Bond dissociation energy of the radical cation dimers of diethyl sulfide, di-n-propyl sulfide and di-n-butyl sulfide

Long-lived radical cation dimers of diethyl sulfide, di-n-propyl sulfide and di-n-butyl sulfide have been prepared via an association reaction of the dialkyl sulfide radical cation with the dialkyl sulfide in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Under low energy collisionally induced dissociation (CID) conditions the studied radical cation dimer dialkyl sulfides, bonded via a two-center three-electron (2c/3e) S-S bond, are found to dissociate exclusively back to the monomer reactants resulting in the detection of the product radical cations of the corresponding dialkyl sulfides. Translational energy resolved CID has led to the determination of S-S bond dissociation energies (BDE) for (Et2S)+.2, (n-Pr2S)+.2, and (n-Bu2S)+.2 of 1.19 ± 0.10 eV, 1.01 ± 0.10 eV, and 0.92 ± 0.10 eV, respectively. These BDEs clearly show a significant trend which indicates that the 2c/3e S-S bond becomes weaker with larger alkyl groups. Weakening of the 2c/3e S-S bond indicates a decrease of the σ/σ* orbital energy splitting of the 2c/3e S-S bond, which evidently is induced by the enhanced electron donation from the larger sulfide alkyl groups. Based on the determined bond dissociation energies, heats of formation of 530, 453 and 373 kJ mol-1 have been estimated for (Et2S)+.2, (n-Pr2S)+.2, and (n-Bu2S)+.2, respectively. Copyright © 1996 Elsevier Science B.V. All rights reserved.