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MO Theory and Covalent Bonding02:40

MO Theory and Covalent Bonding

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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Determination of the Gas-phase Acidities of Oligopeptides
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Gas-phase basicities of polyfunctional molecules. Part 1: Theory and methods.

Guy Bouchoux1

  • 1Laboratoire des Mécanismes Réactionnels, Département de Chimie, Ecole Polytechnique, 91120 Palaiseau, France. bouchoux@dcmr.polytechnique.fr

Mass Spectrometry Reviews
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Summary

This tutorial reviews methods for determining gas-phase basicities and proton affinities, highlighting challenges with polyfunctional molecules and the impact of hydrogen bonding on thermochemistry.

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Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Organic Chemistry

Background:

  • Gas-phase basicity, proton affinity, and protonation entropy are key thermochemical properties.
  • Understanding these properties is crucial for studying molecular interactions and reaction mechanisms.

Purpose of the Study:

  • To provide a tutorial on experimental and theoretical methods for determining gas-phase basicities, proton affinities, and protonation entropies.
  • To emphasize the particularities and limitations of these methods when applied to polyfunctional molecules.
  • To review and classify structural effects and hydrogen bonding on protonation thermochemistry.

Main Methods:

  • Experimental determination of gas-phase basicities, proton affinities, and protonation entropies.
  • Theoretical calculations to model protonation processes and their thermochemical consequences.
  • Analysis of structural effects, substituent effects, and hydrogen bonding.

Main Results:

  • Structural effects, nature of the basic site, and substituent effects significantly influence protonation thermochemistry.
  • Linear correlations exist between gas-phase basicities, ionization energies, and substituent constants.
  • Hydrogen bonding plays a critical role in proton transfer and protonation characteristics, especially in polyfunctional molecules.
  • Complex chemical reactions can occur within protonated species, including bond dissociations, catalysis, tautomerization, and cyclization.

Conclusions:

  • Accurate determination of gas-phase basicities and related properties requires careful consideration of molecular structure and potential complicating factors.
  • Polyfunctional molecules present unique challenges and opportunities in the study of protonation.
  • The interplay of hydrogen bonding and intrinsic molecular properties dictates protonation behavior and subsequent reactivity.