Hartree-Fock, Møller-Plesset calculations and dynamic NMR study of 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one

Abstract

The experimental 1H, 13C NMR spectra of 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one were recorded in CDCl 3 at temperature range 213–323 K. The variable temperature spectra revealed a dynamic NMR effect which is attributed to restricted rotation around the C C double bond. Fast exchange processes of deuterium atoms between CDCl 3 and 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one or fast exchange of proton between nitrogen and oxygen atoms of carbonyl group is also revealed by broadening of N–H (singlet) proton NMR signals. Proton and carbon theoretical chemical shifts of the title molecule were calculated by using RHF and MP2-GIAO levels and different basis sets in gas phase at 298 K. The calculated proton chemical shifts show that the experimental values have no agreement with theoretical values, but for carbon chemical shifts a good agreement achieved by using RHF with 6-31G basis set and MP2/3-21G, 6-31G basis sets. Discrepancies are attributed to either the limitations of calculating program, because the change of the structure while rotation are not considered. The results showed that to select of basis set has more important rule, because RHF–GIAO level calculation with 6-31G basis set in gas phase can excellently reproduce the 13C NMR spectrum. Moreover, MP2/3-21G, 6-31G calculation has not significant influence on 13C NMR chemical shifts with respect to RHF–6-31G.