Ouyang Bin, Howard Brian J
Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, UK OX1 3QZ.
Phys Chem Chem Phys. 2009 Jan 14;11(2):366-73. doi: 10.1039/b814562h. Epub 2008 Nov 5.
High-resolution microwave spectra of the monohydrate and dihydrate of acetic acid were recorded using a pulsed nozzle Fourier transform microwave spectrometer. The rotational and centrifugal distortion constants of these species were determined, which confirms the structures predicted by ab initio calculations that the H(2)O molecules bind to the carboxylic group to form hydrogen-bonded ring complexes. The dependence of the intensity of the rotational transitions on the power of the microwave pulses suggests that both hydrates have small a-and b-dipole moments of less than 0.3 Debye. All rotational transitions were split into two by internal rotation of the methyl group. Analysis of the splitting using both the PAM and the CAM methods allows the orientation and the height of the three-fold barrier to internal rotation (V(3)) of the methyl group to be determined accurately. A consistently declining trend of V(3) from the acid monomer [168.16 cm(-1), B. P. van Eijck, J. van Opheusden, M. M. M. van Schaik and E. van Zoeren, J. Mol. Spectrosc., 1981, 86, 465] through the monohydrate (138.396 cm(-1)) and the dihydrate (118.482 cm(-1)) was observed, which suggests that the amount of decrease of V(3) may be correlated with the strength of hydrogen bonding in these complexes.
使用脉冲喷嘴傅里叶变换微波光谱仪记录了乙酸一水合物和二水合物的高分辨率微波光谱。确定了这些物种的转动常数和离心畸变常数，这证实了从头算计算预测的结构，即H₂O分子与羧基结合形成氢键环络合物。转动跃迁强度对微波脉冲功率的依赖性表明，两种水合物的a-和b-偶极矩都很小，小于0.3德拜。所有转动跃迁都因甲基的内转而分裂为两个。使用PAM和CAM方法对分裂进行分析，可以准确确定甲基内转三重势垒（V₃）的取向和高度。观察到从酸单体[168.16 cm⁻¹，B.P.范·艾杰克、J.范·奥菲斯登、M.M.M.范·沙伊克和E.范·佐伦，《分子光谱学杂志》，1981年，86卷，465页]到一水合物（138.396 cm⁻¹）和二水合物（118.482 cm⁻¹），V₃呈现出持续下降的趋势，这表明V₃的下降量可能与这些络合物中氢键的强度相关。
