Double proton transfer in crystals of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a] pyrimidine (hppH): C-13 and N-15 CPMAS NMR study of (hppH)(2)

In this paper, we report an example of intermolecular solid-state proton transfer in the bicyclic guanidine, hppH. A combination of X-ray crystallography, CPMAS NMR (C-13 and N-15) and theoretical calculations allows us to determine that a double proton transfer takes place in the (hppH)(2) dimer with an activation energy of about 50 kJ mol(-1). According to the B3LYP/6-311++G(d,p) calculations, the double proton transfer occurs non-symmetrically through a zwitterion.