Origin of the Hadži ABC structure: An ab initio study

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Strong hydrogen bonds create broad vibrational spectral features, sometimes exceeding 1000 cm⁻¹.
• The Hadži ABC structure, a three-peaked feature in certain acid homodimers, has remained unexplained since the 1950s.

Purpose of the Study:

• To provide the first *ab initio* calculation explaining the origin of the Hadži ABC structure.
• To reproduce the complex vibrational spectrum of strongly hydrogen-bonded dimers from first principles.

Main Methods:

• Reduced dimensionality calculation incorporating four vibrational modes.
• Inclusion of an adiabatically separated dimer stretch mode to model hydrogen bond modulation.
• Calculation of vibrational spectra for both protonated and deuterated isotopologues.

Main Results:

• Successfully reproduced the three-peak structure and broadness of the Hadži ABC feature.
• Identified Fermi resonances of in-plane and out-of-plane bending modes as key contributors.
• Demonstrated the role of the dimer stretch mode in coupling the OH stretch with overtone bending modes, explaining spectral shifts.

Conclusions:

• The Hadži ABC structure arises from a complex interplay of vibrational modes, including Fermi resonances and dimer stretching.
• This study provides a first-principles explanation for a long-standing spectroscopic puzzle in hydrogen-bonded systems.
• Fermi resonances, previously underestimated, play a substantial role in shaping the observed vibrational spectra.