The multiple CF bond character of PhCFMe (+) ions has been examined by means of theory, vibrational spectroscopy of the gaseous ions, and unimolecular decomposition chemistry. Atoms in Molecules analysis of DFT wave functions gives a CF bond order of n = 1.25 (as compared with n = 1.38 for Me 2CF (+), relative to n = 1 for fluoromethane and n = 2 for diatomic CF (+)), which is consistent with calculations of adiabatic CF stretching frequencies (nu CF). Experimental gas phase IR spectra, recorded by means of resonant multiphoton dissociation (IRMPD) using a free-electron laser connected to an FTICR mass spectrometer, show good agreement with predicted band positions for five deuterated isotopomers of PhCFMe (+). Metastable ion decompositions of deuterated analogues of PhCFMe (+) show the same HF/DF loss patterns as those produced by IRMPD. The evidence supports the conclusion that PhCFMe ions retain structural integrity until they become sufficiently excited to dissociate, whereupon they undergo intramolecular hydrogen scrambling that is competitive with HF/DF expulsion. Relative rates of hydrogen transposition and unimolecular dissociation are extracted from relative experimental fragment ion abundances. The predominant decomposition pathway is inferred to operate via a five-center transition state, as opposed to a four-center transition state for HF loss from gaseous Me 2CF (+).

The thermal chemistry of cis- and trans-1,4-difluoro-2-butenes on Pt(111) single-crystal surfaces was characterized by temperature programmed desorption (TPD) and reflection−absorption infrared spectroscopy (RAIRS). The study was motivated by the possibility of differentiating the two isomers directly by mass spectrometry, something that is not possible with the nonfluorinated olefins. On the clean surface a preference was identified for the conversion of the cis isomer to its trans counterpart, the opposite to what had been previously reported with regular 2-butenes on hydrogen (or deuterium)-predosed Pt(111). Indeed, the formation of trans-1,4-difluoro-2-butene from the adsorbed cis isomer occurs at around 260 K, as identified directly by RAIRS and in the gas phase by mass spectrometry. Some cis is also formed from the trans, but only in a small yield and at much higher temperatures (∼315 K). The preference for trans-to-cis conversion with the nonfluorinated olefins may therefore be explained by a reversal in adsorption stability induced by coadsorbed hydrogen. It was also determined that significantly more hydrogenation of the fluorinated olefins to their fluorinated alkanes occurs upon coadsorption with hydrogen or deuterium compared with what is seen with the regular olefins. Finally, the inductive effect of the fluorine substitutions leads to some changes in the high-temperature conversion of the adsorbed species.

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Concerted unimolecular eliminations from ionized sec-alkyl aryl ethers (ROAr (+*)) display a preference for producing double bonds with trans geometry. This preference can be assessed quantitatively, provided that a regioselective variant can be found. Expulsion of neutral alkenes via syn-elimination to give ionized ArOH does not exhibit a pronounced preference with regard to the direction of elimination. By contrast, ionized 2-hexyl p-trifluoromethylphenyl ether eliminates neutral ArOH regioselectively, giving ionized 2-hexenes rather than ionized 1-hexene. Vicinally monodeuterated 2-hexyl and 3-hexyl ethers were prepared as pure diastereomers. Metastable ion decompositions of their gaseous radical cations are compared over two different time windows. The regioselectivity for the 2-hexyl ether allows the geometric preference for the double bonds to be estimated based on the difference between the erythro and threo monodeuterated diastereomers ( trans/ cis = 2.0 for producing ionized 2-hexene from parent ions with the lowest internal energies). The 3-hexyl ethers and ionized 2- and 3-phenoxyoctanes also undergo stereoselective elimination but give experimental values that reflect their lack of regioselectivity. Examination of erythro/ threo combinations shows that GC/MS/MS has the ability to quantitate stereochemistry in mixtures containing both positional and stereoisomers.

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