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

Abstract

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 C 2 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 φ min at the minima are compared with the angles φ 0 and φ e made by the O· · · H bond with the C 2 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 V HF (φ) of the HF molecule with φ. The angles φ min generated by this model are also compared with φ 0 and φ e . 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.

Nonbonding pairs in cyclic thioethers: Electrostatic modeling and ab initio calculations for complexes of 2,5‐dihydrothiophene, thietane, and thiirane with hydrogen fluoride

“…The positions of the minima in the V HF ( ϕ ) vs ϕ functions are in good agreement with the directions of maximum nucleophilicity exhibited in the MESP diagrams of the cyclic thioethers. In the simplest electrostatic model, the minima | ϕ min | in the V ( ϕ ) vs ϕ functions occur at smaller values, the reason for which has been discussed elsewhere . In the thioether⋯HF complexes, the ab initio calculations reveal that atoms S⋯HF deviate from collinearity by an angle θ in a direction that suggests a secondary interaction of the nucleophilic region of F with electrophilic H atoms attached to ring carbon atoms.…”

“…(The equatorial n ‐pair of thietane has been excluded from this discussion because of the distorting effect that the secondary interaction mentioned in Section 3.2 has on the potential curve when unmoderated by exchange repulsion). In the corresponding series in which the ring heteroatom is oxygen, namely 2,5‐dihydrofuran, oxetane and oxirane, and in which a similar decrease of ring angle obtains, there is evidence of a significant increase in the inter‐ n ‐pair angle as the ring angle decreases . Presumably, if the n ‐pairs on S are in two sp hybrids and the CS bonds involve two orthogonal p orbitals on S, there is only small scope for changing the hybridisation at S as the ring angle deceases.…”

“…Along this series, the internal ring angle COC decreases from 108° through ~90° to ~60°. The extended electric dipole moment model of HF suggested that the angle 2 ϕ between the n ‐pairs on O increases as the ring angle COC decreases.…”

“…[5][6][7][8][9][10][11] The relative orientations of hydrogen-bonded complexes in the gas-phase has been the subject of a number of systematic studies, both experimentally (by microwave and infrared spectroscopy) and theoretically, see Refs. [9,[12][13][14] for examples and further references. In particular, the use of hydrogen fluoride as a probe molecule has allowed for explorations of the nucleophilic regions of Lewis bases that act as hydrogen bond acceptors.…”

“…[13] The barrier height (2.9 kJ mol −1 ) for curve (C) in Figure 1 is also similar to that (1.5 AE 0.8 kJ mol −1 ) [18] determined spectroscopically. The same model has been applied recently [14] to the series of cyclic ethers 2,5-dihydrofuran, oxetane, and oxirane and their complexes with HF. Along this series, the internal ring angle COC decreases from 108 through~90 to~60 .…”