NMR Shielding Constants

chemical shift

Gas-phase and solution-phase NMR shielding constants are available for closed-shell and unrestricted open-shell wave functions [19].1 Keyword nmr = 1 in the gen section. Shielding constants are returned as atom properties (in ppm) in the Maestro output file. You can use these values for atom selection, for example, or you can display them in labels.

Shieldings are calculated for all atoms, including those with ECPs. Shielding constants for atoms whose core is represented by an ECP should be ignored as erroneous, because the main contributions come from the core tail of the valence orbitals, which is largely absent at ECP centers. Chemical shifts derived from these shielding constants might display the correct trends, but are likely to have the wrong magnitude.

Chemical shifts are calculated for ¹H, ¹³C, ¹⁵N, ¹⁹F, and ³¹P nuclei from the shieldings, using a linear fit to experimental data, following refs. [273], [274], [317–317. The shifts for C are corrected if Cl atoms are directly attached to the C atom. No corrections are available for other heavy atoms, so shifts for C bonded to atoms heavier than Cl should be treated with caution. The shifts are added to the structures as an atom property (NMR Chemical Shifts) in the output structure file.

The chemical shift calculations are available for the 6-31G, 6-311G, LACVP, and LAV2P basis sets with polarization (* or **) and optionally with diffuse functions, and also for the cc-pVDZ and MIDIX basis sets. The following density functionals can be used: M06-2X, PBE, PBE0, BLYP, B3LYP, O3LYP, B3PW91, PW6B95, and B97 (with or without a posteriori corrections).

If you want to generate Boltzmann-averaged chemical shifts for C and H atoms over a set of conformers, you can do so from the command line with the spectroscopy.py script. For more information, see spectroscopy.py: Calculation of Boltzmann-Averaged Properties. The script also generates Boltzmann-averaged H-H and H-F distances.

If you want to calculate chemical shifts directly, you will need to calculate NMR shielding constants for the reference molecules for each element of interest, in the same basis set and with the same method as for the molecule of interest.