Isomer-specific detection in the UV photodissociation of the propargyl radical by chirped-pulse mm-wave spectroscopy in a pulsed quasi-uniform flow

Abstract

Isomer-specific detection and product branching fractions in the UV photodissociation of the propargyl radical is achieved through the use of chirped-pulse Fourier-transform mm-wave spectroscopy in a pulsed quasi-uniform flow (CPUF). Propargyl radicals are produced in the 193 nm photodissociation of 1,2-butadiene. Absorption of a second photon leads to H atom elimination giving three possible CH isomers: singlets cyclopropenylidene (c-CH) and propadienylidene (l-CH), and triplet propargylene (HCCCH). The singlet products and their appearance kinetics in the flow are directly determined by rotational spectroscopy, but due to the negligible dipole moment of propargylene, it is not directly monitored. However, we exploit the time-dependent kinetics of H-atom catalyzed isomerization to infer the branching to propargylene as well. We obtain the overall branching among H loss channels to be 2.9% (+1.1/-0.5) l-CH + H, 16.8% (+3.2/-1.3) c-CH + H, and 80.2 (+1.8/-4.2) HCCCH + H. Our findings are qualitatively consistent with earlier RRKM calculations in that the major channel in the photodissociation of the propargyl radical at 193 nm is to HCCCH + H; however, a greater contribution to the energetically most favorable isomer, c-CH + H is observed in this work. We do not detect the predicted HCCC + H channel, but place an upper bound on its yield of 1%.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…The mm-wave spectrum allows discrimination between (and at least approximate quantitation of) the various C3H2 isomers and confirms that H + 3 HCCCH products constitute ~80% of the total dissociation yield. 227 Photofragmentation studies involving the various C3H2 isomers are still rare. Propargylene (HCCCH) has a triplet ground state.…”

Photodissociation transition states characterized by chirped pulse millimeter wave spectroscopy

“…For example, simultaneous broadband detection of isomerizing molecules enables "dynamic rotational spectroscopy" (31,41). The relative transition intensities in CP experiments can be converted to branching ratios of multiple reaction products for chemical kinetics studies (42)(43)(44)(45). A recent study of 193-nm photolysis of vinyl cyanide (VCN) in a roomtemperature reactor revealed a new pathway to HCCCN by measuring the time evolution of the vibrational-level population distribution (VPD) in a CPmmW spectrum (43).…”

Quantum aspects of roaming dynamics

“…Subsequent disappearance of l-C3H2 and appearance of intense c-C3H2 (after 70 µs) is ascribed to H-atom catalyzed isomerization in the high-density region of the flow as discussed previously. 183 Singlet propargylene is also a possible product isomer, but we cannot detect it owing to its negligible dipole moment. We would expect this to undergo H catalyzed isomerization to c-C3H2, which would appear at later times, just as we have shown for the triplet propargylene and l-C3H2.…”

“…The propargyl chloride is seeded at 1% in helium and expanded from a Laval nozzle forming a quasi-uniform flow at a total density of ~10 16 cm -3 and a temperature of products, and the isomer-specific branching is the subject of studies reported elsewhere. 183 The photoproducts are internally cooled in the flow, after which they are excited by π/2 pulses of mmwave radiation tuned to known transitions of C3H2 isomers at time intervals of 10 µs. After each mm-wave pulse, the free induction decay of the excited molecules is acquired and accumulated in the time domain, then Fourier-transformed to give the rotational spectrum from which branching is determined as described previously.…”

“…Results for propargyl chloride are presented along with those for 1,2-butadiene that were previously studied to understand branching in propargyl radical dissociation. 183 These experiments were performed in a quasi-uniform flow at low temperature but in a collisional environment to permit relaxation of the products to the local temperature (~14 K) for detection. In the previous study, propargyl radical was first prepared by photodissociation of 1,2-butadiene.…”