Search results for " Database"

An ab initio study of the relation between NMR chemical shifts and solid-state structures: hexabenzocoronene derivatives

2000

The assignment of solid-state NMR spectra is studied by the use of model systems computed with ab initio methods. The investigated system is a hexabenzocoronene derivative, for which a T-like arrangement of dimer units is found in the solid-state structure. Here, a tetramer model is required to explain the intermolecular interactions influencing the spectrum, whereas a dimer model is found to be inadequate. For the tetramer model, agreement of the computed NMR spectrum with the experimental solid-state magic angle spinning MAS-NMR data is observed. This study implies that the combination of experimental NMR data with quantum chemical calculations can be employed as a useful tool in determin…

Everything you wanted to know about phase and reference frequency in one- and two-dimensional NMR spectroscopy

2019

The fundamental concept of phase discussed in this tutorial aimed at providing students with an explanation of the delays and processing parameters they may find in nuclear magnetic resonance (NMR) pulse programs. We consider the phase of radio-frequency pulses, receiver, and magnetization and how all these parameters are related to phases and offsets of signals in spectra. The impact of the off-resonance effect on the phase of the magnetization is discussed before presenting an overview of how adjustment of the time reference of the free induction decay avoids first-order correction of the phase of spectra. The main objective of this tutorial is to show how the relative phase of a pulse an…

Proceedings of the 16th International Symposium on Spatial and Temporal Databases

2019

A method for measurement of spin-spin couplings with sub-mHz precision using zero- to ultralow-field nuclear magnetic resonance.

2017

We present a method which allows for the extraction of physical quantities directly from zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) data. A numerical density matrix evolution is used to simulate ZULF NMR spectra of several molecules in order to fit experimental data. The method is utilized to determine the indirect spin-spin couplings ($J$-couplings) in these, which is achieved with precision of $10^{-2}$--$10^{-4}$ Hz. The simulated and measured spectra are compared to earlier research. Agreement and precision improvement for most of the $J$-coupling estimates are achieved. The availability of an efficient, flexible fitting method for ZULF NMR enables a new generation of…

Fourier transform nmr investigations of organotin compounds

1980

Abstract NMR spectra have been obtained for fiteen title compounds, mainly with R = Me. Tin chemical shifts were found to lie within the expected ranges, but to vary little with R or, apparently, with change in ring size. While the magnitute of 1 (SnSe) was as predicted, the of 1 (SnTe) was more than twice as large as that previously observed for (Me 3 Sn) 2 Te. In compounds Me 6 Sn( 3 X 3 , 2 J(SnMSn) varied systematically with variations in x, but within relatively narrow limits (195-250 Hz); a much more drastic variation was observed for Me 6 Sn 3 X 2 , possibly due to bond angle changes at Sn and X.

CPMAS 13C NMR Characterization of Leaves and Litters from the Reafforestated Area of Mustigarufi in Sicily (Italy)

2010

Reafforestation is generally based on the planting of exotic fast growing tree species suitable for adapting to even harsh environments. Once the introduced plants ameliorate soil conditions, they can be progressively replaced by au- tochthonous plant species. Reafforestation is applied worldwide. However, only few studies on the effect of reafforesta- tion on lands from Mediterranean regions are available. This paper reports the characterization by cross polarization 13C NMR spectroscopy of fresh leaves and superficial litters from a reafforestated area in central Sicily (Italy). NMR assign- ment is attempted. A differentiation among the molecular systems within leaves and litters is also …

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.

Experimental and theoretical investigations of structural isomers of dichalcogenoimidodiphosphinate dimers: dichalcogenides or spirocyclic contact io…

2007

A synthetic protocol for the tert-butyl-substituted dichalcogenoimidodiphosphinates [Na(tmeda){(EPtBu(2))(2)N}] (3 a, E=S; 3 b, E=Se; 3 c, E=Te) has been developed. The one-electron oxidation of the sodium complexes [Na(tmeda){(EPR(2))(2)N}] with iodine produces a series of neutral dimers (EPR(2)NPR(2)E--)(2) (4 b, E=Se, R=iPr; 4 c, E=Te, R=iPr; 5 a, E=S, R=tBu; 5 b, E=Se, R=tBu; 5 c, E=Te, R=tBu). Attempts to prepare 4 a (E=S, R=iPr) in a similar manner produced a mixture including HN(SPiPr(2)). Compounds 4 b, 4 c and 5 a-c were characterised by multinuclear NMR spectra and by X-ray crystallography, which revealed two alternative structures for these dimeric molecules. The derivatives 4 b,…

Structural study of ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates: conformation and transmission of substituent effects acro…

2001

Nine new ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates were prepared by a [4 + 2] cycloaddition and their FTIR, 1H, 13C and 15N NMR spectra were measured and assigned. Single crystals were grown for five compounds and their X-ray data were obtained. The electronic structure and the conformations were calculated by the semi-empirical AM1 method. Using correlations between the spectral, empirical and theoretical structural data, the transmission of substituent effects and the preferential conformation connected with the consecutive double nitrogen inversion and regarding the mutual orientation of NH and CO bonds were investigated. The results are compared with those f…

Areas of research and clinical approaches to the study of liver abscess

2016

AIM To analyze the research activity on liver abscess (LA) and identify the main topic clusters in the area. METHODS We identified all documents using the medical subject heading "LA" indexed in the MEDLINE database between 2001 and 2015. We performed a descriptive bibliometric analysis, characterizing the evolution of scientific activity, the publication types of the documents, the document categories of clinical interest (case reports, clinical trials, evaluation studies, meta-analysis, observational studies, practice guidelines and validation studies) and the geographic distribution of the research. We also carried out an analysis of networks and research clusters in order to identify th…