D. Dinu, M. Podewitz, H. Grothe, K. Liedl, T. Loerting:
"Toward Elimination of Discrepancies between Theory and Experiment: Anharmonic Rotational−Vibrational Spectrum of Water in Solid Noble Gas Matrices";
Journal of Physical Chemistry A,
Rotational-vibrational spectroscopy of water in solid noble gas matrices has been studied for many decades. In spite of that, discrepancies persist in 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 cm-1 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, e.g., concerning the ν3 triad in HDO. We identify new transitions, e.g., the ν2 101←110 transition in D2O and H2O or the ν3 000←101 transition in D2O, and reassign bands, e.g., 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, proves 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.
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