Molecular Orbital Theory (MOT)

Unit I: Atomic and Molecular Structure & Advanced Materials

• Molecular orbital theory was developed by R.S. Mulliken and F. Hund.
• Valence electrons are associated with all nuclei in a molecule according to Molecular Orbital Theory (MOT).
• Atomic orbitals of similar energy and symmetry combine to form molecular orbitals through the Linear Combination of Atomic Orbitals (LCAO).
  • If ΨA and ΨB are wave functions, then ΨM.O = ΨA ± ΨB
  • Bonding and anti-bonding molecular orbitals are formed.
• Bonding Molecular Orbitals (Ψb = ΨA + ΨB) result from the additive overlap of atomic wave functions.
• Anti-bonding molecular orbitals (Ψa = ΨA - ΨB) arise from the subtractive overlap.
• The number of molecular orbitals formed equals the number of atomic orbitals participating.
• Bonding molecular orbitals have lower energy than individual atomic orbitals, stabilizing the molecule when electrons are present.
• Anti-bonding molecular orbitals have higher energy, destabilizing the molecule when electrons are present.
• Non-bonding molecular orbitals do not participate in bonding and have the same energy as individual atomic orbitals.
• Molecular orbitals are polycentric.
• Atomic orbitals lose their identity upon molecular orbital formation.
• Electrons fill molecular orbitals following the Aufbau principle, Pauli’s exclusion principle, and Hund's rule.
• Filling order from H2 to N2: σ1s, σ1s, σ2s, σ2s, π2px = π2py, σ2pz, π2px= π2py, σ*2pz
• Filling order from O2 to Ne2: σ1s, σ1s, σ2s, σ2s, σ2pz, π2px = π2py, π2px= π2py, σ*2pz
• Paramagnetic molecules/ions have unpaired electrons, while diamagnetic ones have all electrons paired.
• Bond order indicates stability and strength; calculated as ½ (Nb - Na), where Nb is the number of electrons in bonding M.O and Na is the number of electrons in anti-bonding M.O.
  • Zero bond order means the molecule doesn't exist.
  • Positive bond order indicates a stable molecule.
  • Bond orders of +1, +2, +3 correspond to single, double, and triple bonds, respectively.
  • Bond order is directly proportional to bond stability and inversely proportional to bond length.
• Bonding molecular orbitals are formed by additive overlapping (Ψb = ΨA + ΨB), electrons contribute to bond formation, have low energy and high stability, are represented as σ, π, etc., and have higher electron density between nuclei.
• Anti-bonding molecular orbitals are formed by subtractive overlapping (Ψa = ΨA - ΨB), electrons don't contribute to bonding, have high energy and low stability, are represented as σ*, π*, etc., and have lower electron density.
• Homodiatomic molecules are composed of the same atoms (e.g., O2, H2).
• Dihelium (He2) does not exist as its bond order is zero.
• O2 is paramagnetic due to unpaired electrons; N2 is diamagnetic with all electrons paired.
• Bond orders and stability decrease in the order: O2 > O2- > O2-2; bond lengths increase in the reverse order.
• Hetero-diatomic molecules contain two different atoms.
• Molecular orbital diagrams for heteronuclear molecules are skewed toward the more electronegative atom.
• The more electronegative atomic orbital is closer to bonding M.O. MO Diagrams of NO
• NO is paramagnetic with a bond order of 2.5. MO Diagrams of CO
• CO is diamagnetic with a bond order of 3. σ(1s 2) < σ ∗(1s 2) < σ(2s 2) < π(2px) 2 =π(2py) 2 < σ(2pz) 2 < σ∗(2s 2) π ∗(2px) 0 π∗(2py) 0
• Linear combination to form σsp z molecular orbital. Molecular Properties
• It is a stable molecule.
• HF is diamagnetic in nature.
• Liquid Crystals (LCs) are in a mesomorphic state between liquid and solid.
• Liquid Crystals are rod or disc shaped.
• Liquid Crystals are anisotropic in nature.
• Each molecule is oriented at an angle to the director.
• Strong dipole moments are present in these molecules.
• Molecules align parallel to each other due to intermolecular attraction.
• A Liquid Crystal molecule consists of a rigid, aromatic mesogen and flexible, aliphatic ends.
• Essential requirements for a molecule to be a liquid crystal: rod or disc shape, anisotropy, rigidity in central region, flexible ends.
• Classification of liquid crystals includes thermotropic (temperature-dependent) and lyotropic (solvent-dependent) types; thermotropic further divided into calamatic (rod-shaped) and discotic (disc-shaped).
• Nematic LCs have orientational order but no positional order, flow like liquids, and can be aligned by electric or magnetic fields.
• Cholesteric LCs add a chiral twisting agent to nematic LCs, forming a helical structure with a pitch affected by temperature and pressure.
• Smectic LCs have both orientational and positional order, arranged in layers, with limited mobility and high viscosity.
• Discotic LCs are disc-shaped, forming nematic or columnar phases with electrical semi-conductivity along the stacking direction.
• Lyotropic LCs are solvent-dependent, consisting of amphiphiles in a solvent, forming lamellar, hexagonal, and cubic phases.
• Applications include cosmetics, pharmaceuticals, LCDs, drug delivery, thermometers, and detection of radiation.
• Industrially important materials based on liquid crystals are nonmaterial, polymer blends, reinforced materials, elastomers, flexible glass are follows.
• Liquid Crystal polymer (LCP’s) have excellent mechanical and electrical properties and outstanding chemical resistance.
• Liquid Crystal Elestomers (LCE’s) can be used as optical retarders due to their anisotropic structure because they can control the polymerization state of transmitted light.
• Liquid Crystal Glass are most widely used in digital devices industry.
• Nonmaterial enhanced liquid crystalline Material properties such as threshold voltage, response time, viscosity, dielectric anisotropy, refractive index which are important in the area of LC displays (LCD’s)
• Liquid Crystal as reinforced material provide the strength and stiffness to thermotropic polymer.
• Graphite is an allotrope of carbon with sp2 hybridized carbon atoms forming hexagonal sheets (graphene).
• Graphite layers are attached by weak van der Waals forces (3.41 Ǻ distance).
• Graphite planar, two dimensional structure.
• Graphite is greyish black, greasy, has a high melting point, is slippery, and is a good conductor of electricity.
• Applications includes lubricants, pencil leads, electrodes, crucibles, moderators in nuclear reactors, and conductive ink.
• Fullerenes (e.g., C60) are carbon allotropes with icosahedral symmetry.
• C60 structure includes 12 pentagons and 20 hexagons.
• Diameter of C60 molecule is 7A °.
• Fullerene crystal structure is FCC.
• Fullerenes are prepared by arc discharge method.
• Properties includes semiconductor behavior (can be conductor or super conductor when doped).
• Applications includes antioxidant in health & personal care areas, catalyst, drug delivery, superconductor preparation, optical devices, soft Ferro magnets, and batteries..
• Carbon Nanotubes (CNTs) are cylindrical tubes of rolled-up graphene sheets.
• Carbon nanotubes are stiff as diamond, and are light weight.
• Carbon nanotubes posses 1-D structure and is called Nanowire.
• Single-Walled Carbon Nanotubes (SW-CNTs) and Multi-Walled Carbon Nanotubes (MW-CNTs).
• CNTs are prepared by Electric arc discharge method or the catalytic deccomposition of Methane.
• CNTs are used to make Bullet Proof Jackets.
• CNTs can reduced the weight of aircraft and spacecraft upto 30%.
• Applications of CNTs include composite materials, bulletproof jackets, high-performance transistors, biosensors, and electrodes.
• Nanomaterials have at least one dimension in the nanometer scale (1-100 nm).
• Nanotechnology manipulates materials at the atomic or molecular scale.
• Properties depends on size, composition and structure.
• Nanomaterials can be zero, one, two or three dimensional.
• Nanomaterials synthesis includes Top down approach and Bottom up approach.
• Nanomaterials applications encompass electronics, medicine, engineering, industries, environment, and sensors.

Unit II: Spectroscopic techniques and Applications, Electrochemistry and Batteries, Chemistry of Engineering Materials

• Green chemistry designs processes and products to minimize or eliminate toxic substances.
• Green chemistry is not the same as environmental chemistry
• The 12 principles of green chemistry: preventing waste, maximizing atom economy, avoiding hazardous chemicals, designing safer chemicals, using safe auxiliaries and solvents, increasing energy efficiency, using renewable feedstock, reducing derivatives, using catalysis, designing for degradation, real-time analysis, and preventing accidents.
• Green chemicals provide higher performance with lower environmental impact throughout their life cycle.
• Conventional Synthesis of Paracetamol from Phenol involves three steps to produce p-aminophenol, then Nucleophillic addition reaction to create paracetamol.
• Green Synthesis of Paracetamol from Phenol undergoes acylation reaction to give 4-hydroxy acetophenone, reacts to give corres opnding ketoxime, that undegoes Beckmann rearrangemnt to produce substitute amides called paracetamol.
• Paracetamol is used as an antipyretic and weak analgesic.
• Advantages of Green Chemistry include less harm to plants/animals, lower potential for global warming, less ecosystem disruption, and reduced landfill use.
• Environmental impact of Green Chemistry: proactively prevents pollution, reduces harm to the environment, lowers global warming potential, and leads to cleaner air and water.
• The four R of Green Chemistry: Reduce, Reuse, Recycle, Redesign