Structural study of oxalamide compounds: 1H, 13C, and DFT calculations

The conformational properties of some N-alkyl, N,N′-dialky, and tetraalkyloxalamides have been investigated, in vacuo and in solvent using DFT methods at the B3LYP/6-31G∗∗ computational level. Special emphasis has been given on oxalamides with substituents of the type –CH2CH2OH. In oxalamides with the N–H group (N-alkyl and N,N′-dialky), the most stable conformations are those in which the oxalamide moiety adopts a planar s-trans arrangement and the amide bonds are trans. A different situation appears in the case of tetraalkyloxalamides, in which the oxalamide moiety always adopts a skewed arrangement and there are conformations with similar energy. A careful study of 13C and 1H NMR spectra together with theoretical calculations (GIAO method) allowed the assignment of the signals of these conformers. The presence of the –CH2CH2OH chain produces numerous rotamers. The most stable rotamers, in vacuo, are those with strong intramolecular hydrogen bonds, however in solvent, hydrogen bonds are not crucial to establish the most stable specie and depend on the solvent used.