Novel β-Distonic Radical Cations [C_nH_2n+2S]^•+ (n = 2, 3) Formed upon Decarbonylation of Ionized S-Alkyl Thioformates:  A Mass Spectrometric and ab Initio Study

Abstract

Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H−C(O)SR 1−7. Using a combination of collisional activation (CA) and neutralization−reionization (NR) mass spectra, it is shown that the spectra of the so-formed [M−CO]•+ ions differ from the isomeric alkane thiol ions (RSH•+), if R > CH3. These fragment ions are therefore distonic radical cations, and given the fact that the α-distonic •CH2−S+H2 ions are not present under our experimental conditions, it is proposed that they actually are β-distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2−7 •+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector−quadrupole−sector configuration. In the case of ethyl derivatives (R = C2H5), both the distonic •CH2CH2S+H2 (2b) and conventional CH3CH2SH•+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO•) giving the production of H2S+−NO ions (m/z 64), characterized by high-energy collisional activation. Distonic ions 2b have also been otherwise prepared in an ion−molecule reaction involving the transfer of ionized ethene from •CH2CH2O+CH2 to neutral hydrogen sulfide, H2S. Upon CA, the resulting ions 2b show the same behavior as the product of decarbonylation of the S-ethyl thioformate molecular ions. Upon neutralization−reionization, the distonic isomers of the alkane thiol ions give rise to intense peaks corresponding to H2S•+ and ionized olefins. Portions of the potential energy surface related to the rearrangement and dissociation processes in the [C2H6S]•+ system have also been constructed using ab initio molecular orbital calculations at the QCISD(T)/6-311++G(d,p)//UMP2/6-31G(d,p) level. Theoretical results provide further support for the observation of β-distonic ions.

Electronic Properties of FC(O)SCH2CH3. A Combined Helium(I) Photoelectron Spectroscopy and Synchrotron Radiation Study

“…intramolecular reorganization from the parent molecular ion. This decarbonylation process may involve the formation of β-distonic radical cations, as demonstrated for the case of HC(O)SCH CH 3 10. Branching Ratios (%) for Fragment Ions Extracted from PEPICOFigure 4).…”

“…[9][10][11] Interestingly, the • CH 2 CH 2 SH 2 + β-distonic radical cation was detected as the product of decarbonylation of HC(O)SCH 2 CH 3 in the 70 eV electron-impact mass-spectrum. 10 The interest in this species begins because the close related oxygenated analogue, i.e. the βdistonic radical cation of ethanol, • CH 2 CH 2 OH 2…”

The gas phase sulfur-containing distonic radical cation hc+(oh)sc·h2

“…5 The CA spectra of these two isomers have already been reported 18 and found to be very similar. However, we have observed 12 …”

“…Their behaviour upon electron impact was investigated by using tandem mass spectrometry methodologies and several complex rearrangement reactions were identified and rationalized with the aid of molecular orbital calculations. 9,10,11,12,13 In the present paper, the structural identification of m/z 76 ions having the [C 2 ,H 4 ,O,S] Á composition is reported. These are derived from direct electron ionization of S-methyl thioformate 1 and dissociative ionization (loss of propene) of S-isobutyl thioformate 2.…”

[C2H4OS]>·>+> Radical cations derived from alkyl thioformates: tandem mass spectrometry and molecular orbital calculations

“…Their behavior upon electron ionization was extensively investigated using tandem mass spectrometry methodologies and several complex rearrangement and fragmentation reactions were identified and rationalized with the aid of molecular orbital calculations. [8][9][10][11][12] In the present paper, we report on the structural identification of several m/z 76 ions having [C 2 H 4 OS] ϩ composition derived from direct electron ionization of O-methyl and Smethyl thioformates I and II or dissociative ionization (loss of propene) of O-isobutyl and S-isobutyl thioformates III and IV (Scheme 1). In addition, ab initio molecular orbital calculations have also been carried out in order to assist the interpretation of observed results.…”