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)3P•+
does
not change during the reaction: the CH3S•
abstraction product has the structure
(CH3)3P+−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 reactivity of the prototypical
phosphorus-containing ylidion (α-distonic ion)
•CH2PH3
+
has been
investigated in the gas phase by using a dual cell Fourier-transform
ion cyclotron resonance mass spectrometer. The
ion
•CH2PH3
+
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
+
and
•CH2OH2
+
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.
Together with a calculated heat of formation for
•CH2PH2, this value
yields a heat of formation
of 217 ± 3 kcal mol-1 (at 298 K) for the
distonic ion
•CH2PH3
+.
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