title: NCO(-), a key fragment upon dissociative electron attachment and electron transfer to pyrimidine bases: site selectivity for a slow decay process.
abstract: We report gas phase studies on NCO(-) fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO(-) production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO(-) upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO(-) formation in DEA. Site selectivity in the NCO(-) yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO(-) ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.
authors.affiliation: Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus de Caparica, 2829-516, Caparica, Portugal.
authors.forename: Filipe Ferreira
authors.initials: FF
authors.lastname: da Silva
authors.forename: Carolina
authors.initials: C
authors.lastname: Matias
authors.forename: Diogo
authors.initials: D
authors.lastname: Almeida
authors.forename: Gustavo
authors.initials: G
authors.lastname: García
authors.forename: Oddur
authors.initials: O
authors.lastname: Ingólfsson
authors.forename: Helga Dögg
authors.initials: HD
authors.lastname: Flosadóttir
authors.forename: Benedikt
authors.initials: B
authors.lastname: Ómarsson
authors.forename: Sylwia
authors.initials: S
authors.lastname: Ptasinska
authors.forename: Benjamin
authors.initials: B
authors.lastname: Puschnigg
authors.forename: Paul
authors.initials: P
authors.lastname: Scheier
authors.forename: Paulo
authors.initials: P
authors.lastname: Limão-Vieira
authors.forename: Stephan
authors.initials: S
authors.lastname: Denifl
mesh_terms: D000838:Anions / Q000737:chemistry*; D003485:Cyanates / Q000737:chemistry*; D004583:Electrons; D007477:Ions; D011743:Pyrimidines / Q000737:chemistry*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013941:Thymine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D000838:Anions; D003485:Cyanates; D007477:Ions; D011743:Pyrimidines; C030986:pyrimidine; D013941:Thymine; C002347:3-methylthymine
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