Photoelectron Spectroscopy of Chloro-Substituted Phenylnitrene Anions

Abstract

The 355 nm time-of-flight negative ion photoelectron spectra of (o-, m-, and p-chlorophenyl)nitrene radical anions are reported. Electron affinities are obtained from the photoelectron spectra, and are 1.79 +/- 0.05, 1.82 +/- 0.05, and 1.72 +/- 0.05 eV for the (o-, m-, and p-chlorophenyl)nitrenes, respectively. Singlet-triplet splittings are determined to be 14 +/- 2, 15 +/- 1, and 14 +/- 2 kcal/mol, respectively. The shapes of the photoelectron bands indicate resonance interactions in the singlet states for the ortho- and para-substituted isomers, which is attributed to quinoidal structures of the open-shell singlet states. Reanalysis of the photoelectron spectrum of phenylnitrene anion leads to a revised experimental singlet-triplet splitting of 14.8 kcal/mol in the unsubstituted phenylnitrene.

Singlet stabilization of oxazole- and isoxazolenitrene-n-oxides by radical delocalization

“…The preferential stabilization of the singlet state of the nitrene upon delocalization is proposed [10,26] to be due to decreased electron-pair repulsion in the singlet state, similar to the mechanism proposed by Borden and Davidson [30] to account for the singlet stabilities of disjoint diradicals. Comparison of the triplet and singlet geometries in the oxazole-and isoxazolenitrene-n-oxides reflects the additional delocalization that occurs in the singlet states of these nitrenes as well.…”

“…The experimental value for the energy difference between the triplet and open-shell singlet of phenylnitrene, measured by negative ion photoelectron spectroscopy, is 14.8 kcal/mol [9,10]. Most theoretical studies of nitrenes have focused on understanding the reactivity of phenyl nitrene [2,11,3], and few have examined the effect of substitution on electronic structure [5].…”

“…The spin-state dependence of the radical stabilization has been proposed to occur due to preferential stabilization of the singlet state due to radical delocalization [10,26]. Borden and Davidson [30], have proposed that singlet states of diradicals are preferentially destabilized by electron-pair repulsion.…”

Reactivity of 3- and 4-pyridinylnitrene-n-oxide radical anions

“…Despite their unusual electronic structures, isoelectronic with carbene radical anions [1][2][3][4][5][6], they are surprisingly easily generated in mass spectrometry, cleanly and in high abundance, by simple electron ionization of azide-substituted precursors [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Although azides are considered potentially explosive, given the recent interests in azides as reagents in "click" chemistry [21][22][23][24] they are commonly being synthesized and utilized in chemical applications, and, in principle, any of these derivatives could be used as precursors of nitrene radical anions.…”

“…Although azides are considered potentially explosive, given the recent interests in azides as reagents in "click" chemistry [21][22][23][24] they are commonly being synthesized and utilized in chemical applications, and, in principle, any of these derivatives could be used as precursors of nitrene radical anions. However, despite how easily the ions are to form, few studies of nitrene radical anions have been reported, and many of those reported have focused on the use of the anions as precursors for photoelectron/photodetachment spectroscopy studies of the corresponding nitrenes [9][10][11][12][13][14].…”