THE RADICAL-CATION OF TRIMETHYLPHOSPHINE OXIDE

The structure of the gaseous long-lived radical cation generated upon electron ionization of trimethylphosphine oxide, (CH3)(3)PO, has been investigated by using ion-molecule reactions in a Fourier transform io n cyclotron resonance mass spectrometer. A radical cation with the con nectivity of trimethylphosphine oxide is expected to react by facile e lectron transfer with triethylamine, pyridine and dimethyl disulfide s ince all these reactions are highly exothermic. However, no electron t ransfer reactions were observed. Instead, the radical cation transfers a proton to triethylamine and to pyridine, i.e., acts as a Bronsted a cid. Further, the radical cation abstracts CH3S. from dimethyl disulfi de and hence demonstrates behavior characteristic of a distonic ion wi th a carbon radical center. This reactivity is unprecedented for a rad ical cation such as (CH3P+-O-. with the odd spin located at an oxygen atom. These experimental results indicate that the initially generated radical cation (CH3P+-O-. undergoes unimolecular isomerization to (CH 3P+(OH)CH2. within a millisecond time frame. Ab initio molecular orbit al calculations carried out at the unrestricted second-order Moller-Pl esset (UMP2/6-31G* + ZPVE)level of theory support this conclusion by predicting that (CH3P+(OH)CH2. lies 23 kcal mol(-1) lower in energy th an (CH3P+-O-.. The energy barrier for unimolecular [1,3]-hydrogen atom migration in (CH3P+-O-. is estimated to be 24 kcal mol(-1). This stud y demonstrates that the PO moiety provides a very strong driving force for hydrogen shifts in phosphorus containing radical cations.