Vibronic dynamics of bromine in the condensed phase through Raman spectroscopy.

摘要

Careful analysis of Raman spectra through general principles is used to probe interaction energetics and associated dynamics of bromine in the condensed phase. Through the 30 overtone Raman spectrum of liquid bromine and the UV-Vis spetrum, dynamics of nearly 1 A of the ground state potential are accessed and analyzed in conjunction with the dynamics of the excited state potential. The lifetimes of the vibrational coherences are acquired through the decay in intensity of the overtones and the vibrational dephasing rate is extracted from this data through comparison with simulation. The vibrational dephasing rate follows a classic energy gap law nearly perfectly, despite availability of thermal energy in the solvent bath matched to the vibrational state spacing and strong electronic perturbations to both the ground and electronic state. Also resulting from the long coherence time, an excited Br2 molecule follows expansion of the cage seen through lineskewing in the spectra. These surprising dynamics would be expected for a solid, such as a solvated molecule in a rare gas matrix. The plot completely changes when bromine is solvated in water; the Br2 progression is completely quenched, yet reappears in Br-3 . The only physical explanation of quenching is that the excited Raman wavepacket rapidly evolves off of the Br-Br coordinate onto multiple Br-H 2O coordinates - the hydration shell cannot be considered hydrophobic or hydrophilic, it consists of dynamically equivalent fluxional water molecules. Solvent cage dynamics resurface in the Br-3 spectra where difference transitions between the cage modes and the symmetric stretch coincide with the symmetric stretch progression. Spectroscopic signatures of the cage structure are also found elsewhere in Raman spectra. The band shape and other characteristics are used in the search for a new phase of ice: guest free clathrate hydrates. However, this proves to be challenging experimentally as the extracted cages in guest extracted ice collapse into a hexagonal structure. Energetic interactions are also accessed through macroscopic observables of thermodynamics. The enthalpy of decomposition, Delta Hd, was measured over TS-I clathrate hydrate in equilibria with both ice and liquid water. DeltaHd cannot be fully accounted for with the changes in hydrogen bonding of the equilibria, rather the van der Waals interactions play a significant role.