Hydrogen bonding and charge transfer: Interaction of OH radical with rare gas atoms

Abstract

Hydroxyl radical in solid Ne at 4.2 K is known to rotate freely, vibrationally relax slowly, and have an essentially unperturbed structure. However, the HO structure and dynamics are profoundly affected by solvation in the heavier rare gases Ar, Kr, and Xe. One Ar atom nearest neighbor increases the OH (A ²Σ⁺) vibrational relaxation rate by a factor of ?10³, and A ²Σ⁺↔X ²Π spectra consistant with a linear–linear transition in hydrogen bonded ArHO are observed. The Ar–HO (A ²Σ⁺) well depth is D₀?675 cm⁻¹ with ω_e?203 cm⁻¹ and ω_ex_e?13.1 in solid Ar host; the Kr–HO (A ²Σ⁺) well depth is ?1000 cm⁻¹. The Ar–HO hydrogen bond length is shorter by Δr_e=1.15±0.1 Å in the excited state. The OH (A ²Σ⁺) (0,0) emission band shifts from 3090 in Ne to 4400 Å in Xe. The theory of hydrogen bond formation in both ground and excited states is discussed, with particular reference to the contribution of charge transfer in various environments. Comparison is made with the spectra of hydrogen halides, and the physical origin of rotation–translation coupling theory is discussed. The role of charge transfer complexes in the gas phase quenching of free radical luminescence by rare gas atoms is also considered.