Knowledge Bank

When a Ne:$CH3X(X=F,Cl,Br)$ sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid includes several absorptions which can be assigned to cation products. The identification of these products is aided both by studies of the isotopically substituted methyl halides and by ab initio calculations for both the conventional and the ylidion structures. In each of these systems, a prominent product absorption appears at a frequency intermediate between that of HX and of $HX+$. This and other absorptions which behave similarly on filtered tungsten and mercury-are irradiation of the deposit have been assigned to the ylidion ($H2CXH+$) isomer of the methyl halide cation. Earlier mass spectrometric $studiesa$ had demonstrated $H2CFH+$ to be more stable than $CH3F+$ and had shown that the difference in stability of the two isomers is small for both the chloride and the bromide. An almost complete vibrational assignment has also been obtained for $CH3Br+$.