GIAO‐DFT calculation of ¹⁵N NMR chemical shifts of Schiff bases: Accuracy factors and protonation effects

Abstract

N NMR chemical shifts in the representative series of Schiff bases together with their protonated forms have been calculated at the density functional theory level in comparison with available experiment. A number of functionals and basis sets have been tested in terms of a better agreement with experiment. Complimentary to gas phase results, 2 solvation models, namely, a classical Tomasi's polarizable continuum model (PCM) and that in combination with an explicit inclusion of one molecule of solvent into calculation space to form supermolecule 1:1 (SM + PCM), were examined. Best results are achieved with PCM and SM + PCM models resulting in mean absolute errors of calculated N NMR chemical shifts in the whole series of neutral and protonated Schiff bases of accordingly 5.2 and 5.8 ppm as compared with 15.2 ppm in gas phase for the range of about 200 ppm. Noticeable protonation effects (exceeding 100 ppm) in protonated Schiff bases are rationalized in terms of a general natural bond orbital approach.

Computational 1H NMR: Part 1. Theoretical background

“…As an example of the latter, shown in Figure are the intermolecular complexes of several pyrazines with one molecule of dimethyl sulfoxide in the IEF‐PCM medium 23 – 30 studied by Semenov et al (for details, see original publication). Another illustrative example is the intermolecular cluster of five molecules of nitrometane 31 studied by the same authors, which is widely used for NMR referencing. The structure of this cluster for the mostly computationally achievable five molecules of nitromethane in the IEF‐PCM continuum optimized at the KT3/pcS‐3 level is shown in Figure .…”

“…Salient interatomic distances are given in Angstroms. Reproduced from Semenov et al with the permission of John Wiley and Sons…”

“…[24] As an example of the latter, shown in Figure 6 are the intermolecular complexes of several pyrazines with one molecule of dimethyl sulfoxide in the IEF-PCM medium 23-30 studied by Semenov et al [108] (for details, see original publication). Another illustrative example is the intermolecular cluster of five molecules of nitrometane 31 studied by the same authors, [109] which is widely used for NMR referencing. The structure of this cluster for the [102] with the permission of American Chemical Society FIGURE 5 Comparison of the proton shielding constants obtained using the pcS-3/pcS-2/pcS-1 locally dense basis set scheme versus pcS-4 on all atoms for the 1 H NMR shielding constants for a set of 28 benchmark molecules.…”

Computational NMR of Carbohydrates: Theoretical Background, Applications, and Perspectives

“…Different LDBS schemes were systematically employed for the calculation of NMR parameters at the DFT and non-empirical levels of theory in a number of recent publications by Rusakov et al [ 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 ] and Semenov et al [ 80 , 81 , 82 , 83 , 84 , 85 ], as well as in numerous publications by different authors, which are not cited herewith in full in view of their multiplicity.…”

“…Interesting to note, the ongoing from the double zeta cc-pVDZ to the triple zeta cc-pVTZ basis set did not improve in general the quality of the correlations established in that paper [151], as can be seen in the values of RMSD given in the table. (82), sophorose (83), nigerose (84), laminaribiose (85), maltose (86), cellobiose (87), isomaltose (88), and gentiobiose (89). R 1 and R 2 stand for H and OH (or either OH and H) in different anomers.…”