Abstract
We study the environmental effect on molecules embedded in noble-gas (Ng) matrices. The experimental data on HXeCl and HKrCl in Ng matrices is enriched. As a result, the H-Xe stretching bands of HXeCl are now known in four Ng matrices (Ne, Ar, Kr, and Xe), and HKrCl is now known in Ar and Kr matrices. The order of the H-Xe stretching frequencies of HXeCl in different matrices is ν(Ne) < ν(Xe) < ν(Kr) < ν(Ar), which is a non-monotonous function of the dielectric constant, in contrast to the "classical" order observed for HCl: ν(Xe) < ν(Kr) < ν(Ar) < ν(Ne). The order of the H-Kr stretching frequencies of HKrCl is consistently ν(Kr) < ν(Ar). These matrix effects are analyzed theoretically by using a number of quantum chemical methods. The calculations on these molecules (HCl, HXeCl, and HKrCl) embedded in single Ng(') layer cages lead to very satisfactory results with respect to the relative matrix shifts in the case of the MP4(SDQ) method whereas the B3LYP-D and MP2 methods fail to fully reproduce these experimental results. The obtained order of frequencies is discussed in terms of the size available for the Ng hydrides in the cages, probably leading to different stresses on the embedded molecule. Taking into account vibrational anharmonicity produces a good agreement of the MP4(SDQ) frequencies of HCl and HXeCl with the experimental values in different matrices. This work also highlights a number of open questions in the field.
Highlights
Matrix IR spectroscopy was invented as a method to investigate isolated species especially those with low energetic stability and high chemical reactivity as well as chemical and photochemical reactions.1–3 An inert matrix and low temperature make possible to measure vibrational spectra of isolated species over an extended period of time without essential perturbation from the environment
The experience of matrix-isolation research shows that the smallest shift of the vibrational bands occurs for species in a Ne matrix whereas the strongest shift is usually observed for Xe matrices
Tsuge et al have recently used the polarizable continuum model to calculate the properties of HXeBr and HXeCCH in Ng matrixes (Ng = Ne, Ar, Kr, and Xe), and the order of H−Xe stretching frequencies ν(Xe) > ν(Kr) > ν(Ar) > ν(Ne) has been obtained for both molecules, in disagreement with the experimental results
Summary
Matrix IR spectroscopy was invented as a method to investigate isolated species especially those with low energetic stability and high chemical reactivity as well as chemical and photochemical reactions. An inert matrix and low temperature make possible to measure vibrational spectra of isolated species over an extended period of time without essential perturbation from the environment. The calculations with the polarizable continuum model predict exactly this trend.12 This intuitive conclusion seemed to find support in experiments with HXeCl and HXeBr in Ne matrices where the H−Xe stretching frequencies were lower than. Liu et al applied the polarizable continuum model to different Ng molecules to simulate matrix-solvation effects.20 They obtained blue shifts of the H−Xe stretching mode of HXeCl and HXeBr as a result of solvation in a Xe matrix (+120 and +135 cm−1, respectively). Tsuge et al have recently used the polarizable continuum model to calculate the properties of HXeBr and HXeCCH in Ng matrixes (Ng = Ne, Ar, Kr, and Xe), and the order of H−Xe stretching frequencies ν(Xe) > ν(Kr) > ν(Ar) > ν(Ne) has been obtained for both molecules, in disagreement with the experimental results.. We perform simulations of these molecules as well as of HCl in single layer cages of various noble gases Ng optimized with the B3LYP-D, MP2, and MP4(SDQ) methods
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