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Related Concept Videos

IR Spectroscopy: Molecular Vibration Overview01:24

IR Spectroscopy: Molecular Vibration Overview

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When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
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IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

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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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Acid Strength and Molecular Structure03:05

Acid Strength and Molecular Structure

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Binary Acids and Bases
In the absence of any leveling effect, the acid strength of binary compounds of hydrogen with nonmetals (A) increases as the H-A bond strength decreases down a group in the periodic table. For group 17, the order of increasing acidity is HF < HCl < HBr < HI. Likewise, for group 16, the order of increasing acid strength is H2O < H2S < H2Se < H2Te. Across a row in the periodic table, the acid strength of binary hydrogen compounds increases with increasing...
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Molecular Structure and Acidity02:34

Molecular Structure and Acidity

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An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the anion is unstable, then the acid is weaker. Hence, to determine the acidity of the compound, the stability of its conjugate base is studied using various factors.
The size effect explains the change in atomic size on acidity. When comparing the acids formed from elements that belong to the same column in the periodic table, their atomic sizes...
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Molecular Spectroscopy: Absorption and Emission01:14

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Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels.  Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.
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2D NMR: Homonuclear Correlation Spectroscopy (COSY)01:06

2D NMR: Homonuclear Correlation Spectroscopy (COSY)

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Homonuclear correlation spectroscopy, or COSY, is a 2-dimensional NMR technique that provides information about coupled protons. Typically, the geminal and vicinal coupling are observed. For example, consider the COSY spectrum of ethyl acetate, where its 1D proton NMR spectrum is plotted along the vertical and horizontal axes with their corresponding chemical shift scale. Three spots on the diagonal corresponding to the three peaks in the 1D proton spectrum are called diagonal peaks. The COSY...
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Correlation between molecular acidity (pKa) and vibrational spectroscopy.

Niraj Verma1, Yunwen Tao2, Bruna Luana Marcial3

  • 1Computational and Theoretical group (CATCO), Department of Chemistry, Southern Methodist University, 3215 Daniel Ave, Dallas, TX, 75275-0314, USA.

Journal of Molecular Modeling
|February 1, 2019
PubMed
Summary

Predicting molecular acidity (pKa) is challenging. This study introduces a novel method correlating pKa values with local vibrational frequencies, enabling accurate acidity predictions for diverse compounds.

Keywords:
Hammett equationLinear regressionLocal vibrational mode analysisVibrational spectroscopypKa value

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Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Spectroscopy

Background:

  • Molecular acidity, quantified by pKa, is a crucial physicochemical property.
  • Accurate prediction of pKa values remains a significant challenge in computational chemistry.
  • Existing methods often struggle with diverse molecular structures and substituents.

Purpose of the Study:

  • To establish a direct correlation between pKa values and local vibrational frequencies.
  • To develop a predictive model for molecular acidity based on vibrational spectroscopy.
  • To link vibrational spectroscopy data with electronic structure effects influencing acidity.

Main Methods:

  • Investigated 180 molecules across 15 compound groups with varying substituents.
  • Developed a quadratic correlation model using two local vibrational frequencies as predictors.
  • Analyzed the relationship between vibrational modes and electronic structure features affecting pKa.

Main Results:

  • Achieved strong correlations between pKa and local vibrational frequencies.
  • Reported root mean squared errors < 0.11 and mean absolute errors < 0.09 pKa units.
  • Demonstrated the model's applicability to compounds where electronic substituent effects dominate pKa.

Conclusions:

  • A novel, generalizable correlation between pKa and local vibrational modes was established.
  • The model provides a powerful link between the Hammett equation and vibrational spectroscopy.
  • Enables rapid and accurate prediction of pKa values for new chemical entities.