Deconstructing vibrational motions on the potential energy surfaces of hydrogen-bonded complexes

Abstract

Internal vibrations underlie transient structure formation, spectroscopy, and dynamics. However, at least two challenges exist when aiming to elucidate the contributions of vibrational motions on the potential energy surfaces. One is the acquisition of well-resolved experimental infrared spectra, and the other is the development of efficient theoretical methodologies that reliably predict band positions, relative intensities, and substructures. Here, we report size-specific infrared spectra of ammonia clusters to address these two challenges. Unprecedented agreement between experiment and state-of-the-art quantum simulations reveals that the vibrational spectra are mainly contributed by proton-donor ammonia. A striking Fermi resonance observed at approximately 3210 and 3250 cm−1 originates from the coupling of NH symmetric stretch fundamentals with overtones of free and hydrogen-bonded NH bending, respectively. These novel, intriguing findings contribute to a better understanding of vibrational motions in a large variety of hydrogen-bonded complexes with orders of magnitude improvements in spectral resolution, efficiency, and specificity.