Lovas FJ, Hartwig H
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899
J Mol Spectrosc. 1997 Sep;185(1):98-109. doi: 10.1006/jmsp.1997.7371.
The rotational spectrum of (CH3OH)2 has been observed in the 8 to 24 GHz region with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. Previously we demonstrated that each transition of the a-type R(J), Ka = 0 is split into 15 states of the 16 theoretically expected states by tunneling motions. Here we show that the K = 1 states are split into the 16 expected states through the assignment of the Ka = 1 a-type transitions and DeltaKa = 1 b-type transitions. The internal-rotation analysis of the two inequivalent methyl groups presented here was guided by the previous experimental observations and theory for multidimensional tunneling, which predicts 16 tunneling components for each R(J) transition from 25 distinct tunneling motions. The effective barrier to internal rotation for the donor methyl group of (CH3OH)2 is V3 = 183.0 cm-1, and is one-half of the value for the methanol monomer (370 cm-1), while the barrier to internal rotation of the acceptor methyl group is 120 cm-1, one-third of the methanol monomer. The structure of the methanol dimer complex is similar to that of water dimer with a hydrogen bond distance of 1.96(2) A and tilt of the acceptor methanol of 77(2)degrees from the O-H-O axis (one standard deviation uncertainty). This structure shows good agreement with the angular orientation of the methyl groups derived in the internal-rotation analysis. Copyright 1997 Academic Press. Copyright 1997Academic Press
已使用脉冲束法布里 - 珀罗腔傅里叶变换微波光谱仪在8至24 GHz区域观测到(CH3OH)2的转动光谱。此前我们证明，a型R(J)、Ka = 0的每个跃迁通过隧穿运动分裂为理论预期的16个态中的15个态。在此我们表明，通过对Ka = 1的a型跃迁和ΔKa = 1的b型跃迁进行归属，K = 1的态分裂为16个预期态。此处给出的两个不等价甲基的内旋转分析是以前期的实验观测和多维隧穿理论为指导的，该理论预测每个R(J)跃迁有16个隧穿分量来自25种不同的隧穿运动。(CH3OH)2供体甲基的内旋转有效势垒为V3 = 183.0 cm-1，是甲醇单体值（370 cm-1）的一半，而受体甲基的内旋转势垒为120 cm-1，是甲醇单体的三分之一。甲醇二聚体配合物的结构与水二聚体相似，氢键距离为1.96(2) Å，受体甲醇相对于O - H - O轴的倾斜度为77(2)度（一个标准偏差不确定度）。该结构与内旋转分析中得出的甲基的角取向显示出良好的一致性。版权所有1997年学术出版社。版权所有1997年学术出版社
