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 . © 2015 Wiley Periodicals, Inc.