Lecture: IntraMolecular Interactions Distinguish between IntraMolecular (within a molecule) and InterMolecular (between molecules) Ron Reifenberger Birck Nanotechnology Center Purdue University 2012 1
This course is about the Atomic Force Microscope Reflected laser spot Laser Diode Four-Quadrant Photodetector A C B D 4 2 Tip Cantilever (Force Sensor) 3 Sample R Tip 5 Simulations Force 1 Sample 2
Week 1 Force Spectroscopy Instrumentation AFM Topographic Imaging Theory Local Material Property Map Tip-Sample Interaction 3
The Origin of Intramolecular Interactions is Electrostatic: Coulomb s Law Classical Picture Point charges ONLY! The charges are stuck down! In vacuum 1 qq F = = k 4πε o 1 2 1 2 2 2 k = 9 10 Nm / C o 12 2 2 9 2 2 qq ε = 8.85 10 C / Nm Force is a vector With dielectric in between, ε o κε o κ (dielectric constant)>1 4
Energy Is Required to Assemble Charges fixed + q 1 If W>0, then YOU are required to do work + F q 2 W F d Electrostatic Potential Energy = U ( ) W For two point charges a distance apart : 1 4πε 1 2 ( ) [ ] U = o qq in J U useful because F( ) = recover Coulomb ' s Law ( ) 5
Electrostatic Potential Energy is a signed quantity end U() Q fixed + + - 1 Qq U ( ) = [ in J ] πε 4 o begin + - q Push +q charge with external force You do work The system does work PE increases PE decreases 6
IntraMolecular (within a molecule) Electrostatic Potential electron cloud nucleus U() or F() electrostatic forces lead to the formation of stable molecules + + e - repulsive e - attractive Summing the four contributions: eq The chemical bond that forms between two atoms is due to an overall net electrical attraction. Positive U() Repulsive force U () du( ) F ( ) = d Negative U() Attractive force U min F local max For 2 molecule near 300K: U min =4.5 ev and eq =75 pm 7
IntraMolecular Forces Lead to Chemical Bonding (where are the electrons?) Chemical Bonding is a Continuum Totally Covalent: equal electron sharing between identical atoms (di)polar Covalent: unequal electron sharing between dissimilar atoms Ionic: complete electron transfer between dissimilar atoms No permanent electric dipole (non-polar molecule) δ + δ - p Permanent electric dipole moment, p (dipolar molecule) + - Net charge transfer (ion) Note: δ + means slightly positive 8
Electronegativity measures the ability of an atom in a molecule to attract electrons L. Pauling - 1930s Note: C and have about same electronegativity hydrocarbons are non-polar http://users.rcn.com/jkimball.ma.ultranet/biologypages/ 9
Simple examples of non-(di)polar covalent and (di)polar covalent molecules schematic electron cloud Symmetrical pile-up of electrons = non-polar molecule F Asymmetrical pile-up of electrons F δ + δ - Characteristic open shell interaction = δ + δ - p (di)polar molecule 10
Molecular dipole moments the general case molecule oriented in space (x n, y n, n ; a n ) Need to know: (x 1,y 1, 1; a 1 ) x δ q = a e 1 a + 1 i i i y i=1 partial charge on each atom specified by: i=17 i=n... p = x δq p = y δq p = δq p = p + p + p... 2 2 2 x i i y i i i i x y i i i Values for (x i, y i, i; a i ) are obtained from quantum chemistry calculation. If Σa i =0 (electrically neutral molecule), then calculation of (p x, p y, p ) is independent of co-ordinate origin. Stick-ball model: http://scienceray.com/biology/carbohydrates-2/ 11
InterMolecular Interactions Because electrically neutral molecules can have a dipole moment, interactions between molecules (InterMolecular Forces) are nonero. These InterMolecular Forces cause molecules to condense and form a liquid or a solid. The strength of intermolecular forces determines such things as the boiling point, surface tension, and viscosity of liquids. 12
Classifying InterMolecular Interactions Ultimately, all intermolecular forces act between charged species: If molecule has net electrical charge - long range Coulomb interaction If molecule has polaried electrical charge dipole-dipole interaction. Electric field from one molecule may induce small changes in electron distribution of nearby molecule induction. Instantaneous dipole associated with rapid electron movement in one molecule becomes correlated with rapid electron movement in another molecule dispersion. 13