Observation and characterization by 15N CPMAS NMR of a double proton transfer in cyclic dimers of 15N, 15N′-di(4-bromophenyl)-formamidine in the solid state

Using dynamic 15N CPMAS NMR spectroscopy a novel double proton transfer was detected in the cyclic hydrogen bonded dimers of polycrystalline 15N, 15N′-di-(4-bromophenyl)-formamidine. Rate constants of this process, which is degenerate within the margin of error, were obtained by lineshape (k12 = 106.6 exp(−9.7 kJ mol−1/RT) s−1, 110, K ≤ 142 K) and 15N T1 data analysis (minimum at 324 and 9.12 MHz, 1H-15N distance rHN = 1.11 Å, k12 = 1010.9 × exp(−21.5 kJ mol−1/RT)s−1, 240 K ≤ T ≤ 370 K). The non-Arrhenius behavior observed indicates proton tunneling at low temperatures.