Hydrogen bonding lights up overtones in pyrazoles.

The spectral complexity in the NH stretching mode of hydrogen-bonded pyrazoles is traced back to an extensive Fermi resonance system involving combinations and overtones of at least four aromatic ring modes with significant in plane hydride bending character. The couplings are shown to be inherent in the monomer, but hydrogen bonding is required to bring them into resonance with the NH stretching chromophore. A cost-efficient variational "monomers-in-clusters" model is presented and applied to a five-dimensional subspace of pyrazole. Spectra of substituted pyrazoles confirm the robustness of the coupling, which remains dark in strained dimers, but lights up in linearly hydrogen-bonded trimers.