Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 1. Preparation, dissociations, and energetics of 2-hydroxyoxolan-2-yl radical, neutral isomers, and cations

Abstract

Collisional neutralization of several isomeric C 4H 7O 2 cations is used to generate radicals that share some structural features with transient species that are thought to be produced by radiolysis of 2-deoxyribose. The title 2-hydroxyoxolan-2-yl radical ( 1) undergoes nearly complete dissociation when produced by femtosecond electron transfer from thermal organic electron donors dimethyl disulfide and N,N-dimethylaniline in the gas phase. Product analysis, isotope labeling ( 2H and 18O), and potential energy surface mapping by ab initio calculations at the G2(MP2) and B3-PMP2 levels of theory and in combination with Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic calculations are used to assign the major and some minor pathways for 1 dissociations. The major (∼90%) pathway is initiated by cleavage of the ring C-5O bond in 1 and proceeds to form ethylene and ·CH 2COOH as main products, whereas loss of a hydrogen atom forms 4-hexenoic acid as a minor product. Loss of the OH hydrogen atom forming butyrolactone ( 2, ∼9%) and cleavage of the C-3C-4 bonds (<1%) in 1 are other minor pathways. The major source of excitation in 1 is by Franck-Condon effects that cause substantial differences between the adiabatic and vertical ionization of 1 (5.40 and 6.89 eV, respectively) and vertical recombination in the precursor ion 1 + (4.46 eV). +NR + mass spectra distinguish radical 1 from isomeric radicals 2-oxo-( 1H)oxolanium ( 3), 1,3-dioxan-2-yl ( 9), and 1,3-dioxan-4-yl ( 10) that were generated separately from their corresponding ion precursors.