The remarkable decarbonylation of CH3OPO+ and its distonic isomer CH2OPOH+: an experimental and CBS-QB3 computational study

The dissociation chemistry of the title ions was investigated using tandem mass spectrometry-based experiments in conjunction with isotopic labeling and computational quantum chemistry.Upon collisional activation, ions CH3OPO+ (1a) and CH2OPOH+ (1b) readily lose CH2O. This reaction is completely suppressed in the low-energy (metastable) ions, which instead abundantly lose CO. The product ion generated in this decarbonylation reaction is the ylide ion HPOH2+, rather than its more stable isomer H2POH+. Remarkably, the oxygen atoms become equivalent in this reaction: ions CH318OPO+ and CH2OP18OH+ lose C18O and C16O in a 1:1 ratio. The experiments further show that the dissociating ions have a fairly large internal energy content: up to 40kcal/mol for the distonic ion 1b, which, along with the “enol” ion CH2P(OH)O+, represents the global minimum of the CH3O2P+ potential energy surface.The mechanism of this intriguing decarbonylation was studied, using the CBS-QB3 model chemistry to probe the structure and energetics of potential intermediates and their interconversion barriers. It appears that metastable ions 1a and 1b have sufficient internal energy to communicate with a great many isomers. Of these, the cyclic ion P[OCH2O(H)]+ and ion [CH2OH⋅⋅⋅OP]+, a hydrogen-bridged radical cation (HBRC), play a key role in the oxygen equilibration. The HBRC’s [HOP⋅⋅⋅HC(H)O]+ and [HPO(H)H⋅⋅⋅CO]+ are key intermediates in the actual mechanism for the decarbonylation. The latter ion may lose CO to produce HPOH2+ or else isomerize into the ion–dipole complex [H2O⋅⋅⋅P(H)CO]+, whose dissociation into H2O+HPCO+ accounts for the observed minor loss of water.