Off-resonance experiment

Fig. 5 Effect of varying relaxation delays between on- and off-resonance experiments in STD NMR experiments, a Experimental setnp for interleaved measnrements in STD NMR spectroscopy, n represents the nnmber of scans. The inter-scan delay Adi is varied while keeping on- and off-resonance freqnencies constant at -4 and -t300 ppm, respectively, b The resulting STD effects for the 0-methyl group of a-L-Fuc-O-methyl in the presence of RHDV VLPs. The equation that was used for non-linear least squares data fitting is based on the saturation recovery experiment [98], With Ti(iig) = 0.91 s as measured independently (unpublished results) and a Monte Carlo error estimation yields Ti(virus) = 10.06 0.41 s. This value does not directly correspond to a Tl relaxation time of the virus protons, because other factors also influence the observed relaxation [99]. According to these findings a relaxation delay Adi = 25 s was employed in all STD experiments. This results in a recovery of 92% of the virus resonance, and thereby reduces errors in epitope mapping that are introduced otherwise by non-homogeneous recovery of the binding site.

Fig. 6 Binding epitopes for the B-antigen trisaccharide in the presence of RHDV VLPs recorded with a Arfl = 20 xs and b Arfl = 100 s. A setting of Adi = 20 jis for the interpulse relaxation delay corresponds to a total relaxation delay t = 4.7 s since relaxation also occurs during the acquisition time. With Adi = 100 s an increase of STD effects for H2, HI, H4 and H6 " is observed. This is dne to rednced satimation dnring the off-resonance experiment resulting from a sufficient long relaxation delay...

Inhomogeneity of the B, rf field causes nuclei in different parts of the sample to experience values of Bt that are not what is desired. In addition, as we have seen, nuclei that are off resonance experience a Beff that has a magnitude and direction not coincident with the applied B,. Thus the effective pulses may be different from the nominal 90° and 180° pulses that are desired. The result of such pulse imperfections can be particularly troublesome if there is a cumulative effect in successive applications of pulses in a sequence. We describe two ways in which imperfections can be overcome in different situations. 

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

In many cases one finds that considerably more off-resonance experiments are performed than are strictly necessary. This is clearly a considerable waste of spectrometer time when weak samples are involved. In fact two separate coherent experiments will always suffice (195) provided that the decoupler power is known with precision. It is then necessary to solve a quartic equation. Since this will probably be done by iteration it may be helpful to perform one or two additional off-resonance experiments so that spurious solutions can be rejected. 

Fig. 5. (A) Vector picture describing the relationship between the rf field strength and offset frequency in off-resonance experiments such as LGCP and PISEMA. (B) A simple pulse sequence (called LGCP or FFLGCP) to set-up the Lee-Goldburg condition. As explained in the text, this sequence can be used to determine the S-spin-lock field strength that matches ileff./for the effective spin exchange between / and S spins.

General Information. The and l c NMR spectra were recorded at 400 MHz and 100 MHz, respectively, with a Varian XL-400 spectrometer on solutions of the compounds in DCCI3 and with TMS as the internal standard. All NMR spectra were obtained with proton decoupling as were the P NMR spectra which were recorded at 161.9 MHz (referenced to external 85% H3PO4). Chemical shifts downfield from the reference are positive and upfield are negative. Some early NMR experiments were done at 300 MHz. All experiments were performed under N2. Off-resonance experiments confirmed the carbon signals. 

In the C-NMR spectra of the two dehydro compounds (52 and 53), the methyl, methylene, methine, and quaternary carbon signals are easily differentiated by measurements of their partially relaxed FT C-NMR spectra coupled with off-resonance experiments, and the results are summarized in Table VI. In the spectrum of 52, nine methylene and four methine signals are observed, while ten methylene and three methine signals are found in that of 53. 

The latter process, in most cases, becomes superior for the commonly applied low and moderate spin-lock fields and practically does not provide information about the molecular dynamics. In the off-resonance experiment, i -°Jis given by [147]... 

The main advantage of the off-resonance experiment, therefore, is a higher experimentally achievable effective spin-lock field that makes the spin—spin contribution to R p negfigible. An alternative approach is to uti-hze heteronuclear decoupHng during the spin-lock irradiation... 

A quantitative investigation of the H-n.m.r. line shifts induced by Eu(fod)3 in the spectra of the methyl 4,6-Q-benzylidene-2- or 3-deoxy-D-hexopyranosides (11)-(15) has been undertaken. In addition, the C-n.m.r. spectra of these compounds have been completely assigned by use of selective H-spin decoupling and off-resonance experiments. ... 

The structure of thermotropic block copolyesters containing aliphatic unsaturated units was studied using and NMR with selective homo-and heteronuclear irradiations and off-resonance experiments [146]. 

When polypropylenimine is made in this way from racemic 4-methyloxazoline, it is optically inactive and its H NMR spectrum (Figure 1(a)) shows three lines when run in D2O at 60°C [3]. These occur at 54.92 (CH), 53.75 (CH2) and 20.08 (CH3) ppm down-field from TSP (sodium trimethylsilyl-l-propane sulfonate). The assignments are readily made from off-resonance experiments and the line order CH>CH2>CH3 is as expected from the spectra of related polymers [3]. The addition of concentrated hydrochloric acid to the solution causes splitting of all three lines, enabling information on tacticity to be... 

The off-resonance experiment allows the assignment of the olefinic carbon atoms of the isoprenyl units in butyl rubber. The signal at 129.98 ppm appears as a doublet... 

---