Department of Chemistry & Biochemistry, University of Mississippi, University, MS 38677-1848, USA.
SETI Institute, Mountain View, CA 94043, USA.
Molecules. 2023 Feb 13;28(4):1782. doi: 10.3390/molecules28041782.
New high-level ab initio quartic force field (QFF) methods are explored which provide spectroscopic data for the electronically excited states of the carbon monoxide, water, and formaldehyde cations, sentinel species for expanded, recent cometary spectral analysis. QFFs based on equation-of-motion ionization potential (EOM-IP) with a complete basis set extrapolation and core correlation corrections provide assignment for the fundamental vibrational frequencies of the A˜2B1 and B˜2A1 states of the formaldehyde cation; only three of these frequencies have experimental assignment available. Rotational constants corresponding to these vibrational excitations are also provided for the first time for all electronically excited states of both of these molecules. EOM-IP-CCSDT/CcC computations support tentative re-assignment of the ν1 and ν3 frequencies of the B˜2B2 state of the water cation to approximately 2409.3 cm-1 and 1785.7 cm-1, respectively, due to significant disagreement between experimental assignment and all levels of theory computed herein, as well as work by previous authors. The EOM-IP-CCSDT/CcC QFF achieves agreement to within 12 cm-1 for the fundamental vibrational frequencies of the electronic ground state of the water cation compared to experimental values and to the high-level theoretical benchmarks for variationally-accessible states. Less costly EOM-IP based approaches are also explored using approximate triples coupled cluster methods, as well as electronically excited state QFFs based on EOM-CC3 and the previous (T)+EOM approach. The novel data, including vibrationally corrected rotational constants for all states studied herein, provided by these computations should be useful in clarifying comet evolution or other remote sensing applications in addition to fundamental spectroscopy.
探索了新的高级从头算 quartic 力场 (QFF) 方法，这些方法为一氧化碳、水和甲醛阳离子的电子激发态提供了光谱数据，这些阳离子是扩展的、最近的彗星光谱分析的哨兵物种。基于含完全基组外推和核心相关校正的运动方程电离势 (EOM-IP) 的 QFF 为甲醛阳离子的 A˜2B1 和 B˜2A1 态的基本振动频率提供了分配；只有其中三个频率具有实验分配。还首次为这两种分子的所有电子激发态提供了与这些振动激发相对应的转动常数。EOM-IP-CCSDT/CcC 计算支持对水阳离子 B˜2B2 态的 ν1 和 ν3 频率进行暂定重新分配，分别约为 2409.3 cm-1 和 1785.7 cm-1，这是由于实验分配与本文中计算的所有理论水平以及以前作者的工作之间存在重大分歧。与实验值和对变分可及态的高精度理论基准相比，EOM-IP-CCSDT/CcC QFF 对水阳离子电子基态的基本振动频率的一致性达到了 12 cm-1 以内。还探索了使用近似三重态耦合簇方法以及基于 EOM-CC3 和之前的 (T)+EOM 方法的电子激发态 QFF 的更具成本效益的 EOM-IP 方法。这些计算提供的新数据，包括本文研究的所有态的振动校正的转动常数，除了基本光谱学之外，还应该有助于澄清彗星演化或其他遥感应用。
