使用固态NMR张量器对立体化学和分子构造的表征
J K Harper1, A E Mulgrew, J Y Li
1Department of Chemistry, University of Utah, Salt Lake City, 84112, USA.
Journal of the American Chemical Society
|October 5, 2001
概括
固态核磁共振 (NMR) 成功确定了自然产品地形的立体化学. 这种先进的NMR技术对于抗结晶的复杂分子特别有用.
科学领域:
- 有机化学 有机化学
- 固态核磁共振 (NMR) 光谱学 固态核磁共振 (NMR) 光谱学
- 计算化学的计算化学
背景情况:
- 确定原子的精确三维排列 (立体化学) 在自然产品化学中至关重要.
- 像X射线结晶学这样的传统方法需要结晶样本,但这些样本并不总是可以获得的.
- 固态NMR为非晶体化合物的结构阐明提供了潜在的替代方案.
研究的目的:
- 开发和验证固态NMR方法,以确定天然产品的相对立体化学.
- 用自然产品地形作为这种新型NMR方法的模型化合物.
- 用计算化学来确认分子构造和立体化学赋值.
主要方法:
- 固态 (13) C 核磁共振光谱被用来测量实验性化学转移张量主值.
- 进行了初始计算,以生成所有可能的地形立体同位素的理论张量值.
- 用实验和计算的NMR数据进行比较,以确定立体化学和构造性.
主要成果:
- 地形的相对立体化学被确定为2R*,3S,统计概率超过99.5%.
- 该方法成功验证了基团的分子构造,与X射线衍射数据一致.
- 在碳素2和3 (立体中心) 的NMR转移张量值足以进行完整的立体化学和构造性表征.
结论:
- 固态NMR提供了一种可靠的方法来确定自然产品的相对立体化学和分子构造.
- 这种技术对于难以结晶的化合物尤其有价值.
- 该方法在结构性赋值中提供了高准确性和统计信心.
相关概念视频
01:20NMR Spectrometers: Overview
NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field...
01:13¹H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
00:52¹H NMR of Conformationally Flexible Molecules: Temporal Resolution
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
01:12Two-Dimensional (2D) NMR: Overview
The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.
01:15NMR Spectroscopy: Chemical Shift Overview
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
For instance, the proton...
01:15¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.