https://research.ibm.com/publications/proton-transfer-kinetics-in-solids-tautomerism-in-free-base-porphines-by-lesssupgreater15lesssupgreatern-cpmas-nmr
Proton-Transfer Kinetics in Solids: Tautomerism in Free Base Porphines by 15N CPMAS NMR for JACS -...