BCHM-3050 Lecture Notes - Lecture 1: Non-Covalent Interactions, Molecular Symmetry, Covalent Bond
19 May 2018
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Noncovalent interactions
Fundamentally electrostatic in nature
Depend on forces that electrical charges exert
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Individual interactions b/w atoms and molecules
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Covalent v Noncovalent bonds
Covalent bonds: result from sharing electrons
Nonmetals
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Interactions between ATOMS
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Electronegativity plays role
Remember: Up and over to the right
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Polar covalent: unequal sharing of electrons
The electrons flow to one POLE of the molecule as opposed to
the other pole
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Polar covalent bond doesn’t equal polar molecule
Depends on molecular symmetry
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Nonpolar covalent: equal sharing of electrons
Electrons bounce between atoms
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Noncovalent bonds or intermolecular2ws bb bb forces: forces
between two or more molecules
Noncovalent interactions = important
Define structure and function of biomolecules
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Interactions between MOLECULES
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Intermolecular forces:
Hydrogen bonds
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Dipole-dipole
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Van der Waals (London dispersion)
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Covalent and noncovalent bond energies
Environment effects energies b/w atoms or molecules
Covalent bonds are tough to break v non covalent bonds
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Weakest are van der waals
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Type of interaction Approximate Energy (kj/mol)
Charge-Charge 13 to 17
Hydrogen bond 2 to 21
Van der Waals 0.4 to 0.8
Charge-Charge interactions: "ion-ion"
Charge-charge interactions hold the most energy
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Charge-Charge interactions (in general)
All interactions are going to be electrostatic in nature
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Salt bridge: simplest electrostatic interaction b/w charged particles
When ions involved are non metallic
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Coulomb's law
Attraction of oppositely charger ions
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F = force
Q = Charge
R= distance separating charges
Ke = coulomb's constant
Coulomb's law applies in vacuum
Does not happen in a cell
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Charge-Charge interactions (in cell)
A with dot on top = distance
Equivalent to 1/10^10 meters
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In cell charges are screened by medium that exists b/w charges
Ex. Water or other fluids
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Screening effect of medium is represented by E
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e= dielectric constant
E = energy of interaction or energy required to separate two charged particles
Higher the constant the lesser the attraction b/w charged species
Waters is 80. does that mean it has weak attraction b/w atoms?
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Acts like insulation
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Note: e is epsilon
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Water has high dielectric constant
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Dipoles
Dipole: asymmetric internal distribution of charge
Dipole moment: express the magnitude of a molecule's polarity
Stronger dipole = higher boiling point
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Induced dipole interactions: interactions of polarizable molecules
Ex. Anion or cation may induce dipole in polarizable molecule
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Both permanent and induced dipoles give rise to charge separation
One atom will have more negative charge than the other
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Van der Waals
Van der Waals interactions: when molecules w/o covalent bonds b/w
them come so close together that their electron orbitals overlap
Create mutual repulsion
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Repulsion increases as distance b/w centers decreases
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Van der waals radius: defined by the distance between the two
molecules
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Hydrogen bonding
Increase atomic radius (size) increases hydrogen bond distance
Note: longer bond length = stronger bond
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Prevalent in biomolecules and stabilizes structural elements
Specifically water
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Weaker than intramolecular forces b/w doesn’t involve sharing of
electrons
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Hydrogen bond donor: atom to which hydrogen is covalently bonded
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Hydrogen bond acceptor: atom with non bonded electron pair
Ex. Hydrogen
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Chapter 2: Bonds and Interactions
Tuesday, May 15, 2018
4:42 PM
Document Summary
The electrons flow to one pole of the molecule as opposed to the other pole. Noncovalent bonds or intermolecular2ws bb bb forces: forces between two or more molecules. Covalent bonds are tough to break v non covalent bonds. All interactions are going to be electrostatic in nature. Salt bridge: simplest electrostatic interaction b/w charged particles. In cell charges are screened by medium that exists b/w charges. Screening effect of medium is represented by e. E = energy of interaction or energy required to separate two charged particles. Higher the constant the lesser the attraction b/w charged species. Dipole moment: express the magnitude of a molecule"s polarity. Anion or cation may induce dipole in polarizable molecule. Both permanent and induced dipoles give rise to charge separation. One atom will have more negative charge than the other. Van der waals interactions: when molecules w/o covalent bonds b/w them come so close together that their electron orbitals overlap.
