Structures and Proton Transfer for Malonaldehyde Cyclamers

Abstract

In order to elucidate the possibility of hydrogen transfer in the cyclic conjugated hydrogen-bonding system of malonaldehyde enols (the n-meric cyclamers of malonaldehyde, CMAn with n=3-10) DFT calculations were performed using B3LYP functionals with the 6-31G** basis sets. Two stable geometries with forms I and II about the hydrogen bonds in the cyclamers are defined as "[syn-anti]anti" and "[anti-syn]anti," respectively, on the basis of the CC-OH and CC=O...H configurations and the CO...OC conformation. Smaller cyclamers prefer form I, whereas larger ones (n>8) prefer form II. The energy difference between two forms is the smallest for CMA8, where the barrier height for the proton transfer is 9.06 kcal/mol, suggesting that the proton transfer should be possible. In order to probe the accuracy of the present calculations the geometry and the C-13 NMR chemical shifts for the CMA6 clathrate with benzene were examined. Comparison the calculated values with the corresponding experimental ones for the hexameric cyclamer of 1,3-cyclohexanedione enclathrating a benzene molecule yielded satisfactory results.