Dissociation dynamics and thermochemistry of tin species, (CH3)4Sn and (CH3)6Sn2, by threshold photoelectron-photoion coincidence spectroscopy.

The dissociative photoionization of tetramethyltin (Me₄Sn) and hexamethylditin (Me₆Sn₂) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me₄Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E₀ = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me₃Sn(•) loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me₄Sn and Me₆Sn₂ are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol⁻¹: Δ(f)H(m)(o)(Me₃Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me₅Sn₂(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me₃Sn(•),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me₂Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(MeSn(+),g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol⁻¹: BDE(Me₃Sn-Me) = 284.1 ± 9.9; BDE(Me₃Sn-SnMe₃) = 252.6 ± 14.8.