Theoretical gas to liquid shift of ¹⁵N isotropic nuclear magnetic shielding in nitromethane using ab initio molecular dynamics and GIAO/GIPAW calculations

Abstract

Chemical shift requires the knowledge of both the sample and a reference magnetic shielding. In few cases as nitrogen (15N), the standard experimental reference corresponds to its liquid phase. Theoretical estimate of NMR magnetic shielding parameters of compounds in their liquid phase is then mandatory but usually replaced by an easily-get gas phase value, forbidding direct comparisons with experiments. We propose here to combine ab initio molecular dynamic simulations with the calculations of magnetic shielding using GIAO approach on extracted cluster's structures from MD. Using several computational strategies, we manage to accurately calculate 15N magnetic shielding of nitromethane in its liquid phase. Theoretical comparison between liquid and gas phase allows us to extrapolate an experimental value for the 15N magnetic shielding of nitromethane in gas phase between -121.8 and -120.8 ppm.

A Review on Combination of Ab Initio Molecular Dynamics and NMR Parameters Calculations

“…For a long time, in order to do such a conversion, a gas state nitromethane was modeled, which caused some inaccuracies due to the inability to model the intermolecular interactions. In the study of Gerber and Joliboi [41], thanks to compilation of computational methods including the aiMD and the implicit solvent model, the accurate theoretical 15 N spectrum of liquid nitromethane was obtained.…”

Structures and NMR spectra of short amylose-lipid complexes. Insight using molecular dynamics and DFT quantum chemical calculations

“…These discrepancies can be attributed to: 1) the force field parameters that produce structures different from the optimized structure; 2) the environmental effects; 3) unsuitability of the calculated reference (TMS) chemical shielding for chemical shift calculations using MD structures (Cuny et al, 2018;Gerber & Jolibois, 2015). As above, this spectrum may be corrected using a linear fit (δcorrected=0.8963*δtheo+2.7044 with a R 2 =0.9993), leading to an overall improvement, in particular in the upfield shift of the region corresponding to C2, C3 and C5 atoms, toward that observed experimentally (i.e.…”

Influence of Amylose Size on the Structure and NMR Spectrum of Amylose-Palmitic Acid Complexes. Insight from Classical Molecular Dynamics Simulations and DFT Quantum Chemical Computations.

“…The same is true when the dihedral angle around the C-OH bond is modified [35,43]. Therefore, it is difficult to explain in detail, using a simple chemical model, the chemical shift variations for all carbons in the glucose moiety.…”

“…The ones associated to C4 and C6 carbons are located around 88.0 and 66.0 ppm, respectively. These discrepancies can be attributed to some computational issues: 1) force field parameters that produce structures that are different from the optimized one; 2) environemental effects that are not correctly taken into account; 3) by the fact that our reference (TMS) chemical shielding has been determine from a full quantum chemical procedure and might not be suitable for chemical shift calculations using molecular dynamic structures 42,43 . As a consequence, a linear fit between experimental and these simulated data using the three unambigous atoms has been calculated, as previously performed for the full quantum chemical procedure.…”