title: Effect of methylation on relative energies of tautomers and on the intramolecular proton transfer barriers of protonated nitrosamine: A MR-CISD study.
abstract: 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.
authors.affiliation: Departamento de Química, CCEN, Universidade Federal da Paraíba, João Pessoa, PB, 58.059-900, Brazil.
authors.forename: Railton Barbosa
authors.initials: RB
authors.lastname: de Andrade
authors.affiliation: Departamento de Química, CCEN, Universidade Federal da Paraíba, João Pessoa, PB, 58.059-900, Brazil.
authors.forename: Ezequiel Fragoso
authors.initials: EF
authors.lastname: Vieira Leitão
authors.affiliation: Instituto de Química, Universidade Federal do Rio Grande do Norte, Natal, RN, 59072-970, Brazil.
authors.forename: Miguel Angelo Fonseca
authors.initials: MA
authors.lastname: de Souza
authors.affiliation: Departamento de Química, CCEN, Universidade Federal da Paraíba, João Pessoa, PB, 58.059-900, Brazil.
authors.forename: Elizete
authors.initials: E
authors.lastname: Ventura
authors.affiliation: Departamento de Química, CCEN, Universidade Federal da Paraíba, João Pessoa, PB, 58.059-900, Brazil.
authors.forename: Silmar Andrade
authors.initials: SA
authors.lastname: do Monte
mesh_terms: D008745:Methylation; D008956:Models, Chemical; D009602:Nitrosamines / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D009602:Nitrosamines; D011522:Protons
keywords: ab initio; alkylnitrosamines; protonation