Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Chem Phys. 2011 Feb 7;134(5):054316. doi: 10.1063/1.3528688.
The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states. The transitions are fitted to within a few kilohertz of the observed frequencies using a molecule-fixed effective rotational Hamiltonian for the separate A and E vibrational species of the G(12) permutation-inversion symmetry group. Interpretation of the motion problem uses an internal-vibration and overall-rotation angular momentum coupling scheme and full sets of rotational and centrifugal distortion constants are determined. The tunneling frequencies of the proton-transfer motion are measured for the ground A and E methyl rotation states as 250.4442(12) and -136.1673(30) MHz, respectively. The slight deviation of the latter tunneling frequency from being one half of the former, as simple theory otherwise predicts, is due to different degrees of mixing in wavefunctions between the ground and excited states.
使用脉冲喷嘴傅里叶变换微波光谱仪，在 4 到 18GHz 之间记录了由双氢键甲酸和乙酸组成的杂种二聚体的旋转光谱。由于两个同时进行的隧穿运动，每个刚性分子的旋转跃迁分裂成四个，一个是两个酸分子之间的质子转移，另一个是乙酰部分甲基的扭转/旋转。我们给出了基态振动态 J=1 到 J=6 的全谱分配。使用 G(12)置换-反转对称群的 A 和 E 振动物种的分子固定有效转动哈密顿量，将跃迁拟合到观察到的频率的几千赫以内。运动问题的解释使用内部振动和总转动角动量耦合方案，并确定了完整的转动和离心畸变常数集。质子转移运动的隧穿频率分别为基态 A 和 E 甲基旋转态的 250.4442(12)和-136.1673(30)MHz。后一个隧穿频率偏离简单理论预测的前一个隧穿频率的一半，是由于基态和激发态之间波函数的混合程度不同。
