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Related Concept Videos

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

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The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
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IR Frequency Region: Alkyne and Nitrile Stretching01:22

IR Frequency Region: Alkyne and Nitrile Stretching

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Both alkyne (C≡C) and nitrile (C≡N) functional groups contain triple bonds and show stretching absorptions around the wavenumber range of 2100 to 2300 cm−1 in the diagnostic region of the IR spectra.
Comparing the stretching vibrational frequency of  C≡C triple bonds with that of double and single bonds, it is evident that C≡C triple bonds exhibit a higher stretching frequency than C=C double and C–C single bonds. Similarly, the C≡N triple bond...
1.7K
Formal Charges02:42

Formal Charges

41.4K
In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to help predict the most appropriate Lewis structure when more than one reasonable structure exists.
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Acidity of 1-Alkynes02:42

Acidity of 1-Alkynes

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The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
11.5K
Conformations of Cycloalkanes02:29

Conformations of Cycloalkanes

16.7K
Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain — the strain caused by the deviation of bond angles from the ideal 109.5° tetrahedral value for sp3  hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer’s theory that...
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On-line Analysis of Nitrogen Containing Compounds in Complex Hydrocarbon Matrixes
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CHARMM Force Field Parameters for Nitroalkanes and Nitroarenes.

Jeffery B Klauda1, Bernard R Brooks1

  • 1Laboratory of Computation Biology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.

Journal of Chemical Theory and Computation
|December 2, 2015
PubMed
Summary

New CHARMM force field parameters (C27rn) accurately predict properties of nitroalkanes and nitrobenzene. Molecular dynamics simulations show improved conformational stability for nitroalkanes using these novel parameters.

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Area of Science:

  • Computational Chemistry
  • Molecular Dynamics Simulations
  • Physical Chemistry

Background:

  • Developing accurate molecular models is crucial for understanding chemical systems.
  • Existing force fields may not precisely capture the behavior of nitro compounds.

Purpose of the Study:

  • To develop and validate new CHARMM force field parameters (C27rn) for nitro compounds.
  • To improve the accuracy of molecular dynamics simulations for nitroalkanes and nitrobenzene.

Main Methods:

  • Quantum mechanical calculations to determine accurate conformational energies.
  • Adjustment of nonbonded and torsional terms in the CHARMM force field.
  • Molecular dynamics simulations to predict bulk and interfacial properties.

Main Results:

  • The C27rn force field accurately reproduces experimental densities, hydration energies, and conformational preferences.
  • Simulations show a stable gauche conformer for nitroalkanes, consistent with quantum calculations.
  • Excellent agreement was observed for bulk properties (density, compressibility, heat of vaporization) and surface tension.

Conclusions:

  • The C27rn force field provides a significant improvement for simulating nitro compounds.
  • This new parameter set enhances the predictive power of molecular dynamics for nitroalkane and nitrobenzene systems.