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The structural changes occurring in cured phenol-formaldehyde resins when exposed to high temperatures have been investigated by high-resolution 13C solid-state NMR. The initial step in the degradation involves the loss of methylol groupings. Continued degradation results in the appearance of oxidized functionalities, with a simultaneous loss in methylene groupings. The oxidized functionalities have been identifiedas aldehydes, ketones, carboxylic acids, and anhydrides. At higher temperatures the structure becomes mainly aromatics with a few methylene linking groups. When the same resins are heated under vacuum, there is a complete loss of methylols and a reduction in the amount of methylenes along with the appearance of methyl groupings but no oxidation, and the structural and chemical integrity of the sample is largely preserved.

The static 13C nmr powder pattern for solid ammonium thiocyanate is analyzed to obtain the 13C chemical shielding anisotropy, 321 ± 7 ppm, and the 13C–14N dipolar splitting, 1295 ± 25 Hz. Slow magic angle spinning 15N nmr experiments are analyzed to obtain a nitrogen chemical shielding anisotropy of 415 ± 15 ppm. The 13C–14N dipolar splitting leads to an effective C—N bond length of 1.19 ± 0.01 Å, in good agreement with the value of 1.176 Å reported from accurate X-ray and neutron crystallographic studies. In solid NH4NCS absolute values of the average shielding constants [Formula: see text] and ct[Formula: see text] are 52 and 34 ppm, respectively. Comparison of calculated and observed [Formula: see text] values indicates that intermolecular interactions decrease the 13C and 15N shielding constants by approximately 10 and 30 ppm, respectively.

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