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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Extraction: Partition and Distribution Coefficients01:14

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The distribution law or Nernst's distribution law is the law that governs the distribution of a solute between two immiscible solvents. This law, also known as the partition law, states that if a solute is added to the mixture of two immiscible solvents at a constant temperature, the solute is distributed between the two solvents in such a way that the ratio of solute concentrations in the solvents remains constant at equilibrium.
For extracting a solute from an aqueous phase into an...
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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

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A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...
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¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

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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.
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Molecular Orbital Theory II03:51

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基于大小一致的布里卢恩-维格纳分区的近似二阶合集群方法.

Linus Bjarne Dittmer1,2, Nikolay V Tkachenko1, Martin Head-Gordon1,3

  • 1Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, USA.

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概括

我们介绍BWS-CC2,一种新的计算化学方法,它改进了合集群方法 (CC2) 来进行分子性质预测. 这种变体提供了更好的准确性,特别是对于具有挑战性的分子,通过使用精细的分区方法.

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科学领域:

  • 计算化学是一种计算化学.
  • 量子化学是一种量子化学.
  • 方法开发方法的开发方法.

背景情况:

  • 结合集群方法 (CC2) 被广泛用于电子结构计算.
  • 对于不扰乱的哈密尔顿分区的选择会影响CC2的准确性.
  • 梅勒-普莱塞特 (MP) 分区是标准的,但替代方案可能会提供改进.

研究的目的:

  • 引入和评估CC2方法的一个新变体,称为BWS-CC2.
  • 为了比较BWS-CC2与传统CC2和MP2方法的性能.
  • 评估不同分区方案对计算的分子性质的影响.

主要方法:

  • 开发了BWs-CC2,使用两个参数大小一致的Billouin-Wigner (BW-s) 分区.
  • 在535个元素数据库上评估了BWS-CC2,CC2,MP2和BWS-s2的变体.
  • 利用了内部稳定的旋转极化哈特里-福克 (HF) 轨道和aug-cc-pVQZ基础集.

主要成果:

  • 通过单个参数选择,BWs-CC2的性能通常优于CC2和BW-s2.
  • 具有稳定的轨道的CC2在旋转污染系统中显著优于MP2.
  • BWs-CC2对热化学,非共价相互作用,屏障高度和异构化能量有很大的前景.

结论:

  • 该BWs-CC2变体提供了比传统的CC2有基础的改进.
  • 应该重新评估CC2地面状态质量的感知.
  • BWs-CC2表现出更高的准确性,特别是在具有挑战性的分子系统中.