ION MOLECULE REACTIONS OF VIBRATIONALLY STATE-SELECTED NO+ WITH SMALL ALKYL-HALIDES

The effects of vibrational excitation in NO+(v = 0-5) on its reactivity with small alkyl halides (CnH2n+1X; n = 1-3; X = Cl, Br, 1) have been investigated under thermal translational conditions. The method combines resonance enhanced multiphoton ionization to form state-selected NO+(v) and Fourier transform ion cyclotron resonance techniques to trap, react, and detect ions. Besides vibrational quenching of NO+(v > 0), which is found to be very efficient with alkyl halides, three reaction channels are observed: charge transfer, halide transfer, and CnH2nNO+ formation. Branching ratios and rate constants have been determined for the different channels as a function of the NO+(v) vibrational energy. Endoergic charge transfer is efficiently driven by vibrational excitation. Halide transfer is the major channel if it is significantly exothermic for NO+(v = 0). If this is not the case, adding vibrational energy in NO+(v) is only marginally effective in driving this channel. The data suggest that rearrangements in NO+-alkyl halide reaction intermediates and in carbonium ions are very rapid. The CnH2nNO+ formation channel is only observed with n-propyl and isopropyl chloride where it is dominant for NO+(v = 0). Increasing vibrational excitation inhibits C3H6NO+ formation. The results are discussed in terms of possible reaction mechanisms.