Physical and Theoretical Chemistry Laboratory, Department of Chemistry, the University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Phys Chem A. 2010 Apr 1;114(12):4109-17. doi: 10.1021/jp9103849.
Although identical in formula, trans- and gauche-difluoroacetic acids behave as different molecules in rotational spectra due to their distinct sets of rotational constants. In this study, high-resolution microwave spectra of monohydrates, dihydrates, and trihydrates of both trans- and gauche-difluoroacetic acids were recorded using a Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants of these hydrates were obtained with high accuracy via fitting their microwave spectra. It was found that the subtle structural difference between the trans and gauche forms of the acid gives different tunneling motions in their monohydrates. An unusual mixing of wave functions of energetically nondegenerate conformations was observed in the monohydrate of gauche-difluoroacetic acid. Ab initio calculations using the second-order Møller-Plesset perturbation method were also performed to optimize structures and to predict magnitudes of dipole moments of these hydrates. Close agreement between experimental and theoretical rotational constants confirms the feature of hydrogen-bonded ring structures in which all incoming H(2)O molecules bind to the carboxylic group for all hydrates.
尽管反式和 gauche-二氟乙酸的分子式相同，但由于它们的旋转常数不同，它们在旋转光谱中表现为不同的分子。在这项研究中，使用傅里叶变换微波光谱仪记录了反式和 gauche-二氟乙酸的一水合物、二水合物和三水合物的高分辨率微波光谱。通过拟合它们的微波光谱，以高精度获得了这些水合物的旋转和离心畸变常数。研究发现，酸的反式和 gauche 形式之间的细微结构差异导致它们的一水合物中存在不同的隧道运动。在 gauche-二氟乙酸的一水合物中观察到非简并构象的波函数的不寻常混合。使用二级 Møller-Plesset 微扰方法进行的从头算计算也用于优化这些水合物的结构并预测偶极矩的大小。实验和理论旋转常数之间的紧密一致证实了氢键环结构的特征，其中所有进入的 H(2)O 分子都与羧酸基团结合形成所有水合物。
