Direct versus Indirect Photodissociation of Isoxazole from Product Branching: A Chirped-Pulse Fourier Transform mm-Wave Spectroscopy/Pulsed Uniform Flow Investigation

Abstract

The UV photodissociation of isoxazole (c-CHNO) is studied in this work by chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform Laval flow. This approach offers a number of advantages over traditional spectroscopic detection methods due to its broadband, sub-MHz resolution, and fast-acquisition capabilities. In coupling this technique with a quasi-uniform Laval flow, we are able to obtain product branching fractions in the 193 nm photodissociation of isoxazole. Five dissociation channels are explored through direct detection of seven different photoproducts. These species and their respective branching fractions (%) include the following: HCN (53.8 ± 1.7), CHCN (23.4 ± 6.8), HCO (9.5 ± 2.3), CHCN (7.8 ± 2.9), CHCO (3.8 ± 0.9), HCCCN (0.9 ± 0.2), and HNC (0.8 ± 0.2). Guided by previous electronic structure and dynamics simulations, we are able to elucidate the dissociation dynamics that govern the final product branching fractions observed in this work, which differ significantly from previous reports on the thermal decomposition of isoxazole. Interestingly, both direct and indirect dynamics contribute to its dissociation, and clear signatures of both are manifested in the relative branching ratios obtained. Consistent with previous studies on the unimolecular dissociation of isoxazole, our findings also suggest the importance of the open-shell singlet diradicaloid species vinylnitrene in the dissociation dynamics, regardless of the initially populated excited state. This work, taken together with previous investigations, provides a global picture of the complex dissociation pathways involved in the photodissociation of isoxazole.

Vacuum ultraviolet photoionization and ionic fragmentation of the isoxazole molecules

“…The key role of the singlet vinylnitrenes production in the low energy photoreactions of isoxazole was additionally confirmed [25]. The most recent experimental investigation on the fragmentation pathways of isoxazole was performed by Dias et al [24], who observed photodissociation in the UV using a chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform flow. Upon irradiation at 193 nm (6.42 eV), HCN and CH3CO were the predominant dissociation products detected with the branching ratios 53.8 and 23.4%, respectively [24].…”

“…The calculations performed by Bouchoux et al [14] showed that ionization of isoxazole involves isomerization of the parent cation and proceeds through ring-opening by cleavage of the N-O bond. This is also an established initial step for neutral isoxazole dissociation [17], [18], [19], [20], [22], [23], [24], [25], [27].…”

“…The most recent photodissociation experiments [21], [22], [23], [24] shed more light on the underlying physics of fragmentation mechanisms of isoxazole molecule. In the studies of the vacuum-ultraviolet photodissociation of neutral isoxazole in the energy range of 14−50 eV, Wasowicz et al [21], [22] exploited photon-induced fluorescence spectroscopy to identify the dissociation products by detecting their fluorescence emission.…”

“…The most recent experimental investigation on the fragmentation pathways of isoxazole was performed by Dias et al [24], who observed photodissociation in the UV using a chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform flow. Upon irradiation at 193 nm (6.42 eV), HCN and CH3CO were the predominant dissociation products detected with the branching ratios 53.8 and 23.4%, respectively [24]. A few fragments with branching fractions below 10%, namely HCO, CH2CN, CH2CO, HCCCN, HCN, and HNC were also recorded [24].…”

“…Upon irradiation at 193 nm (6.42 eV), HCN and CH3CO were the predominant dissociation products detected with the branching ratios 53.8 and 23.4%, respectively [24]. A few fragments with branching fractions below 10%, namely HCO, CH2CN, CH2CO, HCCCN, HCN, and HNC were also recorded [24]. Based on previous calculations, it was suggested [24] that the observed relative branching ratios can be only explained when the proper contribution of the direct or indirect ultrafast photodissociation processes are considered.…”