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.
Highlights
Many electronic quantities of a molecule undergo small but specific changes upon isotopic substitution [1]
In a recent paper [30], we presented the difference-dedicated (DD) VPT2 approach, which provides values for nuclear magnetic resonance (NMR) isotopic shifts at VPT2 quality, that is, without any assumptions on the structure of the vibration hole, at a cost comparable to the Local Mode Zero-Point Level (LMZL) or other comparable a priori local methods
In standard VPT2, the NMR isotope shift between isotopologues A and B is represented as the sum of a harmonic part connecting the curvature of the chemical-shielding surface with the harmonic vibrations, and an anharmonic part connecting the anharmonic part of the vibrations with the gradient of the σ surface: n
Summary
Many electronic quantities of a molecule undergo small but specific changes upon isotopic substitution [1]. In a recent paper [30], we presented the difference-dedicated (DD) VPT2 approach, which provides values for NMR isotopic shifts (or isotope effects on other quantities) at VPT2 quality, that is, without any assumptions on the structure of the vibration hole, at a cost comparable to the LMZL or other comparable a priori local methods. We will make use of this possibility and investigate the structure and locality of the vibration hole both for the systems investigated in Reference [30] and for a number of molecules studied recently by Hansen et al [20] where the substitution site is involved in an intra-molecular hydrogen bond.
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