Experimental and theoretical study of the energetics of trialkylsulfonium ions

The clustering reaction of dimethyl sulfide with t-butyl cation was examine d using high pressure mass spectrometry. A lower limit of 581 +/- 10 kJ/mol for the standard enthalpy of formation of trimethylsulfonium ion at 298 K was obtained from the energetics of the clustering reaction and derived ent halpies of formation for t-butyl cation (711 +/- 5 kJ/mol), t-butyl-dimethy lsulfonium ion (492 +/- 6 kJ/mol) and 2,3,3-trimethyl-2-butylcation (t-buty ldimethylcarbenium ion, 622 +/- 7 kJ/mol). A second estimate of 595 +/- 11 kJ/mol was extrapolated from trends in the methyl cation affinities of anal ogous oxygen, nitrogen and sulfur compounds. G2(MP2) calculations combined with isogyric reactions lead to a predicted standard enthalpy of formation of 593 +/- 10 kJ/mol for trimethylsulfonium ion and a methyl cation affinit y of 463 +/- 11 kJ/mol for dimethylsulfide. The computational results lie a t the lower end of the range of 615 +/- 15 kJ/mol reported for this enthalp y of formation based on an appearance potential measurement of dimethylsulf ide radical cation from trimethylsulfonium ion. (C) 1999 Elsevier Science B .V. All rights reserved.