Dimethyl disulfide and dimethyl diselenide are known to readily undergo charge exchange with gaseous conventional radical cations containing oxygen, nitrogen, and sulfur functionalities. In sharp contrast, the radical cations of trimethylphosphine and trimethyl phosphite rapidly abstract CH3S. and CH3Se. groups from dimethyl disulfide and dimethyl diselenide, respectively, in a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer, These sorts of abstraction reactions have been reported earlier only for distonic radical cations (ions with spatially separated charge and radical sites). Isomerization of the organophosphorus radical cations to their distonic forms prior to or during the reaction was ruled out by demonstrating that the connectivity in (CH3)(3)P-.+ does not change during the reaction: the CH3S. abstraction product has the structure (CH3P+-SCH3. Instead, the abstraction reactions are likely initiated by thermoneutral charge exchange. The neutral phosphorus compound then replaces a CH3S. or CH3Se. group in ionized dimethyl disulfide and ionized dimethyl diselenide, respectively, In support of this mechanism, three different neutral phosphorus compounds were shown to replace CH3S. in the radical cation of dimethyl disulfide. Phosphorus radical cations with high recombination energies were found to react with dimethyl disulfide by exclusive charge exchange, Hence, the abstraction reactions require a radical cation with a recombination energy close to the ionization energy of dimethyl disulfide (8.1 eV) and dimethyl diselenide (7.9 eV). Further, the reactions seem to be limited to phosphorus-containing ions since radical cations with nitrogen and sulfur functionalities do not undergo these reactions even when their recombination energies are close to 8.1 eV.

The structure of the gaseous long-lived radical cation generated upon electron ionization of trimethylphosphine oxide, (CH3)3PO, has been investigated by using ion-molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer. A radical cation with the connectivity of trimethylphosphine oxide is expected to react by facile electron transfer with triethylamine, pyridine and dimethyl disulfide since all these reactions are highly exothermic. However, no electron transfer reactions were observed. Instead, the radical cation transfers a proton to triethylamine and to pyridine, i.e., acts as a Brønsted acid. Further, the radical cation abstracts CH3S from dimethyl disulfide and hence demonstrates behavior characteristic of a distonic ion with a carbon radical center. This reactivity is unprecedented for a radical cation such as (CH3)3P+-O with the odd spin located at an oxygen atom. These experimental results indicate that the initially generated radical cation (CH3)3P+-O undergoes unimolecular isomerization to (CH3)2P+(OH)CH2 within a millisecond time frame. Ab initio molecular orbital calculations carried out at the unrestricted second-order Møller-Plesset (UMP2/6-31G** + ZPVE) level of theory support this conclusion by predicting that (CH3)2P+(OH)CH2 lies 23 kcal mol−1 lower in energy than (CH3)3P+-O. The energy barrier for unimolecular [1,3]-hydrogen atom migration in (CH3)3P+-O is estimated to be 24 kcal mol−1. This study demonstrates that the PO moiety provides a very strong driving force for hydrogen shifts in phosphorus containing radical cations.

The reactivity of the prototypical phosphorus-containing ylidion (alpha-distonic ion) (CH2PH3+)-C-. has been investigated in the gas phase by using a dual cell Fourier-transform ion cyclotron resonance mass spectrometer. The ion (CH2PH3+)-C-. and its more stable conventional isomer CH3PH2.+ show distinctly different reactivities toward neutral reagents. This observation contrasts the facile interconversion of the analogous sulfur- and oxygen-containing distonic ions (CH2SH2+)-C-. and (CH2OH2+)-C-. with their conventional isomers CH3OH.+ and CH3SH.+, respectively, within collision complexes in the gas phase. Bracketing experiments yield a proton affinity of 190.4 +/- 3 kcal mol(-1) for the phosphorus atom in (CH2PH2)-C-.. Together with a calculated heat of formation for (CH2PH2)-C-., this value yields a heat of formation of 217 +/- 3 kcal mol(-1) (at 298 K) for the distonic ion (CH2PH3+)-C-..

Using the technique of neutralization-reionization mass spectrometry (NRMS) it could be shown that the elusive phosphorotrithious acid (HS)(3)P is a stable molecule in the rarefied gas phase. A triple propene elimination from the molecular ions of the dipropyl ester of propylphosphonotrithioic acid, PrP(S)(SPr)(2), (70 eV EI) yields m/z 130 radical cations of composition ''[H3PS3](.+)''. Analysis of their collisional activation (CA) mass spectrum using thermochemical data shows that these ''[H3PS3](.+)'' ions have the structure [(HS)(3)P](.+) rather than that of the tautomer [(HS)(2)P(=S)H](.+). Subjected to a NRMS experiment, these ions retain their structure and are cleanly reduced to (HS)(3)P. The results are entirely compatible with ab initio MO-calculations executed at the HF/3-21G* and HF/6-31G** levels of theory (GAUSSIAN 92 system of programs). The calculations predict that [(HS)(3)P](.+) and [(HS)(2)P(=S)H](.+) and their respective neutral counterparts are local minima which are separated by high potential energy barriers. Neutral (HS)(3)P is calculated to be lower in energy than its tautomer (HS)(2)P(=S)H but upon ionization this order of stability is reversed.