HELIUM NANODROPLET ISOLATION SPECTROSCOPY AND AB INITIO CALCULATIONS OF HO$_3$-(O$_2$)$_N$ CLUSTERS (N=0-4)

Abstract

The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the $\textit{in-situ}$ association reaction between OH and O$_2$. The infrared spectrum in the 3500-3700 cm$^{-1}$ region reveals bands that are assigned to the $\nu_1$ (OH stretch) fundamental and $\nu_1+\nu_6$ (OH stretch plus torsion) combination band of the $\textit{trans}$-HOOO isomer. The helium droplet spectrum is assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. \textbf{128}, 244313 (2008)]. Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O$_2$ van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction. There is also no spectroscopic evidence for the formation of \textit{cis}-HOOO, which is predicted by theory to be nearly isoenergetic to the \textit{trans} isomer. Stark spectroscopy of the \textit{trans}-HOOO species provides vibrationally averaged dipole moment components that qualitatively disagree with predictions obtained from CCSD(T) computations at the equilibrium, planar geometry, indicating a floppy complex undergoing large-amplitude motion about the torsional coordinate. Under conditions that favor the introduction of multiple O$_2$ molecules to the droplets, bands associated with larger H/DOOO-(O$_2$)$_n$ clusters are observed shifted ~1-10 cm$^{-1}$ to the red of the \textit{trans}-H/DOOO $\nu_1$ bands. Detailed \textit{ab initio} calculations are carried out for multiple isomers of \textit{cis}- and \textit{trans}-HO$_3$-O$_2$, corresponding to either hydrogen or oxygen bonded van der Waals complexes. Comparisons to theory suggest that the structure of the HO$_3$-O$_2$ complex formed in helium droplets is a hydrogen-bonded $^4$A$^{\prime}$ species consisting of a \textit{trans}-HO$_3$ core. The computed binding energy of the complex is approximately 240 cm$^{-1}$. Despite the weak interaction between \textit{trans}-HO$_3$ and O$_2$, non-additive red shifts of the OH stretch frequency are observed upon successive solvation by O$_2$ to form the larger clusters with \textit{n}$>$1.