title: Adjacent Lone Pair (ALP) Effect: A Computational Approach for Its Origin.
abstract: The adjacent lone pair (ALP) effect is an experimental phenomenon in certain nitrogenous heterocyclic systems exhibiting the preference of the products with lone pairs separated over other isomers with lone pairs adjacent. A theoretical elucidation of the ALP effect requires the decomposition of intramolecular energy terms and the isolation of lone pair-lone pair interactions. Here we used the block-localized wavefunction (BLW) method within the ab initio valence bond (VB) theory to derive the strictly localized orbitals which are used to accommodate one-atom centered lone pairs and two-atom centered σ or π bonds. As such, interactions among electron pairs can be directly derived. Two-electron integrals between adjacent lone pairs do not support the view that the lone pair-lone pair repulsion is responsible for the ALP effect. Instead, the disabling of π conjugation greatly diminishes the ALP effect, indicating that the reduction of π conjugation in deprotonated forms with two σ lone pairs adjacent is one of the major causes for the ALP effect. Further electrostatic potential analysis and intramolecular energy decomposition confirm that the other key factor is the favorable electrostatic attraction within the isomers with lone pairs separated.
authors.affiliation: The 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.|Department of Chemistry, Western Michigan University, Kalamazoo, MI, 49008, USA.
authors.forename: Huaiyu
authors.initials: H
authors.lastname: Zhang
authors.affiliation: The 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. weiwu@xmu.edu.cn.
authors.forename: Wei
authors.initials: W
authors.lastname: Wu
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, MI, 49008, USA.
authors.forename: Basil M
authors.initials: BM
authors.lastname: Ahmed
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, MI, 49008, USA.
authors.forename: Gellert
authors.initials: G
authors.lastname: Mezei
authors.affiliation: Department of Chemistry, Western Michigan University, Kalamazoo, MI, 49008, USA. ymo@wmich.edu.
authors.forename: Yirong
authors.identifier: 0000-0002-2994-7754
authors.initials: Y
authors.lastname: Mo
keywords: adjacent lone pair effect; conjugation; electrostatic interactions; energy decomposition; valence bond theory