Gas-Phase Photolysis and Radiolysis of Cyclopropane

The gas-phase photolysis of c-C3H6–O2, c-C3H6–NO, and c-C3H6–H2S mixtures has been investigated at 1470 Å (8.4 eV), 1236 Å (10 eV), and at 1048–67 Å (11.6–11.8 eV), that is, at photon energies below and above the ionization energy of cyclopropane (I.E. = 10.06 eV). Examination of the products formed in the gas-phase experiments at 1470 and 1236 Å as well as of those formed in a few solid-phase photolysis experiments indicate that the major primary act of the neutral excited cyclopropane molecule can be written as follows: c-C3H6 + hv→CH2CH2CH2*. The highly excited trimethylene decomposes by the following two processes; (a) CH2CH2CH2*→C2H4+CH2 and (b) CH2CH2CH2*. There is, however, also evidence for rearrangement CH2CH2CH2*→CH3CHCH2* which, at low densities, is always followed by fragmentation of CH3CHCH2*. The primary process c-C3H6 + hv→H2+C3H4 is of negligible importance, but detachment of H atoms before or after ring opening does occur, and increases in importance, at higher energies. At 1236 Å the quantum yield of excited cyclopropane molecules which decomposes is within experimental error equal to unity. In the photoionization region alkyl radicals such as C2H5 and CH3CH2CH2 which were entirely absent at the lower energies are formed. These alkyl radicals which are also formed in the radiolysis are ascribed to ion–molecule reactions involving C3H6+. Photoionization and radiolysis of c-C3H6–NO mixtures yielded evidence for the occurrence of the charge transfer processes: C3H6++NO [or (CH3)3N]→C3H6+NO+ [or (CH3)3N+] and (NO+)* + c-C3H6→C3H6+ + NO, where (NO+)* is a vibrationally excited NO+ ion. The C3H6+ ions involved in these events ultimately yield propylene as a neutral product.