Abstract
Rotational–vibrational spectroscopy of water in solid noble gas matrices has been studied for many decades. Despite that, discrepancies persist in the literature about the assignment of specific bands. We tackle the involved rotational–vibrational spectrum of the water isotopologues H216O, HD16O, and D216O with an unprecedented combination of experimental high-resolution matrix isolation infrared (MI-IR) spectroscopy and computational anharmonic vibrational spectroscopy by vibrational configuration interaction (VCI) on high-level ab initio potential energy surfaces. With VCI, the average deviation to gas-phase experiments is reduced from >100 to ≈1 cm–1 when compared to harmonic vibrational spectra. Discrepancies between MI-IR and VCI spectra are identified as matrix effects rather than missing anharmonicity in the theoretical approach. Matrix effects are small in Ne (≈1.5 cm–1) and a bit larger in Ar (≈10 cm–1). Controversial assignments in Ne MI-IR spectra are resolved, for example, concerning the ν3 triad in HDO. We identify new transitions, for example, the ν2 101 ← 110 transition in D2O and H2O or the ν3 000 ← 101 transition in D2O, and reassign bands, for example, the band at 3718.9 cm–1 that is newly assigned as the 110 ← 111 transition. The identification and solution of discrepancies for a well-studied benchmark system such as water prove the importance of an iterative and one-hand combination of theory and experiment in the field of high-resolution infrared spectroscopy of single molecules. As the computational costs involved in the VCI approach are reasonably low, such combined experimental/theoretical studies can be extended to molecules larger than triatomics.
Highlights
Water in the gas phase is of key importance in several fields, including engineering, meteorology, astrochemistry, and physical chemistry
Our results demonstrate that deviations of computed vibrational spectra to experimental spectra are minimized when anharmonicity and mode coupling are incorporated with a multimode potential energy surfaces (PES) representation and the time-independent nuclear Schrödinger equation is solved with the variational vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI) approach
From our matrix isolation infrared (MI-IR) experiments, we gain profound understanding of matrix-shifts where Ne shifts are rather small with a difference to the gas phase of only 1.5 cm−1 on average
Summary
Water in the gas phase is of key importance in several fields, including engineering, meteorology, astrochemistry, and physical chemistry. The spectroscopic complexity of water in the gas-phase is reflected in a comprehensive summary by the IUPAC.[6−9] In 1957, Pimentel et al isolated water in an inert N2 matrix at 20 K,10 providing the first matrix isolation infrared (MI-IR) spectrum. After this pioneering study, single molecule spectroscopy of water has seen a bloom[11,12,21,13−20] where MI has emerged as the standard for the study of the water monomer, the water dimer, and higher oligomers. The rotation of the water molecule in a cage of noble gas atoms and matrix
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