Couplings Across the Vibrational Spectrum Caused by Strong Hydrogen Bonds: A Continuum 2D IR Study of the 7-Azaindole–Acetic Acid Heterodimer

Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. However, structures with medium and strong hydrogen bonds are difficult to study due to their characteristically broad vibrational bands and large anharmonicity. This is further complicated by strong interactions between the high-frequency hydrogen-bonded vibrational modes, fingerprint modes, and low-frequency intradimer modes that modulate the hydrogen-bonding. Understanding these structures and their associated dynamics requires studying much of the vibrational spectrum. Here, mid-IR continuum spectroscopy of the cyclic 7-azaindole–acetic acid (7AI–AcOH) heterodimer reveals the vibrational relaxation dynamics and couplings of this complex hydrogen-bonded system. Within this dimer, the NH bond of 7AI exhibits a band at 3250 cm–¹ caused by a medium strength hydrogen bond, while the strongly hydrogen-bonded OH modes of acetic acid exhibit a broad double-peaked vibrational feature spanning 1750 to 2750 cm–¹. Transient IR and 2D IR experiments were performed using three excitation frequencies, centered on the high-frequency OH and NH modes, and probed with a mid-IR continuum to measure the spectral response from 1000 to 3500 cm–¹. While the NH stretch is observed to relax in 300 fs, the strongly hydrogen-bonded OH modes relax within the time resolution of the experiment (sub-100 fs). The difference in the strength of the hydrogen bonds is also reflected in the coupling pattern in the fingerprint region observed with 2D IR spectroscopy. Here the NH is strongly coupled to fingerprint modes involving the 7AI monomer, while the OH vibrations are strongly coupled to vibrational modes across the entire dimer. Together, the results show strong coupling and rapid energy transfer across the hydrogen-bonded interface and through the structure of the 7-azaindole–acetic acid heterodimer, highlighting the need to study the full vibrational spectrum for strongly hydrogen-bonded systems.