HFCN open-shell isomers in solid argon. II. Excited-state tunneling isomerization HFC=N•→FC•=NH

Abstract

Photochemical reaction of the π-type fluoromethylene amidogen radical HFC=N• was studied by infrared, electroparamagnetic resonance, and laser-induced fluorescence spectroscopies in solid argon matrices. Laser excitation of the 0–0 band of the first excited state of HFC=N• at 488.1 nm leads to complete conversion to another structural isomer, the σ-type fluoroiminomethyl radical FC•=NH. The quantum yield of the photoisomerization reaction is ΦH≈0.5 at temperatures T⩽30 K. Experiments using pulsed laser excitation show that the rate constant for hydrogen atom transfer is kH=5×104 s−1 at T⩽30 K, whereas the rate constant of the analogous process in DFC=N• is only kD=30 s−1 under the same experimental conditions. The large kinetic isotope effect kH/kD≈1400 and absence of temperature dependence of kH show that the mechanism of photoisomerization is intramolecular tunneling transfer of the hydrogen atom in the first excited à 2A″ state. Simple one-dimensional consideration of the tunneling transfer of an H or D atom fails to describe the observed values of kH and kD. Therefore, multidimensional character of the hydrogen atom tunneling transfer should be taken into account, and a multidimensional potential energy surface of the excited state is required to describe correctly the chemical dynamics of the HFCN molecular system.