Formation and hydrolysis of gas‐phase [UO₂(R)]⁺: R═CH₃, CH₂CH₃, CH═CH₂, and C₆H₅

Abstract

Abstract The goals of the present study were (a) to create positively charged organo‐uranyl complexes with general formula [UO 2 (R)] + (eg, R═CH 3 and CH 2 CH 3 ) by decarboxylation of [UO 2 (O 2 C─R)] + precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas‐phase H 2 O. Collision‐induced dissociation (CID) of both [UO 2 (O 2 C─CH 3 )] + and [UO 2 (O 2 C─CH 2 CH 3 )] + causes H + transfer and elimination of a ketene to leave [UO 2 (OH)] + . However, CID of the alkoxides [UO 2 (OCH 2 CH 3 )] + and [UO 2 (OCH 2 CH 2 CH 3 )] + produced [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + , respectively. Isolation of [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + for reaction with H 2 O caused formation of [UO 2 (H 2 O)] + by elimination of ·CH 3 and ·CH 2 CH 3 : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO 2 (O 2 C─CH═CH 2 )] + and [UO 2 (O 2 C─C 6 H 5 )] + , caused decarboxylation to leave [UO 2 (CH═CH 2 )] + and [UO 2 (C 6 H 5 )] + , respectively. These organometallic species do react with H 2 O to produce [UO 2 (OH)] + , and loss of the respective radicals to leave [UO 2 (H 2 O)] + was not detected. Density functional theory calculations suggest that formation of [UO 2 (OH)] + , rather than the hydrated U V O 2 + , cation is energetically favored regardless of the precursor ion. However, for the [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + precursors, the transition state energy for proton transfer to generate [UO 2 (OH)] + and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO 2 (H 2 O)] + . The situation is reversed for the [UO 2 (CH═CH 2 )] + and [UO 2 (C 6 H 5 )] + precursors: The transition state for proton transfer is lower than the energy required for creation of [UO 2 (H 2 O)] + by elimination of CH═CH 2 or C 6 H 5 radical.