Heteronuclear Decoupler Modes

The standard 13 C experiment leaves the decoupler on continuously to take advantage of the NOE enhancement decoupler on at very low power during the relaxation delay) and to get decoupling of Jch C1 H decoupler on at low power during the acquisition of the FID). Even when decoupling is not used during acquisition (i.e., when you want to observe fully 

A During relaxation delay (NOE) B During FID acquisition (decoupling) 

Varian uses the decoupler mode (dm) parameter to determine when the decoupler should be turned on and off during the pulse sequence. Time periods A, B, C,. .. are defined during any pulse sequence, and the definition of these time periods can be observed by entering dps (display pulse sequence). For the general ID sequence called s2pul (simple 2-pulse), the periods A, B, and C are defined as follows  

Relaxation delay Pulse Delay Pulse Acquisition 

The B period is irrelevant in a simple 13C experiment because pi and d2 are usually set to zero. 

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In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

The proton-decoupled proton spectra allowed a distinction to be made between homo-and heteronuclear spin couplings, and Laurie and coworkers also demonstrated nulling of residual solvent resonances during the 2D /-resolved NMR of uridine in aqueous solution, wrote software for 45° tilting of the 2D spectra, and developed experimental protocols for multiple data-acquisition and processing, and a method for acquisition of the 2D /-resolved spectra in phase-sensitive mode. Lately, the 2D /-resolved technique has been less used, as it yields little evidence for spectral assignments. 

Figure 5.57. Pulse sequences for heteronuclear shift correlation experiment (A) normal mode (B) employing homonuclear broad-band decoupling (COLOC).

Spectroscopic Measurements. and NMR spectra were obtained on either a Bruker AC-250 or an AM-360 spectrometer operating in the FT mode. Si NMR spectra were recorded on an IBM Bruker WP-270-SY spectrometer. Five percent w/v solutions of copolymer in chloroform-c/ were used to obtain NMR spectra. NMR spectra were run with broad band proton decoupling. A heteronuclear gated... 

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