Joint multinuclear NMR chemical shift correlations with DFT/GIAO computed data are beneficial

What is it about?

Organic chemists and natural product researchers often encounter substantial difficulties in analyzing and assigning the resonance signals observed in NMR spectra performed in solutions for the substances under their study. A typical problem is the presence of numerous heteroatoms at the expense of protons in the (bio)organic molecules under study (proton-poor systems). The latter limitation can often be circumvented by using properly referenced NMR spectra taken in the same deuterated solvent. [It may be useful here to familiarize yourself with two later papers by Nazarski (Molecules 2023, 28, 4369 and Magn. Reson. Chem. 2024, 62, 535–543)]. Performing 1H/13C NMR spectra for a series of synthesized bicyclic aromatic nitrogen heterocycles 1-11 allowed the authors to propose a linear multinuclear correlation equation, involving the H, C, and N nuclei, between the experimental NMR chemical shifts and those GIAO-calculated at the DFT level.

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Why is it important?

An advantage of the use of isodensity-based SCRF radii (called IDSCR radii, introduced by Fang and coworkers in 2013) over typical UFF-radii-based solute cavities in GIAO/DFT NMR calculations is presented for the 13C and especially 15N chemical shifts for several bicyclic aromatic nitrogen heterocycles that were studied in CDCl3 solution mimicked within the classical IEF-PCM solvation scheme. Another interesting and important finding was that the use of IDSCR radii at the modeling stage of the studied systems and in the subsequent single-point frequency calculations performed at the same theoretical level did not observe small spurious imaginary vibrational modes. Such small negative harmonic frequencies were obtained for some studied molecules using the UFF-radii defaulted in the Gaussian 09 program. This situation was explained by numerical inaccuracies in the calculation performed.

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