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

The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The only primary photofragments observed were HCCH (acetylene) and C(6)H(4). Some of the C(6)H(4) molecules were found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of DeltaH(f) < or = 160 +/- 4 kcal mol(-1) was determined for 1,3,5-hexatriyne from the energetic threshold for this process. This experimentally determined value agrees well with our ab initio calculations performed at the G2 level of theory. Angular distribution measurements for the HCCH + C(6)H(4) channel yielded an isotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded no evidence for the phenyl + ethynyl or the atomic hydrogen elimination channels even though these were observed in the pyrolytic studies of phenylacetylene [Hofmann, J.; Zimmermann, G.; Guthier, K.; Hebgen, P.; Homann, K. H. Liebigs Ann. 1995, 631, 1995. Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zimmermann, G. Liebigs Ann. 1995, 637, 1995].

Photodissociation of ethylene sulfide at 193 nm: a photofragment translational spectroscopy study with VUV synchrotron radiation and ab initio calculations.

Photodissociation of ethylene sulfide at 193 nm has been studied using photofragment translational spectroscopy and ab initio theoretical calculations. Tunable synchrotron radiation was used as a universal but selective probe of the reaction products to reveal new aspects of the photodissociation dynamics. The channel giving S + C2H4 was found to be dominated by production of ground-state sulfur atoms (S(3P):S(1D) = 1.44:1), mostly through a spin-forbidden process. The results also suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B(1u)) and allow insight into the energy of the latter species near its equilibrium geometry, in which the two methylene groups occupy perpendicular planes. In addition, a channel leading to the production of H2S with C2H2 also has been observed. Our experimental results are supported and elaborated by theoretical calculations.

Franck-Condon Factors and Analysis of Several Electronic Systems of C2

The D1Sigma+u-X1Sigma+g (Mulliken) and E1Sigma+g-A1Piu (Freymark) systems of C2 have been analyzed. Molecular constants have been determined for the D1Sigma+u and E1Sigma+g states. Using these molecular constants, Franck-Condon factors (FCFs) have been calculated for the previously observed Mulliken and D1Sigma+u-B'1Sigma+g band systems. FCFs for two new electronic transitions, D1Sigma+u-C1Pig and E1Sigma+g-D1Sigma+u, have also been calculated. Knowledge of FCFs will help spectroscopists in the search for and identification of these bands. Copyright 1998 Academic Press.