Interacciones moleculares

1. Interacciones moleculares Interacciones de Van der Waals

2. cy>cx q dif. de cargas r distancia entre cargas m = q . r m MOMENTOS DIPOLARES La separación de cargas parciales crea un dipolo eléctrico d+ d-

3. Electric dipole moments: polyatomic molecules

4. MOLÉCULAS POLARES Y APOLARES DIPOLO TOTAL EN NH3 DIPOLO EN ENLACE DIPOLO TOTAL EN H2O NO HAY DIPOLO TOTAL EN CH4 AUNQUE EXISTEN DIPOLOS EN ENLACES

5. SÓLIDOS MOLECULARES FUERZAS INTERMOLECULARES ENLACES DE VAN DER WAALS • Entre moléculas discretas (dipolos inducidos) • Actúan a larga distancia • No son dirigidas • Débiles - - + + - + - +

6. ENLACES DE VAN DER WAALS VdW (débil) 10-100 Kj.mol-1 Covalente (fuerte) 50-1000 Kj.mol-1

7. - - - - d+ d- - - - + + - - - + Fuerzas Moleculares de Van der Waals (3) • El momento dipolar fugaz (instantáneo, transitorio) se origina por distribuciones asimétricas de las nubes electrónicas alrededor de los núcleos en moléculas no polares. • La moléculas no polares (incluso átomos con capas electrónicas cerradas (Ar)), responden a campos eléctricos (momentos dipolares) de moléculas vecinas, dando dipolos inducidos. + -

8. - + - + + - + - Uniones Intermoleculares de Van der Waals (1) • Los dipolos inducidos se forman igualmente en respuesta a los dipolos permanentes y a los dipolos transitorios. • Los movimientos moleculares reorientan los dipolos en ambas moléculas. • Las uniones intermoleculares de Van der Waals explican la condensación del H2 y del Ar y el estado líquido del benceno a temperatura ambiente.

9. Uniones Intermoleculares de Van der Waals (2) • Dipolo inducido • El dipolo permanente de la molécula de agua produce un dipolo inducido (de menor  de carga) en una molécula contigua (un grupo =CH2 en este caso). • Una molécula o un grupo apolar (como el =CH2) produce dipolos espontaneamente.  - O _ + H H  +  +  - C _ + H H  +  +

10. H H O O H H Interacciones electrostáticas d+ d- d+ d- Entre dipolos permanentes (moléculas polares) C C O O - - - - d+ d- Entre dipolos instantáneos (ej.: Gases nobles) + - - d- - - d+ - - d- d+ + - - Entre dipolos inducidos (ej.: moléculas apolares en agua)

11. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—van der Waals Forces • van der Waals forces are also known as London forces. • They are weak interactions caused by momentary changes in electron density in a molecule. • They are the only attractive forces present in nonpolar compounds. Even though CH4 has no net dipole, at any one instant its electron density may not be completely symmetrical, resulting in a temporary dipole. This can induce a temporary dipole in another molecule. The weak interaction of these temporary dipoles constituents van der Waals forces.

12. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—van der Waals Forces • All compounds exhibit van der Waals forces. • The surface area of a molecule determines the strength of the van der Waals interactions between molecules. The larger the surface area, the larger the attractive force between two molecules, and the stronger the intermolecular forces.

13. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—van der Waals Forces • van der Waals forces are also affected by polarizability. • Polarizability is a measure of how the electron cloud around an atom responds to changes in its electronic environment. Larger atoms, like iodine, which have more loosely held valence electrons, are more polarizable than smaller atoms like fluorine, which have more tightly held electrons. Thus, two F2 molecules have little attractive force between them since the electrons are tightly held and temporary dipoles are difficult to induce.

14. Interactions between dipoles E a 1/r6 => van der Waals interaction E a 1/T => greater thermal motion overcomes the mutual orientating effects of the dipoles at higher T

15. Modelling the total interaction The sum of the repulsive interaction with n = 12 and the attractive interaction given by: is called the Lennard-Jones (12,6)-potential.It is normally written in the form:

16. Modelling the total interaction The two parameters are e (epsilon), the depth of the well, and s, the separation at which V = 0. The Lennard-Jones potential models the attractive component by a contribution that is proportional to 1/r6, and a repulsive component by a contribution proportional to 1/r12

17. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—Dipole-Dipole Interactions • Dipole—dipole interactions are the attractive forces between the permanent dipoles of two polar molecules. • Consider acetone (below). The dipoles in adjacent molecules align so that the partial positive and partial negative charges are in close proximity. These attractive forces caused by permanent dipoles are much stronger than weak van der Waals forces.

18. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—Hydrogen Bonding • Hydrogen bonding typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.

19. Introduction to Organic Molecules and Functional Groups Intermolecular Forces—Hydrogen Bonding Note: as the polarity of an organic molecule increases, so does the strength of its intermolecular forces.

20. Uniones Intermoleculares de Van der Waals (3) • Las fuerzas de van der Waals generan interacciones moleculares que no perturban la reactividad química de las moléculas involucradas. • Reconocidas en el siglo XIX como responsables de las desviaciones del comportamiento ideal de los gases reales ( P = [nRT/V - nb] - [n/V]2 ) y de la cohesión de los gases eléctricamente neutros (como el Argón). • Estas cargas eléctricas interaccionan entre sí y son responsables de la cohesión de la materia, especialmente en el estado líquido y en los sistemas biológicos.

21. Uniones intermoleculares en agua a 30 °C Interacción Tipo kJ/mol Ión/dipolo Na+ ... H2O 60 2 Dipolos permanentesH2O...H2O (estructura del H2O) 20 2 Dipolos permanentes=CO...HN= (unión peptídica)15 Dipolos: permante e inducidoH2O...CH2=10 2 Dipolos inducidos=H2C...CH2= (London)4 2 Dipolos inducidosAr...Ar (London)4