Characterization of iso-CF_2I_2in frequency and ultrafast time domains

摘要

The photolysis of diiododifluoromethane (CF_2I_2) in condensed phases was studied by a combinationof matrix isolation and ultrafast time-resolved spectroscopy, in concert with ab initio calculations.Photolysis at wavelengths of 355 or 266 nm of CF_2I_2:Ar samples (1:5000) held at 8 K yieldediso-CF_2I_2(F_2C-I-I), a metastable isomer of CF_2, characterized here for the first time. Theinfrared (IR) spectra of this isomer were recorded in matrix experiments, and the derived positionsof the C-F stretching modes are in very good agreement with the predictions of high level ab initiocalculations, which show that the iso-form is a minimum on the CF_2I_2ground state potential energysurface. The formation of this isomer following 350 nm excitation of CF_2I_2in room temperatureCC1_4solutions was monitored through its intense C-F stretching mode by means of ultrafasttime-resolved IR absorption. Together, matrix isolation and ultrafast IR absorption experimentssuggest that the formation of iso-CF_2I_2occurs via recombination of CF_2I radical and I atom.Ultrafast IR experiments detect a delayed rise of iso-CF_2I-I absorption, placing an upper limit of400 fs for the C-I bond dissociation and primary geminate recombination processes. The productabsorption spectrum recorded 1 ns after 350 nm excitation of CF_2I_2in solution is virtually identicalto the visible absorption spectrum of iso-CF_2I_2trapped in matrix isolation experiments [withsubtracted I_2(X) absorption]. The formation of this isomer in solution at room temperature has directdynamic implications for the ultrafast production of molecular iodine from electronically excited CF_2I_2.