Calculation of Ion–Molecule Reaction Rate Constants from Ion Cyclotron Resonance Spectra: Methyl Fluoride

Abstract

A general method is given for obtaining ion–molecule reaction rate constants from low-pressure ion cyclotron resonance signal intensity measurements; the analysis can be applied to any kinetic scheme including those with tertiary and higher-order products. Steps in the analysis include determining the number of ions in a particular region of the analyzer, calculating from the zero-pressure ICR lineshape the power absorption from ions in that region, and then summing such contributions over all regions of the analyzer. To illustrate the analysis, explicit ICR intensity expressions are derived for a kinetic scheme with stable tertiary species. These expressions are used to determine the rate constants for the ion–molecule reactions in methyl fluoride. The resultant rate constants CH3F++CH3F→CH3FH++CH2F, k = 13.6 × 10−10cm3molecule−1·sec−1; CH3F++CH3F→C2H4F++HF+H, k = 0.96 × 10−10cm3molecule−1·sec−1; and CH3FH++CH3F→C2H6F++HF, k = 8.0 × 10−10cm3molecule−1·sec−1 are independent of pressure over the range from 2 to 5 × 10−5 torr. The theory accurately predicts the relative signal intensities in methyl fluoride for each ion up to a pressure of 5 × 10−5 torr, where the tertiary ion signal intensity is 30% of the total intensity of all species present.