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

Reactions of methane cations, CH, with H-2 HD and D-2 have been studied in a variable temperature 22-pole ion trap from room temperature down to 15 K. The formation of CH5+ in collisions with H-2 is slow at 300 K, but it becomes faster by at least one 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 restrictions must be responsible for the negative temperature dependence. In collisions of CH4+ with HD, the CH5+ product ion (68% at 15 K) prevails over CH4D+ (32%). Reaction of CH4+ with D-2 is found to be much slower than with H-2 or HD. The rate coefficient for converting CH4+ into CH3D+ by H-D exchange has been determined to be smaller than 10(-12) cm(3)/s, indicating that scrambling 4 in the CH6divided by Complex is very unlikely. (C) 2003 Elsevier B.V. All rights reserved.