Presentation on theme: "Crystal Binding (Bonding) Continued"— Presentation transcript:
1 Crystal Binding (Bonding) Continued More on Van der Waals& Hydrogen Bonding
2 Van Der Waals BondingThis WEAK dipole interaction is the origin of the van der Waals bond, which is therefore characterized by a LOW cohesive energy, and so LOW melting temperaturesAnother type of van der Waals bonding is observed for certain POLAR molecules, that have a PERMANENT dipole momentSPONTANEOUS DIPOLE FORMATION IN ONE ATOMMAY INDUCE AN EQUAL AND OPPOSITE DIPOLE INANOTHER ATOM NEARBY SO CAUSING ATTRACTION THIS IS THE VAN DER WAALS BOND
3 A DIPOLE CONSISTS OF EQUAL AND OPPOSITE CHARGES SEPARATED BY Van Der Waals BondingAlthough IONIZED atoms are electrically NEUTRAL, electrons within them are in a CONSTANT state of motion, and so may MOMENTARILY form small charge DIPOLESThe direction and magnitude of this dipole CONSTANTLY fluctuates, but may INDUCE similarly fluctuating dipoles in other atomsDIPOLE FORMS+e-eA CHARGE DIPOLE MAY SPONTANEOUSLY FORM IN A NEUTRAL ATOM DUE TO THE MOTION OF ELECTRONS AROUND THE NUCLEUSA DIPOLE CONSISTS OF EQUAL ANDOPPOSITE CHARGES SEPARATED BYSOME DISTANCE
4 Van Der Waals Bonding typical values are n = 12, m = 6 Mathematically this potential energy variation can be APPROXIMATED as The CONSTANTS A and n are associated with REPULSIVE forces. The constants B and m are associated with ATTRACTIVE forces. At EQUILIBRIUM these forces BALANCE and we may write ro and E(ro) are the equilibrium SEPARATION and ENERGY For a stable bond to form E(ro) must be NEGATIVE thus m < ntypical values aren = 12, m = 6
6 Intermolecular Forces The origin of intermolecular forcesThe classification of intermolecular forcesVan der Waal’s forceHydrogen bondingExplore an example in depth to show the significance of existence of intermolecular forces.
7 The Origin of Intermolecular Forces It is weak electrostatic force of attraction that exist an area of negative charge on one molecule and an area of positive charge on a second molecule.What causes intermolecular forces?Molecules are made up of charged particles: nuclei and electrons. When one molecule approaches another, there is a multitude of interactions between the particles in the two molecules. Each electron in one molecule is subject to forces from all the electrons and the nuclei in the other molecule.
8 Intermolecular force is weak compared to covalent bond Intermolecular force is weak compared to covalent bond. It is relatively weak interactions that occur between molecules.There are 2 types of intermolecular forces (both of them are electrostatic attraction between dipoles formed by uncharged molecules.)1. Van der Waals' force2. Hydrogen bondingVan der waals’ force is formed by dipoles. There are 3 types of dipoles:1. Permanent dipoles2. Instantaneous dipoles3. Induced dipoles
9 Permanent DipoleThese molecules have a permanent separation of positive and negative charge.A simple example is HCl -The pair of electrons in the covalent bond between hydroge and chlorine isunequally shared due to the difference in electronegativity between hydrogen andchlorine. Chlorine has a greater electronegativity compared to hydrogen and henceChlorine tends to attract the bonded electron pair to itself. chlorine becomes slightlynegatively charged (-), hydroge atom has a partial positive charged (+) .Theunsymmetrical distributed charge on the HCl molecule produces a permanentdipole.
10 Instantaneous Dipole Induced Dipole Instantaneous dipole is due to the fluctuation of electron clouds on non-polar molecules, positive and negative charges exist temporarily.Induced DipoleInduced dipole exists when a permanent dipole or instantaneous dipole comes close to a non-polar molecule, the non-polar molecule will be induced to form a dipole temporarily.
11 Classification diagram of intermolecular forces
12 Dipole-Dipole Interactions Dipole-dipole interactions exist between molecules which are permanent dipole. They tend to orientate themselves that the attractive forces between molecules are maximized while repulsive forces are minimized.In the illustration :the H end of HCl is permanently slightly positive charge. The Cl end of HCl has a permanent slight negative charge, the "H" in one molecule is attracted to the "Cl" in a neighbor.
13 Instantaneous Dipole-Induced Dipole Interactions Also known as London forces or Dispersion ForcesInstantaneous dipole-induced dipole Interactions exist in non-polar molecules. These forces result from temporary charge imbalances. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. The nucleus of one atom attracts electrons form the neighboring atom. At the same time, the electrons in one particle repel the electrons in the neighbor and create a short lived charge imbalance.These temporary charges in one molecule or atom attract opposite charges in nearby molecules or atoms. A local slight positive charge + in one molecule will be attracted to a temporary slight - negative charge in a neighboring molecule.Note: dispersion forces operate in all molecules whether they are polar or non-polar.
14 Dipole-Induced Dipole Interactions Also known as induction force.When a polar molecule approaches a nonpolar molecule, the permanent dipole on the polar molecule can distort the electron cloud of the nonpolar molecule, forming an induced dipole.
15 Van der Waals Radius & Covalent Radius Van der Waals radius is one half of the distance between the nuclei of two atoms in adjacent molecules.Covalent radius is one half of the distance between two atoms in the same molecules.Van der Waals’ radius of a non-metal is always larger than the corresponding covalent radius because the covalent radius because covalent bond is much stronger than van der Waals’ forces.
16 Aided by polar or partial polar covalent bonds. 4) Van der Waals BondsWeakest bondUsually between neutral molecules (even large ones like graphite sheets)Aided by polar or partial polar covalent bonds.Even stable A-A bonds like O2 or Cl2 will get slightly polar at low T & condense to liquid & ordered solid as vibration slows & polarityWeakness of the bond is apparent in graphite cleavage
17 Van der Waals Bondingcreated by weak bonding of oppositely dipolarized electron cloudscommonly occurs around covalently bonded elementsproduces solids that are soft, very poor conductors, have low melting points, low symmetry crystals
18 Hydrogen BondingH+Electrostatic bonding between an H+ ion with an anion or anionic complex or with a polarized moleculesWeaker than ionic or covalent; stronger thanVan der WaalsClose packing ofpolarized moleculesAnionspolarized H2OmoleculeIce
19 Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces.
20 van der Waals Forces Dipole-dipole interactions Hydrogen bonding London dispersion forces
21 Ion-Dipole Interactions A fourth type of force, ion-dipole interactions are an important force in solutions of ions.The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.
22 Dipole-Dipole Interactions Molecules that have permanent dipoles are attracted to each other.The positive end of one is attracted to the negative end of the other and vice-versa.These forces are only important when the molecules are close to each other.
23 Hydrogen BondingThe dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.We call these interactions hydrogen bonds.
24 Hydrogen BondingHydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine.Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.
25 Hydrogen BondingHydrogen bond is a electrostatic force of attraction existing between polar hydrogen(+) and electronegative atom(-) of dipoles.The hydrogen bond is weaker than the covalent bond, but relatively strong compared to van der Waals’ force.Hydrogen bonding is a unique type of intermolecular molecular attraction. There are two requirements. 1. The first is a covalent bond between a H atom and either F, O, or N (These are the three most electronegative elements.) 2. The second is an interaction of the H atom in this kind of polar bond with a lone pair of electrons on a nearby atom of F, O, or N.
26 Hydrogen Bonding in an Ice Crystal Ice has a lower density than water as ice has an open structure. In ice, each molecule is tetrahedral bonded to other molecules by hydrogen bond.
27 Hydrogen Bond in WaterMany other unique properties of water are due to the hydrogen bonds. For example, ice floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. The unique physical properties, including a high heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding.
28 Hydrogen Bonding in DNA Hydrogen bonds play an important role in the ‘base-pairing’ duplication of DNA (A-T,C-G). Matching of the bases produces an accurate duplicate of the original DNA chain.
29 Lennard-Jones Potential Attraction due to instantaneous dipole of moleculesPair-wise non-bonded interactions O(N2)Short range forceUse cut-off radius to reduce computationsReduced complexity close to O(N)
33 Minimum interaction energy and its distance Some Lennard-Jones potential examples of application are listed in the Table. Even this crude interaction model has extensive applications. This model can explain many properties of gases, solids and liquids quite well.Minimum interaction energy and its distancer0(A)(J)He2.2110 -22H22.74Ar3.215N23.713CO24.540
34 The Lennard-Jones Potential The short range 1/r12repulsive termThe long range1/r6 attractive termContributions:Dipole-dipole (including H-bonding)Induced dipoleLondon dispersion attractionu(r)/εεr/σ123
35 Example – Lennard-Jones (LJ) clusters 21/6σETwo atoms:RRRεrepulsiondispersion (van der Waals)Multiple atoms - assume pairwise additive:a123Isomersdifferent minima on potential energy surfacenumber of isomers grows exponentially with # of atomsa and b – permutation-inversion isomersEa = Eb ≠ Ecb132c
36 Lennard-Jones Potential physisorption-DHads < 35 kJmol-1potential energyrepulsive Coulombic interactionsPauli repulsionattractive van der Waals interactionszDHads
37 Intermolecular Potentials Electric dipole ~ r -32 dipoles ~ r -6Repulsive nuclear forces ~ r -12Total Potential = (Attractive) + (Repulsive)“Lennard-Jones 6-12 Potential”Dense Gases, Liquids and SolidsNo long range interactionsInteract via elastic collisions“Hard Sphere” PotentialGasesCarey, V.P., Statistical Thermodynamics and Microscale Thermophysics, New York: Cambridge University Press, 1999.