Microwave spectra of two carboxylic acid anhydrides: Acetic anhydride and acetic difluoroacetic anhydride

IF 1.4 4区 物理与天体物理 Q4 PHYSICS, ATOMIC, MOLECULAR & CHEMICAL Journal of Molecular Spectroscopy Pub Date : 2024-07-01 DOI:10.1016/j.jms.2024.111926
Nathan Love, Kenneth J. Koziol, Kaitlyn Belmont, Kenneth R. Leopold
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Abstract

Microwave spectra of acetic anhydride, D6-acetic anhydride, and acetic difluoroacetic anhydride have been observed in a supersonic jet. In conjunction with accompanying DFT and MP2 calculations, these systems are shown to adopt a nonplanar configuration in which the C=O groups point in approximately orthogonal directions. Methyl group internal rotation was fully analyzed for both species. The observed conformation of these systems appears to result from an interaction between a CH3 hydrogen (in acetic anhydride) or the CF2H hydrogen (in acetic difluoroacetic anhydride) with the carbonyl group to which it is not directly bound, forming a six-membered ring. The fitted rotational constants for both systems are in reasonably good agreement with calculated values, but for acetic anhydride, the agreement is somewhat worse than that previously observed for a series of syn anhydrides. The calculations indicate a pronounced flexing of the heavy atom frame as the CH3 group in the six-membered ring undergoes internal rotation, and this likely influences the level of agreement between the theoretical and vibrationally averaged experimental constants. The other CH3 group does not interact with a carbonyl oxygen because of its orientation in the molecule, and its internal rotation does not induce similar changes in the molecular frame. In the acetic difluoroacetic anhydride, it is the CF2H hydrogen that interacts with its remote carbonyl oxygen, leaving the internally rotating CH3 group unaffected by participation in a six-membered ring and giving rise to much smaller deviations in the rotational constants as it moves along its internal rotation coordinate. Correspondingly better agreement between experimental and theoretical spectroscopic constants is obtained.

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两种羧酸酐的微波光谱:醋酸酐和二氟乙酸酐
在超音速射流中观测到了乙酸酐、D6-乙酸酐和乙酸二氟酐的微波光谱。结合相应的 DFT 和 MP2 计算,这些系统被证明采用了非平面构型,其中 C=O 基团指向近似正交的方向。对这两种物质的甲基内旋进行了全面分析。观察到的这些体系的构象似乎是 CH3 氢(乙酸酐中)或 CF2H 氢(乙酸二氟乙酸酐中)与未直接结合的羰基相互作用的结果,形成了一个六元环。这两个体系的拟合旋转常数与计算值相当吻合,但醋酸酐的吻合程度比以前观察到的一系列合成酸酐的吻合程度要差一些。计算结果表明,当六元环中的 CH3 基团发生内旋转时,重原子框架会发生明显的弯曲,这可能会影响理论常数与振动平均实验常数之间的一致程度。另一个 CH3 基团因其在分子中的取向而不与羰基氧相互作用,其内旋也不会引起分子框架的类似变化。在乙酸二氟乙酸酐中,是 CF2H 氢与其远处的羰基氧相互作用,使得内部旋转的 CH3 基团不受参与六元环的影响,并且当它沿着内部旋转坐标移动时,旋转常数的偏差要小得多。实验和理论光谱常数之间的一致性也相应得到改善。
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来源期刊
CiteScore
2.70
自引率
21.40%
发文量
94
审稿时长
29 days
期刊介绍: The Journal of Molecular Spectroscopy presents experimental and theoretical articles on all subjects relevant to molecular spectroscopy and its modern applications. An international medium for the publication of some of the most significant research in the field, the Journal of Molecular Spectroscopy is an invaluable resource for astrophysicists, chemists, physicists, engineers, and others involved in molecular spectroscopy research and practice.
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