Theoretical studies on the structure of 1,3-diol systems: Part 1. Monomers, dimers and monohydrates

Abstract

The structures and stabilities of the isolated 1,3-propanediol (PDL) monomers and dimers, and their monohydrates were investigated by the force field versions MMP2 and PIMM90, the semiempirical AM1 and PM3 methods, and the point charge model LEP. The calculations on the monomers were performed in order to test the applicability of these methods for diol systems. Especially the PIMM90, AM1 and PM3 results on PDL show (in agreement with ab initio data and experimental findings) that intramolecular hydrogen bonding is a significant structural element in such systems. The most stable structures of the dimers are characterized by a remarkable retention of the intramolecular hydrogen bonds and the formation of up to three additional intermolecular ones. The aim of the calculations on the monohydrates of PDL and (PDL) 2 was to simulate the function of water at the aggregation of diol head groups in bilayers in a very simple way. Within the PIMM90, PM3 and AM1 methods a different arrangement and function of the water molecule in the formation of most stable monohydrate structures results. From the studies on the PDL systems it follows that intramolecular hydrogen bonding is overestimated by the AM1 method in comparison with intermolecular hydrogen bonding. Especially the calculations on systems with intennolecular interactions show that the PIMM90 findings are in better agreement with the PM3 results than with the AM1 ones.