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

低压（0.01 - 0.5 atm）气相高分辨NMR方法是研究未扰动态（即不含溶剂和分子间相互作用的状态）结构的有用工具。本研究中，我们将我们的新方法推广到~ 1H NMR和~ 1F <19>~ 1C <13>NMR，并在结构问题上作了一些应用：1）我们成功地发展了~ 1F核NMR的新观测方法<19>，并能获得很高分辨率的NMR谱; 2）将变温~ 1H和~ <19>1F气相NMR方法应用于1，2-二氟乙烷的“gauche效应”研究，我们确定了在未扰动状态下的偏位和反式构象之间的焓差和熵差。由此测定的吉布斯自由能差DELTAG（G-T）为-0.81 kcal/mol，这与来自MP3/6-311++G^** 从头计算的值-0.85 kcal/mol非常一致。3）以甲醇和二甲醚为例，<13>采用10 mm的样品管，在270 MHz下成功地获得了高分辨的~ 1C低压气相NMR谱。对于一般的应用，观测方法还需要进一步改进，但它可以成为非微扰态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.