Catalyzed isomerization and decarbonylation of ionized formic acid and dihydroxycarbene

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 HCO2D.+, 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 [H2OH+...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 [(OCOH)-C-....H+...OH2] intermediate which dissociates to form [H2OH+...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. (C) 2003 Elsevier B.V. All rights reserved.