Proton Dynamics in Interacting Hydrogen Bonds in the Solid State: Proton Tunneling in the NHO Hydrogen Bonds of N,N′-Di(2-Hydroxy-1-Naphthylmethylene)-p-Phenylenediamine

Combination of comprehensive investigations of the spin-lattice relaxation rate of proton and low temperature ¹⁵N-CP/MAS NMR spectrum provides unique information of proton dynamics in two interacting NHO hydrogen bonds of solid N,N′-di(2-hydroxy-1-naphthylmethylene)-p-phenylenediamine (DNP). It was evidenced from the ¹H-NMR relaxation measurement that tunneling mechanism operates for the proton transfer in the hydrogen bonds. The tunneling phenomenon is closely related to the very small energy differences among the four tautomeric states accompanied with the proton transfer in the two NHO hydrogen bonds. The very small values of the energy difference, in spite of the chemically asymmetric NHO hydrogen bond, were revealed by the ¹⁵N-CP/MAS NMR spectrum. This is a unique character of solid DNP. It was also suggested from the derived energy scheme of the four tautomers and activation energies of the proton transfer that an interaction exists between the two NHO hydrogen bonds linked by π-electronic molecular frame. This means that the information of one NHO hydrogen bond, i.e. OH-form or NH-form, propagates to the other hydrogen bond and the proton transfer in the first hydrogen bond induces the change of the potential function for the proton transfer in the second hydrogen bond.