Theoretical determination of the barriers to internal rotation in hydrogen peroxide

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Abstract
The cis and trans barriers to internal rotation in hydrogen peroxide have been accurately determined within the Hartree–Fock approximation using a [4s3pld/2slp] basis set. The calculated trans barrier (384 cm−1) differs from experiment (386 cm−1) by only 2 cm−1. The cis barrier is not well determined by experiment, and the calculated value (2921 cm−1) may well be the most accurate value reported to date. The effects of geometry optimization and polarization functions were also studied. The calculations clearly illustrate the importance of determining the Hartree–Fock geometry for a suitable molecular configuration. With the exception of the OOH bond angle, optimization of the geometrical parameters as the molecule rotates was found to be of little importance; the increase of 6° in &OOH upon rotation from the cis to the trans configuration lowers the cis barrier by 653 cm−1. It was found that the inclusion of oxygen 3d and hydrogen 2p functions was essential for describing the trans barrier, without which the barrier height was just 54 cm−1. In addition, polarization functions lowered the cis barrier by 675 cm−1.