Gas-phase High Resolution NMR and its Application to Structural Chemistry

低气压(0.01-0.5atm)气相高分辨核磁共振方法是研究非微扰态结构的有用工具，即无溶剂和分子间相互作用的无扰状态。在这项研究中，我们将我们的新方法推广到^&lt；19&gt；F和^&lt；13&gt；C核磁共振，并对结构问题进行了一些应用。1)我们成功地开发了^&lt；19&gt；F原子核核磁共振的新观测方法，并能够获得非常高分辨率的核磁共振谱。2)应用可变温度^1H和^&lt；19&gt；用气相核磁共振法研究了1，2-二氟乙烷的“高支效应”，测定了高支异构体和反式异构体在无扰状态下的焓和熵差。由此确定的Gibbs自由能差DELTAG(G-T)为-0.81kcal/mol，与MP3/6-311++G^&lt；**&gt；从头算得到的-0.85kcal/mol值符合得很好。3)以甲醇和二甲醚为例，用10 mm的样品管，在270 MHz频率下获得了分辨率较好的~(13)&gt；C低压气相核磁共振谱。对于一般的应用，观察方法还需要进一步改进，但它可以成为无扰状态下结构化学的有用工具。

Low pressure (0.01 - 0.5 atm) Gas-phase high-resolution NMR method is a useful tool for the structural study of unperturbed states, i.e. states free from solvent and intermolecular interactions. Their data can directly be compared with the predictions from quantum chemical and molecular mechanics calculations since these methods also calculate for a molecule in an isolated state.In this study, we extended our novel method for the ^1H NMR to ^<19>F and ^<13>C NMR, and made some applications to the structural problems.1) We have succeeded in developing new observation method for ^<19>F nucleus NMR, and was able to obtain NMR spectra of very high resolution.2) Applying the variable temperature ^1H and ^<19>F gas-phase NMR methods to the investigation of "gauche-effect" in 1,2-difluoroethane, we determined enthalpy and entropy differences between gauche and trans conformers in the unperturbed states. The Gibbs free energy difference thus determined, DELTAG(G-T), is -0.81 kcal/mol, which is in good agreement with the value -0.85 kcal/mol from MP3/6-311++G^<**> ab initio calculations. The 1-4 through bond interaction between H and F atoms was found to play the most important role in the gauche-effect.3)Taking methanol and dimethyl ether as examples, we succeeded in obtaining a well-resolved ^<13>C low pressure gas-phase NMR spectra at 270 MHz with 10 mm sample tube. Method of observation should further be improved for general applications, but it could become a useful tool for the structural chemistry in the unperturbed state.