Limitations of the moderated nuclear recoil technique for investigating thermal hydrogen abstraction reactions by atomic fluorine

A sensitive technique has been developed for characterizing the nonthermal H18F yields formed from the dilute concentrations of hydrogen donors present in moderated nuclear recoil (MNR) 18F experiments. The relative nonthermal mean collisional reactivities of cyclopentane and of C1-C6 alkanes present in excess C2F6 have been measured using this method. The nonthermal reactivities per carbon-hydrogen bond in these substances exhibit +-10% maximum variations about the constant average value that is characteristic of unbranched C2-C6 alkanes. The MNR method can be utilized for the investigation at enhanced sensitivity of thermal hydrogen abstraction reactions by atomic 18F. Quantitative relative-to-absolute scaling relationships have been used for the estimation of absolute rate constants for many such reactions. The advantages, limitations, and physical basis of the MNR technique are considered