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Propagation of Uncertainty from Systematic Error01:10

Propagation of Uncertainty from Systematic Error

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The atomic mass of an element varies due to the relative ratio of its isotopes. A sample's relative proportion of oxygen isotopes influences its average atomic mass. For instance, if we were to measure the atomic mass of oxygen from a sample, the mass would be a weighted average of the isotopic masses of oxygen in that sample. Since a single sample is not likely to perfectly reflect the true atomic mass of oxygen for all the molecules of oxygen on Earth, the mass we obtain from this...
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Propagation of Uncertainty from Random Error00:59

Propagation of Uncertainty from Random Error

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An experiment often consists of more than a single step. In this case, measurements at each step give rise to uncertainty. Because the measurements occur in successive steps, the uncertainty in one step necessarily contributes to that in the subsequent step. As we perform statistical analysis on these types of experiments, we must learn to account for the propagation of uncertainty from one step to the next. The propagation of uncertainty depends on the type of arithmetic operation performed on...
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Uncertainty: Overview00:59

Uncertainty: Overview

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In analytical chemistry, we often perform repetitive measurements to detect and minimize inaccuracies caused by both determinate and indeterminate errors. Despite the cares we take, the presence of random errors means that repeated measurements almost never have exactly the same magnitude. The collective difference between these measurements - observed values - and the estimated or expected value is called uncertainty. Uncertainty is conventionally written after the estimated or expected value.
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Standard Entropy Change for a Reaction03:00

Standard Entropy Change for a Reaction

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Entropy is a state function, so the standard entropy change for a chemical reaction (ΔS°rxn) can be calculated from the difference in standard entropy between the products and the reactants.
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Reaction Mechanisms03:06

Reaction Mechanisms

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Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
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MO Theory and Covalent Bonding02:40

MO Theory and Covalent Bonding

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The molecular orbital theory describes the distribution of electrons in molecules in a manner similar to the distribution of electrons in atomic orbitals. The region of space in which a valence electron in a molecule is likely to be found is called a molecular orbital. Mathematically, the linear combination of atomic orbitals (LCAO) generates molecular orbitals. Combinations of in-phase atomic orbital wave functions result in regions with a high probability of electron density, while...
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相关实验视频

Updated: Jul 3, 2025

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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基于分子动力学反应的二项不确定性分析分析.

Sofya P Tyukina1,2, Julia A Velmiskina1, Artem O Dmitrienko1,3

  • 1N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, 119991 Moscow, Russian Federation.

The journal of physical chemistry letters
|February 15, 2024
PubMed
概括

分子动力学模拟可以预测复杂反应的产物比率. 新的方法估计了这些预测的可靠性,揭示了许多过去的研究缺乏足够的模拟运行准确度.

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科学领域:

  • 计算化学的计算化学
  • 化学动力学 化学动力学

背景情况:

  • 基于分子动力学 (MD) 的反应分析对于偏离过渡状态理论 (TST) 的过程至关重要.
  • 两态反应的特点是过渡后的状态分叉,从单个过渡状态形成多个产物,违背了TST预测.
  • 目前的MD方法可以预测产品比率,但缺乏可靠性估计.

研究的目的:

  • 开发一种方法来估计基于MD的产品比率预测的不确定性.
  • 为了确定在双模反应中准确预测所需的MD运行次数.
  • 评估先前发表的关于双管道反应的MD研究的可靠性.

主要方法:

  • 使用MD模拟来采样反应路径并预测产品比率.
  • 开发了一种新的统计方法,类似于模拟卷,用于量化预测不确定性.
  • 对已发表的文献进行分析,以评估最近研究中使用的MD运行次数.

主要成果:

  • MD模拟运行显示的统计行为类似于随机机会事件 (例如,子滚动).
  • 建立了一种简单的方法来估计预测不确定性和所需的模拟运行.
  • 大多数最近的MD研究 (过去5年) 使用了不足的运行,导致预测产品比率的潜在错误>50%.

结论:

  • 开发的方法提供了一种可靠的方式来评估基于MD的产品比率预测的准确性.
  • 由于采样不足,许多先前对两种方式反应的研究可能具有不可靠的产品比率预测.
  • 未来的MD研究必须包含适当的抽样和不确定性量化,以获得可靠的结果.