A DFT and AIM analysis of the spin–spin couplings across the hydrogen bond in the 2‐fluorobenzamide and related compounds

Abstract

In 1975 a large number of coupling constants were measured in 2-fluorobenzamide labeled with (15)N. Some of them were assigned to couplings through intramolecular N-H...F hydrogen bonds (HBs). These couplings change dramatically when CDCl(3) is replaced by DMSO-d(6). In this theoretical paper we provide density functional theory (DFT) calculations that justify the existence of a weak HB in the absence of solvent, while solvents that act as HB acceptors break down the intramolecular hydrogen bond (IMHB) of 2-fluorobenzamide. Atoms in molecules (AIM) analyses and Steiner-Limbach plots were used to analyze the structure of the compounds.

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

“…The values of the 2h J(F,H) coupling constant increase in the absolute size as the ρ electron density increases. [28] A decrease of the 1h J (N,H) and 2h J(N,N) coupling constants in 1-and 2-Z,Z appears to be connected with a decrease of the ρ H· · ·N and ∇ 2 ρ H· · ·N parameters as compared with 1-and 2-Z,E. In other words, an electronic channel for transmission of the spin-spin information through a hydrogen bridge is depleted on going from the simple to bifurcated hydrogen bond.…”

“…It occurs due to the formation of the intermolecular N-H· · ·O hydrogen bond with the DMSO molecule. [28] The attenuation of the trans-hydrogen couplings in 1-and 2-Z,Z can be supposed to originate from the weakening of the N-H· · ·N bridge as result of the bifurcated hydrogen bonding. In consequence of the weakening, the r(H· · ·N) and r(N· · ·N) distances in 1-and 2-Z,Z become longer in comparison with the molecules 1-and 2-Z,E having simple hydrogen bond.…”

“…Such conclusion agrees with calculations of the topological parameters in the other molecules having bifurcated hydrogen bonds. [31] The trans-hydrogen bond coupling constants have been recognized [28] to be related to the magnitude of ρ parameter. The values of the 2h J(F,H) coupling constant increase in the absolute size as the ρ electron density increases.…”

Conformational analysis, part 43. A theoretical and LIS/NMR investigation of the conformations of substituted benzamides

“…Alkorta et al [48] have shown by DFT calculations that introducing one solvent molecule (acetone) into a 2-fluorobenzamide calculation retains the intramolecular NH..F hydrogen bond but adding a further solvent molecule breaks the hydrogen bond. The intramolecular NH..F hydrogen bond in the trans form restricts any intermolecular hydrogen bond with the solvent to only one of the NH 2 atoms resulting in ca.…”

Computational NMR Spectroscopy

“…In the case of benzene as solvent, we have calculated the aromatic solvent-induced shifts (ASIS) with a considerable success [188]. In the case of fluorobenzamide (66), we have calculated the effect of water and acetone on the chemical shifts using the 1: 1 and 1: 2 complexes [19]. One can consider the solid state as a special case of solvation; only, instead of solvent there are other molecules in the unit cell.…”