Effect of methylation on relative energies of tautomers and on the intramolecular proton transfer barriers of protonated nitrosamine: A MR-CISD study

MR-CISD, MR-CISD+Q, and MR-AQCC calculations have been performed on the minima and transition states (corresponding to intramolecular proton transfer between the protonation sites) of the ground state of protonated nitrosamine and N,N-dimethylnitrosamine. Our highest level results (MR-AQCC/cc-pVTZ) for the smaller system indicate that protonation on the N amino (2a) is practically as favorable as the most favorable protonation on the O atom (1a). They also suggest that protonation on the nitroso N atom (2c) is ∼14.5 kcal/mol less favorable than 1a. Results obtained at the MR-CISD+Q/cc-pVTZ level indicate that the effect of methylation on the relative energies of the tautomers is, in order of importance, 2a > 2c and increases their energies by ∼17.5 and 4.8 kcal/mol, respectively. They also indicate that methylation alters significantly the intramolecular proton transfer barriers. The largest differences between the common geometric parameters of both systems have been found for 2a.