Deprotonation of Isoxazole: A Photoelectron Imaging Study

Abstract

We report a photoelectron imaging study of gas-phase deprotonation of isoxazole in which spectroscopic data are compared to the results of electronic structure calculations for the anion products corresponding to each of three possible deprotonation sites. The observed photoelectron spectra are assigned to a mixture of the anion isomers. Deprotonation at the most acidic (C5) and the least acidic (C4) positions yields the respective C5- and C4-isoxazolide anions, while the reaction at the intermediate-acidity C3 site leads to a cleavage of the O–N bond and an opening of the ring in the anion. Following photodetachment, the ground states of neutral C5- and C4-isoxazolyl are assigned to be σ radicals (X 2A′), while the ground-state neutral derived from the ring-open C3-anion is a π radical (X 2A″). The relative intensities of the spectral bands exhibit sensitivity to the ion source conditions, giving evidence of competing and varying contributions of the dominant C5 and C3, as well as possible C4, deprotonation pathways.

Diradical Interactions in Ring-Open Isoxazole

“…This approach to modeling photoelectron spectra has been previously tested on other systems. 17,28,29 The VDEs and the full widths at half-maxima (FWHM) of the Gaussian parts of the four WSG functions are included in the table insets in Figure 2b,c. The parameters of WSG(1) and WSG(2) were adjusted independently in both cases.…”

“…Although the final state in molecular anion photodetachment is always a superposition of partial waves with different l values, in the absence of detailed information about the partial-wave composition, the above scaling function is most appropriate at low energy, where this effect is most pronounced. This approach to modeling photoelectron spectra has been previously tested on other systems. ,, …”

“…In the present work, we turn to analogous, but (as it turns out) significantly different, interactions in oxazole’s isomer, isoxazole. Oxazole and isoxazole are distinct closed-shell molecules with divergent chemical properties, including their respective acidities, basicities, deprotonation, and protonation behaviors. , As illustrated in Figure b, electron attachment to isoxazole ( 1 or isoxazole-1) also results in the opening of the ring, but it is the O–N, rather than O–C, bond that breaks in this case . As with oxazole, photodetachment of the ring-open isoxazole anion ( 2a , hereafter termed isoxazole-2a) leaves a neutral diradical, isoxazole-2, which is sampled at the anion geometry, with O· and ·N ∼3 Å apart.…”

Anion of Oxalyl Chloride: Structure and Spectroscopy

“…In previous studies, we modeled congested bands using the Wigner-scaled Gaussian (WSG) function − The Gaussian part of WSG approximates the Franck–Condon envelope peaking at the transition’s VDE, while ( h ν – eBE) P = ε P multiplied by the Heaviside function Θ­( h ν – eBE) is a Wigner prefactor accounting for the eKE-dependent scaling of the electronic cross section. With P = l + 1/2, the electronic term corresponds to the Wigner threshold law, , σ ∝ ε l +1/2 , where l is the orbital angular momentum quantum number of the outgoing electron.…”

Stepwise Microhydration of Isoxazole: Infrared Spectroscopy of Isoxazole-(Water)n≤2 Clusters in Helium Nanodroplets

“…39 Previous microwave spectroscopy, 40 infrared (IR) spectroscopy, 41,42 and ab initio calculations 41 reported the geometrical and vibrational parameters of neutral isoxazole in its ground electronic state. Gas-phase photoelectron imaging study revealed that the hydrogen (C5H) adjacent to ring O is more acidic than the hydrogen (C3H) close to skeletal N. 43 Past microwave investigation of the isoxazole−argon (iso- xazole−Ar) complex showed that the nonpolar Ar ligand binds to the aromatic π electron cloud and slightly moves to the N− O bond because of the exchange repulsion forces. 44 However, changing the solvent to polar aprotic CO causes a substantial modification of the solvent-binding motif.…”