Examples of various approaches to determining gaseous ion structures are described. Two cases are presented in which infrared multiple photon dissociation (IRMPD) spectroscopy provides characteristic information. Comparison with DFT-calculated normal modes differentiates isomers. The IRMPD spectrum of the even-electron C8D8F+ product ion from the ion–molecule reaction of CF3+ with perdeuterated acetophenone exhibits a degree of resolution sufficient to identify C6D5CFCD3+ and rule out ring-fluorinated isomers. The dimerization of phenylacetylene with its molecular ion has been explored, with the conclusion that a mixture of product ions is formed. The best match to the experimental spectrum is achieved by superimposing the bands calculated for ionized 1-phenylazulene and ionized 1,2-diphenylcyclobutadiene.
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