ROESY- rotating frame overhauser experiments

ROESY Rotating-frame Overhauser effect spectroscopy. A variation (one and two dimensional) on the nuclear Overhauser experiment (NOE). The techniques have the advantage of being applicable for all sizes of molecule. See Laboratory frame model. 

We described the basic aspects of NOESY in Section 10.1 as an introductory example of a 2D experiment. NOESY is very widely used in measuring macro-molecular conformation, as we see in Chapter 13. However, as shown in Fig. 8.4, the H— H nuclear Overhauser enhancement 17 varies from its value of +0.5 in small molecules to a limiting value of — 1 in large polymers with very long Tc, and at intermediate values of rc the NOE may vanish. An alternative is to use the NOE measured in the rotating frame, as this quantity is always positive. By analogy to NOESY, this technique has the acronym ROESY (rotating frame Overhauser enhancement spectroscopy),... 

A complex, multiply bridged 1,2,3,4-adduct of C60 (168) (Scheme 1.11) including a noninherently chiral addition pattern as well as a multitude of stereogenic centers in the addend moiety was obtained in a tandem reaction between the alkaloid scandine and Ceo-324 The sequence included a photoin-duced addition of the tertiary amine subunit of the alkaloid and a [2 + 2] cycloaddition of its vinyl group to the adjacent intrahexagonal formal double bond of the fullerene. The structural elucidation included 1H-1 H COSY-, HMQC- (heteronuclear multiple quantum coherence), HMBC-, and ROESY-(rotating frame Overhauser enhancement spectroscopy) NMR experiments and... 

NMR experiments include COSY, TOCSY, Cheteronuclear NMR experiments, NOESY (nuclear overhauser enhancement spectroscopy) and ROESY (rotating frame overhauser effect spectroscopy) as well as other two- and three-dimensional methodologies (Fossen and Andersen, 2006). 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

The best way to avoid the l-cox problem (and weak nOes) is to use a rotating frame experiment, for instance ROESY (Rotating frame Overhauser Effect Spectroscopy) also named CAMELSPIN by its inventors (77). The (oXg dependence of rOes is complex and one may simply remember that rOes are always positive, never null. The ROESY sequence is similar to the sequence of HOHAHA the main difference is the power of the spinlock which is generated by a long soft pulse rather than by a WALTZ sequence. In ROESY experiments, rOe cross peaks may be accompanied by Hart-mann-Hahn correlations which are easily distinguished by their opposite sign (in phased experiments) (78). 

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]. 

In Chapter 3 (Section 3.16), there is a description of the nuclear Overhauser effect difference experiment, an experiment that provides information about H— H through-space proximity. Review of this section is helpful before proceeding here. The ROESY experiment, rotating-frame Overhauser effect spectroscopy, is a useful 2-D analogue of the nuclear Overhauser effect difference experiment. This experiment is useful for molecules of all sizes whereas the related experiment, NOESY (nuclear Overhauser effect spectroscopy), is not very useful with small molecules. NOESY is used primarily with biological macromolecules. Both NOESY and ROESY experiments correlate protons that are close to each other in space, typically 4.5 A or less. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

The ROESY experiment combines ideas drawn from the TOCSY and NOESY experiments. Protons are correlated to one another via ROEs (rotating frame Overhauser effect) [35, 36, 52, 53]. ROEs are developed in the ROESY experiment through the use of a spin-locking field in a manner analogous to the propagation of magnetization in the homonuclear TOCSY experiment (see Fig. 8.16). The dura-... 

Chiappe et al. reported the use of rotating-frame Overhauser spectroscopy (ROESY) and HOESY to investigate two pyrazolium-based ILs with different anions [83]. Using homo- and heteronuclear NOE, they showed the presence of aggregation, how sensitive the aggregation is to steric hindrance, and the nature of anions. Specifically they found that aggregation motives are in a head-to-tail and head-to-head manner. From the HOESY experiments, the researchers probed the formation of loose ion pairs, which are also sensitive to steric effects. Castner s group has utilized H— F HOESY to probe-specific cation—anion interactions for isoelectronic... 

NOE becomes negligible and no cross-peaks are observed), which correlates well with IR measurements and theoretical calculations on the same system By deuteriating 95% of the cations and repeating the experiment, they determined the existence of intermolecular NOE transfer. In this work, the rotating-frame Overhauser effect spectroscopy (ROESY) experiment was performed, as it is more suitable for very viscous materials and requires shorter mixing time (reducing the overall experiment time). For a full description of the ROESY experiment, the reader is referred to the excellent text by Jacobsen et al. [34]. 

INADEQUATE is an acronym for Incredible Natural Abundance Double Quantum Transfer Experiment, t NOESY and ROESY are the acronyms for Nuclear Overhauser Effect SpectroscopY and Rotation-frame Overhauser Effect SpectroscopY, respectively. 

At 500 MHz, moderate-sized (more than six residues) oligosaccharides lie within the spin-diffusion limit. However, for smaller molecules, as the value of the function (OqT (where (Oq is the Larmor frequency of protons, and is the correlation time of the molecule) approaches 1 then the value of the NOE tends towards 0. Cross-peak intensities of NOESY spectra of smaller oligosaccharides (2-5 residues) may thus become too small to measure accurately. In such cases, the rotating frame Overhauser effect spectroscopy (ROESY, originally referred to as CAMELSPIN) experiment is commonly used to measure NOE values. To reduce the appearance of HOHAHA-like cross-peaks, a low power spin-lock field should be used, and the transmitter carrier offset to the low-field end of spectrum. The offset dependency of cross-peak intensities should also be removed by 90° pulses at either end of spin-lock period. 

NOESY, total correlation spectroscopy (TOCSY), and rotating frame Overhauser enhancement spectroscopy (ROESY). These experiments are all quite easy to perform, and adequate data are usually obtained with suhmilligram quantities of material on a 300-MHz spectrometer. 

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

Fattorusso and co-workers identified a component of wormwood called artar-borol. Correlation spectroscopy (COSY) and rotating frame nuclear overhauser effect spectroscopy (ROESY) experiments allowed for deduction of four possible diastereomeric structures of artarborol, 2-5. Low energy conformers of 11-14 were obtained through a molecular mechanics (MM) search. These conformers were screened to identify those having a dihedral angle of around 90 for the C-8 and C-9 protons due to a low coupling constant between these protons. Only conformers of 11 and 13 satisfied this criterion. Next, five low energy conformers, two... 

The elucidation of the scalar coupling network by the correlation experiments is, apart from small molecules, not sufficient for the unambiguous, sequential and stereo-specific assignment. The complementary information of spatially adjacent protons is obtained via cross-relaxation experiments, the laboratory-frame nuclear Overhauser enhancement spectroscopy (NOESY) and the rotating-frame nuclear Overhauser effect spectroscopy (ROESY). These experiments provide also the distance restraints for the structure determination and help to recognize exchange processes. 

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