The gas phase ion chemistry and proton affinity of hexamethyldisiloxane studied by high pressure mass spectrometry

Abstract

The ion chemistry of hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, HMDS) has been studied under chemical ionization conditions at ion source temperatures of 300–600 K and pressures of 2–4 Torr. Highly exothermic proton transfer to HMDS from CH5+ and C2H5+ leads mainly to loss of CH3 but with decreasing exothermicity the yield of HMDSH+ increases such that transfer from t-C4H9+ (ΔH0 = −7.5 kcal mol−1) yields almost exclusively HMDSH+. Although HMDSH+ transfers (CH3)3Si+ rather than a proton to most reference bases, the proton affinity of HMDS has been determined from van't Hoff plots using the equilibrium method with methylaromatics as reference bases. PA(HMDS) = 203.4 kcal mol−1 is in excellent accord with an earlier estimate of 203 kcal mol−1 obtained by the bracketing method. The rate constants for these proton transfer reactions show very large negative temperature coefficients in the exothermic directions which are consistent with the reactions of charge delocalized ions and/or reactions in which considerable loss of rotational freedom occurs along the reaction coordinate. The rate constants in the reverse directions have a positive temperature coefficient only when the endothermicity is significant (>2.5 kcal mol−1).