Ultrafast N-H vibrational dynamics of cyclic doubly hydrogen-bonded homo- and heterodimers

Abstract

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore, they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen-bonded donor and acceptor species (i.e., NH...O --> N...HO). Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton-transfer dynamics. Here, we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low-frequency interdimer stretch and twist modes and not to interference between multiple Fermi resonances that dominate the substructure of the linear spectra.

Figure 1

The structures of the four dimers calculated using Gaussian 98 or 03.

Figure 2

FTIR spectra of the four dimers in CCl₄. The black, red, green and blue line is the FTIR spectrum of the 7AI homo-dimer, the PQ homo-dimer, the 7AI-Ac hetero-dimer and the PQ-Ac hetero-dimer, respectively.

Figure 3

TG and PS measurements of the four dimers. The TGs were measured at magic angle (MMZZ) while the PSs were recorded using all parallel (ZZZZ) polarization. The black lines are the measured data and the red lines are the exponential fits as described in the text. The green lines are the residuals of the TG measurements.

Figure 4

Low-frequency FTIR spectra and beat frequencies. (a) The FIR absorption spectra (black lines) are compared to the low-frequency modes obtained in the Gaussian calculations (red sticks). (b) Fourier components of the residual beat patterns of both TG and PS measurements shown in black and green, respectively. These are compared to the inter-dimer modes shown as red sticks. For the three planar dimers, the inter-dimer stretch mode is given the intensity 1 and the two inter-dimer twist modes are given the intensity 0.5. For the non-planar PQ homo-dimer, the monomer plane-bending modes also modulate the NH vibrations and the two additional asymmetric plane-bending modes are shown as the higher frequency red sticks of 0.5 intensity. The CCl₄ Raman modes are shown as blue sticks.