Solvent frequency

Soft-pulse multiple irradiation In this method, pre-saturation is done using shaped pulses having a broader excitation profile. Therefore, it is a more suitable method for the suppression of multiplets. This technique is very effective, easy to apply and easy to implement within most NMR experiments. In aqueous solutions, however, slowly exchanging protons would be detectable due to the occurrence of transfer of saturation. In addition, the spins with resonances close to the solvent frequency will also be saturated. 

Here cos is the solvent frequency and we have assumed the solvent s equilibrium position to vary linearly with the solute coordinate x in the neighborhood of the barrier top ... 

The very strongly H-bound carboxylic acid dimer has a very broad absorption at 2500 cm 1 for the OH bond. Because H bonding is dependent on concentration and on the polarity and H-bonding properties of the solvent, frequency shifts due to H bonding are quite variable. 

The Markovian description, inherent in Eqs. (2.1) and (2.2), is unrealistic for most chemical situations as it assumes that the relaxation time of the medium surrounding the molecule is faster than all molecular time scales. This assumption obviously breaks down in the low-friction regime where the escape rate is dominated (or at least influenced) by the well dynamics (that is, intramolecular motion), since typical molecular frequencies are of the same order or larger than intermolecular (solvent) frequencies. Even for higher friction, where the escape is dominated by the dynamics in the barrier regime, the Markovian assumption can fail because the characteristic barrier time... 

Presaturation may result in significant bleaching out of the resonances near the solvent frequency (e.g. protein C H protons if irradiating at the water frequency) in the resulting spectrum (see Fig. 6). The saturation can also be spread to labile protons via chemical exchange and, especially in macro-... 

The jump-and-retum (JR) pulse sequence consists of two hard tt/2 pulses that are opposite in phase, at the solvent frequency. The first -nil pulse brings, in the rotating frame, all of the spins to the +y direction, then during a subsequent delay t the water protons remain directed along +y whilst the other spins rotate in the plane by m, where w is the frequency offset from the water (i.e. pulse) frequency. The second ttH pulse is then applied to bring all of the spins into the z-x plane, thus the amplitude response is sin wt. t is chosen so that... 

In binomial composite pulse sequences the amplitudes of the pulses are proportional to the binomial coefficients of -t-1 square pulses separated by equal interpulse delays, r. These sequences provide broad regions of near-zero excitation at the solvent frequency while the solute resonances are... 

The solvent mass and the force constant are related to the solvent frequency tu by... 

The simplest, most robust and most widely used technique is presaturation of the solvent [62]. This is simple to implement, may be readily added to existing experiments and leaves (non-exchangeable) resonances away from the presaturation frequency unperturbed. It involves the application of continuous, weak rf irradiation at the solvent frequency prior to excitation and acquisition (Fig. 9.23a), rendering the solvent spins saturated and therefore unobservable (Fig. 9.24). Invariably resonances close to the solvent frequency also experience some loss in intensity, with weaker irradiation leading to less spillover but reduced saturation of the solvent. Longer presaturation periods improve the suppression at the expense of extended experiments so a compromise is required and typically 1-3 s are used trial and error usually represents the best approach to optimisation. Wherever possible the same rf channel should be used for both the presaturation and subsequent proton pulsing, with appropriate transmitter power switching. 

Concerns have been raised about the genotoxicity of the individual components of Stoddard solvent. Frequency of mutations induced by methylazoxymethanol (MAM), a complete carcinogen in several species, was increased (p<0.05) in V79 Chinese hamster cells by n-decane, while treatment with this alkane alone did not cause mutagenesis (Lankas et al. 1978). This suggests that n- decane may be a promoter. Further study is needed to determine whether the effects observed with individual components would also occur in the Stoddard solvent mixture. 

The PFG NMR solvent suppression approach as discussed in this work removes all solvent peaks, impurities from the solvents, residual monomers, and small molecules reaction byproducts. As a result, the PFG NMR method permits the acquisition of a clean spectrum of polymers by LC-NMR and GPC-NMR. One can detect a signal such as the methoxy signal of pMMA (3.6 ppm) in THF which also resonates at about the same frequency. This is not possible with other well established techniques such as WET since the suppression is applied at selected solvent frequencies and all signals, solvent and polymer, in the vicinity of those frequencies will be eliminated. 

Here, the solvent frequency scale is approximated to be cojl in the Ohmie spectral density. In the nonadiabatic limit (g l) the Zusman equation (eqn (12.41)) prediets the classical FGR ... 

Polymer Moderate Frequency High-ij Solvent Frequency --10 Hz L0W-1 Solvent Ref. 

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