Protonation and methylation of thiophenol, thioanisole and their halogenated derivatives: Mass spectrometric and computational study

Abstract

The protonation and methylation of thiophenol (C6H5SH), thioanisole (C6H5SCH3) and 4-bromo derivatives have been studied using both tandem mass spectrometric techniques and ab initio quantum chemical calculations. Protonated 4-bromo thio-compounds produced by chemical ionization (CI) are found to be collisionally dehalogenated in an rf-only quadrupole collision cell in the low (20-30eV) kinetic energy regime giving essentially thiophenol or thioanisole radical cations. This is indicated by MS/MS/MS experiments performed in an hybrid sector-quadrupole-sector mass spectrometer. B3LYP and CCSD(T) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of either (bromo)thiophenol or thioanisole takes place on the ring; the C4-protonated thiophenol lies about 13kJmol-1 below the S-protonated isomer. However, under similar conditions, protonated thioanisole is also readily demethylated generating thiophenol radical cation, but no isomer has been detected. On the other hand, experimental and theoretical results reveal a regiospecific cationization (methylation) at the sulfur atom of the title compounds. The proton and methyl cation affinities are estimated as follows: PA (thiophenol)=812±10kJmol-1, MCA (thiophenol)=397±10kJmol-1, PA (thioanisole)=839±10kJmol-1, and MCA (thioanisole)=454±10kJmol-1. The available experimental value of 873kJmol-1 for PA (thioanisole) appears to be overestimated. Calculated PAs at various sites of halogeno (F, Cl and Br) thiophenols (o, m, p) are also reported. © 2003 Elsevier Science B.V. All rights reserved.