Impact of remote protonation on 13C CPMAS NMR quantitation of charred and uncharred wood

The impact of inefficient cross polarization (long T-CH values), caused by long C-13-H-1 internuclear distances, on C-13 CPMAS NMR spectra of charred and uncharted woods is determined by simultaneously fitting data from complementary variable spin lock and variable contact time experiments. As expected, the impact is minimal for uncharted woods, but is very significant for the charred woods. Quantification of the decrease in CPMAS signal intensity caused by both inefficient cross polarization and rapid T1pH relaxation is achieved using an advanced spin counting methodology, for which the term "spin accounting" is proposed. C-13 CPMAS NMR observabilities determined using the spin accounting methodology were close to 100% for the uncharred samples, and 69-82% for the charred samples. This represents a large improvement on the 30-40% observabilities determined using other spin counting techniques. Furthermore, it is shown that remote protonation and rapid T1pH relaxation are roughly equally responsible for the low signal intensity of standard (1 ms contact time) C-13 CPMAS spectra of charcoal. (C) 2002 Elsevier Science (USA).