA) code was employed to study the conformational properties of the 8α-hydroxy-germacradiene-6,12-olide isomers. This procedure was extended to the trans,cis- and trans,trans-1(10),4-isomers of 8α,15-dihydroxy-germacradiene-6,12-olides and 8α-hydroxy-15-oxo-germacradiene-6,12-olides, to investigate structural and energetic analogies between these and the 8α-hydroxy-germacradiene-6,12-olide species. The calculated 13C NMR spectra, at the hybrid DFT mPW1PW91 level, showed a sound correlation with the corresponding experimental spectra, providing a valid support to the reliability of the calculated structures and to the consistence of our confor…

Die Kooperativität bei der CO2-Hb-Bindung: Messungen mit Hilfe der 13C-NMR-Spektrometrie / The Cooperativity of the CO2 Hb Binding: Measurement by 13…

1979

The detailed structure of the CO2 Hb binding curve is of considerable physiological interest, because the carbamino derivates, which are formed by the reaction of CO2 with Hb, are respon­sible for the well known Haldane effect. Under physiological conditions CO2 binds to the four terminal NH2-groups of the protein chains of Hb and it is generally assumed, that there exists no cooperativity between these groups. To examine whether this assumption is correct or not, we have measured the CO2Hb binding curve over a wide PCO2-range (from 0 kPa to about 70 kPa) by using 13C-NMR spectrometry. Morrow and coworkers (1976) have shown that carbamino-Hb causes a specific NMR signal. The quantitative an…

Exciton interactions in self-organised bacteriochlorophyll a - aggregates

2002

Exciton interactions of self-organised bacteriochlorophyll a - aggregates in non-polar solution linked via water and dioxane have been studied. Absorption and CD spectra of the aggregates show large red shifts typical of J-aggregates. Femtosecond excitation of the Qy band of the aggregates is followed by wavelength dependent non-exponential picosecond relaxation and anisotropy decay takes place in subpicosecond time scale. To explain these observations exciton theory and semi-empirical MO/CI calculations, that constitute the basis of the CIEM-approach developed by Linnanto et al. (J. Phys. Chem. B, 1999, 103B, 8739) was used. Structural models of aggregates were created by using the molecul…

Complete assignments of1H and13C NMR spectra of four hispanane diterpenoids

2000

An NMR study of four hispanane diterpenoids is described. In addition to conventional 1D NMR methods, 2D shift-correlated experiments [1H,1H-COSY, 1H,1H-TOCSY, 1H,13C-HSQC-1J(C,H). 1H,13C-HMBC-nJ(C,H) (n = 2 and 3)] and 2D 1H,1H-NOESY were used for the complete and unambiguous 1H and 13C chemical shift assignments of these diterpenoids. Copyright © 2000 John Wiley & Sons, Ltd.

1H,13C and15N NMR chemical shifts of substituted pyrazolo[1,5-a]pyrimidines

2002

Exciton Interactions and Femtosecond Relaxation in Chlorophyll a−Water and Chlorophyll a−Dioxane Aggregates

1998

Chlorophyll a (Chl a) in hydrocarbon solution with a small amount of dioxane or water shows red-shifted absorption bands at 686 nm and at 700 nm (dioxane) and at 745 nm (water), indicative of self-organized aggregate structures in solution. To study the relationship between the structure and spectral properties of the aggregates, several one-dimensional model structures of Chl a−dioxane and Chl a−water aggregates were computed by the molecular mechanics method. Three overall structures ranging from stick to a ring shape were energetically favored for the dioxane system. All these structures contain structural heterogeneity that consists of repeating dimers that further form tetramer substru…

Ab-initio crystal structure analysis and refinement approaches of oligo p-benzamides based on electron diffraction data.

2012

Ab-initio crystal structure analysis of organic materials from electron diffraction data is presented. The data were collected using the automated electron diffraction tomography (ADT) technique. The structure solution and refinement route is first validated on the basis of the known crystal structure of tri-p-benzamide. The same procedure is then applied to solve the previously unknown crystal structure of tetra-p-benzamide. In the crystal structure of tetra-p-benzamide, an unusual hydrogen-bonding scheme is realised; the hydrogen-bonding scheme is, however, in perfect agreement with solid-state NMR data.

Solid state NMR studies of gels derived from low molecular mass gelators

2016

The emergence of NMR crystallography provides a unique opportunity to study solids, gels and xerogels, thereby providing ample information to elucidate molecular packing in the native gel. This review details the importance as well as the application of solid state NMR spectroscopy combined with other analytical tools to study gels derived from low molecular mass organo- and hydrogelators.

Spin Crossover in Fe(II) Molecular Compounds — Mössbauer and µSR Investigations

2002

The compound [Fe(ptz)6](C104)2 (ptz = 1-propyl-tetrazole) displays a complete and gradual spin crossover centred around 125 K as evidenced by magnetic and muon measurements over the temperature range ∼ 4.2–300 K. Although the crystal structure reveals only one crystallographic site, line broadening is observed in the Mossbauer spectra in the vicinity of the spin transition. The muon spin relaxation behaviour of this compound indicates that a structural transformation rather than dynamic processes may account for the observed spectral features. Both the Mossbauer and muon measurements are consistent with a mixture of high and low spin Fe ions in the transition region.

1986

The molecular order and mobility of two liquid-crystalline polyacrylates with phenyl benzoate moieties as mesogenic side groups and (CH2)m spacers (m = 2 and m = 6) were investigated by pulsed 2H NMR. The mesogenic side groups were isotopically labelled at the terminal phenylene ring. In the glassy state the orientational distribution function was characterized from the angular dependence of the 2H NMR line shape of samples macroscopically ordered in their nematic phase by the 8,4 T magnetic field of an NMR spectrometer. It was found to be Gaussian with widths of ± 18,5° and ± 10,5° for the m = 2 frozen nematic and m = 6 frozen smectic system, respectively. The phenylene rings undergo 180° …