![]() This seems counterintuitive, yet the charge distribution in naked NO 3 − is anisotropic and reminiscent of a π-hole ( Fig. ![]() We wondered to what extend this rationale applies to nitrate anions and if perhaps this anion might function as a π-hole to enable so-called (pseudo)anti-electrostatic interactions 13, 14. The magnitude of such π-holes can be enhanced when the negative charge is diluted over a larger area, for example, if the O-atoms interact with water or NaCl ( Fig. ![]() 1a) can be directional in the solid state 8, 12. It is known that π-holes in nitro-compounds such as nitrobenzene ( Fig. In analogy, a π-hole can be seen as a region of electropositive potential on a molecule that is roughly located on an unpopulated π* antibonding orbital of a π bond, for example, on carbonyls or π-acidic aromatics like hexafluorobenzene 8, 9, 10, 11. Typical examples of such σ-holes can be found along the O–H/C–Br vectors in phenol or bromobenzene 8. A σ-hole can be seen as a region of electropositive potential on a molecule that is roughly located on the unpopulated σ* antibonding orbital of a covalent bond. Both have recently been contextualized as instances of ‘σ-hole bonding’ 5, 6, 7. Well-known intermolecular forces include hydrogen and halogen bonding. Molecular recognition phenomena are of pivotal significance in biology and define the field of supramolecular chemistry 1, 2, 3, 4.
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