INTRAMOLECULAR HYDROGEN-BONDING AND MOLECULAR-STRUCTURE OF 2,5-DIHYDROXYTEREPHTHALALDEHYDE AND 4,6-DIHYDROXYISOPHTHALALDEHYDE - A GAS-PHASEELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL STUDY

The molecular structure of 2,5-dihydroxyterephthalaldehyde has been de termined from a joint electron diffraction/ab initio investigation, an d the molecular structure of 4,6-dihydroxyisophthalaldehyde has been o btained from ab initio calculations at the MP2/6-31G level. There is considerable intramolecular hydrogen bonding in these structures manif ested by the O ... H and O ... O distances as well as by the structura l changes in the rest of the molecule. These changes are consistent wi th the notion of resonance-assisted hydrogen bonding. The hydrogen bon ding is somewhat stronger in 4,6-dihydroxyisophthalaldehyde than in 2, 5-dihydroxyterephthalaldehyde, and this difference may be linked to th e difference in the mutual positioning of the interacting formyl and h ydroxy groups in these molecules.