Molecular Orbital Theory (MOT)

Molecular Orbital Theory (MOT)

• MOT involves drawing molecular orbital diagrams and understanding the theoretical postulates.
• MOT was developed by two scientists.
• Molecular orbitals are formed from atomic orbitals of similar energy and symmetry.
• Atomic orbitals combine through linear combination to form molecular orbitals.
• When atoms combine, the concept of electron wave nature is considered.
• Atoms A and B have wave functions denoted as ψA and ψB respectively.
• Molecular orbitals (MOs) are formed via additive and subtractive combinations of atomic orbitals.
• Additive combination leads to bonding molecular orbitals.
• Bonding orbitals are denoted by ψb = ψA + ψB.
• Subtractive combination gives antibonding molecular orbitals.
• Antibonding orbitals are represented as ψ* = ψA - ψB.
• The number of formed molecular orbitals always equals the number of combining atomic orbitals.
• Bonding molecular orbitals have lower energy than individual atomic orbitals.
• Electrons in bonding MOs stabilize the molecule.
• Antibonding MOs are higher in energy compared to atomic orbitals.
• Electrons in antibonding MOs destabilize the molecule.
• Atomic orbitals lose their identity upon forming molecular orbitals.
• Filling of electrons in MOs follows Aufbau principle, Pauli exclusion principle, and Hund's rule.
• Aufbau principle: Fill orbitals in increasing energy order.
• Pauli principle: Maximum of two electrons per orbital.
• Electrons must have opposite spins (+1/2 and -1/2).
• Hund's rule states that electrons fill each orbital half-filled before pairing occurs.
• Polynuclear: Molecules have multiple atomic nuclei.
• For N2 molecule with two N atoms, there are two different nuclei in the molecule, therefore called "poly".
• For molecules with more than 14 electrons, the filling order is: σ, π, π*, σ*.
• Molecules with unpaired electrons in MOs are paramagnetic.
• Molecules with all paired electrons are diamagnetic.
• Bond length is inversely proportional to bond order.
• Higher bond order means shorter bond length and vice versa.
• Bond order is directly proportional to stability and bond dissociation energy.
• Higher bond order equals greater stability and higher bond dissociation energy.
• Bond order = 1/2 (Number of bonding electrons - Number of antibonding electrons).
• Negative or zero bond order indicates an unstable, non-existent molecule.
• Positive bond order indicates a stable molecule.
• Bond order values of 1, 2, and 3 indicate single, double, and triple bonds, respectively.
• Atomic orbitals (AOs) are less stable than molecular orbitals (MOs).
• AOs have simple shapes, while MOs have complex shapes.
• AOs are represented as s, p, d, f, etc.
• MOs are represented as sigma (σ) and pi (π).
• Antibonding orbitals are denoted as σ* and π*.
• Bonding MOs result from additive overlapping.
• Antibonding MOs result from subtractive overlapping.
• Bonding MOs have symbol ψb = A+B (additive overlap).
• Antibonding MOs have symbol ψ* = A-B (subtractive overlap).
• Electrons in bonding MOs contribute to bond formation.
• Electrons in antibonding MOs do not contribute to bond formation.
• Bonding MOs have lower energy and thus higher stability.
• Antibonding MOs have higher energy and lower stability.
• Maximum electron density is between the nuclei in bonding MOs.
• Electron density is lower between the nuclei in antibonding MOs.

Molecular Orbital Diagrams

• Molecular orbital diagrams construction and interpretation for N2, O2, F2, HF, ClF, and Be2 molecules discussed in detail.
• To draw the MO diagram for N2, determine the electronic configuration and atomic number of N.
• N has an atomic number of 7, so electronic configuration is 1s² 2s² 2p³.
• Total number of electrons in N2 is 14 (7 from each N atom).
• Draw the diagram with increasing energy levels from bottom to top: 1s, 2s, 2p.
• 1s orbitals form sigma (σ) and sigma star (σ*) orbitals.
• Atomic orbitals combine to form molecular orbitals.
• Draw atomic orbitals for each atom on the sides and molecular orbitals in the middle.
• Energy levels are equal for both atoms in homonuclear molecules.
• For N2 below 14 electrons, the filling order is: π, σ, π*, σ*.
• The configuration can be written as (σ1s)² (σ1s)² (σ2s)² (σ2s)² (π2px)² (π2py)² (σ2pz)².
• Follow the 21 21 sequence pattern for energy levels.
• After drawing the diagram, calculate the bond order.
• Bond order = ½ (Number of bonding electrons - Number of antibonding electrons).
• For N2, bond order = ½ (10 - 4) = 3, indicating a triple bond.
• Determine the magnetic behavior based on unpaired electrons.
• If the final molecular orbital is filled with paired electrons, it is diamagnetic.
• If there are unpaired electrons, the molecule is paramagnetic.
• O2 molecule has 8+8 = 16 electrons (more than 14).
• It follows a 1 2 2 1 pattern for filling.
• Draw electronic configuration of O (1s² 2s² 2p⁴).
• O2 is paramagnetic.
• Total number of electrons in F2 is 18.
• F2 also follows a 1 2 2 1 sequence pattern for filling.
• HF and ClF diagrams are heteronuclear.
• HF: H (1s¹) and F (1s² 2s² 2p⁵).
• The 1s of H interacts with the 2p of F.
• Draw the diagram with appropriate energy levels on each side.
• Show bonding and non-bonding orbitals.
• H is an electron donor, whereas, F is more electronegative.
• ClF: Follow the same steps as HF, with Cl electronic configuration.
• Always remember that the more electronegative atom would be towards the lower side.
• Be2 needs only σ levels to be drawn.
• Always level diagrams accordingly.

Graphite

• Graphite is an allotrope of carbon.
• Each carbon atom in graphite is sp² hybridized.
• Carbon atoms join through covalent bonds to form hexagons.
• The fourth electron of each carbon atom is free to move within the layer.
• Hexagons join together to form a plane or sheet (graphene).
• Layers of graphite are attached by weak van der Waals forces.
• Graphite has a gray to black color and greasy feel.
• Graphite has high melting point due to strong covalent bonds.
• Graphite is slippery due to layers sliding over each other.
• Graphite is thermodynamically more stable than diamond.
• Two forms of graphite exist: alpha and beta.
• In alpha graphite, layers are arranged in the sequence ABAB.
• In beta graphite, layers are sequenced ABCABC.
• Graphite is a conductor due to free electrons.
• Applications of graphite: lubricant, pencil leads.

Fullerenes

• Fullerenes are another allotrope of carbon.
• Discovered in 1985.
• Each carbon atom is sp² hybridized.
• Forming three sigma bonds and having one unpaired electron.
• Consist of pentagons and hexagons.
• The most stable fullerene is C60, containing 12 pentagons and 20 hexagons.
• Carbon atoms combine to form close spherical structures.
• Carbon atoms do not touch each other.
• The bond length is 1.45 Å.
• Preparation of fullerenes:
  • Graphite rod kept in an inert atmosphere.
  • Electric current passed, causing evaporation.
  • Sublimation gives a mixture of fullerenes.
• Properties of fullerenes:
  • Mustard-colored solid and looks brown or black as thickness increases.
  • Basically a semiconductor, but on doping it becomes a conductor.
  • Shows poor aromatic nature.
  • The strongest known non-material to man.
  • Unlike graphite and diamond.

Nanomaterials

• Nanomaterials have at least one dimension in the nanometer scale (1 nm = 10⁻⁹ m).
• Types of nanomaterials based on dimension: 0D, 1D, 2D, 3D.
• Synthesis approaches: top-down and bottom-up.
• Top-down approach: breaking down bulk material into nanoparticles.
• Bottom-up approach: assembling atoms/molecules into nanostructures.
• Applications: various fields including electronics, medicine, and energy.

Carbon Nanotubes (CNTs)

• Discovered in 1991.
• There are single-walled (SWCNTs) and multi-walled (MWCNTs) types.
• SWCNTs have a one-dimensional structure and exist as armchair configurations.
• MWCNTs contain several nested carbon nanotubes and two diameters (inner, outer).
• Properties of carbon nanotubes:
  • High tensile strength (20 times stronger than steel).
  • Used in making bridges, aircraft materials.
  • Good conductors of heat and electricity.
• Applications of CNTs:
  • Bulletproof jackets.
  • Nano-scale electric motors.
  • Catalysts.
  • Wind mills.

Liquid Crystals

• Liquid crystals exhibit properties between conventional liquids and solid crystals.
• Liquid crystals flow like liquids, but molecules are oriented like crystals.
• When a solid is heated, there is a transition point.
• That is where the turbidity occurs, which is the liquid crystal.
• Melting point when it becomes a liquid.
• Liquid crystalline state is called the mesomorphic state (intermediate).
• Liquid crystals are also called mesophases.
• Key requirements for liquid crystals: positional order, orientational order.
• Positional order: extent to which molecules exhibit translational symmetry.
• Orientational order: tendency of molecules to align along a direction.
• Essential requirements for a molecule to be a liquid crystal:
  • Presence of carboxylic group at the end.
  • Anisotropic in nature (properties dependent on direction).
  • Functional groups should not be bulky.
  • Presence of unsaturation, flexible end chains.
• Characteristics for liquid crystals:
  • Strong dipole moment.
  • Molecules align parallel due to intermolecular forces (π-π interactions).
• Classification: thermotropic and lyotropic.
• Thermotropic: temperature-dependent, formed by temperature changes.
• Example: LCD screens, classified into nematic, smectic, and chiral phases.
• Thermatropic are classfied into: Nematic, smectic and Chiral.
  • Nematic- headlight like.
  • Smectic- Grease like.

Types of liquid crystals:

• Discusses types of liquid crystals: nematic, smectic A, chiral nematic, discotic.
• Nematic: Thread like, no positional order but orientational order, flows easily.
• Smectic A: High Viscosity, molecules in layers, do not flow as easily as Nematic.
  • Nematic: Has only orientational order, flows easily, no layers.
  • Nematic uses: LCD
  • Smectic: Molecular structure is defined as Layar.
  • Smectic: Touch screen mobile.
• Chiral nematic: molecules contain a chiral center, tilted arrangement in layers.
• Discotic is when molecules are disk type.
• Discotic liquid crystals contain column-like structures.
• Discotic structure is based on Columar face.
• Discotic molecule is also based on dependence.
• Discotic liquid: Solvent concentration and concentration is dependent.

Types of Liquid Crystals: Lyotropic

• Lyotropic liquid crystals are solvent-dependent, formed by adding solvents to solids.
• Solvent is added until solids reach critical concentration.
• Further addition of solvent gives isotropic solution.

The difference between thermatropic and lyotropic:

• Thermatropic is dependant on temperature.
• Lyotropic is dependant on concentration.

Applications of Liquid Crystals:

• Decorative cosmetics (nail polish, eyeshadow).
• Drug delivery systems (pharmaceuticals).
• Body care cosmetics.
• Photo-voltaic devices

Green Chemistry

• Goal: designing chemical products/processes to prevent pollution.
• Prevent waste, maximize atom economy, design safe chemicals.
• Use safer solvents reaction conditions, increase energy efficiency.
• Use renewable feed-stocks, avoid chemical derivatives.
• Use catalytic reagents, design for degradation.
• Real-time analysis for pollution prevention, safe design for accident prevention.
• Maximize atom economy: maximizing incorporation of starting materials into product.
• Minimize hazardous chemicals in synthesis.
• Reduce toxicity: The chemical being syntheszed can be harmful and toxic to humas, animas, and all sorts of life forms that can cause severe damage.
• Safer chemical design: chemicals are safe and do not explode.
• Avoid using solvents: Avoid using solvents as chemicals that reduce safety in production.
• Energy efficiency: chemical process do not have to performed under pressure and high atmospheric temperature.
• A Green stock is a stock made of renewable compounds.
• Catalysis should be used instead of Stoichometry.
• There cannot be any accidents along the product chain.
• Green Chemist defined: Green Chemistry and chemical engineering designs to produce the most least harm to environment.

Green Chemicals

• One that promotes a proper and functional output across an environment.

Important Definitions

• Green chemical that provides performance using environmentally benign materials throughout its life cycle.
• Green chemistry is the design of chemical products/processes that reduce or eliminate hazardous substances.

Green Synthesis

• Focuses on green and friendly processes in order to produce the least harm to the environm

Common Green Chemical - Analyszing Green Syntesis: Adipic acid

• Adipic acid is an important precursor to Nylon.
• Discussed: adipic acid synthesis (conventional vs. green routes), conventional method details.

Conventional Synthesis of analysis:

• This is made from benzene and it can have harmful side affects.
• Reduction of benzene, oxidation of cyclohexanone to adipic acid.
• Chemical is used to break down cyclohexanone oxidation to produce cyclohexanol.
• Reduction can create alcohol and oxidation can result in adipic acid.

Green Synthesis - Analysis

• Use the reduction of glucouse, resulting in reduction of enzyme.
• Has Hydroxyl.
• Use the reduction of glucouse, resulting in reduction of enzyme.
• Then you can start to oxidize it with hno3.

Common Green Method - Analysis: Paracetamol Synthesis

• Details paracetamol synthesis (conventional vs. green routes).
• Involves nitration of phenol, reduction to para-aminophenol.

Convectional Synthesis involves:

• Need to start off with Phenol.

Green synthesis:

• Involves hydroquinine.

Environmental Green Chemistry benefits

• Reducing harm by chemical and enviornmental risks overall.
• Minimize the potential for global warming.
• Reduce of the destruction of ecosystems.