The structure of F2O2: Theoretical predictions and comparisons with F2 and F2O

Exploring the structural and conformational properties of dioxygen dihalides (halogen = F, Cl, Br)

The structural and conformational properties of dioxygen difluoride (1), dioxygen dichloride (2), and dioxygen dibromide (3) have been investigated by means of the hybrid density functional theory (B3LYP) and the hybrid meta exchange-correlation functional (M06-2X) with the aug-cc-pVQZ, aug-cc-pVTZ, cc-pVTZ, and 6-311++G** basis sets and natural bond orbital interpretation. The results obtained showed that the rotation at the O–O bond by passing from the plane symmetrical trans-(C2h) (or cis-(C2v)) form leads to the O–F bonds breaking. The natural bond order analysis revealed that the O–O bonds in the C2h and C2v forms of compound 1 possess double bond characters and the small bond orders between the oxygen and fluorine atoms results from the strong electron delocalization between the lone pairs of the fluorine atoms (LP4F) and the antibonding orbitals of the adjacent O–O double bond (π*O–O). The lengthening and shortening of the O–F and O–O bonds, respectively, in the trans-(C2h) or cis-(C2v) forms of compound 1 can be interpreted by the decrease of the bonding interactions between the fluorine and oxygen orbitals. The profiles of the orbital amplitudes (or electron densities) of O–O and O–F bonds in the C2h and C2v forms of compound 1 revealed that there are strong electronic repulsions between the πO–O and the lone pairs of oxygen atoms with the lone pairs of fluorine atoms, leading to the dissociation of O–F bonds. It is worth noting that there are strong hyperconjugative interactions between LP2O and the antibonding orbitals of adjacent O–F bonds (σ*O–F) in the skew (C2) form of compound 1, leading to the decrease of the O–O bond length and the increase of the F–O bond length by increasing the πO-O bonding and the σ*O–F antibonding orbital occupations. The increase of the electron delocalization from LP3F to σ*O–O with the decrease of the σ*O–O–σO–O energy gap (results from the increase of the O–O bond length) justifies the similarity between the adiabatic O–O bond dissociation energies in compound 1 and HOOH. The electron delocaizations from LP2O2 to σ*O3–X (X = F (1), Cl (2), Br (3)) decrease drastically from the skew ground state (C2) forms of compound 1 to compound 2 but increase slightly from compound 2 to compound 3, causing the significant increase of the O–O bond length ongoing from compound 1 to compound 2 and the slight decrease from compound 2 to compound 3. Effectively, the conformational properties of compounds 1–3 can be interpreted with the principle of maximum softness.

Gas phase structures of peroxides: experiments and computational problems

Gas-phase structures of several organic and inorganic peroxides X-O-O-X and X-O-O-X', which have been determined experimentally by gas electron diffraction and/or microwave spectroscopy, are discussed. The OO bond length in these peroxides varies from 1.481(8) Å in Me3 SiOOSiMe3 to 1.214(2) Å in FOOF and the dihedral angle ϕ(XO-OX) between 0° in HC(O)O-OH and near 180° in Bu(t) O-OBu(t) . Some of the peroxides cause problems for quantum chemistry, since several computational methods fail to reproduce the experimental structures. Extreme examples are MeO-OMe and FO-OF. In the case of MeO-OMe only about half of the more than 100 computational methods reported in the literature reproduce the experimentally determined double-minimum shape of the torsional potential around the OO bond correctly. For FO-OF only a small number of close to 200 computational methods reproduce the OO and OF bond lengths better than ±0.02 Å.