Monitoring Thermodynamic Equilibrium Processes At 10 K: Conformational Isomerization And Photochromism Of O4+ In Argon Matrices.

Bands corresponding to structural isomers of matrix-isolated O$_{4}$$^{+}$ are observed upon deposition of ions into argon matrices doped with moderate (0.1-1\%) concentrations of O$_{2}$. These bands have been assigned based on previous matrix isolation spectroscopy,\footnote{Jacox, M.E.; Thompson, W.E.; J. Chem. Phys. 100, 750 (1994); Zhou, M.-F., et al.; J. Chem. Phys. 110, 9450 (1999).} as well as high-level computational studies.\footnote{Lindh, R.; Barnes, L.; J. Chem. Phys. 100, 224 (1994).} In the current work, these bands are observed upon co-deposition of Cu$^{-}$ and Ar$^{+}$ ions at low-energies. The Cu$^{-}$ is present only as a non-interacting counter-ion, as is verified by studies using exclusively high-energy Ar$^{+}$ beams; in this case, the spectroscopy of the O$_{4}$$^{+}$ species is completely equivalent, however there is now also an intense peak corresponding to O$_{4}$$^{-}$ counter-ion species. Following deposition at 20 K, the matrices are cooled to 10 K, where the FTIR spectra show a band at 1119 \wn for the trans-O$_{4}$$^{+}$ isomer, and a doublet at 1329/1331 \wn, corresponding to the cyclic-O$_{4}$$^{+}$ isomer, based on earlier work. There is also a band at 1186 \wn that was previously assigned to a larger O$_{6}$$^{+}$ complex. A temperature series taken in 1 K increments between 10 and 20 K reveal two reversible interconversion processes: the 1119 \wn band decreases between 10 and 14 K while a new band grows in at 1242 \wn, and the 1186 band shows a similar interconversion between 11 and 16 K with the 1331 \wn peak of the cyclic-O$_{4}$$^{+}$ doublet, while the 1329 \wn peak diminishes and broadens over the same temperature range. The interconverting peak pairs can be converted into equilibrium constants based on relative changes in integrated intensities, and the associated van't Hoff plots show linear trends with $\Delta$H values in the range expected based on computational work. Finally, the 1186 \wn and 1331 \wn peak pair exhibit strong photochromism at 10 K: irradiation with red light converts 1186 to 1331, while irradiation with blue light shifts the equilibrium in the other direction. In both cases the phenomena is completely reversible and reproducible, with the original intensity ratio being restored after a few minutes once the light has been switched off. Irradiation has no effect on the other spectra bands, indicating that the 1329 and 1331 \wn bands cannot arise from the same species, as previously thought