Studying Ultrafast Vibrational dynamics of Intramolecular Hydrogen Bonds using Broadband Infrared Pump-Probe Spectroscopy.
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The concept of hydrogen bonding has been intimately involved in the bonding and arrangement pattern of various molecular assemblies including complex protein structure. There are also indications that hydrogen bonds play an important role also in biological electron transfer across long distances. The vibrational dynamics of intramolecular hydrogen bonds pose experimental and theoretical challenges due to anharmonic couplings between the high frequency hydrogen bond stretching (νOH) mode and low frequency skeletal motions. This dissertation presents work done by using ultrafast Infrared pump-probe spectroscopy to study the vibrational dynamics of two intramolecular hydrogen bonded complexes, 10-Hydroxybenzo[h]quinolone (HBQ) and 2-(2′-Hydroxyphenyl)benzothiazole (HBT) dissolved in carbon tetrachloride. One of the biggest difficulty in studying the νOH stretching vibration is that they have complex and broad lineshapes in the infrared region (IR) that are difficult to decipher. To overcome this difficulty, we have developed a tunable broadband mid-infrared (BBIR) source with the spectral range of 2-8 microns. The broadband pulse enables us to perform novel experiments in multiple domains (both in infrared and electronic) to understand energy transfer dynamics and vibrational couplings of very broad peaks. The tunable BBIR source is generated by a filamentation process of 800nm and 400nm pulses in a gas cell. To generate compressed pulses for the experiment, correction of higher order dispersion effects is necessary. Hence a compressor was built with a deformable mirror for the manipulation of spectral phase to make transform limited pulse. An iterative genetic algorithm was written with a simple single valued feedback for the optimal mirror compression. Nearly transform limited pulses were measured with the following characterization techniques to extract electric field: cross correlation Frequency Resolved Optical Gating (XFROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER). The compressed broadband IR (BBIR) source, when used as part of a third order nonlinear infrared pump-probe spectroscopy, was able to give us information into the electronic ground state potential energy surface of the two model systems. .We pump the νOH and probe across 1800 cm-1 to 3300 cm-1 using the broadband IR pulse. The average lifetime of the OH mode for the HBQ and HBT systems was calculated to be 1.65 ps and 1.30 ps respectively. Both systems exhibit large anharmonicities (> 500 cm-1), which are characteristic of strongly hydrogen bonded systems. Additionally, we observe the coherent beating of low-frequency modes of 248 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 prove 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.
- Chemistry