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

Abstract

Weak π hydrogen bonded solute-solvent complexes are studied with ultrafast two dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy, temperature dependent IR absorption spectroscopy, and density functional theory calculations. Eight solute-solvent complexes composed of a number of phenol derivatives and various benzene derivatives are investigated. The complexes are formed between the phenol derivative (solute) in a mixed solvent of the benzene derivative and CCl 4 . The time dependence of the 2D-IR vibrational echo spectra of the phenol hydroxyl stretch is used to directly determine the dissociation and formation rates of the hydrogen bonded complexes. The dissociation rates of the weak hydrogen bonds are found to be strongly correlated with their formation enthalpies. The correlation can be described with an equation similar to the Arrhenius equation. The results are discussed in terms of transition state theory.

Solute−Solvent Complex Kinetics and Thermodynamics Probed by 2D-IR Vibrational Echo Chemical Exchange Spectroscopy

“…[17][18][19]21 The exchange kinetics can be extracted from analyzing the time dependent peak volumes. 17,21,22 If spectral diffusion is fast compared to the chemical exchange rate, the peak intensities can be used to obtain the chemical exchange dynamics. 17,21 The 2D IR signal is affected by four dynamic processes: chemical exchange, vibrational relaxation of the OD stretch excitation, orientational relaxation of the species, and the spectral diffusion.…”

“…4 Recently nonlinear IR techniques especially 2D IR methods [5][6][7][8][9][10][11][12][13] have overcome the difficulties. [14][15][16][17][18][19][20][21] The techniques have an intrinsic fast temporal resolution that can be <100 fs. In addition, perturbations of the dynamics using 2D-IR methods have been shown to be negligible, 17 and there are well defined internal tests to determine if the true equilibrium dynamics are in fact being observed.…”

“…20 We found that there is a clear correlation between the enthalpies of formation of the complexes and the dissociation time constants (inverse of the dissociation rate constants), and the correlation can be described by an equation similar to the Arrhenius equation. 20,21 It was determined that the solute-solvent complex dissociation time constant is linearly related to exp(-∆H 0 /RT) where ∆H 0 is the complex enthalpy of formation, R is the gas constant, and T is the absolute temperature. This observation is interesting and important.…”

Theoretical vs. Experimental Ir Frequency Shifts Upon Π- Hydrogen Bonding: Complexes of Substituted Phenols With Hexamethylbenzene

“…The experimentally determined electrostatic interaction energy compared well with the theoretically derived from DFT computations. Vibrational echo spectroscopy study on the same interaction revealed that the time constant of the complex was only 8 picoseconds [28,29]. In a previous work, we presented results from an IR spectroscopic and theoretical investigation for a series of 20 π-hydrogen-bonded complexes of monosubstituted phenols and benzene [16].…”

“…This method has also been instrumental in studies of π-hydrogen bonds [6][7][8]. The advances of computational quantum chemistry have opened possibilities for gaining deeper insights into the nature of this type of noncovalent interactions that are of key importance in biology, chemistry, and materials science [9][10][11][12][13][14].…”