Gas-Phase Radiolysis and Photolysis of Neopentane

Abstract

The photolysis of neo-C5H12—neo-C5D12 mixtures has been investigated at 1470 and 1236 Å. From the products formed in the absence and presence of NO or H2S, the occurrence of the following fragmentation processes could be derived: neo−C5H12*→CH4+iso−C4H8CH3+H+iso−C4H8CH3+CH3+C3H6.These modes of decomposition also prevail when neopentane is excited by collision with electrons accelerated by an electrical field applied during radiolysis. At 1236 Å, the relative probabilities of occurrence of Processes (1), (2), and (3) are approximately 1.0, 4.5, and 1.1. From the isotopic analysis of the products formed in the direct and inert-gas-sensitized radiolysis of neo-C5H12—neo-C5D12 mixtures in the presence of NO, O2, or I2 as free-radical scavengers, it could be derived that the fragment ions C2H3+, C2H5+, C3H5+, and C3H7+ are formed and undergo hydride-ion transfer reactions with neopentane to form C2H4, C2H6, C3H6, and C3H8, respectively. Assuming that this is the only mode of reaction of these ions in the system, ion-pair yields of 0.028, 0.068, 0.062, and 0.015 are obtained for the above listed ions at a pressure of 30 Torr. The propyl ion, which has a secondary propyl structure, undergoes a hydride transfer reaction with neopentane at a rate which is slower by more than one order of magnitude than the analogous reactions of the ethyl or allyl ions. It is observed that this relatively slow ion—molecule reaction is inhibited when an electric field is applied to the system, probably because of the collection of propyl ions on the cathode. In agreement with mass-spectrometric results, it is suggested that propyl ions are formed mainly as a result of the decomposition of the C5H11+ ion which is formed in the hydride-ion transfer processes. The C4H9+ ion does not react with neopentane but does, under certain conditions, yield isobutene by an unspecified process. The ion-pair yield of isobutene increases with increasing pressure and with diminishing recombination energy of added inert gas. In the direct radiolysis, the ion-pair yields of the fragment ions formed with appearance potentials above that of C4H9+ are seen to diminish when the pressure is increased, while, in the inert-gas-sensitized radiolysis, most of these ions increase with an increase of the recombination energy of the inert-gas ions, Xe+, Kr+, Ar+, Ne+.