The ultraviolet photochemistry of phenylacetylene and the enthalpy of formation of 1,3,5-hexatriyne

The ultraviolet photochemistry of phenylacetylene was studied in a molecula r beam at 193 nm. The only primary photofragments observed were HCCH (acety lene) and C6H4 Some of the C6H4 molecules were found to decompose to 1,3,5- hexatriyne and molecular hydrogen. An enthalpy of formation of DeltaH(f) le ss than or equal to 160 +/- 4 kcal mol(-1) wad determined for 1,3,5-hexatri yne from the energetic threshold for this process, This experimentally dete rmined value agrees well with our ab initio calculations performed at the G 2 level of theory. Angular distribution measurements for the HCCH + C6H4 ch annel yielded an isotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded n o evidence for the phenyl + ethynyl, dr the atomic hydrogen elimination cha nnels even though these:were observed in the pyrolytic studies of phenylace tylene [Hofmann,J.; Zimmermann, G.; Guthier; K.; Hebgen, P.; Homann, K. ii. Liebigs Ann. 1995, 631, 1995, Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zim mermann, G. Liebigs Ann. 1995, 637, 1995].