The primary reactions of the lowest energy triplet states of diacetylene (C4H2*) with 1,3-butadiene (C4H6) in a helium buffer are characterized with a molecular beam pump−probe technique. Triplet diacetylene is prepared in the early portions of a molecular expansion by laser excitation of the 2 061 0 band of the 1Δu ← X1Σ+ g transition in C4H2 at 231.5 nm, which rapidly interconverts to high vibrational levels of the lowest energy triplet surfaces. The subsequent reactions with C4H6 are allowed to proceed for 20 μs while the expansion traverses a short ceramic reaction tube or slit channel. Primary products are observed by quenching secondary processes as molecular collisions cease outside the tube. The major photochemical products C6H6 and C8H6 are detected in a linear time-of-flight mass spectrometer using both vacuum ultraviolet photoionization and resonant two-photon ionization (R2PI). R2PI spectra of the C6H6 and C8H6 products unambiguously identify them as benzene and phenylacetylene, respectively. Based on deuterium substitution experiments, a mechanism for these ring-forming reactions is proposed. The potential importance of these reactions for forming aromatics in sooting flames and planetary atmospheres is discussed.

The chemistry of the triplet metastable state of diacetylene (C 4 H 2 *) with ethene, propene, and propyne in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. An ultraviolet photoexcitation laser counterpropagates the molecular expansion through a short reaction tube, exciting the C 4 H 2 1 ∆ u r 1 ∑ g + 2 1 0 6 1 0 and 6 1 0 transitions at 231.5 and 243.1 nm, respectively. Efficient intersystem crossing forms the metstable triplet state from which reaction occurs. The short length of the tube (8 mm) serves to quench the reaction after 10-30 µs so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuum ultraviolet light and mass-analyzed in a linear time-of-flight mass spectrometer. H 4 ) are consistent with poly-yne, enyne, and cumulene products. Percent product yields are determined assuming equal photoionization cross sections for the products. Relative photoionization cross sections at 118 nm for a series of model alkene, alkyne, enyne, diene, and diyne compounds are determined to test the variations in photoionization cross section expected for the products. Relative rate constants for the reactions (scaled to k(C 4 H 2 * + C 4 H 2 ) ) 1.00) with ethene, propene, and propyne are extracted from concentration studies, determining values of 0.24 ( 0.01, 0.32 ( 0.01, and 0.42 ( 0.02 in helium buffer, respectively. Isotopic studies employing deuterated reactants are used to constrain the mechanisms for the reactions. Most of the major products are proposed to follow formation of an unbranched or branched chain adduct which subsequently decomposes by loss of interior atoms to form a stable poly-yne or en-yne product. Two schemes are proposed to account for formation of the isotopically labeled C 5 H 4 and C 5 H 3 products in the C 4 H 2 * + CH 3 C 2 H reaction. Only one of these mechanisms appears to be operative in the C 4 H 2 * + CH 3 CHdCH 2 reaction.

The ultraviolet and infrared spectra of the C6H6−(C4H2) n complexes with n = 1 and 2 have been formed and studied in a supersonic expansion using resonant two-photon ionization (R2PI), resonant ion-dip infrared spectroscopy (RIDIRS), and IR−UV hole-burning spectroscopy. A T-shaped structure is deduced for the complex, with the C4H2 centered, end on, over the benzene ring. This C−H···π H-bond shifts the frequency of the vibronic transitions of C6H6 by 161 cm-1 to the blue. The acetylenic CH stretch fundamental of C4H2 is localized and split by formation of the π H-bond. The H-bonded CH stretch fundamental is lowered in frequency by about 40 cm-1, increased in intensity by more than a factor of 2, and split by a Fermi resonance that is turned on by the complexation. The C6H6−(C4H2)2 complex has a structure that makes the S1−S0 origin transition weakly allowed and possesses an infrared spectrum that has acetylenic CH stretch absorptions due to free CH, aromatic π-bound CH, and a more weakly π-bound CH. Its structure is tentatively assigned as a cyclic, “pinwheel” structure.

The reactions of metastable diacetylene with benzene and toluene are explored using a molecular beam pumpprobe time-of-flight mass spectrometer. Diacetylene is laser-excited to the 2 1 0 6 1 0 band of the 1 ∆ u r X 1 Σ + g transition, whereupon rapid intersystem crossing occurs to the lowest triplet states. The triplet state diacetylene then reacts with either benzene or toluene as the gas mixture traverses a short reaction tube (∼20 µs). The reactions are quenched as the gas mixture expands into the ion source region of a time-of-flight mass spectrometer where the primary photoproducts are detected using vacuum ultraviolet (VUV) photoionization or resonant two-photon ionization (R2PI). The major products from the reaction of diacetylene and benzene have molecular formulas C 8 H 6 and C 10 H 6 , and are identified as phenylacetylene and phenyldiacetylene using R2PI spectroscopy. The major products from metastable diacetylene's reaction with toluene are C 9 H 8 and C 11 H 8 . The C 9 H 8 product is confirmed as a mixture of o-, m-, and p-ethynyltoluene, with the ortho product dominating. Mechanisms for the formation of the above products are proposed based on deuterium substitution studies of the reactions. The potential importance of these reactions is discussed as they relate to hydrocarbon growth in sooting flames. † Part of the special issue "C. Bradley Moore Festschrift".

The detection of vapors of dinitrobenzenes and dinitrotoluenes by UV cavity ringdown spectroscopy (CRDS) was investigated. Absorption cross sections at 248 nm were estimated by measurements on saturated vapors and compared with solution-phase values. The computed subparts per 10(9) detection sensitivity with no effort at preconcentration was demonstrated through measurements on diluted flows. The factors affecting measurements on 1 atm total pressure were considered, and it was demonstrated that Rayleigh scattering by air will reduce the detection sensitivity by 5%-10%. The UV absorption spectra of these compounds are broad, resulting in a relatively poor selectivity for single-wavelength measurements with the UV CRDS technique.