The remarkable decarbonylation of CH 3OPO [rad]+ and its distonic isomer CH 2OPOH [rad]+: an experimental and CBS-QB3 computational study

Abstract

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 CH 3OPO + ( 1a) and CH 2OPOH + ( 1b) readily lose CH 2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OH 2 +, rather than its more stable isomer H 2POH +. Remarkably, the oxygen atoms become equivalent in this reaction: ions CH 3 18 O P O + and CH 2 O P 18 OH + lose C 18 O and C 16 O in a 1:1 ratio. The experiments further show that the dissociating ions have a fairly large internal energy content: up to 40 kcal/mol for the distonic ion 1b, which, along with the “enol” ion CH 2P(OH)O +, represents the global minimum of the CH 3O 2P + 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[OCH 2O(H)] + and ion [CH 2O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OH 2 + or else isomerize into the ion–dipole complex [H 2O⋅⋅⋅P(H)CO] +, whose dissociation into H 2O+HPCO + accounts for the observed minor loss of water.