The basis of spin decoupling

Since different molecular processes may simultaneously contribute to the spin relaxation in LC, the relaxation rates due to various relaxation mechanisms are additive if the motions can be decoupled on the basis of sufficiently different correlation times 30... 

The 100 MHz H-NMR spectrum of polypropylene oxide is too complex to be analyzed accurately even when decoupled from methyl protons. It is reasonable to ascribe the complexity of the spectrum to the spin coupling between hydrogen atoms linked to contiguous carbon atoms in the main chain. If this interpretation is correct, the spectrum should be simplified by substituting the methine proton by a deuterium atom. In fact, the spectrum of polypropylene oxide-a-d was simpler than that of the undeuterated one. Methyl protons lie at higher field and methylene protons at lower field. The stereoregularity was analyzed on the basis of the spectrum of the methylene proton absorption other than that of the methyl proton absorption. 

However, this model is not entirely consistent with the CRAMPS-determined H dipolar-dephasing behavior (39), the main basis for distinguishing between the clustered and isolated protons in H CRAMPS spectra. This model is also inconsistent with related H spin-diffusion behavior, as reflected in some preliminary 1H-29Si dipolar-dephasing experiments, 29Si-detected H spin-diffusion results, and 29Si CP-MAS spectra detected in the absence of H decoupling. Such experiments,... 

We report here the application of the solid state carbon-13 1 1R with cross-polarization (CP), high power proton decoupling, and magic-angle sample spinning (MAS) to study the curing of epoxy resins. On the basis of our preliminary studies, we show here the application of solid state carbon-13 NMR spectroscopic data to determine some of the Important parameters such as the gelation point and effective functionality of the epoxy monomers. 

T2, the spin-spin relaxation time. Carbons that are attached directly to hydrogen (primary, secondary, and tertiary carbons) experience strong interactions and decay more rapidly than carbons without attached hydrogen (quaternary carbons). When the decoupler is switched on after a time, tt, the resultant signal is due primarily to quaternary (nonprotonated) carbons. However, methyl groups (CH3) are not completely suppressed because of their rapid rotation in the solid state and appear in the aliphatic carbon signal. These carbons can be distinguished from the other quaternary aliphatic carbons on the basis of chemical shifts. 

Spectra were observed at 60 macrocycles/s and 100 macrocycles/s both at room temperature and at high temperatures, and spin-decoupling experiments were done. The difference in the chemical shifts of the two meso methylene protons at 60 macrocycles/s was found to be approximately 7 cycles/s for the isotactic three-unit model, whereas it was approximately 16 cycles/s for the isotactic two-unit model or heterotactic three-imit model. PVC spectra can be reasonably interpreted on the basis of this result. Observed values of vicinal coupling constants of model compounds were interpreted as the weighted means of those for several conformations, and the stable conformations of the models determined. 

These represent the nuclear spin Zeeman interaction, the rotational Zeeman interaction, the nuclear spin-rotation interaction, the nuclear spin-nuclear spin dipolar interaction, and the diamagnetic interactions. Using irreducible tensor methods we examine the matrix elements of each of these five terms in turn, working first in the decoupled basis set rj J, Mj /, Mi), where rj specifies all other electronic and vibrational quantum numbers this is the basis which is most appropriate for high magnetic field studies. In due course we will also calculate the matrix elements and energy levels in a ry, J, I, F, Mf) coupled basis which is appropriate for low field investigations. Most of the experimental studies involved ortho-H2 in its lowest rotational level, J = 1. If the proton nuclear spins are denoted I and /2, each with value 1 /2, ortho-H2 has total nuclear spin / equal to 1. Para-H2 has a total nuclear spin / equal to 0. 

It is possible to work in a basis set in which the nuclear spins are coupled, both to each other and also to./ the analysis of the spectrum by Trischka [50] was accomplished using a coupled basis set for weak field experiments, and a fully decoupled basis for the strong field experiments. We will evaluate the matrix elements in both bases. 

---