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A hidden secret of chemistry: Why do certain lone pairs of electrons lead to molecular chirality?
In chemistry, a lone pair of electrons is a pair of valence electrons that is not shared with any other atom. These unshared electrons are sometimes called unshared pairs or non-bonding pairs, and they are usually located in the outermost electron shell of an atom. The presence of lone pairs of electrons is not just a consideration of the electron configuration, but also has a significant impact on the geometry and chemical properties of molecules, especially in terms of molecular chirality.
The presence of lone pairs of electrons can affect the geometric structure of molecules and lead to the formation of chiral molecules.
According to the valence shell electron pair repulsion theory (VSEPR), lone pairs of electrons show a significant negative polarity due to their high charge density, and they are usually closer to the nucleus than the electrons of the bonding pair. This repulsion, in turn, reduces the angle between the bond pairs. For example, the oxygen atoms of the water molecule have two lone pairs of electrons, which results in the H-O-H bond angle between the hydrogen atoms being about 104.5 degrees, which is lower than the ideal tetrahedral geometry of 109 degrees. The strong repulsion of the lone pair of electrons pushes the hydrogen atoms further away.
In addition to affecting the geometry, lone pairs of electrons can also contribute to the dipole moment of the molecule. For example, ammonia (NH3) forms a polar N-H bond because nitrogen has a higher electronegativity than hydrogen, and the lone pair of electrons further strengthens the effect of this dipole moment. In contrast, nitrogen fluoride (NF3) has a lower dipole moment due to the higher electronegativity of fluorine, which reflects the role of lone pairs of electrons in different structures.
Lone pairs of electrons can give molecules different polar characteristics, thereby affecting their chemical properties.
In some cases, lone pairs of electrons not only help form molecular chirality, but can also create new chemical structures. For example, when three different groups are attached to one atom, if that atom has a lone pair of electrons, that atom will form a chiral center. We see this phenomenon in amines, phosphanes, and many other compounds. However, due to the low inversion energy barrier of nitrogen, these chiral molecules often interconvert rapidly at room temperature, making them difficult to separate.
In addition, divalent ions of heavy metals such as lead and tin also exhibit stereochemical effects of lone pair electrons. The ns2 lone pair electrons of these heavy metals can affect their coordination structure, leading to asymmetric crystal shapes. Recent studies have found that the behavior of this lone pair of electrons may not be related to previous explanations of heavy metal hybridization, but is instead affected by the electronic state of the ligand.
The lone pair electrons of heavy metals can lead to distortion of the coordination structure, demonstrating their complex chemical properties.
In solution chemistry, the participation of lone pairs of electrons can also lead to the formation of acid-base reactions. When an acid is dissolved in water, the lone pair of electrons on the oxygen atom attracts ionized hydrogen (hydrogen ion) to form hydronium ion (H3O+). This process, and the interaction between lone pairs of electrons in molecules, clearly demonstrates the indispensable role of lone pairs of electrons in chemical reactions.
In introductory chemistry courses, the lone pair of electrons in the water molecule is often described as "rabbit ears", which somewhat visualizes the existence and impact of this pair. However, in more advanced chemical research, there are more complex explanations that analyze the behavior of these lone pairs through the symmetry of the molecules.
Furthermore, the properties of lone pair electrons not only affect the geometric shape of the compound, but are also closely related to interactions such as hydrogen bonds within the molecule. In water, hydrogen bonds form due to the high availability of these lone pairs of electrons. This phenomenon may also be one of the sources of water's unique physical and chemical properties.
When it comes to describing molecular structures, there is still debate in the chemical community about how to accurately describe molecules containing lone pairs of electrons. Could the intimate connection between the structure and properties of these molecules lead to new understandings?
