Dynamical interrogation of the hydration cage of bromine in single crystal clathrate hydratesversuswater

Abstract

We report transient grating measurements carried out on single crystals of bromine clathrate hydrates and on bromine dissolved in water. In all cases, excitation into the B-state of Br₂ leads to prompt predissociation, followed by cage-induced recombination on the A/A′ electronic surfaces. In liquid water, the vibrationally incoherent recombinant population peaks at t = 1 ps and decays with a time constant of 1.8 ps. In the hydrate crystals, the recombination is sufficiently impulsive to manifest coherent oscillations of the reformed bond. In tetragonal TS-I crystals, with the smaller cages, the recombination is fast, t = 360 fs, and the bond oscillation period is 240 fs. In cubic CS-II crystals, the recombination is slower, t = 490 fs, and the visibility of the vibrational coherence, which shows a period of 290 fs, is significantly reduced due to the larger cages and the looser fit around bromine. The mechanical cage effect is quantified in terms of the recombination time-distribution, the first three moments of which are associated with size, structural rigidity, and anelasticity of the cage. In the crystalline cages, the distribution is symmetric about the mean: mean time t_m = 300 fs, 400 fs and standard deviation σ = 70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 5¹²6² polyhedra in TS-I and 5¹²6⁴ polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by t_m = 555 fs and σ = 400 fs, is strongly skewed (γ₁ = 1.88) toward delayed recombination—the effective liquid phase hydration shell is larger than that in a hydrate phase, structurally disordered, and anelastic. Information about dipolar disorder, comparable in all three media, is extracted from electronic predissociation rates of the B-state, which is sensitive to the symmetry in the guest–host interaction.