Spin densities, conformations and rotational energy barriers around the C(O)N bond in acylaminoxyl (RC(O)N(O)R′; R,R′H,CH 3) radicals: ab initio and density functional study

Abstract

Systematic ab initio (at UHF and UMP2 levels of theory) and density functional calculations (using non-local exchange and correlation functionals) were carried out for the C- and N-methyl substituted formylaminoxyl derivatives using a 6–31 + G ∗ basis. For the formylaminoxyl radical, larger bases (up to 6–311 + + G ∗∗, to calculate the geometry and Chipman's [6s,3p,2d] basis for spin densities) and CCSD and CCSD(T) types of wavefunction were also applied. The core part (the OCNO group) was found to be planar in their most stable (E) conformation in every case. Some small deviation from this planarity could be observed for their less stable (Z) conformation. The energy gap between these two stable conformers was in the range 4–9 kcal mol −1 depending on the compound and the level of theory, the most “crowded” example ( N-acetyl- N-methylaminoxyl) exhibiting the largest energy difference. The rotational energy barriers around (O)CN(O) were less influenced by the methyl substitution. At the transition state, the aminoxyl moiety had lost its planarity. The (O)CN(O) bond was substantially longer in the transition state (by ~0.05 Å) than in either of the energy minima, indicating a partial double-bond character in the latter cases. As a consequence of this delocalisation of the SOMO orbital, the relatively small a N hyperfine coupling constant (hfcc) can be exfplained by a smaller spin polarization at the N nucleus. However, one cannot neglect the effect of local planarity of the aminoxyl group on this hfcc as shown by the comparison between the rigid (keeping the radical center planar) and relaxed rotor approaches upon rotation around the (O)CN(O) bond.