STRAIN ENERGIES IN CYCLIC ON, N = 3-8

We report the results of geometry-optimized ab initio SCF MO calculati ons of total energies and structures of O(n) rings, n = 2-8, and H2Om chains, m = 0-5, at RHF and MP2 levels using the 6-31G* basis set. Fr om the total energies we calculate O(n) strain energies on the basis o f energy changes for several model reactions that convert O(n) rings i nto H2Om chains. We introduce a generalization of the homodesmotic rea ction that we call the s-homodesmotic reaction, O(n) + nH2Os+1 --> nH2 Os+2, where s = -1 is an isogyric reaction, s = 0 is isodesmic, s = 1 is homodesmotic, s = 2 is hyperhomodesmotic, and so on. Larger values of s permit the conversion of O(n) rings into longer H2Os+2 chains whi ch should contain conformational effects not present in short chains w ith small s. For large s, calculated strain energies approach a consta nt value and results based on RHF energies approach those from MP2 ene rgies. Unlike the cycloalkanes for which strain energies decline quick ly with increasing ring size, we find the strain energy Of O4 to be la rger than that Of O3 for all models tested. Beyond O4, strain energies of larger rings decline through the series as expected. The large str ain energy of O4 is probably due to large lone pair-lone pair interact ions which are not present in the cycloalkanes. We estimate heats of f ormation for O(n) rings and H2O(m) Chains. For m greater-than-or-equal -to 3 total energies of H2O(m)+1 chains appear to be adequately approx imated by a constant plus the total energy of H2Om.