Solvent effect on the tautomers' stabilities of protonated N,N-dimethylnitrosamine: The role of hydrogen bonds network

DFT calculations have been applied in order to study the free energies of the structures corresponding to the three different protonation sites of N, N-dimethylnitrosamine (DMNA). The solvent effect has been taken into account through the study of clusters consisting of protonated DMNA and up to four explicit water molecules, either in the absence or in the presence of a continuum (CPCM) solvation model. Addition of water molecules has been done by a careful screening procedure through which all important hydrogen bonds are likely to be considered. Protonation of DMNA makes all their lone pairs no longer available for hydrogen bond formation with water molecules, such that hydrogen bonds have been observed, for almost all structures, only between water molecules and between one water molecule and the protonated DMNA, in this latter case intermediated by the proton. The stabilities of the solvated structures are governed not only by the number of hydrogen bonds but also by the positions of the water molecules involved in these bonds, as well as by which of them donate or accept H atoms. Our results indicate that oxygen protonation is the most favorable one, regardless of the presence of water molecules. In vacuum protonation at the N-amino (2a) is approximately as favorable as protonation at the N nitroso (2c). However, in water the former protonation is by far the less favorable one. Our best estimates for the Delta G values in bulk solvent are: Delta G(2a) approximate to 17.9, Delta G(1c) approximate to 4.3, and Delta G(2c) approximate to 4.9 kcal/mol.