Variable temperature ion trap studies of CH4++H2, HD and D2: negative temperature dependence and significant isotope effect

Abstract

Reactions of methane cations, CH þ , with H2, HD and D2 have been studied in a variable temperature 22-pole ion trap from 9 room temperature down to 15 K. The formation of CH þ in collisions with H2 is slow at 300 K, but it becomes faster by at least one 0 order of magnitude when the temperature is lowered to 15 K. This behavior is tentatively explained with a longer complex lifetime at low temperatures. However, since tunneling is most probably not responsible for product formation, other dynamical or statistical 2 restrictions must be responsible for the negative temperature dependence. In collisions of CH þ with HD, the CH þ product ion (68% at 15 K) prevails over CH4D þ (32%). Reaction of CH þ with D2 is found to be much slower than with H2 or HD. The rate coefficient 4 for converting CH þ into CH3D þ by H–D exchange has been determined to be smaller than 10 � 12 cm 3 /s, indicating that scrambling 5 in the CH þ complex is very unlikely.