JOURNAL OF MASS SPECTROMETRY J. Mass Spectrom. 2006; 41: 470–476 Published online 23 February 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jms.1005 The atmospheric pressure Meerwein reaction Eduardo C. Meurer and Marcos N. Eberlin ∗ Institute of Chemistry, State University of Campinas, UNICAMP, 13083-970, Brazil Received 28 December 2005; Accepted 2 January 2006 We have already shown that the in-vacuum gas-phase Meerwein reaction of (thio)acylium ions is general in nature and useful for class-selective screening of cyclic (thio)epoxides. Herein we report that this gas-phase reaction can also be performed efficiently at atmospheric pressure under both electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) conditions. This alternative expands the range of molecules that can be reacted by gas-phase Meerwein reaction. Phenyl epoxide, thiirane, 3- methoxy-2,2-dimethyloxirane, propylene oxide, 2,2 ′ -bioxirane, trans-1,3-diphenyl-2,3-epoxypropan-1-one, epichloridrine and propylene oxide are shown to react efficiently in both ESI and APCI conditions. Tetramethylurea (TMU) and (thio)TMU were both used as dopants, being co-injected with either toluene, acetonitrile or methanol solutions of the (thio)epoxides, with similar results. In both ESI and APCI, (thio)TMU is protonated preferentially, and these labile species dissociate promptly to yield (CH 3 ) 2 N–C + O and (CH 3 ) 2 NCS + , which are the least acidic and most reactive (thio)acylium ions so far tested in the gas-phase Meerwein reaction. Under the low-energy ESI conditions set to favor both the formation of the (thio)acylium ion and ion/molecule reactions, (CH 3 ) 2 NCO(S) + react competitively with (thio)TMU to form acylated (thio)TMU and with the (thio)epoxide to form the characteristic Meerwein products. Enhanced selectivity in structural characterization or for the screening of (thio)epoxides is achieved by performing on-line collision-induced dissociation of Meerwein products, particularly for the more structurally complex (thio)epoxides. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: class-selective ion/molecule reactions; gaseous acylium ions; (thio)epoxides; Meerwein reaction; ESI mass spectrometry INTRODUCTION (Thio)epoxides are very common reagents or key inter- mediates in a wide variety of synthetically important condensed-phase reactions. 1 In the gas phase, the Meer- wein reaction with (thio)epoxides 2 (Scheme 1) is one of the most favored reactions of gaseous acylium ions. 3 This class- selective reaction 4 displays many features common to those of transacetalization reactions of (thio)acylium ions with (thio)acetals and analogs 5 and is analogous to the condensed- phase reaction described in 1955 by Meerwein. 6 It promotes expansion of the (thio)epoxide ring and is useful for both the characterization and selective screening of (thio)epoxides, including explosives and warfare agents. 7 To perform such reactions in vacuum for mass-selected (thio)acylium ions, the neutral (thio)epoxides must be sufficiently volatile for its vapor to fill the collision cell region inside the mass spectrom- eter at the required concentration. Lack of proper volatility has therefore been a major limitation for the more general L Correspondence to: Marcos N. Eberlin, Institute of Chemistry, State University of Campinas, UNICAMP, 13083-970, Brazil. E-mail: eberlin@iqm.unicamp.br Contract/grant sponsor: Research Support Foundation of the State of S˜ ao Paulo (FAPESP). Contract/grant sponsor: Brazilian National Research Council (CNPq). X = O or S + O X R R 2 R 1 X R 2 O R 1 + X R 2 R 1 + O X R R 2 R 1 + R O R C Scheme 1 applications of the gas-phase Meerwein reaction for heavier and less volatile (thio)epoxides. In the gas phase, the intrinsic reactivity of gaseous (thio)acylium ions has been systematically investigated and Copyright  2006 John Wiley & Sons, Ltd.