The observed and calculated ¹H and ¹³C chemical shifts of tertiary amines and their N‐oxides

Abstract

A series of model tertiary amines were oxidized in situ in an NMR tube to amine N-oxides and their (1)H and (13)C NMR spectra were recorded. Next, the chemical shifts induced by oxidation (Δδ) were calculated using different GIAO methods investigating the influence of the method [Hartree-Fock (HF), Moeller-Plesset perturbation, density functional theory (DFT)], the functional applied in the DFT (B3LYP, BPW, OPBE, OPW91) and the basis set used [6-31G*, 6-311G**, 6-311 + + G** and 6-311 + + G(3df,3pd)]. The best results were obtained with the HF/6-311 + + G** and OPBE/6-311 + + G** methods. The computation/experiment comparison approach was used for the configuration prediction of chiral amine N-oxides-(R) and (S)-agroclavine-6-N-oxide.

13C GIAO DFT calculation as a tool for configuration prediction of N–O group in saturated heterocyclic N‐oxides

“…Unfortunately, there, we have not taken into account that N‐oxidation of the amines with MCPBA leads to the fast equilibrium mixture of amine oxides and m ‐chlorobenzoate salts ( N ‐hydroxyammonium‐type salts) of the corresponding amine oxides, whose chemical shifts differ somewhat from those of the free amine oxides. Thus, the chemical shifts we reported in Pohl et al . for 5a–9a are, in reality, the averaged shifts of 5a–9a and 5b–9b (for details, refer to the following discussion).…”

“…In our previous contribution, we screened the computation methods on model amines 5–9 and amine oxides 5a–9a (Fig. ).…”

“…Therefore, prior to the NMR data calculation, it is necessary to perform a conformation analysis. The conformation analysis for model compounds 5–9 and 5a–9a has been already published in our previous paper, and we used the conformations population for the nuclear shielding calculation. There are two possible conformers in the case of protonized N ‐oxides 7b–9b ( anti and gauche , Fig.…”

“…The molecular geometries were optimized at the B3LYP/6‐31G* level of theory in vacuum . The conformation analysis of model compounds was performed in our previous paper, and all calculated NMR data presented here are weighted averaged values according to the conformer population. The nuclear magnetic shielding constants were calculated by GIAO DFT using the OPBE/6‐31 G* functional basis set setup.…”

“…Therefore, reproducible experimental data are obtained when the acid is neutralized. This omission led to incorrect experimental chemical shifts of model compounds in our previous paper . Hence, the corrected experimental NMR data together with improved experimental procedure providing reproducible values are described in the present work.…”