MP2 calculation of ⁷⁷Se NMR chemical shifts taking into account relativistic corrections

Abstract

The main factors affecting the accuracy and computational cost of the Second-order Möller-Plesset perturbation theory (MP2) calculation of (77) Se NMR chemical shifts (methods and basis sets, relativistic corrections, and solvent effects) are addressed with a special emphasis on relativistic effects. For the latter, paramagnetic contribution (390-466 ppm) dominates over diamagnetic term (192-198 ppm) resulting in a total shielding relativistic correction of about 230-260 ppm (some 15% of the total values of selenium absolute shielding constants). Diamagnetic term is practically constant, while paramagnetic contribution spans over 70-80 ppm. In the (77) Se NMR chemical shifts scale, relativistic corrections are about 20-30 ppm (some 5% of the total values of selenium chemical shifts). Solvent effects evaluated within the polarizable continuum solvation model are of the same order of magnitude as relativistic corrections (about 5%). For the practical calculations of (77) Se NMR chemical shifts of the medium-sized organoselenium compounds, the most efficient computational protocols employing relativistic Dyall's basis sets and taking into account relativistic and solvent corrections are suggested. The best result is characterized by a mean absolute error of 17 ppm for the span of (77) Se NMR chemical shifts reaching 2500 ppm resulting in a mean absolute percentage error of 0.7%.

Correlated ab initio calculations of one‐bond 31P77Se and 31P125Te spin–spin coupling constants in a series of PSe and PTe systems accounting for relativistic effects (part 2)

“…We noted previously that this level of theory, while the most accurate among those available, still does not guarantee the reliability of SSCCs. However, based on vast previous experience, [28,[65][66][67] the relativistic corrections to NMR properties, reflected as the difference between the relativistic and nonrelativistic values calculated under the same conditions, do reflect the influence of relativistic effects satisfactorily. A practical meaning of this fact is vital; however, the theoretical explanation goes far beyond the scope of this paper, and we have elected to just take advantage of this knowledge here.…”

“…[61] This idea was successfully applied in calculations of NMR parameters by numerous authors. [62][63][64] We understand that the idea does not reflect the true nature, but our previous calculations of SSCCs involving either selenium nuclei or tellurium nuclei [28,[65][66][67] have also indicated that this assumption leads to satisfactory estimations. To provide a rather reliable description of the electron correlation, we have used, as a starting point, the combination of the SOPPA (CC2) method for the evaluation of 1 J( 77 Se, 31 P) and 1 J( 125 Te, 31 P) SSCCs at the nonrelativistic level of theory and the fourcomponent DFT-KT1 approach for the relativistic corrections.…”

Four‐component relativistic DFT calculations of 77Se NMR chemical shifts: A gateway to a reliable computational scheme for the medium‐sized organoselenium molecules

“…In view of the utmost interest and a vast amount of experimental data on 77 Se NMR chemical shifts, ranging inbetween about −700 and +2200 ppm and providing a gateway to structural elucidation of organoselenium compounds, the development of the efficient computational protocol for 77 Se NMR shielding constants (converted to the corresponding chemical shifts scale) remains an important problem. A number of computational approaches, either at the nonrelativistic, or relativistic levels were used by different authors for the calculations of on 77 Se NMR chemical shifts, and these papers are discussed in our recent publication . Based on these results, the contribution of relativistic effects totals to about 230–260 ppm (15%) for shielding constants or about 20–30 ppm (5%) in the corresponding chemical shifts scale.…”

“…Based on these results, the contribution of relativistic effects totals to about 230–260 ppm (15%) for shielding constants or about 20–30 ppm (5%) in the corresponding chemical shifts scale. In that paper, we proposed a hybrid computational scheme using the calculation of the nonrelativistic counterpart at the correlated MP2 level while relativistic corrections were evaluated within the uncorrelated four‐component relativistic random phase approximation (RPA) level in its modern implementation . Both methods, MP2 and RPA, belong to the pure nonempirical approaches based on the wave function formalism with the former taking into account electronic correlation at the second‐order excitation theory level and with the later ignoring correlation effects.…”

“…Both methods, MP2 and RPA, belong to the pure nonempirical approaches based on the wave function formalism with the former taking into account electronic correlation at the second‐order excitation theory level and with the later ignoring correlation effects. Proposed computational scheme is characterized by a mean absolute error (MAE) of only 17 ppm for the span of 77 Se NMR chemical shifts reaching 2500 ppm resulting in a mean absolute percentage error of only 0.7% However, in spite of a very high accuracy of the aforementioned MP2 + 4RPA hybrid scheme, the latter is extremely computationally demanding and time consuming. Another minor point of this scheme lies in the fact that relativistic counterpart is calculated without taking into account correlation effects with the latter aparently contributing to relativistic correction.…”