Ultrafast Dynamics of Solute-Solvent Complexation Observed at Thermal Equilibrium in Real Time

Abstract

In general, the formation and dissociation of solute-solvent complexes have been too rapid to measure without disturbing the thermal equilibrium. We were able to do so with the use of two-dimensional infrared vibrational echo spectroscopy, an ultrafast vibrational analog of two-dimensional nuclear magnetic resonance spectroscopy. The equilibrium dynamics of phenol complexation to benzene in a benzene-carbon tetrachloride solvent mixture were measured in real time by the appearance of off-diagonal peaks in the two-dimensional vibrational echo spectrum of the phenol hydroxyl stretch. The dissociation time constant tau(d) for the phenol-benzene complex was 8 picoseconds. Adding two electron-donating methyl groups to the benzene nearly tripled the value of tau(d) and stabilized the complex, whereas bromobenzene, with an electron-withdrawing bromo group, formed a slightly weaker complex with a slightly lower tau(d). The spectroscopic method holds promise for studying a wide variety of other fast chemical exchange processes.

Probing dynamics of complex molecular systems with ultrafast 2D IR vibrational echo spectroscopy

“…The rate of chemical exchange can be determined by observing the growth of the off-diagonal peaks in the 2D vibrational echo spectrum. 29,31,36,100 2. Organic solute-solvent complex formation and dissociation.…”

“…Such weak interactions will produce solutesolvent complexes that are short lived. 29,36,100 Although short lived, the dissociation and formation of organic solute-solvent complexes can influence chemical processes such as reactivity.…”

“…2D IR experiments can also be useful as a tool for chemical structural analysis by revealing the relationship among different mechanical degrees of freedom of a molecular or biomolecular system. 2D IR vibrational echo experiments have been successfully applied to study fast chemical exchange reactions and solution dynamics, 29,31,36 water dynamics, 37,38 hydrogen network evolution, 28 intramolecular vibrational energy relaxations, 34 protein structures and dynamics, 30,39,40 and mixed chemicals analysis. 41 Since the first fast condensed matter vibrational echo experiments in 1993, 20 the field has advanced a great deal.…”

Hydrogen Bond Lifetimes and Energetics for Solute/Solvent Complexes Studied with 2D-IR Vibrational Echo Spectroscopy

“…2 Measurements of the lifetimes of relatively weak hydrogen bonds (< 5 kcal/mol ) at room temperature under thermal equilibrium conditions have recently become possible because of the advent of ultrafast IR techniques. [6][7][8][9][10][11] To the best of our knowledge, no systematic study on the relationship between the lifetimes and strength of hydrogen bonds has been reported.…”

Dynamics of a Multimode System Coupled to Multiple Heat Baths Probed by Two-Dimensional Infrared Spectroscopy

“…Especially, objects under study in two-dimensional infrared (2D-IR) spectroscopy have definitely spread very wide: structures and dynamics of peptides, [9][10][11][12][13][14][15] conformational changes in protein, [16][17][18] water dynamics, 19 hydrogen-bond dynamics, 20-25 solute-solvent interactions, 26,27 quantum tunneling processes, [28][29][30] and fast chemical exchange in molecular complexes. [31][32][33] Multidimensional vibrational spectroscopy is the optical counterpart of multidimensional NMR such as COSY and NOESY, which are powerful tools for organic structural analysis and structural biology. 34 In 2D-NMR and 2D-IR experiments, the data sets are represented in the 2D frequency space.…”