Abstract
11 aryl–lone pair and three aryl–anion π –hole interactions are investigated, along with the argon–benzene dimer and water dimer as reference compounds, utilizing the local vibrational mode theory, originally introduced by Konkoli and Cremer, to quantify the strength of the π –hole interaction in terms of a new local vibrational mode stretching force constant between the two engaged monomers, which can be conveniently used to compare different π –hole systems. Several factors have emerged which influence strength of the π –hole interactions, including aryl substituent effects, the chemical nature of atoms composing the aryl rings/ π –hole acceptors, and secondary bonding interactions between donors/acceptors. Substituent effects indirectly affect the π –hole interaction strength, where electronegative aryl-substituents moderately increase π –hole interaction strength. N-aryl members significantly increase π –hole interaction strength, and anion acceptors bind more strongly with the π –hole compared to charge neutral acceptors (lone–pair donors). Secondary bonding interactions between the acceptor and the atoms in the aryl ring can increase π –hole interaction strength, while hydrogen bonding between the π –hole acceptor/donor can significantly increase or decrease strength of the π –hole interaction depending on the directionality of hydrogen bond donation. Work is in progress expanding this research on aryl π –hole interactions to a large number of systems, including halides, CO, and OCH3− as acceptors, in order to derive a general design protocol for new members of this interesting class of compounds.
Highlights
The term ’π–hole interaction’ was coined by Murray and Politzer [1,2,3,4], and is described as a noncovalent interaction (NCI) between a region of positive electrostatic potential (ESP) located on a π–bond (i.e., a ’π–hole’) [5], and a lone–pair donor [6,7,8], anion [9,10], or other electron rich species [11,12]; where the π–hole is perpendicular to the molecular framework and electrons from the π–hole acceptor interact with an empty π ∗ orbital of the donor
Given the fact that the aforementioned parameters are not reliable descriptors of bond strength, our results provide a much needed perspective on the matter
In addition to quantification of π–hole interaction strength in terms of k a, this work confirms an interplay between three key factors which can influence bond strength and can be insightful for the design of materials with specific properties
Summary
The term ’π–hole interaction’ was coined by Murray and Politzer [1,2,3,4], and is described as a noncovalent interaction (NCI) between a region of positive electrostatic potential (ESP) located on a π–bond (i.e., a ’π–hole’) [5], and a lone–pair (lp) donor [6,7,8], anion [9,10], or other electron rich species [11,12]; where the π–hole is perpendicular to the molecular framework and electrons from the π–hole acceptor interact with an empty π ∗ orbital of the donor. Some classic examples of π–hole interactions involving aryl groups include the benzene/hexafluorobenzene–water complexes, where an oxygen–lp interacts favorably with the center of the aromatic ring [13,14,15,16,17,18,19,20] This special type of interaction has been identified in several important and highly relevant areas of modern chemical research, including drug targets [21,22], biological systems [23,24], and molecular crystals/solid state chemistry [25,26,27,28,29,30]. Crystals 2020, 10, 556 polarizable atoms, as these properties improve accessibility, size, and positive ESP of a π–hole [34,35,36,37].
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