The Structure of the “Vibration Hole” around an Isotopic Substitution—Implications for the Calculation of Nuclear Magnetic Resonance (NMR) Isotopic Shifts

Abstract

Calculations of nuclear magnetic resonance (NMR) isotopic shifts often rest on the unverified assumption that the “vibration hole”, that is, the change of the vibration motif upon an isotopic substitution, is strongly localized around the substitution site. Using our recently developed difference-dedicated (DD) second-order vibrational perturbation theory (VPT2) method, we test this assumption for a variety of molecules. The vibration hole turns out to be well localized in many cases but not in the interesting case where the H/D substitution site is involved in an intra-molecular hydrogen bond. For a series of salicylaldehyde derivatives recently studied by Hansen and co-workers (Molecules 2019, 24, 4533), the vibrational hole was found to stretch over the whole hydrogen-bond moiety, including the bonds to the neighbouring C atoms, and to be sensitive to substituent effects. We discuss consequences of this finding for the accurate calculation of NMR isotopic shifts and point out directions for the further improvement of our DD-VPT2 method.

Halogen Bond of Halonium Ions: Benchmarking DFT Methods for the Description of NMR Chemical Shifts

“…Herein, we report the comprehensive assessment of the accuracy of DFT methods and two wave functions (HF and MP2) for the description of NMR chemical shifts of three-center, four-electron halogen-bond systems, that is, the exceptionally strong halogen-bond complexes of halonium ions. For this investigation, we used the three-center, four-electron halogen-bond model systems that have so far been experimentally most extensively studied (Figure ) ,, and have also been used as benchmark systems in various contexts, ,, providing ample and reliable experimental data for comparison. As the counterion has previously been demonstrated to not influence [N–I–N] + halogen bonds significantly, it was omitted in the current calculations …”

Isotope Effects on Chemical Shifts in the Study of Hydrogen Bonds in Small Molecules

“…Gräfenstein has developed a difference dedicated second-order vibrational perturbation theory to calculate isotope effects [79]. This was applied to a series of o-hydroxybenzaldehydes [80]. Ab initio path integral molecular dynamics (PIMD) calculations showed a barrier-less proton transfer and a C 2 V symmetry of the hydrogen bond.…”