Download PDF by Harold M. McNair, James M. Miller: Basic gas chromatography

By Harold M. McNair, James M. Miller

ISBN-10: 047117260X

ISBN-13: 9780471172604

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Example text

To understand the nature of NOE, we have to look at a two-spin system I and S with spin 12. e. 6(b) will be used. In this Figure, the single-, double-, and zero-quantum transitions are labelled as W1I or W1S, W2, and W0, respectively. Ws represent transition probabilities or rates at which certain transitions can take place. W1I or W1S correspond merely to spin–lattice relaxation of the two spins I and S. W2, and W0 cannot be observed because they have much lower probability to occur (remember that these transitions are forbidden), but they are allowed for relaxation.

Zz-NOESY Solvent suppression; separation of chemical exchange and NOE. Distance information between protons; 3D structure for small and medium sized molecules. g. 13 C–1H or 31P–1H); provides distances between quaternary carbons and protons. Study of chemical and conformational exchange processes; it is applicable equally well for two-sites and multi-sites exchange processes. This pulse sequence is almost identical as the one used for TOCSY. To avoid TOCSY artifacts, the power used to achieve spin-lock is reduced; cross peaks are always positive.

G. protons. The magnitude of NOE depends strongly on the difference (W2 À W0). When W2oW0, then NOE is negative, and it is positive when W24W0. The relative magnitude of W2 and W0 depends on the molecular motion. A microscopic measure of the rotational motion of a molecule is the so-called molecular correlation time, tc (the time it takes a molecule to rotate one radian, 3601/2p); tc is related to the transition probabilities Ws and the resonance frequencies oI and oS of nuclei I and S, respectively, with rather complex equations.

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Basic gas chromatography by Harold M. McNair, James M. Miller

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