NON COVALENT BONDS

VIPIN MOHAN2011-09-112College of AgricultureVellayani, TVm

COVALENT INTERACTIONS• Covalent interactions (bonds) provide the glue that holds biopolymers together.

Covalent bond energies are on the order of 100 kcal/mole.

NON COVALENT INTERACTIONS• A non-covalent interaction differs from a covalent bond in that it does

not involve the sharing of electrons, but rather involves more dispersed variations of electromagnetic interactions between molecules or within a molecule.

• The energy released in the formation of non-covalent interactions is typically on the order of 1-5 kcal/mol .

•  Non-covalent interactions can be generally classified into 4 categories: electrostatic, π-effects, van der Waals forces, and hydrophobic effects.

• In fact, van der Waals forces are responsible for why geckos can walk up and down walls!

• Non-covalent forces drive spontaneous folding of proteins and nucleic acids and mediate recognition of complementary molecular surfaces.• Noncovalent forces dictate conformation and interaction in

biological systems. • Non-covalent interactions are the dominant type of interaction between 

supermolecules in supermolecular chemistry.

• 1 Electrostatic Interactions•  Ionic•  H-bonding•  Halogen Bonding

• 2 Van der Waals Forces•  Dipole-Dipole•  Dipole-Induced Dipole•  London Dispersion Forces

• 3 π-effects•  π-π Interaction•  Cation-π & Anion-π•  Polar-π

• 4 Hydrophobic effect

IONIC• It involve the attraction of ions or molecules with full permanent charges

of opposite signs.• Ionic interactions occur between cations and anions.• These bonds are non-directional, and strength depends on the

distance of separation (r) according to 1/r2. Strength also depends on the medium (dielectric constant), and is less in polar than nonpolar solvents.

H-BONDING

• A hydrogen bond (H-bond), is a specific type of dipole-dipole interaction that involves the interaction between a partially-positive hydrogen atom and a highly electronegative atom .

• It is technically not a covalent bond, but instead electronegative, partially-negative oxygen, nitrogen, sulfur, or fluorine is classified as a very strong dipole-dipole (non-covalent) interaction.

• Most commonly, the strength of hydrogen bonds lies between 0 - 4 kcal/mol, but can sometimes be as strong as 40 kcal/mol

HALOGEN BONDING• Halogen bonding is a type of non-covalent interaction which does

not involve the formation nor breaking of actual bonds, but rather is similar to the dipole-dipole interaction known as hydrogen bonding.• In halogen bonding, a halogen atom acts as an electrophile, or

electron-seeking species, and forms a weak electrostatic interaction with a nucleophile, or electron-rich species. • The nucleophilic agent in these interactions tends to be highly

electronegative (such as oxygen, nitrogen, or sulfur), or may be anionic, bearing a negative formal charge. • As compared to hydrogen bonding, the halogen atom takes the

place of the partially-positively charged hydrogen as the electrophile.

VAN DER WAALS FORCES• Van der Waals Forces are a subset of electrostatic interactions involving

permanent or induced dipoles (or multipoles). These include the following:

• permanent dipole-dipole interactions, alternatively called the Keesom force

• dipole-induced dipole interactions, or the Debye force• induced dipole-induced dipole interactions, commonly referred to as 

London dispersion forces• Note: Although hydrogen bonding and halogen bonding are both forms of

dipole-dipole interactions, these are typically not classified as Van der Waals Forces by convention.

Dipole-Dipole

Dipole-dipole interactions are electrostatic interactions between permanent dipoles in molecules. These interactions tend to align the molecules to increase attraction (reducing potential energy). Normally, dipoles are associated with electronegative atoms, including (but not limited to) oxygen, nitrogen, sulfur, and fluorine.

DIPOLE-INDUCED DIPOLE

• A dipole-induced dipole interaction (Debye force) is due to the approach of a molecule with a permanent dipole to another non-polar molecule with no permanent dipole.

• This approach causes the electrons of the non-polar molecule to be polarized toward or away from the dipole (or "induce" a dipole) of the approaching molecule.

•  

LONDON DISPERSION FORCES

• London dispersion forces are the weakest type of non-covalent interaction. • They are also known as "induced dipole-induced dipole interactions",

and form from molecules that inherently do not have permanent dipoles.• They are caused by the temporary repulsion of electrons away from

the electrons of a neighboring molecule, leading to a partially-positive dipole on one molecule and a partially-negative dipole on another molecule.

• Hexane is a good example of a molecule with no polarity or highly electronegative atoms.

π-EFFECTS

• π-effects can be broken down into numerous categories, including ,  π-π interactions, cation-π & anion-π interactions, and polar-π interactions.• In general, π-effects are associated with the interactions of molecules

with the π-systems of conjugated molecules such as benzene.

π-π INTERACTION• π-π interactions are associated with the interaction between the π-orbitals

of a molecular system.•  For a simple example, a benzene ring, with its fully conjugated π cloud,

will interact in two major ways and one minor way’ with a neighboring benzene ring through a π-π interaction.

• The two major ways that benzene stacks are edge-to-face, with an enthalpy of ~2 kcal/mol, and displaced (or slip stacked), with an enthalpy of ~2.3 kcal/mol. Interestingly, the sandwich configuration is not nearly as stable of an interaction as the previously two mentioned due to high electrostatic repulsion of the electrons in the π orbitals.

CATION π&ANION π

• Cation-π interactions involve the positive charge of a cation interacting with the electrons in a π-system of a molecule. • This interaction is surprisingly strong (as strong or stronger

than H-bonding in some contexts),  and has many potential applications in chemical sensors.•  For example, the sodium ion can easily sit atop the π cloud of

a benzene molecule, with C6 symmetry (for more on point groups and molecular symmetry.

•Anion-π interactions are very similar to cation-π interactions, but reversed.• In this case, an anion sits atop an electron-poor π-system,

usually established by the placement of electron-withdrawing substituents on the conjugated molecule.

POLAR-π• Polar-π interactions involve molecules with permanent dipoles (such as

water) interacting with the quadrupole moment of a π-system (such as that in benzene .• While not as strong as a cation-π interaction, these interactions can be

quite strong (~1-2 kcal/mol), and are commonly involved in protein folding and crystallinity of solids containing both hydrogen bonding and π-systems.•  In fact, any molecule with a hydrogen bond donor (hydrogen bound to a

highly electronegative atom) will have favorable electrostatic interactions with the electron-rich π-system of a conjugated molecule.

HYDROPHOBIC EFFECT

• The hydrophobic effect is the desire for non-polar molecules to aggregate in aqueous solutions in order to separate from water. • This phenomenon leads to minimum exposed surface

area of non-polar molecules to the polar water molecules (typically spherical droplets), and is commonly used in biochemistry to study protein folding and other various biological phenomenon. • olive oil in water

Thank you