Abstract
Abstract: We have investigated, by means of ab initio and DFT calculations, the magnitude of through-space spin-spin couplings ( J CH and J HH ) in CH/π bonded van der Waals dimers involving acetylene, and in a structurally related covalent compound (4-ethynylphenanthrene). Within regions where the interaction is stabilizing J HH couplings are very small (< 0.1 Hz) for all complexes. In the acetylene-methane complex J CH is also very small, whereas in the acetylene-benzene complex and the acetylene dimer it shows a relatively large dependence on the tilt angle from the T-shaped arrangement, for which the smallest values are calculated, to a parallel slipped arrangement where J CH is ca. 0.5 Hz. The relative magnitude of through-space couplings in 4-ethynylphenanthrene and a tilted acetylene-benzene dimer, featuring the same internuclear arrangement as in model dimers, are compared. Keywords: Through-space coupling, NMR, ab initio , DFT. Introduction Although spin-spin coupling is normally considered as a signature of covalent bonding, several cases have been known for a long time where non-negligible couplings are observed between nuclei (notably
Highlights
Spin-spin coupling is normally considered as a signature of covalent bonding, several cases have been known for a long time where non-negligible couplings are observed between nuclei separated by many covalent bonds
In this paper we extend our investigation to model van der Waals complexes involving triple bonds and to a stable, rigid covalent molecule (4-ethynylphenanthrene) where a C-H bond is forced to point towards the π system of the triple CC bond, so that through-space coupling may be detectable
For the acetylene dimer 3, a comparison is made with results obtained at the ab initio SOPPA level with the aug-cc-pVDZ-su1 basis set, extensively employed by Pecul and coworkers to this purpose,[17] which is an augmentation of the cc-pVDZ basis set with a diffuse function, plus a tight s function, with all s functions deconstructed
Summary
Spin-spin coupling is normally considered as a signature of covalent bonding, several cases have been known for a long time where non-negligible couplings are observed between nuclei (notably 19F) separated by many covalent bonds. Nuclear spin-spin JCH and JHH couplings were calculated between the hydrogen of the acceptor CH bond and the carbon or hydrogen atoms of the donor π system, which belong to two different molecules.
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