Deuterium isotope effects on ¹³C chemical shifts of nitromalonamide

Abstract

The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH...O hydrogen bond as well as a NH...O-N=O hydrogen bond.

Isotope Effects on Chemical Shifts in the Study of Intramolecular Hydrogen Bonds

“…Series 1 is; Data from Refs. [ 5 , 37 , 42 , 43 ] series 2 citrinin [ 44 ]; series 3 is data for mono anions of succinic acid [ 37 ].…”

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

“…The 1 H chemical shift of acetyl acetone enol ( 3 ) is experimentally determined as 15.5 ppm and the anisotropy effect estimated as 0.4 ppm [ 12 ] Here, we use the value 15 ppm as a lower limit for strong O–H···Y hydrogen bonds. The upper value 19 ppm is taken from nitromalonamide enol ( 4 ) [ 31 ]. Larger shifts are considered to correspond to systems with very strong hydrogen bonds.…”

Isotopic splitting patterns in the 13C NMR spectra of some partially deuterated 1‐aryl‐2‐(phenyldiazenyl)butane‐1,3‐dione and 4‐hydroxy‐3‐(phenyldiazenyl)‐2H‐chromen‐2‐one: evidence for elucidation of tautomeric forms

“…The 15 N NMR data is also a powerful tool for the confirmation of hydrazo forms according to the splitting of the exchangeable proton upon nitrogen atom in hydrazo form . Another way to confirm the predominant tautomeric form is carbon‐13 splitting via partial deuteration (for current research work, see later).…”

“…One of the most distinguished works about deuterium isotope effects on 13 C chemical shifts have been reported by Hansen et al , who worked on deuterium isotope effects on chemical shifts in intramolecular hydrogen‐bonded systems, especially resonance assisted hydrogen bonding . In those cases, it has been shown that the isotope effect may be transmitted across the hydrogen bonds and may affect the chemical shifts in hydrogen‐bonded systems .…”

“…In those cases, it has been shown that the isotope effect may be transmitted across the hydrogen bonds and may affect the chemical shifts in hydrogen‐bonded systems . Also the variable‐temperature nuclear magnetic resonance (NMR) study of the enol forms of benzoylacetones and corresponding isotope effect, deuterium isotope effects on 13 C chemical shifts of nitromalonamide, 10‐Hydroxybenzo[ h ]quinolines, enaminocarbonyl derivatives of Meldrum's and Tetronic acid, o ‐hydroxy Schiff bases by applying factor analysis and tautomerism of an intramolecular O‐H...O...H‐N‐charge of the 15 N labeled Schiff base 3‐carboxy‐5‐methyl‐salicylidenaniline have been studied by means of the 1 H and 15 N NMR spectroscopy . Deuterium isotope effects in 13 C NMR spectra of intramolecularly hydrogen‐bonded salicylaldehyde‐4‐phenylthiosemicarbazone has also been studied by means of NMR spectroscopy by Novak et al …”