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

低压（0.01 ~ 0.5 atm）气相高分辨率核磁共振方法是研究无扰动状态（即无溶剂和分子间相互作用的状态）结构的有用工具。他们的数据可以直接与量子化学和分子力学计算的预测进行比较，因为这些方法也可以计算孤立状态下的分子。在这项研究中，我们将我们的新方法扩展到^<19>F和^<13>C NMR，并在结构问题上进行了一些应用。1)成功开发了^<19>F核磁共振观测新方法，获得了非常高分辨率的核磁共振波谱。2)应用变温^1H和^<19>F气相核磁共振方法研究1,2-二氟乙烷中的“间扭效应”，确定了间扭构象和反扭构象在无扰动状态下的焓和熵差。由此确定的吉布斯自由能差DELTAG（G- t）为-0.81 kcal/mol，与MP3/6-311++G^<**>从头计算得到的-0.85 kcal/mol值很吻合。H和F原子之间的1-4 - through键相互作用在扭扭效应中起着最重要的作用。3)以甲醇和二甲醚为例，用10 mm样管在270 MHz下获得了分辨率较好的^<13>C低压气相核磁共振波谱。为了广泛应用，观察方法还有待进一步改进，但它可以成为研究无扰动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.