SLOW DISSOCIATIONS OF THIOPHENOL MOLECULAR ION - STUDY BY TRPD AND TPIMS

Dissociation of thiophenol molecular ion to yield competitive product ions at m/z 84 (C4H4S+) and 66 (C5H6+) was studied by several techniques. Time-resolved photodissociation at 308 nm (4.20 eV ion internal energy) in the ICR ion trap gave rate constants of 1.3 ⨯ 105 s-1 for m/z 84 formation and 8 ⨯ 104 s-1 for m/z 66 formation, and at 355 nm (3.66 eV ion internal energy) it gave 2.8 ⨯ 103 s-1 for m/z 84 formation, with m/z 66 not observed at this wavelength. Time-resolved photoionization mass spectrometry at 800 μs trapping time gave thresholds of 2.9 eV for m/z 84 formation and 3.2 eV for m/z 66 formation; the photoionization efficiency curves cross at approximately 4 eV internal energy so that m/z 66 is more abundant at energies above this; at 20-μs trapping a kinetic shift of approximately 0.2 eV was observed relative to 800-μs trapping. MIKES spectra indicated a low average kinetic energy release for metastable ion decomposition, ⟨T⟩ = 52 ± 5 meV, for m/z 66, and a higher kinetic energy release, ⟨T⟩ = 151 ± 5 meV, for m/z 84. RRKM modeling of these dissociations was successful, using a very loose transition state (ΔS‡(1000 K) = 14 eu) and a critical energy of 3.2 eV for m/z 66 formation, a tighter transition state (ΔS‡(1000 K) = 2.74 eu) and a critical energy of 2.9 eV for m/z 84 formation. With use of TRPD as an ion-thermometric tool, collisional cooling of hot thiophenol ions by collisions with neutral thiophenol was observed and a collisional cooling rate constant of 4 ⨯ 10-10 cm3·molecule-1·s-1 was measured. © 1990, American Chemical Society. All rights reserved.