Dynamical interrogation of the hydration cage of bromine in single crystal clathrate hydrates versus water

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 tm=300 fs, 400 fs and standard deviation sigma=70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 5(12)6(2) polyhedra in TS-I and 5(12)6(4) polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by tm=555 fs and sigma=400 fs, is strongly skewed (gamma1=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.

Cross-nucleation between clathrate hydrate polymorphs: Assessing the role of stability, growth rate, and structure matching

“…47,48,75 The melting temperatures of sII and sI bromine hydrates are predicted to be only 2 K and 5 K, respectively, below that of the stable TS crystal. 54 Our analysis suggests that TS bromine clathrate can cross-nucleate to sII but probably not to sI.…”

“…The highest growth rate of sII wins over the driving force favoring sI clathrate at 320 K. crystals. [46][47][48]75 We investigate the growth of TS under various conditions to examine the possibility of cross-nucleation that may facilitate the formation of sII and sI from the TS phase in bromine hydrates.…”

Halogen Bonds in Clathrate Cages: A Real Space Perspective

“…The nature of guest‐host interactions in the case of dihalogen molecules in water clathrate cages has been the subject of recent debate in both experimental and theoretical studies . The initial interpretation of the observed electronic shifts in ultraviolet and visible spectra of bromine clathrate assumed that halogen bonding could not be responsible since all the water lone‐pairs are involved in hydrogen bonding of the water lattice and thus not available for halogen bonding .…”

“…[1] Even though its full understanding is relatively recent, its relevance is already widely recognized in very different fields, such as biomolecules, crystal engineering, or functional systems design. [2] The nature of guest-host interactions in the case of dihalogen molecules in water clathrate cages 1 has been the subject of recent debate in both experimental [3][4][5] and theoretical studies. [6][7][8][9] The initial interpretation of the observed electronic shifts in ultraviolet and visible spectra of bromine clathrate assumed that halogen bonding could not be responsible since all the water lone-pairs are involved in hydrogen bonding of the water lattice and thus not available for halogen bonding.…”

Ground state analytical ab initio intermolecular potential for the Cl2-water system

“…Recent experiments using a wide range of spectroscopic tools performed in the groups of Janda and Apkarian have provided great insight into dihalogen molecule dynamics and their behavior as sensitive probe to their aqueous environment. [11][12][13][14][15] Molecular modeling can provide fundamental informations to characterize the water structure around the halogen solute and to understand the parameters governing it (orientation of the solute in the cage, configuration of the solute at the clathrate/ice surface, etc. ).…”