Regularities and\nAnomalies in Neon Matrix Shifts of\nHydrogen-Bonded O–H Stretching Fundamentals

O–H bond stretching vibrations in hydrogen-bonded\ncomplexes\nembedded into cryogenic neon matrices are subtly downshifted from\ncold gas phase reference wavenumbers. To the extent that this shift\nis systematic, it enables neon matrices as more universally applicable\nspectroscopic benchmark environments for quantum chemical predictions.\nOutliers are indicative of either an assignment problem in one of\nthe two cryogenic experiments or they reveal interesting dynamics\nor structural effects on the complexes as a function of the environment.\nWe compile 6 literature-known pairs of experimental data in jet and\nneon matrix expansions and realize a 6-fold expansion of that number\nthrough targeted matrix isolation and/or slit jet expansion spectroscopy\npresented in this work. In many cases, the neon matrix shift is less\nthan the uncertainty of the currently best-performing blind quantum\nchemical predictions for the gas phase, but in specific cases, it\nmay exceed the currently achievable theoretical accuracy. Some evidence\nfor a positive correlation of the matrix shift with the hydrogen bond\nshift is found, similar to observations for helium nanodroplets. Outliers\nin particular for water acting as a donor are discussed, and in a\nfew cases they call for a future reinvestigation. Substantial improvement\nin the correlation of the matrix shift with the hydrogen bond shift\nis achieved for ketone monohydrates by removing a vibrational resonance.\nNew insights into nitrile hydration isomerism are obtained, and the\nlinear OH stretching spectrum of the jet-cooled ammonia–water\ncomplex is presented for the first time. Vibrational spectroscopy\nin weakly perturbing solid rare gas quantum matrices for the benchmarking\nof gas phase theory and future explicit theoretical treatments of\nthe quantum matrix environment to better understand the outliers are\nboth encouraged.