Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine.
J Chem Phys. 2010 Nov 14;133(18):184307. doi: 10.1063/1.3493336.
We present here the first experimental and theoretical study of the microwave spectrum of 5-methyltropolone, which can be visualized as a seven-membered "aromatic" carbon ring with a five-membered hydrogen-bonded cyclic structure at the top and a methyl group at the bottom. The molecule is known from earlier studies in the literature to exhibit two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is particularly interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the molecule, which then triggers a 60° internal rotation of the methyl group. Measurements were carried out by Fourier-transform microwave spectroscopy in the 8-24 GHz frequency range. Theoretical analysis was carried out using a tunneling-rotational Hamiltonian based on a G(12)(m) extended-group-theory formalism. Our global fit of 1015 transitions to 20 molecular parameters gave a root-mean-square deviation of 1.5 kHz. The tunneling splitting of the two J=0 levels arising from a hypothetical pure hydrogen-transfer motion is calculated to be 1310 MHz. The tunneling splitting of the two J=0 levels arising from a hypothetical pure methyl top internal-rotation motion is calculated to be 885 MHz. We have also carried out ab initio calculations, which support the structural parameters determined from our spectroscopic analysis and give estimates of the barriers to the two large-amplitude motions.
我们在此呈现了对 5-甲基色酮微波光谱的首次实验和理论研究，该分子可被视为一个具有七个成员的“芳香”碳环，顶部具有一个五元氢键环状结构，底部具有一个甲基基团。从文献中的早期研究可知，该分子存在两个大振幅运动，即分子内的氢转移和甲基扭转。前者的运动特别有趣，因为氢原子从羟基转移到羰基会诱导分子的互变异构，从而引发甲基的 60°内旋转。测量是在 8-24GHz 的微波频率范围内通过傅里叶变换微波光谱法进行的。理论分析是基于基于 G(12)(m)扩展群论形式的隧穿-转动哈密顿量进行的。我们对 1015 个跃迁进行了 20 个分子参数的全局拟合，得到了 1.5kHz 的均方根偏差。由假设的纯氢转移运动引起的两个 J=0 能级的隧穿分裂计算值为 1310MHz。由假设的纯甲基顶部内旋转运动引起的两个 J=0 能级的隧穿分裂计算值为 885MHz。我们还进行了从头算计算，这支持了我们从光谱分析中确定的结构参数，并对两个大振幅运动的势垒进行了估计。
