National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia.
Spectrochim Acta A Mol Biomol Spectrosc. 2021 May 15;253:119551. doi: 10.1016/j.saa.2021.119551. Epub 2021 Feb 5.
The infrared spectra of the long-chain perfluorocarboxylic acid monohydrates differ markedly from those of the anhydrous dimers. Consequently, the structure of the solid perfluorocarboxylic acid monohydrates must differ from any known dimer-containing carboxylic acid crystals. Consideration of the significant features of the infrared spectra of the long-chain perfluorocarboxylic acid monohydrates, supplemented by their Raman spectra, and comparison with the spectra of auxiliary substances have led us to conclude that the rather strong neutral carboxyl-hydroxyl to water bonding can best explain the observations. The infrared spectra indicate the presence of fairly short hydrogen bonds connecting the water molecules to the carbonyl groups. In the construction of the hydrogen bonding pattern of the perfluorocarboxylic acid monohydrates, the oxalic acid dihydrate plays the key role. The striking similarity between the infrared spectra of the oxalic acid dihydrates and the perfluorocarboxylic acid monohydrates in the regions characteristic of water and OH⋯O vibration suggests that the structure of the hydrated carboxyl groups is the same in both crystals. These regions are characterized by the sharp doublet at 3539 cm and 3464 cm, which is due to the HO ν and ν stretching vibrations, respectively, and the broad absorption between 3000 cm and 1500 cm with the intense band at 1970 cm, both associated with the vibration of the OH⋯O group. The later peak consists of two band components at near 1980 cm and 2020 cm. These band components show different behaviour when the temperature, polarization or deuteration is changed. In general, the infrared spectra of long-chain perfluorocarboxylic acids represent the system with very short hydrogen bonds connecting the water molecules to the carboxylates. This hydrogen bond pattern should be very similar to that found in the crystals of α-oxalic acid dihydrate.
长链全氟羧酸单水合物的红外光谱与无水二聚体明显不同。因此，固态全氟羧酸单水合物的结构必然与任何已知的含二聚体羧酸晶体不同。考虑到长链全氟羧酸单水合物的红外光谱的显著特征，辅以拉曼光谱，并与辅助物质的光谱进行比较，我们得出结论，相当强的中性羧基-羟基与水的键合可以最好地解释这些观察结果。红外光谱表明存在相当短的氢键，将水分子连接到羰基上。在全氟羧酸单水合物的氢键模式构建中，草酸二水合物起着关键作用。草酸二水合物和全氟羧酸单水合物的红外光谱在水和 OH⋯O 振动特征区域非常相似，这表明在这两种晶体中，水合羧基的结构是相同的。这些区域的特征是在 3539 cm 和 3464 cm 处的尖锐双峰，分别归因于 HO ν 和 ν 伸缩振动，以及在 3000 cm 和 1500 cm 之间的宽吸收，其中在 1970 cm 处存在强烈的带，两者都与 OH⋯O 基团的振动有关。后一个峰由近 1980 cm 和 2020 cm 处的两个带组成。这些带组件在温度、极化或氘化变化时表现出不同的行为。一般来说，长链全氟羧酸的红外光谱代表了通过非常短的氢键将水分子连接到羧酸盐上的系统。这种氢键模式应该与 α-草酸二水合物晶体中发现的非常相似。
