Structure and stability of X4Y4H4 (XY = CC, BN)

Ab initio calculations have been carried out to study the structures and relative stabilities of the planar eight-membered ring B4N4H4 and its isoelectronic species C8H4 at the HF/6-31G*, MP2/6-31G*, MP2/6-311G**, and MP4SDQ/6-31G* levels. The analyses of Milliken population, vibration frequencies, x-molecular orbital components, and orbital energy levels were used to evaluate the relative stabilities of these two similar systems. The homodesmotic reactions were also taken to be a useful index of relative stability for X4Y4H4 (XY = CC, BN) and gave the resonance energies with MP4SDQ/6-31G* of C8H4 (-37.2 kcal/mol) < B4N4H4 (-29.2 kcal/mol). Furthermore, we calculated the thermodynamic functions of these reactions to discuss the influence of temperature. It is concluded that B4N4H4 may exist in theory and could be a little more stable than C8H4. (C) 2001 John Wiley & Sons, Inc.