Carbon polarity

Assuming further that decoupling has been turned on a long time ago before the carbon-13 measurement so that a steady state is reached, one ends up with a steady state carbon polarization, I, obtained by setting dh /dt to zero ... 

Cross-polarization is based on the notion that the vast proton spin system can be tapped to provide some carbon polarization more conveniently than by thermalization with the lattice (7). Advantages are two-fold the carbon signal (from those C nuclei which are indeed in contact with protons) is enhanced and, more importantly, the experiment can be repeated at a rate determined by the proton longitudinal relaxation time Tin, rather than by the carbon T c (I)- There are many variants (7) of crosspolarization and only two common ones are described below (12,20). 

The term Umpolung is most often used to describe the reversal of the carbon polarity when going from a carbonyl group to the carbanion of a dithioacetal. 

Pa PACE PAGES PAHO PALE PAR PARCS PBL PCM PDV PhA PIK PIRA PIRATA POC POLDER Partial pressure in the atmosphere Permafrost And Climate in Europe Pilot Analysis of Global EcoSystems Pan American Health Organization Paleoclimates of Arctic Lakes and Estuaries Photosynthetic Active Radiation Paleoenvironmental ARCtic Science Planetary Boundary Layer Parallel Climate Model Pacific Decadal Variability Phytogenic Aerosol Potsdam-Institut fur Klimafolgenforschung Petroleum Industry Research Associates Pilot Research moored Array in the Tropical Atlantic Permanganate Oxidizable Carbon POLarization and Directionality of the Earth s Reflectances Princeton Ocean Model... 

Sn . Tertiary carbon, polar solvent. Very slow reaction rate. 

The carbon rf Is phase modulated to prevent an accumulation of carbon polarization thereby helping to ensure simple kinetics describing the to transfer (2 ). A direct difference experiment between single- and double-cross polarization procedures (Figure 2) results In the accumulation of a signal arising exclusively from those nitrogens directly bonded to Data collection for difference spectra typically require 12-24 hours. 

Fig. 44a-c. The way in which an electric field set up by a single electron transfer from lithium to carbon polarizes the valence electrons of the remaining Li atoms engaged into covalent bonding with carborf Arrows indicate how the Li electron will move under the infiuence of a field, (a) (RLi) case (b) CLf with no d-orbital involvement, (c) Cli with d-orbital (and higher energy orbital) involvement... 

Thus the carbon polarization obtained in the CP process is ca. 4x that established in Hq (i.e., ... 

The interpretation of carbon Tip data is complicated by the fact that spin-spin (cross-relaxation) processes as well as rotating frame spin-lattice processes may contribute to the relaxation. This arises because the proton dipolar state is strongly coupled to the lattice. During the period in which carbon magnetization decays with H] h off, carbon polarization can decay by motional processes (Tjp) or by polarization transfer to the proton dipolar state (T p ), then to the lattice via the proton dipolar spin-lattice process Schaefer et al., ... 

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