Andrew M. McAnoy

Ion-Molecule Reactions of O,S-Dimethyl Methylphosphonothioate: Evidence for Intramolecular Sulfur Oxidation during VX Perhydrolysis

The alkaline perhydrolysis of the nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) was investigated by studying the ion-molecule reactions of HOO(-) with O,S-dimethyl methylphosphonothioate in a modified linear ion-trap mass spectrometer. In addition to simple proton transfer, two other abundant product ions are observed at m/z 125 and 109 corresponding to the S-methyl methylphosphonothioate and methyl methylphosphonate anions, respectively. The structure of these product ions is demonstrated by a combination of collision-induced dissociation and isotope-labeling experiments that also provide evidence for their formation by nucleophilic reaction pathways, namely, (i) S(N)2 at carbon to yield the S-methyl methylphosphonothioate anion and (ii) nucleophilic addition at phosphorus affording a reactive pentavalent intermediate that readily undergoes internal sulfur oxidation and concomitant elimination of CH(3)SOH to yield the methyl methylphosphonate anion. Consistent with previous solution phase observations of VX perhydrolysis, the toxic P-O cleavage product is not observed in this VX model system and theoretical calculations identify P-O cleavage to be energetically uncompetitive. Conversely, intramolecular sulfur oxidation is calculated to be extremely exothermic and kinetically accessible explaining its competitiveness with the facile gas phase proton transfer process. Elimination of a sulfur moiety deactivates the nerve agent VX and thus the intramolecular sulfur oxidation process reported here is also able to explain the selective perhydrolysis of the nerve agent to relatively nontoxic products.

Do the interstellar molecules CCCO and CCCS rearrange when energised

Neutrals CCCO, CC(13)CO, CCCS and CC(13)CS have been prepared by one-electron vertical (Franck-Condon) oxidation of the precursor anion radicals (CCCO)(-*), (CC(13)CO)(-*), (CCCS)(-*) and (CC(13)CS)(-*)respectively in collision cells of a reverse sector mass spectrometer. Ionisation of the neutrals to decomposing cations shows the neutrals to be stable for the microsecond duration of the neutralisation-ionisation ((-)NR(+)) experiment. No rearrangement of the label in energised CC(13)CO or CC(13)CS occurs during these experiments. In contrast, minor rearrangement of (CC(13)CO)(+*) is observed [(CC(13)CO)(+*)-->(OCC(13)C)(+*), while significant rearrangement occurs for (CC(13)CS)(+*) [(CC(13)CS)(+*)-->(SCC(13)C)(+*)]. Theoretical calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory show that the cationic rearrangements occur by stepwise processes via key rhombic structures. Overall, the degenerate processes result in O and S migration from C-3 to C-1. The cations (CCCO)(+*) and (CCCS)(+*) require excess energies of > or = 516 and > or = 226 kJ mol(-1) respectively to effect rearrangement.

The formation of neutral CCC and its radical cation from the CCC radical anion in the gas phase. A joint experimental and theoretical study

The radical anion [CC13C]−˙ has been produced by treatment of [(CH3)3SiCC13C(NNHSO2C6H4-p-CH3)Si(CH3)3] with HO−/F− in the ion source of a mass spectrometer. The stable anion undergoes vertical two-electron oxidation [charge reversal (−CR+)] in a collision cell to give [CC13C]+˙ which cyclises to the more stable [cyclo-CC13C]+˙ over a barrier of only 11 kJ mol−1 [calculated at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory], effectively scrambling the three carbon atoms of the cation radical. One-electron Franck–Condon oxidation of [CC13C]−˙ yields neutral CC13C. Theoretical calculations suggest that neutral CCC may undergo a degenerate rearrangement through a cyclic C3 transition state if the excess energy of CCC is ≥104 kJ mol−1 (at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory). It is likely that at least a proportion of the CC13C neutrals formed from [CC13C]−˙ should have sufficient energy to effect this reaction, resulting in the scrambling of the 13C label. The neutralisation/reionisation (−NR+) spectrum of [CC13C]−˙ ([CC13C]−˙ → CC13C → [CC13C]+˙) shows a pronounced peak corresponding to the parent cation, confirming that neutral CC13C is stable for the time of the NR experiment (10−6 s). However due to total scrambling of the label in the cation, possible scrambling in the neutral CCC molecule cannot be probed by this experiment. The corresponding −NR− experiment of [CC13C]−˙ showed a recovery signal but the sensitivity of the instrument was not sufficient to detect the decomposition fragments of the final radical anion.

Neutral cumulene oxide CCCCO is accessible by one-electron oxidation of [CCCCO]−˙ in the gas phase

Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory indicate that doublet [CCCCO]-* is a stable species which should undergo collision-induced Franck-Condon vertical oxidation under neutralisation-reionisation conditions (-NR+) to produce both triplet CCCCO (ground state) and singlet CCCCO. Some of the neutral CCCCO species formed (particularly the triplet) should be stable for the microsecond duration of the NR experiment, whereas others will be energised (particularly the singlet) and should decompose to C3 and CO. The [CCCCO]-* radical anion has been formed in the ion source of the mass spectrometer by the reaction CH3OCH2C[triple bond]C-CO-CH(CH3)2 + O-* --> [CCCCO]-* + CH3O* + H2O + (CH3)2CH*. The -NR+ spectrum of [CCCCO]-* shows a recovery signal at m/z 64 corresponding to ionised CCCCO, together with a pronounced peak at m/z 36 (CCC+*) produced by ionisation of CCC (formed by the reaction CCCCO --> CCC + CO). The experimental observations are in agreement with the predictions of the theoretical study.

Generation of transient neutrals in the gas phase from anionic precursors. Does energised CNCCO rearrange to NCCCO

The stability and reactivity of the neutral species CNCCO generated by one electron oxidation of the anion [CNCCO](-) have been investigated by a combination of theoretical calculations (carried out at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory) and tandem mass spectrometric experiments. Some of the neutrals formed in this way are stable for the microsecond duration of the experiment, but others are energised. The neutrals which are energised may either (i) dissociate [CNCCO --> CNC + CO (+92 kJ mol(-1))], and/or (ii), undergo the isonitrile to nitrile rearrangement to yield NCCCO energised neutrals (barrier 133 kJ mol(-1), reaction exothermic by 105 kJ mol(-1)). Some of these rearranged neutrals NCCCO have excess energies as high as 238 kJ mol(-1) and will dissociate [NCCCO --> NCC + CO (+203 kJ mol(-1))].

The gas-phase rearrangement of HCCBCCH to a trans pentacyclic isomer. A joint experimental and theoretical study

A theoretical study has reported that CCBCC will rearrange to planar cyclo-C4B if the excess energy of CCBCC is ≥ 67 kJ mol−1 [calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-31G(d) level of theory]. Cyclo-C4B lies only 4.5 kJ mol−1 above CCBCC. Although we were unable to prepare an ionic precursor (of CCBCC) in sufficient yield to allow the production of neutral CCBCC by neutralisation–reionisation, the analogue HCCBCCH can be prepared by one-electron vertical (Franck–Condon) reduction of cation [HCCBCCH]+. Some of the HCCBCCH neutrals formed in this way are stable for the microsecond duration of the +NR+ experiment, whereas others rearrange. Calculations indicate that energised HCCBCCH may rearrange over a barrier of 203 kJ mol−1 to cyclo-C4H2B, which lies 105 kJ mol−1 above HCCBCCH. The cyclic isomer is not planar like cyclo-C4B, but has trans geometry. The cyclic species is formed with an excess energy of 98 kJ mol−1, and may undergo facile and degenerate formation of the alternative trans structure through a planar C4H2B transition state (+17 kJ mol−1). If the cyclic isomer has an excess energy of ≥ 133 kJ mol−1, it may ring open to give an open chain C4H2B isomer containing a terminal B atom.

Rearrangements of transient neutral molecules in the gas phase. Does the conversion of CCCHO to HCCCO involve oxygen or hydrogen migration

Stable (CC13CHO)− may be formed in the chemical ionisation ion source of a VG ZAB 2HF mass spectrometer by the SN2(Si) reaction between Me3SiC≡C13CHO and F−. Vertical (Franck–Condon) one-electron oxidation of (CC13CHO)− in the first of the tandem collision cells of a VG ZAB 2HF mass spectrometer gives CC13CHO. Some of these neutrals have sufficient excess energy to effect rearrangement to HCC13CO, some of which are energised and decompose to HCC• and 13CO. Thus, the neutral rearrangement exclusively involves H migration: no products from O migration are detected. The corresponding two-electron oxidation of (CC13CHO)− gives mainly unrearranged (CC13CHO)+. A minority of these cations are energised and rearrange by H and O migration to yield (HCC13CO)+ and (OCC13CH)+, respectively. All experimental observations are backed up by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory.

Loss of CO2 from the ortho isomer of deprotonated methyl phenyl carbonate involves a methyl migration

Abstract The collision induced decompositions of the ortho-, meta , and para - (M − H) − anions of methyl phenyl carbonate were studied to see whether there is loss of CO 2 occurring by the benzyne cine-substitution {(C 6 H 4 ) − − OCO 2 Me → [(C 6 H 4 )MeOCO 2 − ] → (C 6 H 4 ) − − OMe + CO 2 }. Loss of CO 2 is observed from the ortho isomer, but the process does not involve a benzyne cine substitution. It is a methyl migration through a six-centre transition state to give a cresol (M − H) − ion, probably ortho -MeC 6 H 4 − O − . Theoretical calculations [at the B3LYP/6-311++G( d,p )//HF/3-21+G( d ) level of theory] indicate the methyl migration is a stepwise process, with the barrier for the first (and rate determining) step being 191 kJ mol −1 . The overall process is calculated to be exothermic by 276 kJ mol −1 .