Double proton transfer in crystals of 1,3,4,6,7,8‐hexahydro‐2<i>H</i>‐pyrimido[1,2‐<i>a</i>] pyrimidine (hppH):¹³C and¹⁵N CPMAS NMR study of (hppH)₂

Abstract

Abstract 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 ( 13 C and 15 N) 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. Copyright © 2009 John Wiley &amp; Sons, Ltd.