Vacuum Ultraviolet Photochemistry. II. Photolysis of Ethylene

Abstract

The direct photolyses of CH2CD2 and trans-CHDCHD were carried out at room temperature in the vacuum ultraviolet region. Kr, Xe, and Hg resonance radiations were used for photolyses. Products at the Kr lines were mainly hydrogen, acetylene, ethane, and n-butane. The ratio of acetylene/hydrogen was 2.8±0.2 at 1236 A. The photolysis of CH2CD2 gives 40% H2, 40% HD, and 20% D2. The acetylene was 12% C2H2, 62% C2HD and 26% C2D2. The relative amounts of H2, HD and D2 depend upon whether a hydrogen molecule is eliminated from two (type I process) or one (type II process) carbon atom. The remaining vinylidene rapidly rearranges to acetylene. The photolysis of trans-CHDCHD gives 17% H2, 73% HD, and 10% D2. This shows that hydrogen is formed from the trans as well as from the cis position and that the isotope effect for the type I process is slightly smaller than that for the type II process. Nearly free rotation of the molecule in its excited state is suggested. The percentages of the hydrogen isotopes produced are almost independent of exciting wavelength in both isotopic ethylenes. A comparison is made with Hg(3P1)-sensitized photolysis.

Photodissociation of ethylene at 193 nm

“…The HD to D2 isotope effect ratio of 1.5 is exactly what was previously observed and predicted [15,16]. This gives further credibility to the mechanism.…”

“…No 1,2 H exchange, however, was observed [15,16] . The vinylidene channel is favored over the acetylene by a ratio of approximately 3 to 2 [16].…”

“…The vinylidene channel is favored over the acetylene by a ratio of approximately 3 to 2 [16]. There is an isotope effect of = 1:1.5:2 with respect to D2:HD:H2 elimination for both channels ia and ib, although Okabe and McNesby noted that there seems to be a slightly smaller isotope effect in (ia) [15,16].…”

“…A summary of the known energetics for various dissociation channels is given in Figure 1 [2]. The earliest work was mainly concerned with identifying the primary decomposition channels [14][15][16][17][18][19]. The general consensus was that atomic (ii) and molecular (i) elimination producing Hand H2 are of approximately equal importance [17,18,19b In the molecular elimination, there appeared to be two different primary products, acetylene (ia) and vinylidene (ib) [14,16].…”

“…The elimination of a single H atom (iia) and the simultaneous loss of two H atoms (iib), were difficult to distinguish because the vinyl radicals created with vibrational energy exceeding the dissociation energy of H-CHCH (only 35 ± 3 kcaI/mole [2]) are apparently very short-lived with respect to further loss of an H atom [16][17][18][19]. Back and Griffiths [18] and Hara and Tanaka [19], however, "detected" stabilized C 2 H 3 by observing products that could only corne from reaction of the vinyl radical, so there is no question that iia occurs.…”