Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor

Abstract

Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1-10 μbar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CHCHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 μs time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.

Photodissociation transition states characterized by chirped pulse millimeter wave spectroscopy

“…Using the ATcT results from the SI appendix, table S5 of ref. 43, we find that this reaction is slightly endothermic: Δ r H°(0K) = 5.01 kcal/mol. According to the photofragment translational spectroscopy study of Hall and coworkers (12), the average kinetic energy release in the CH 2 CHCN → CH 2 CCN + H reaction is near 4 kcal/mol, with its distribution extending to 10 kcal/mol.…”

“…A fundamental difficulty in the use of CPmmW spectroscopy is understanding how the observed intensity of only one rotational transition relates to a vibrational population. The relevant equations require the existence and knowledge of the rotational temperature (43). The contribution to our measured VPD from TSs that release products with very high rotational excitation (23,24,26,54), which are not thermalized to the ambient temperature in the detection volume, will be underestimated relative to nascent product populations in the low-J states (43).…”

“…Photolysis of ethylene (13) and its substituted analogs, VCN (12,22,(46)(47)(48)(49)(50)(51)(52), vinyl chloride (15,23,25,26), vinyl bromide (17,24,53,54), vinyl iodide (55), and propene (56), has been extensively studied during the last 4 decades. The 193-nm photolysis (57) of VCN has been observed (12,22,43,47,49) and calculated (43,48,50,52) to take place via a dizzying array of reaction pathways. Since many of these pathways lead to difficult-todistinguish final product channels, it has been challenging to obtain definitive experimental evidence in support of a particular model or theoretical description.…”

“…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).…”

“…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). Precise determination of (averaged) molecular structure remains at the core of these new directions, while broad spectral range and line intensities at ∼20% relative accuracy enable new chemical applications.…”