Coupled spin systems parameter sets

NMR-SIM is very powerful simulation tool based on a density matrix approach and is designed to make low demands on computer resources. The spin system parameters consist of a small basic set of spin parameters chemical shift, weak/strong scalar J coupling, dipolar coupling, quadrupolar coupling, longitudinal and transverse relaxation time. Currently the calculated coherence transfer processes are limited to polarization transfer and cross polarization. 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

In both the one-dimensional carbon- or proton-detected INEPT polarization transfer experiment and the two-dimensional proton-detected experiment, one needs to optimize the polarization transfer delay with an estimate of the coupling constant. In addition, the refocusing delay. A, must be set separately for an IS and IS2 spin system. For an IS spin system, the optimum delay is A = (l/4)Jis, whereas for an IS2 system the delay should be set to A = (l/8)Jis to get maximum polarization transfer. If one is interested in, for instance, the determination of relaxation parameters for carbons carrying two protons as well as for CH carbons, this requires separate experiments. 

Siloxanes represent another large class of silicon compounds for which 29Si-NMR data would be useful. The compound HMe2SiOSiMe2H was chosen as a representative siloxane with a more complex spin system due to the Si-H moieties. The proton coupled spectrum (Fig. 4) consists of a major doublet (J = 204 Hz), for which INEPT and DEPT parameters were set, and a minor septet of doublets. In the INEPT spectrum (Fig. 4a) the major doublet shows the expected INEPT intensities (+1 — 1), while the minor splittings show normal coupling intensities. A subtle but important distortion is present in the INEPT spectrum the expected septet of doublets... 

---