The influence of solvent molecules on NMR spectrum of barbituric acid in the DMSO solution

Abstract

This work shows the modification of barbituric acid (BA) chemical shifts by dimethylsulphoxide (DMSO) molecules. The discussed changes are caused by creation of the H-bonded associates formed by barbituric acid with DMSO in solution. Free molecule of barbituric acid, the cluster of BA with two DMSO molecules and two different clusters of BA with four DMSO units are taken into consideration. The chemical shifts of these systems have been calculated and the obtained results have been compared with experimental data. Theoretical calculations predict a significant downfield shift for imino protons of barbituric acid involved in intermolecular-N-H...DMSO hydrogen bonds. The influence of the solvent molecules on other nuclei chemical shifts, especially protons of barbituric acid methylene group, is also reported. The calculations have involved Hartree-Fock and several Density Functional Theory methods. All methods correctly describe experimental 1 H and 13 C NMR spectra of barbituric acid. The best consistence between experiment and theory is observed for the BLYP functional. Four approximations of magnetic properties calculations embedded in the Gaussian'98 package have been tested. The results of the performed calculations indicate that from a practical point of view the GIAO method should be preferred. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2006.