Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy

Abstract

In this study we use ultrafast infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of two intramolecular hydrogen bonded complexes, 10-hydroxybenzo[h]quinoline (HBQ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) dissolved in carbon tetrachloride. We pump the OH stretching (νOH) mode and probe across 1800 cm−1–3300 cm−1 using a broadband IR probe pulse. Both systems exhibit large anharmonicities (>250 cm−1), which are characteristic of medium strong hydrogen bonded systems. Additionally, we observe the coherent beating of low-frequency modes of 245 cm−1 and 118 cm−1 across the νOH stretch of HBQ and HBT, respectively. Anharmonic frequency calculations at the DFT level identify these low-frequency structural modes as in-plane bending modes, which modulate the intramolecular hydrogen bonding distance. These findings provide evidence that the anharmonic nature of the fundamental νOH stretch is primarily dictated by the coupling to low-frequency structural modes and these motions play an important role in the hydrogen bonding dynamics.