C[sbnd]F bond breaking by bare actinide monocations in the gas phase: a relativistic DFT study

Abstract

Investigations of the CF bond activation by actinide monocation An+ (An=Ac, Th, Pa, U, Np, Pu and Am) are carried out using relativistic density functional theory (DFT) computations. Originally, the aim of the study is to compare the ability of different actinide ions to break strong bonds particularly in the context of accidental radioactive dissemination. The An+ reaction with the fluorinated hydrocarbon CH3F was selected as a representative system in this context. Unexpectedly, the considered An+ were found to react differently. Via linear transit (LT) and intrinsic reaction coordinate (IRC) calculations, three reaction mechanisms for the CF bond activation, leading to the An–F+ formation, were revealed; the first one, i.e. ‘harpoon’ mechanism which was observed in the case of Pu+, Am+, while the second called ‘insertion-elimination’ mechanism concerned the case of Th+, Pa+, U+ and Np+. DFT computations highlight the particular case of the Ac+ system which presents two different mechanisms according to its spin state: a mechanism qualified as ‘harpoon-like’ for the triplet state and an ‘insertion-elimination’ mechanism for the singlet state. The activation barrier for the fluorine elimination from CH3F is weak for all the studied systems, from 0.9 kcal/mol for Th+ to 8.2 kcal/mol for Am+. Th+ is found as the most effective ion to activate the CF bond and a considerable exergonic character (−81.5 kcal/mol) for this reaction is expected. The performed orbital, population and charge analyses permitted to reveal the role of the actinide 7s, 6d and 5f orbitals and of electron transfers during the reaction.