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Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin.

Huaiyu Zhang1,2, Wei Wu3, Basil M Ahmed2

  • 1The State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, Fujian Provincial Key Laboratory of Theoretical and Computational, Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, P. R. China.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|May 4, 2016
PubMed
Summary

The adjacent lone pair (ALP) effect in nitrogenous heterocycles is not due to lone pair repulsion. Reduced π conjugation and favorable electrostatic attraction in separated lone pair isomers explain the ALP effect.

Keywords:
adjacent lone pair effectconjugationelectrostatic interactionsenergy decompositionvalence bond theory

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Area of Science:

  • Theoretical Chemistry
  • Quantum Chemistry
  • Organic Chemistry

Background:

  • The adjacent lone pair (ALP) effect describes product preference for separated lone pairs over adjacent ones in nitrogenous heterocycles.
  • Understanding the ALP effect requires detailed analysis of electron pair interactions and intramolecular energies.

Purpose of the Study:

  • To theoretically elucidate the underlying causes of the adjacent lone pair (ALP) effect.
  • To investigate the roles of lone pair-lone pair interactions, π conjugation, and electrostatic forces in the ALP effect.

Main Methods:

  • Utilized the block-localized wavefunction (BLW) method within ab initio valence bond (VB) theory.
  • Derived strictly localized orbitals for lone pairs and bonds to analyze electron pair interactions.
  • Performed electrostatic potential analysis and intramolecular energy decomposition.

Main Results:

  • Direct calculation of two-electron integrals showed that lone pair-lone pair repulsion does not cause the ALP effect.
  • Disabling of π conjugation significantly reduced the ALP effect, particularly in deprotonated forms with adjacent σ lone pairs.
  • Favorable electrostatic attraction within isomers featuring separated lone pairs was identified as another key contributing factor.

Conclusions:

  • The adjacent lone pair (ALP) effect is primarily driven by the reduction of π conjugation and favorable electrostatic attractions, not lone pair-lone pair repulsion.
  • Theoretical analysis using BLW/VB methods provides a detailed understanding of electron pair interactions governing the ALP effect in heterocyclic systems.