Slow multiphoton excitation as a probe of bimolecular and unimolecular reaction energetics. Multiphoton dissociation of proton-bound alcohol dimers

Abstract

Proton bound dimers of simple aliphatic alcohols (R/sub 1/OH)H/sup +/(R/sub 2/OH) where R/sub 1/ = R/sub 2/ = CH/sub 3/, C/sub 2/H/sub 5/, i-C/sub 3/H/sub 7/, n-C/sub 3/H/sub 7/, s-C/sub 4/H/sub 9/, t-C/sub 4/H/sub 9/, and t-C/sub 5/H/sub 11/ are intermediates formed in gas phase reactions of the corresponding protonated alcohols with neutral alcohols. Products from the decomposition of these chemically activated species are compared with those obtained by slow multiphoton excitation. In addition, it is shown that isomeric ions are distinguished by formation of different sets of decomposition products. Also, we report the first observation of multiple sets of products arising from low-power infrared activation. This result is due to an exothermic isomerization in the transition state which makes accessible more than one reaction pathway. Use of infrared multiphoton excitation as a complementary technique to collisional activation of ions in mass spectrometric analysis of mixtures is discussed. For this study, ions are formed, stored, and detected by using techniques of ion cyclotron resonance (ICR) spectroscopy.