Gas-phase IR spectra of carbon dioxide and methane are nowadays well understood, as a consequence of their pivotal roles in atmospheric- and astrochemistry. However, once those molecules are trapped in noble gas matrices, their spectroscopic properties become difficult to conceptualize. Still, such spectra provide valuable insights into the vibrational structure. In this study, we combine new matrix-isolation infrared (MI-IR) spectra at 6 K in argon and neon with in vacuo anharmonic spectra computed by vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI). The aim is to separate anharmonicity from matrix effects in the mid-infrared spectra of ¹²C¹⁶O₂, ¹²CH₄, and ¹²CD₄. The accurate description of anharmonic potential energy surfaces including mode-coupling allows to reproduce gas-phase data with deviations of below 3 cm⁻¹. Consequently, the remaining difference between MI-IR and VSCF/VCI can be attributed to matrix effects. Frequency shifts and splitting patterns turn out to be unsystematic and dependent on the particular combination of analyte and noble gas. While in the case of neon matrices these effects are small, they are pronounced in xenon, krypton, and argon matrices. Our strategy allows us to suggest that methane rotates in neon matrices – in contrast to previous reports.

中文翻译：

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从基质分离的二氧化碳和甲烷的红外光谱中的基质效应分解非谐性和模式耦合。

由于二氧化碳和甲烷在大气化学和天体化学中的关键作用，二氧化碳和甲烷的气相红外光谱现在已经得到很好的理解。然而，一旦这些分子被困在惰性气体基质中，它们的光谱特性就变得难以概念化。尽管如此，这样的光谱还是提供了对振动结构的宝贵见解。在这项研究中，我们将氩气和氖气中 6 K 的新基质隔离红外 (MI-IR) 光谱与通过振动自洽场 (VSCF) 和振动构型相互作用 (VCI) 计算的真空非谐波光谱相结合。目的是将¹² C ¹⁶ O ₂ , ¹² CH ₄的中红外光谱中的非谐性与基质效应分开，和¹² CD ₄。包括模式耦合在内的非谐势能面的准确描述允许重现偏差低于 3 cm ^-1的气相数据。因此，MI-IR 和 VSCF/VCI 之间的剩余差异可归因于基质效应。频移和分裂模式被证明是不系统的，并且取决于分析物和惰性气体的特定组合。虽然在氖矩阵的情况下这些影响很小，但它们在氙、氪和氩矩阵中很明显。我们的策略使我们能够建议甲烷在氖矩阵中旋转 - 与之前的报告相反。