Mo/ller–Plesset treatment of electron correlation effects in (HOHOH)−

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Abstract
The effects of electron correlation on the calculated properties of the (HOHOH)− anion are studied using Mo/ller–Plesset (MP) perturbation theory. With this technique, inclusion of corrections up to third order are shown to provide results quite similar to those obtained with an extensive CI approach when equivalent basis sets are used. Barriers to proton transfer between the two oxygen atoms at a fixed R(OO) distance are computed with a number of basis sets ranging from split‐valence 4–31G to triple‐valence with polarization functions on all atoms, 6–311G**. Each successive enlargement of the basis set leads to a greater barrier. The second‐order correction to the energy reduces the Hartree–Fock barrier dramatically while subsequent inclusion of the third‐order energy results in an increase over the MP2 barriers. MP3 formalism is found capable of accurately reproducing CI results for both the barrier height and functional dependence of the correlation energy upon the proton position. The potential energy surface is calculated as a function of both the R(OO) distance and the position of the central proton. At the Hartree–Fock level, all basis sets yield a surface with two minima separated by a saddle point, representing the transition state for adiabatic proton transfer. The surface is flattened a great deal by inclusion of second‐ and third‐order corrections such that the barrier to proton transfer is considerably below the estimated zero vibrational level for protonic motion. Electron correlation effects are also responsible for an increase of about 3 kcal/mol in the hydrogen‐bond energy of the (HOHOH)− complex.