Photolysis of cis-2-butene at 7.1 eV

Abstract

Publication costs assisted by Université du Québec a ChicoutimiThe photolysis of cis-2-butene (C2B) was carried out in a static system using nitrogen resonance lines at 7. 10-7.11 eV (174.5-174.3 nm). The main fragmentation processes of the photoexcited (C4H8)** molecule are as follows: C4H8-2** -*• C4H6 -I-2H (H2), = 0.40, and C4H8-2** -* C3H5 + CH3, = 0.38. The energy partitioning between CH3CH=CH and CH3 radicals is discussed. The excited vinylic radicals either decompose yielding acetylene, or isomerize (which is followed by aliene formation), or are stabilized by collisions. The kinetics of the (s-C4H9)* radical decomposition provide some information on the energy distribution of the atomic hydrogen present in the studied system. The stabilization of C4H8-2** molecules and the formation of isomers are inefficient processes between 6.6 and 4 X 104 N m"2 (0.05 and 300 Torr).

Excited radicals in the gas phase photolysis of butene isomers at different photon energies: energy distribution in reaction products

“…In the C2B and IB photolytic systems, two pressure regions, low and high, were distinguished and analysed (Figs. 2-5) in the same way as was done in our previous works (3,5). On the contrary, the good linearity of the plot [@(allene) -0.021-I vs. NB pressure, covering a broad pressure region, was attained (Fig.…”

“…Two factors may be cited to account for this observation: (i) virtually monoenergetic H-atoms are produced during the photolysis and (ii) the energy of the H atoms is higher than their average thermal energy (5) (the thermal energy distribution of the NB molecules contributes less to the overall energy distribution function of s-C,H," radicals). The higher excess energy on the s-C,H,* rad~cals than that expected from the exothermicity process [3], involving thermalized species, has previously been concluded (5). Now, as shown in Fig.…”

“…1-6 were used to determine the decomposition rate constants of the excited radicals (3,5) and they are given in Table 1. It was assumed that the deactivation of the excited radicals : occurs at each collision, and the collision numbers were calculated in a normal [vay (9).…”

“…However, the Stern-Volmer treatment of the primary photoexcited species is limited to large molecules of numer- '1976/1977 ous oscillators, and/or at relatively low photon energies. For example, the collisional stabilization of photoexcited cis-2-butene (C2B) molecules reported to occur at 6.7 eV (2) is no longer observed at 7.1 eV (3).…”

“…In the 7.i eV photolytic butene systems (nitrogen resonance lines, 174.3-174.5 nm), we have shown that the C,H,* radicals decompose, yielding C,H, hydrocarbon products, whose yield is strongly dependent on the total pressure. In particular, the resulting [O(allene)]-I vs. butene pressure plots turn out to be nonlinear, and this observation was attributed to the formation and decomposition of two distinct types of the C,H,#: radicals of reasonably narrow energy content (3,5).…”