Structure, Vibrational Spectra, and Unimolecular Dissociation of Gaseous 1-Fluoro-1-phenethyl Cations

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 Me2CF+, rela...

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Published in:The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2008-10, Vol.112 (43), p.10774-10783
Main Authors: Oomens, Jos, Kraka, Elfi, Nguyen, Michael K, Morton, Thomas Hellman
Format: Article
Language:English
Subjects:
A: Kinetics, Spectroscopy
Cations - chemistry
Ethane - analogs & derivatives
Ethane - chemistry
Gases - chemistry
Hydrocarbons, Fluorinated - chemistry
Mass Spectrometry - methods
Models, Chemical
Molecular Structure
Spectrophotometry, Infrared - methods
Vibration
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Summary: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 Me2CF+, relative to n = 1 for fluoromethane and n = 2 for diatomic CF+), which is consistent with calculations of adiabatic CF stretching frequencies (νCF). 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 Me2CF+.
ISSN:1089-5639
1520-5215
DOI:10.1021/jp804706z