NMR spectroscopy in environmental chemistry: 1H and 13C NMR chemical shift assignments of chlorinated dibenzothiophenes based on two‐dimensional NMR techniques and ab initio MO and DFT/GIAO calculations

Abstract

ABSTRACT:1 H and 13 C NMR spectra for seven chlorinated dibenzothiophenes (DBTs) were measured. Complete 1 H and 13 C NMR chemical shift assignments for 2,8-and 3,7-dichloro-and 2,3,6,8-, 2,3,7,8-and 2,4,6,8-tetrachloro congeners are based on z-gradient selected inverse (proton detected) two-dimensional heteronuclear chemical shift correlation experiments, 1 H, 13 C HMQC and HMBC. For 1,2,3,4,6,7,8-heptachloro-DBT and octachloro-DBT, where polarization transfer techniques from proton to (all) carbons are not possible, the 13 C NMR chemical shift assignments are based on theoretical calculations using ab initio MO and DFT/GIAO methods at the HF/6-311GŁ and BPW91/6-311G Ł levels. The observed HMBC correlations for the heptachloro congener are in good agreement with a theoretically predicted shift order, providing evidence for the reliability of the theoretical method.

Theoretical Study of 13C Chemical Shifts Structures of Some ortho‐Substituted 3‐Anilino‐2‐nitrobenzo[b]thiophenes and 2‐Anilino‐3‐nitrobenzo[b]thiophenes and Comparison with Experimental

“…The predicted 13 C chemical shifts are derived from equation δ＝σ 0 ＋σ where δ is the chemical shift, σ is the absolute shielding, and σ 0 is the absolute shielding of the standard TMS (in this case δ 183.6). 24 Linear regression analysis between the experimental 13 The correlations between the experimental values in solvent DMSO-d 6 and CDCl 3 of the 13 C chemical shifts and those calculated using GIAO and CSGT for the carbons of compound G of 3-anilino-2-nitrobenzo[b]thiophenes (2o) are shown in Figure 3. It is seen that chemical shifts for the all carbons agree quite well with the experimental values.…”

Spectroscopic QSAR Methods and Self-Organizing Molecular Field Analysis for Relating Molecular Structure and Estrogenic Activity

“…Thus, in contrast to MO energies and vibration frequencies, high-level ab initio/density functional calculations must be employed to predict NMR chemical shifts correctly. As in our previous works, − a multiphase procedure (beginning from the MM+ optimized geometry and proceeding via AM1 and 3-21G* to the 6-31G* level) was adopted, in which the initial force constants were taken from a previous, lower level calculation. It appears that this approach has a clear advantage over direct calculations in which the MM+ optimized structure as such is used as an input for the 6-31G* calculation.…”

“…The first part involved obtaining sufficiently accurate determinations of the molecular geometries. We have shown previously that the 6-31G* basis set can provide reliable geometries for dibenzothiophenes, 31 camphenes, 32 and chloroterpenes, 33 and this method was also employed here. The second part consisted of a GIAO calculation for determining the chemical shifts, which were derived by calculating first the absolute NMR shielding for each nucleus and then taking the difference between the calculated value and the shielding value for the same nucleus of a suitable reference molecule, e.g., tetramethylsilane (TMS), calculated by reference to exactly the same model chemistry.…”