Theoretical determination of molecular structure and conformation. II. Hydrogen trioxide—a model compound for studying the conformational modes of geminal double rotors and five membered rings

Abstract

The internal rotational surface of hydrogen trioxide has been calculated with the Hartree–Fock method using basis sets with and without polarization functions and optimizing all structural parameters. These calculations have been repeated at the level of Rayleigh–Schrödinger–Mo/ller–Plesset (RS–MP) perturbation theory in order to study the role of correlation effects on barrier values and structures. The relative energies as well as the computed geometrical parameters underline the importance of polarization functions. Although the intrapair correlation contributions significantly stabilize the planar conformations of H2O3, the net effect of correlation effects on the conformational potential is moderate because of well-balanced interpair contributions of opposite sign. As in the case of H2O2, the correlation effect on the relative energies further decreases if the basis set is improved. On the other hand, reliable structural parameters can only be achieved at the RS–MP level by employing a large augmented basis. The calculated equilibrium RS–MP structural parameters of H2O3 are: R (OO) =143.9 pm, R (OH) =97.2 pm, α (OOO) =106.3°, β (OOH) =100.2°, ϑ=78.1°. Bond angles and OO bond lengths are strongly coupled to the rotational angles. A simultaneous rotation of both OH bonds is hindered by large barriers of 22.5 and 11.5 kcal/mole. These potential maxima can be surrounded in a stepwise flip-flop rotation of the OH groups. The barriers of this rotational process turn out to be the saddlepoint energies (6.5 and 5.4 kcal/mole) of the internal rotational surface. It is shown that a close relationship exists between the flip-flop rotation of the geminal double rotor and the pseudorotation of five membered rings. Using the calculated conformational potential V (ϑ1,ϑ2), the prediction is made that the envelope form of 1,2,3-trioxolane is more stable than the corresponding twist form.