Vibrational excitation of the allene–fluorine reaction in cryogenic matrices: Possible mode selectivity

The infrared induced reaction of allene (C3H4) and fluorine in N2, Ar, Kr, and Xe matrices at 12 K has been studied. The reaction is promoted by the selective vibrational excitation of C3H4 ⋅ F2 reactive pairs and the course of the reaction followed by infrared spectroscopy. Four products are detected, consistent with the concerted addition of F2 across the C–C double bond to form vibrationally excited cis- and gauche-CH2CFCH2F which may then eliminate HF to yield either CH≡CCH2F ⋅ HF or CH2=C=CHF ⋅ HF pairs. The relative abundance of each product depends markedly upon the matrix. Selective vibrational photoisomerization of CH2=CFCH2F was used to distinguish and assign spectral features due to the cis- and gauche-rotameters as well as to provide evidence about the torsional barrier in the matrix. In a particular matrix, the allene–fluorine reaction product distribution is independent of the exciting frequency, with measurements at 1679, 1953, 1999, and 3076 cm−1. However, the relative quantum yield at 3076 cm−1 is three orders of magnitude higher than observed at the lower frequencies. Furthermore, the quantum yields at 1680, 1953, and 1999 cm−1 show differences that implicate mode selectivity in the allene–fluorine reaction. The CH2 rocking overtone 2ν9 at 1999 cm−1 and perhaps as well the CH2 bending overtone 2ν10 at 1679 cm−1 show a higher reaction propensity than that of the asymmetric skeletal stretching vibration ν6 at 1953 cm−1.