Proximity Effects on Nuclear Spin-Spin Coupling Constants. 1. J(CH) Couplings in the Vicinity of an Atom Bearing Lone Pairs

Abstract

Proximity effects on lJ(CH) couplings are studied from a theoretical point of view in the following systems:CH4/FH (A) and H*O/HCN (B) which form dimers by hydrogen-bond interactions. 'J(CH) couplings for different intermolecular distances are calculated for the C-H bond facing the atom bearing lone pairs. While in the former system, this coupling is increased owing to the proximity to the F atom; in the latter this coupling is decreased owing to the proximity to the 0 atom. These opposite trends are accompanied by slight shortening and lengthening, respectively, of the corresponding C-H bond lengths. As part of this work, measurements of 'J(CH) couplings in 9 4 1,3-dioxolan-2-y1)-1,2,3,4-tetrafluorotriptycene (I) and l-formyl-2-hydroxy-8-fluoronaphthalene (11) and J ( N H ) in o-fluorobenzamide (III), showing proximity effects as those described above are carried out. These results suggest that J(XH) couplings are adequate probes to distinguish two different types of X-H.-Y hydrogen bonds.

Electronic Mechanisms of Intra and Intermolecular J Couplings in Systems with C-H···O Interactions

“…Correlated results found in this work for 1 J(CH) in systems I and II complement previous findings [13,15] with new interesting qualitative features which are related to the expected behavior of the π- …”

“…couplings and C, H and O magnetic shielding constants [13,14,15]. In previous work, the change in J couplings was analyzed at the TDA [14] and CHF [13] levels since it was assumed that, even though it is well known that 1 J(CH) couplings are very sensitive to correlation effects, the changes in those couplings in the presence of the specific C-H...O interaction could probably be well reproduced at the uncorrelated level.…”

Solvent Effects on Nuclear Magnetic Resonance 2J(C,Hf) and 1J(C,Hf) Spin–Spin Coupling Constants in Acetaldehyde

“…For C(sp 3 )H and C(sp 2 )H bonds the electrostatic part of this proximate effect causes an increase in its corresponding 1 J(C,H) coupling [46][47] while the charge transfer contribution causes a decrease in such a coupling [2].…”

The CH‥O H-Bond as a Determining Factor in Molecular Structure

“…The sp 2 -hybridized C of the vinyl [41] or phenyl group [42][43][44][45] could also donate a proton, as could other aromatic systems such as imidazole [46], pyrimidine [47] or nucleic acid bases [48][49][50]. Other work concluded that the very simple CH 4 could engage in a HB [51][52][53][54][55][56][57][58][59][60][61], albeit a weak one. An early study of chlorosubstituted methanes [62] indicated that the CH··O HB with water would be progressively strengthened by each exchange of H with Cl.…”