Catalyzed isomerization and decarbonylation of ionized formic acid and dihydroxycarbene
Résumé
Catalyzed conversion of HCOOH*+ into HOCOH* + was studied in the cell of a FT-ICR in the presence of different molecules. The reaction of HCOOH*+ with SO2, whose proton affinity (PA) lies between that of the HOCO* radical at the carbon and at the oxygen sites, yields the HOCOH*+ carbene isomer as proved by its characteristic reaction with cyclopropane. When the PA of the catalyst lies above the highest PA of both sites of the HOCO* radical, formation of HOCOH*+ cannot be observed since its final state lies above that corresponding to protonation of the catalyst. However, reactions of DCO2H*+ and of HCO2D*+, which protonate several catalysts in an identical ratio which is very near of 1/1 at the beginning of the reaction, indicates that both ions, DCO2H*+ and of HCO 2D*+, convert into ion DO-C-OH*+ within a complex prior to protonation. The reactions of HCOOH*+ and HOCOH*+ with water were also more particularly studied by using theoretical calculations. Both reactions lead to protonated water and to the ionized water dimer which has been shown to possess the [H 2OH+?OH*] structure. The first step of the process is the conversion of the [HO(O)C*? H+?OH2] complex into [HOCO*?H +?OH2]. This latter complex undergoes two main pathways: on the one hand, it leads to protonation of water; on the other hand, it isomerizes to the [O*COH?H+?OH2] intermediate which dissociates to form [H2 OH+?OH*] with CO loss. Formation of H3O+ and [H2OH+?OH*] being rapid, the [HOCO*?H+?OH2] complex does not dissociate to yield the ionized carbene product which was not detected. Since the catalyzed isomerization of the 1,2-H transfer, converting HCOOH* + into HOCOH*+, is only observed within the corresponding complexes, this is a typical case of hidden isomerization. Finally, the differences in the unimolecular fragmentations of ionized formic acid and of its water solvated ion were explained. © 2003 Elsevier B.V. All rights reserved.