Origin of 17O NMR chemical shifts based on molecular orbital theory: paramagnetic terms of the pre-α, α and β effects from orbital-to-orbital transitions, along with the effects from vinyl, carbonyl and carboxyl groups

Abstract

¹⁷ O NMR chemical shifts (δ(O)) were analysed based on the molecular orbital (MO) theory, using the diamagnetic, paramagnetic and total absolute magnetic shielding tensors (σ^d(O), σ^p(O) and σ^t(O), respectively). O²⁻ was selected as the standard for the analysis. An excellent relationship was observed between σ^d(O) and the charges on O for O⁶⁺, O⁴⁺, O²⁺, O⁰ and O²⁻. The data from H₂O, HO⁺, HO⁻ and H₃O⁺ were on the correlation line. However, such relationship was not observed for the oxygen species, other than above. The pre-α, α and β effects were evaluated bases on σ^t(O), where the pre-α effect arises from the protonation to a lone pair orbital on O²⁻, for an example. The 30–40 ppm and 20–40 ppm (downfield shifts) were predicted for the pre-α and β effects, respectively, whereas the values for the α effect was very small in magnitude, where the effect from the hydrogen bond formation should be considered. Similarly, the carbonyl effect in H₂CO and the carboxyl effects in H(HO)CO were evaluated from MeOH, together with H₂CCHOH from CH₃CH₂OH. Very large downfield shifts of 752, 425 and 207 ppm were predicted for H₂CO*, H(HO)CO* and H(HO*)CO, respectively, together with the 81 ppm downfield shift for H₂CCHO*H. The origin of the effect were visualized based on the occupied-to-unoccupied orbital transitions. As a result, the origin of the ¹⁷O NMR chemical shifts (δ(¹⁷O)) can be more easily imaged and understand through the image of the effects. The results would help to understand the role of O in the specific position of a compound in question and the mechanisms to arise the shift values also for the experimental scientists. The aim of this study is to establish the plain rules founded in theory for δ(¹⁷O), containing the origin, which has been achieved through the treatments.