Nuclear difference spectroscopy

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

Total assignment of the H and 13C NMR chemical shifts as well as the relative configuration of the Diels-Alder adducts 33-35 was accomplished with the help of 2D (111-111 COSY, H-111 NOESY (NOESY = nuclear Overhauser enhancement spectroscopy), H- C XHCORR (XHCORR = nucleus X-hydrogen correlation), H-13C COLOC) and NOE difference spectroscopy <1996JHC697>. 

The use of reference deconvolution for the correction of artefacts in nuclear Overhauser effect difference spectroscopy [9] is illustrated by the spectra of fig. 3. The experimental technique used here differs slightly from that normally encountered in using a control spectrum in which the preirradiation is gated off rather than shifted in frequency, and in keeping the decoupler and transmitter at the same frequency. These modifications were... 

The above data clearly suggest a dibenzazonine structure with two methoxyl and two hydroxyl groups as substituents, their locations being determined by nuclear Overhauser effect difference spectroscopy experiments (Fig. 1). Several derivatives of crassifolazonine (2) were prepared and characterized (2a-2c). Final proof for the proposed structure of crassifolazonine (2) was obtained by its total synthesis (15). 

Fig. 1. Nuclear Overhauser effect difference spectroscopy of crassifolazonine (2).

The assignment of the H-3 and H-5 signals in several A-substituted nitropyrazoles was made by the use of NOE (Nuclear Overhauser Enhancement) difference spectroscopy [290], NMR spectroscopy was employed for the investigation of metallotropic transitions in nitropyrazole organomercury derivatives [291], By means of II NMR spectroscopy the solvation effects of 4-substituted-l,3,5-trimethylpyrazoles in binary solvents (CC14/C6H6, benzene molar fractions) were studied [292], The author of this... 

The decomposition of Co2(CO)8 in faujasites has been studied in some detail. Low-temperature spin-echo ferromagnetic nuclear resonance spectroscopy shows that very small Co particles are formed in supercages of zeolite NaX by microwave plasma activation at low temperatures (86). In situ far-infrared spectroscopy revealed that adsorbed Co2(CO)s interacts with accessible supercage cations in NaY and CoY (239). Carbonyl complexes of different Co nuclearity, such as Co4(CO)i2 and Co(CO)4, are also formed (227,228). In HY the Co atoms are oxidized to Co ions by the zeolite protons. 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

Several physical methods have been employed to ascertain the existence and nature of ICs infrared (IR) absorption spectroscopy nuclear magnetic resonance (NMR) spectroscopy,14 including JH nuclear Overhauser effect (NOE) difference spectroscopy, H 2-D rotating-frame Overhauser effect spectroscopy (2-D ROESY),15 and solid-state 13C cross-polarization/magic angle spinning (CP/MAS) spectroscopy 16 induced circular dichroism (ICD) absorption spectroscopy 17 powder and singlecrystal X-ray diffraction 18 and fast atom bombardment mass spectrometry (FAB MS). 

Nuclear magnetic resonance spectroscopy plays a major part in the elucidation of the structures of these bases, and continues to be the most valuable technique in structural work in the series. Pulsed nmr difference spectroscopy of small nuclear Overhauser effects between aromatic protons and o-methyl and /-methyl groups has permitted differentiation between two possible isomeric structures for dihydrodaphnine (D. Neuhaus et al.. Tetrahedron Letters, 1981, 2933. 

The traditional nuclear Overhauser difference spectroscopy can be replaced by DPFGSE-NOE (Double Pulsed Field Gradient Spin Echo method-NOE spectrometry) experiment. The desired NOE effects can be recorded without interference from other signals and without phase distortion [66]. 

NOEDIF nuclear Overhauser effect difference spectroscopy... 

The most interesting work is the determination of stereochemistry of 5a and 5)3 diastereomers of (2a,3)5,3aa)-ethylperhydro-2-(l,2,3,4-tetraacetoxybutyl)-5,6,6-trimethylpyrrolo[l,2-Z)]isoxazole-3-carboxylate by use of ( H- H)COSY, ( C- H) HETCOR, and nuclear Overhauser effect difference spectroscopy (NOEDS) techniques <91MRC645>. 

Heteronuclear Multiple Quantum Correlation) and HMBC (Heteronuclear Multiple Bond Correlation). Application of nuclear Overhauser effect (nOe) difference spectroscopy and nuclear Overhauser effect spectroscopy (NOESY) complete the analysis, giving atomic spatial relationships. Sensitivity problems can be alleviated using Homo Hartmann-Hahn spectroscopy (HOHAHA or TOCSY, Total Correlation Spectroscopy). For weak nOes a rotating frame experiment, i.e. ROESY (Rotating frame Overhauser Effect Spectroscopy) is useful, and may be the best experimental method to sequence chains of sugars [5]. 

Sanders, J.K.M. and Mersh, J.D., Nuclear Magnetic Double Resonance The Use of Difference Spectroscopy. Prog. Nucl. Magn. Reson. Spectrosc., Vol. 151983, Pergamon Press Oxford, p. 353. 

Sanders JKM and Mersh JD (1982) Nuclear magnetic double resonance the use of difference spectroscopy. Progress in NMR Spectroscopy 15 353—400. 

The Nuclear Overhauser Difference Spectroscopy (NOED) spectra were recorded with an average of more than 512 scans, with 64k data points each. Irradiation duration was always 10 seconds and the temperature was 25 C. The solvents used were CDCI3 and C D, with Si(CH3)4 as the internal standard. Residual oxygen was purged from the samples before measurement using the standard freeze-thaw cycle method. For calculating the structures a force field program was used [72]. 

One of the most useful proton nuclear magnetic resonance ( H-NMR) one-dimensional (a single-frequency axis) techniques to be applied in structure determination is difference spectroscopy. This approach involves subtracting a spectrum obtained under the conditions of some perturbation from a normal, unperturbed spectrum. The resulting difference spectrum reveals the spectral changes caused by the perturbation, whereas those signals that remain unaffected are nulled. The perturbation can be a decoupling irradiation, a nuclear Overhauser enhancement, or even a population transfer experiment (135, 328). 

---