Electrostatic Potential and a Simple Extended Electric Dipole Model of Hydrogen Fluoride as Probes of Non-Bonding Electron Pairs in the Cyclic Ethers 2,5-Dihydrofuran, Oxetane and Oxirane

J Hill,Anthony Legon

doi:10.3390/cryst7090261

Abstract

The electrostatic potential near to the oxygen atom in each of the cyclic ethers 2,5-dihydrofuran, oxetane and oxirane has been calculated by using a distributed multipole analysis (DMA) of each molecule. The electrostatic potential energy V(φ) of a unit non-perturbing positive charge was calculated (via the DMA of the cyclic ether molecule) as a function of the angle φ between the C2 axis of the cyclic ether and a vector of length r from the O atom to the unit charge. The resulting potential energy functions each has two equivalent minima. The angles φmin at the minima are compared with the angles φ0 and φe made by the O⋯H bond with the C2 axes in the cyclic ether⋯HF complexes, as determined by rotational spectroscopy and ab initio calculations at the CCSD(T)-F12c/cc-pVTZ-F12 level of theory, respectively. An electrostatic model of cyclic ether⋯HF complexes in which the DMA of the cyclic ether interacts with a simple extended electric dipole representation of HF is also used to calculate the variation of the potential energy VHF(φ) of the HF molecule with φ. The angles φmin generated by this model are also compared with φ0 and φe. The extent to which the electrostatic potential and the extended electric dipole HF model can be used as probes for the directions of non-bonding electron pairs carried by O in these cyclic ethers is discussed.

Highlights

The hydrogen bond and the halogen bond are the best-known of non-covalent interactions and have recently been defined by IUPAC Working Parties [1,2]
A simple model can be applied to account for the bond, namely that an electrophilic region of the molecule acting as a Lewis acid A interacts with a nucleophilic region of a molecule acting as a Lewis base B
The electrostatic potential energy V(φ) of a non-perturbing unit positive charge has been calculated as a function of the angle φ at various fixed distances r from the O atom in the three cyclic ethers

Summary

Introduction

The hydrogen bond and the halogen bond are the best-known of non-covalent interactions and have recently been defined by IUPAC Working Parties [1,2]. A simple model can be applied to account for the bond, namely that an electrophilic region of the molecule acting as a Lewis acid A (e.g., the hydrogen atom participating in a hydrogen bond or a halogen atom in a halogen bond) interacts with a nucleophilic region of a molecule acting as a Lewis base B (e.g., a non-bonding electron pair or a π-bonding electron pair). We shall focus attention on the nucleophilic region of the Lewis base B that accepts the hydrogen atom on hydrogen bond formation, the non-bonding electron pairs (n-pairs) of B. The systematic variation of B led to a set of Crystals 2017, 7, 261; doi:10.3390/cryst7090261 www.mdpi.com/journal/crystals simple rules [6,7] for predicting the angular geometry (i.e., the relative orientation of B and HX) of a

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