Hydrogen bonding lights up overtones in pyrazoles.

Area of Science:

• Molecular Spectroscopy
• Quantum Chemistry
• Hydrogen Bonding

Background:

• The NH stretching mode in pyrazoles exhibits complex spectral features.
• Fermi resonance, a phenomenon where vibrational modes couple, is implicated in this complexity.

Purpose of the Study:

• To elucidate the origins of spectral complexity in the NH stretching mode of hydrogen-bonded pyrazoles.
• To investigate the role of Fermi resonance and hydrogen bonding in spectral characteristics.

Main Methods:

• A cost-efficient variational "monomers-in-clusters" model was developed.
• The model was applied to a five-dimensional subspace of pyrazole.
• Spectra of substituted pyrazoles were analyzed.

Main Results:

• Spectral complexity arises from Fermi resonance involving combinations and overtones of aromatic ring modes.
• Hydrogen bonding is crucial for bringing these ring modes into resonance with the NH stretching chromophore.
• The coupling is inherent in the pyrazole monomer but requires hydrogen bonding to become spectroscopically active.

Conclusions:

• The Fermi resonance system involving ring modes is robust in pyrazoles.
• Hydrogen bonding is a key factor in activating the NH stretching spectral complexity.
• The phenomenon is observed to be 'dark' in strained dimers but 'lights up' in linearly hydrogen-bonded trimers.