Abstract: Ab initio molecular orbital calculations were performed on various structures of the acetaldehyde dimer at STO-3G and 6-31G levels. All the fully optimized structures of the dimer are cyclic structures with two C—H…O hydrogen bonds. At 6-31G level, the most stable geometry of the dimer is a symmetric one and the C—H…O hydrogen bonds are composed of hydrogen in—CHO group and carbonyl oxygen. The binding energy is —20.17 kJ·mol~(-1), which is in reasonable agreement with the estimated experimental value of -22.39±0.15 kJ·mol~(-1). The STO-3G basis set underestimates the binding energy and fails to give consistent description of the relative stabilities of various structures of the dimer with the 6-31G basis set.

Key words: Acetaldehyde dimer, Ab initio molecular orbital, Stabllities of structures