Theoretical determination of molecular structure and conformation. I. The role of basis set and correlation effects in calculations on hydrogen peroxide

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Abstract
Equilibrium structure and barriers to internal rotation of hydrogen peroxide have been accurately determined with the Hartree–Fock method and Rayleigh–Schrödinger perturbation theory using a (9s5p1d/4s1p) [4s3p1d/2s1p] contracted and (11s6p2d/6s2p) uncontracted basis set. Extensive rescaling of the contracted basis accompanied by complete geometry optimization leads to barrier values of 0.7 (trans) and 8 (cis) kcal/mole. Results obtained with the uncontracted basis indicate an improvement of the barriers to 1.1 and 7.4 kcal/mole comparable to the refined experimental values of Ewig and Harris. Inclusion of correlation does not change the barriers significantly. The latter, however, is necessary to obtain correct equilibrium parameters. The computed bond lengths [R (OO) =1.451 Å, R (OH) =0.967 Å] and angles [α (OOH) =99.3° and ϑ (HOOH) =119.3°] are in good agreement with experiment while near HF values lead to a false structure [R (OO) =1.390 Å, R (OH) =0.943 Å, α (OOH) =102.9°, ϑ (HOOH) =111.2°]. The importance of optimum scaled polarization functions in the perturbation approach is demonstrated.