Deuterium isotope effects on 13C and 15N chemical shifts of intramolecularly hydrogen-bonded enaminocarbonyl derivatives of Meldrum’s and Tetronic acid

Secondary deuterium isotope effects on 13C and 15N nuclear shieldings in a series of cyclic enamino-diesters and enamino-esters and acyclic enaminones and enamino-esters have been examined and analysed using NMR and DFT (B3LYP/6-31G(d,p)) methods. One-dimensional and two-dimensional NMR spectra of enaminocarbonyl and their deuterated analogues were recorded in CDCl3 and CD2Cl2 at variable temperatures and assigned. 1JNH coupling constants for the derivatives of Meldrum’s and tetronic acids reveal that they exist at the NH-form.It was demonstrated that deuterium isotope effects, for the hydrogen bonded compounds, due to the deuterium substitution at the nitrogen nucleus lead to large one-bond isotope effects at nitrogen, 1Δ15N(D), and two-bond isotope effects on carbon nuclei, 2ΔC(ND), respectively. A linear correlations exist between 2ΔC(ND) and 1Δ15N(D) whereas the correlation with δNH is divided into two. A good agreement between the experimentally observed 2ΔC(ND) and calculated dσ13C/dRNH was obtained. A very good correlation between calculated NH bond lengths and observed NH chemical shifts is found. The observed isotope effects are shown to depend strongly on Resonance Assisted Hydrogen bonding.