Photoinduced C–H bond fission in prototypical organic molecules and radicals

Abstract

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

“…This process is exoergic at all wavelengths studied, though the adiabatic S 1 PES will likely exhibit a barrier at short R C-C bond extensions as the Rydberg function acquires increasing s* antibonding valence character. 6 The CH * 3 radicals would be unstable with respect to H + 1 CH 2 (ã) products. 31 Again, the H atoms would carry most of any kinetic energy release, so the translational energy distributions of any 1 CH 2 (ã) fragments formed in this way should broadly mirror that of their CH * 3 precursor.…”

“…Analogy with CH 4 suggests that this energy disposal is a consequence of the nuclear motions that promote C-H bond ssion by non-adiabatic coupling to the S 0 PES. 6,35 Most of the 'C 2 H 5 ' products assumed in deriving the P(TKER) distribution have sufficient internal energy to dissociate furtherby loss of another H atom (net reaction (2)), or H 2 (net process (9)), or both (net channel (10)), or to two CH x species (e.g. via net channels (6) or (11) in Table 1).…”

“…6,35 Most of the 'C 2 H 5 ' products assumed in deriving the P(TKER) distribution have sufficient internal energy to dissociate furtherby loss of another H atom (net reaction (2)), or H 2 (net process (9)), or both (net channel (10)), or to two CH x species (e.g. via net channels (6) or (11) in Table 1). 42 However, the smoothly varying P(TKER) distributions shown in Fig.…”

“…If C-C bond ssion completes aer non-adiabatic coupling to the S 0 PES, the resulting CH # 3 fragments most likely decay to H + 3 CH 2 (X) products (i.e. net reaction (6)). This reaction is not included in the current model and, according to the present analysis, will have signicantly higher quantum yield than reaction (5).…”

“…For example, the dominant C 2 H 6 and C 2 H 2 generation mechanisms are assumed to involve secondary reactions following photolysis of CH 4 . [6][7][8] But both the Cassini-Huygens y-by of Jupiter and terrestrial measurements reveal very different meridional and latitudinal distributions for C 2 H 6 and C 2 H 2 . Such would be surprising if both species are tightly experimental and computational methods used in this study; ionisation energies and dissociative ionisation thresholds for various C x H y (x ¼ 1, 2, y # 6) species of present interest; mass spectra showing CH 2 (m/z 14) and CH 3 (m/z 15) products from photolysis of C 2 H 6 at l ¼ 125.6, 121.6, 118.2 and 112.0 nm using two different FEL pulse energies; CH 2 + and CH 3 + ion images analysed to prepare the P(TKER) and b(TKER) distributions shown in Fig.…”