Intermolecular Forces and Dipole Moments

Fundamentals of Medicinal and Pharmaceutical Chemistry

• Presented by Professor Celine J. Marmion
• Date: 14th October 2024
• Course: FUNCHEM.8 Intermolecular interactions: The medical importance of water in the body

Nobel Prize in Chemistry 2024

• Discoveries recognized this year focus on protein construction and prediction.
• Heiner Linke, Chair of the Nobel Committee for Chemistry highlighted these discoveries.
• David Baker (2003): Designed a novel protein with unique structure.
• Demis Hassabis and John Jumper (2020): Developed AlphaFold2, an AI model predicting protein structures.
• AlphaFold2 used by over 2 million people from 190 countries.

Learning Outcomes

• Understand electronegativity and dipole formation in molecules
• Predict dipole moments of molecules, considering polarity and shape
• Characterize ion-dipole, dipole-dipole, dipole-induced dipole, and instantaneous dipole-induced dipole interactions
• Explain the role of hydrogen bonding in physical properties (melting/boiling points)
• Differentiate types of intermolecular forces based on strength
• Analyze medical, biochemical, and pharmacological implications of intermolecular forces.

Intermolecular Forces

• Attractive forces between molecules.
• Responsible for bulk properties of matter (melting & boiling point).
• Significantly weaker than intramolecular forces (hold atoms together)
• Example: 41 kJ needed to evaporate one mole of water at its boiling point compared to 930 kJ to break the O-H bonds.

Intermolecular Forces: State of Matter

• Boiling point/melting point relate to intermolecular force strength.
• The state of matter depends on intermolecular force strength.

Basis of Intermolecular Interactions

• Magnets demonstrate fundamental requirements.
• Attraction between oppositely charged regions.

Dipole

• Polar covalent bond: Unequal electron sharing.
• Molecules with both positive and negative poles are polarized.
• Electronegativity difference drives unequal electron sharing.
• Dipole moment is a measure of polarity, measured in Debye (D) units.

Dipole Moments

• Diatomic molecules with identical atoms have zero dipole moments.
• Diatomic molecules with different atoms have a non-zero dipole moment.
• Dipole moment of polyatomic molecules depend on bond polarity and molecular shape.

Shape and Polarity

• CO2, linear molecules, have equal and opposite dipole moments resulting in zero net dipole moment.
• Bent molecules like CO2 have non-zero dipole moment.

Molecule and Dipole Moments

• Specific values for H2O (1.94D), CO2 (0D), NH3 (1.16D), and CH4 (0D) are provided.

Different Types of Intermolecular Forces

• Lists dipole-dipole, ion-dipole, ion-induced dipole, dipole-induced dipole, instantaneous dipole-induced dipole, hydrogen bonds, and ion-ion forces.

Dipole-Dipole Forces

• Attractive forces between polar molecules.
• The larger the dipole moment, the stronger the force.
• Alignment of polar molecules in liquids is not rigid, but similar.

Ion-Dipole Forces

• Attractive forces between ions and polar molecules.
• Strength depends on ion charge/size and dipole moment/size of molecule.
• Hydration of ions is an example.

Ion-Dipole Forces: Variable Magnitude

• Mg²⁺ ions have a higher charge and smaller ionic radius than Na⁺, leading to stronger interaction with water molecules.

Ion-Dipole Forces

• Illustration of ion-dipole interactions in water, showing how water molecules surround and interact with ions.

Ion-Induced Dipole Forces

• An ion induces a dipole moment in a nonpolar molecule via its electric field.
• Polarizability—ease of distortion of electron cloud—influences the strength of this force.

Dipole-Induced Dipole Forces

• A polar molecule induces a dipole moment in a nonpolar molecule via its electric field.
• Strength depends on the polar molecule's dipole moment and polarizability of nonpolar molecule

Instantaneous Dipole-Induced Dipole Forces

• Instantaneous dipoles can induce dipoles in neighboring atoms or molecules.
• Strength is proportional to polarizability of the molecules involved; increases in strength with increasing molar mass.

Instantaneous Dipole-Induced Dipole Forces (Dispersion Forces)

• Strength increases with molar mass due to larger electron cloud.
• Significantly impact many substances, including nonpolar molecules.

Hydrogen Bonds

• Strong type of dipole-dipole interaction between a highly polar molecule with H bonded to highly electronegative atoms (N, O, or F), and the other electronegative atom.
• Hydrogen is the 'donor' atom, and the other highly electronegative atom is the 'acceptor'.
• Only H directly attached to N,O or F will form hydrogen bond.

Hydrogen Bonds

• H must be attached directly to N, O, or F.

Ion-Ion Forces

• Strong electrostatic forces between ions.
• Responsible for ionic lattice structures in compounds like NaCl and MgSO4.

Relative Strength of Intermolecular Forces

• Dispersion forces can vary greatly in strength.
• Can exceed the strength of dipole-dipole forces in some cases.

Properties of Water

• Water's properties arise from intermolecular interactions.
• Hydrogen bonding gives water a high boiling point (more kinetic energy required).

Heat Capacity of Water

• Water has unusually high heat capacity due to hydrogen bonding.
• Specific heat capacity (energy to raise 1g of substance by 1°C) is high compared to other liquids.
• Water's heat capacity influences the body's temperature regulation.

Medical and Biological Relevance of Intermolecular Forces: Sickle Cell Anemia

• Sickle cell anemia is a genetic disease where a single amino acid substitution in hemoglobin distorts red blood cells
• This distorts the haemoglobins structure distorts red blood cells.
• This abnormal shape is caused by the hydrophobic valine replacing the polar glutamic acid causing them to precipitate in low oxygen conditions ( low O2 in blood),

Sickle Cell Anemia Explained

• The difference between normal and sickle cell hemoglobin (HbA and HbS) lies in a single amino acid (Glutamic acid replaced by Valine).
• This mutation creates a hydrophobic region in HbS, leading to aggregation and precipitation.

Glutamic Acid versus Valine

• Valine, a nonpolar amino acid, replaces glutamic acid, a polar amino acid.
• This substitution creates a nonpolar region, promoting HbS aggregation.

Structure of DNA

• DNA's double helix structure is stabilized by extensive hydrogen bonding between base pairs.
• Details on base pairings (A-T, C-G) and their arrangement.

Drug Design

• Modern drugs target specific sites or receptors.
• Drug structures match target structures to maximize intermolecular interactions and binding

Conclusion

• Intermolecular forces are crucial to biological and chemical processes.
• For molecules, as for people, love is the greatest thing.