Alkyl peroxides: Effect of substituent groups on the torsional mode around the O&bond;O bond.

We present here a systematic study by quantum mechanical methods of structural and energetic properties for a series of substitutions by alkyl groups of the hydrogens in hydrogen peroxide. The emphasis is on the torsion around the peroxidic bond, which leads to the chirality changing stereomutation. The dihedral angle dependence of the geometrical features and of the dipole moment is discussed with reference to previous experimental and theoretical information, and with respect to the preceding paper on hydrogen peroxide (Maciel et al., Chem Phys Lett, 2006, 432, 383). This information is of interest for chiral separation experiments as well as in view of a possible dynamical mechanism for chirality exchange by molecular collisions. The cis and trans barriers appear to vary remarkably upon substitution by alkyl groups (methyl, ethyl, n- and iso-propyl, sec- and tert-butyl hydroperoxides), the most important property being their geometrical dimensions. As the latter increase, tendency for the equilibrium configuration towards the trans structure increases, so that the trans barrier becomes negligible for dimethyl and diethyl peroxides and for n- and iso-butyl hydroperoxides, giving essentially achiral molecules. For the chiral ones (HOOH, CH3OOH, and C2H5OOH) torsional level energies and eigenfunctions are calculated and their distribution as a function of temperature determined. Their use is exemplified by a calculation of the dipole moment of hydrogen peroxide at room temperature, reconciling previous disagreement between theory and experiment. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]