Microhydration of Methylated Pyrazoles and Imidazoles: Robust Vibrational Resonances in Red-Shifted Water

Abstract Mono- and dihydrates of 11 N-methylated pyrazoles and imidazoles are studied as vacuum-isolated complexes at low temperature by linear infrared spectroscopy to reveal the interplay between the hydrogen-bonded water OH stretching fundamentals and anharmonic resonance partners. The water bending overtones, with and without additional stretching motion of a water unit against the diazole, are identified as the dominant resonance partners. Methyl substitution at the ring allows to tune the water OH stretch wavenumber through these resonances. A simple nested 2 × 2 model Hamiltonian, in combination with harmonic DFT predictions, provides an effective joint description of the anharmonic spectra of all 22 investigated hydrogen-bonded complexes, including resonance-induced intensity redistribution patterns. Future multidimensional anharmonic quantum calculations can test the extent to which the proposed empirical model is successful for the right reasons and the extent to which it may just mimic other vibrational effects. Model-based blind and thus unbiased predictions for 6 experimentally uncharacterized N-alkylated pyrazoles and imidazoles may allow quantification of the predictive power of the DFT-supported anharmonic model Hamiltonian in a future investigation.