Puentes de Hidrógeno

Intermolecular Forces 11.2 Intermolecular forces are forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) Generally, intermolecular forces are much weaker than intramolecular forces. “Measure” of intermolecular force boiling point melting point  H vap  H fus  H sub

Types of Intermolecular Forces 2. Ion-Dipole Forces Attractive forces between an ion and a polar molecule 11.2 Ion-Dipole Interaction

11.2

Types of Intermolecular Forces 3. Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

Covalent Bond and Ionic Bond Ionic bond: electron transfer Covalent bond: electron sharing

Sodium Chloride: ionic bond formation

The geometry of covalent bonds

Types of Intermolecular Forces 4. Dispersion Forces – van der Walls forces/London forces (weakest) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules 11.2 ion-induced dipole interaction dipole-induced dipole interaction

Early evidence of hydrogen bonding 11.2

Types of Intermolecular Forces 1. Hydrogen Bond (strongest) 11.2 The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND. A H … B A H … A or A & B are N, O, or F

Hydrogen Bond 11.2

The strongest H-bonds are linear Hydrogen bonds are directional, with the strongest bonds formed when the –D––HA– atoms are arranged linearly. The histogram below shows the distribution of hydrogen bond angles in crystal structures of small molecules.

H-bonds direction, lengths, angles, and strength H-bond possesses direction and is generally asymmetric Bond strength depends on angle and length Small changes from linearity (20º) have minor effect in strength H-bond strength, on the other hand,decreases exponentially with distance There is a tradeoff between H-bond and covalent bond strength stronger the H····O bond, the weaker the O-H covalent bond, and shorter the O····O distance Weakening of the covalent bond is a good indicator of strengthening of H-bond energy Cooperativity and anti-cooperativity in H-bonding

Hydrogen bonding in water H-bond partly electrostatic (90%), partly covalent (10%) Approximated by the following bonds:  a (A-H B);  b (A - H + ---B: ionic);  c (A - H----B + : covalent);  d (A + H- ---B: ionic);  e (H - A---B + : covalent)  =  a +  b +  c +  d +  e X-Ray spectroscopic evidence suggests that these interactions shift within a femtosecond Nuclear quantum effects strengthen the H-bond by increasing the dipole moment –Donor hydrogen stretches away from its oxygen –Acceptor lone-pair stretches away from it oxygen –Both oxygens being pulled towards each other Cute Movie:

How would you calculate the strength of an H-bond? Between 3 and 9 kcal/mol Dissociation rate constants of 4 X to 2 X 10 6 s -1 from transition state theory R-O-H-----ORH ROH + ROH K eq =[ROH] 2 / [R-O-H-----ORH] But it is more complicated than this for biological systems in water If K eq =1,  G=0; if K eq = 10,  G=1.3 kcal/mol, if K eq = 100, then  G= 2.6 kcal/mol

Different types of H-bonds Common elements that form H-bonds –S, O, N, F CH H-bond RHO----RCOH  + COR’  -cation interaction

Low Barrier Hydrogen bonds (LBHB) H-bond strength depends on its length, linearity, microenvironment, and the pK a values of the the H sharing components H-bonds in water are relatively weak because of pK a mismatch between H 3 O + (-1.7) and H 2 O (15.7) –The proton in the structure is tightly associated with the OH- as a water molecule In gas phase, dielectric constant is low –Hydrogen bonds between heteroatoms with matched pK a values can be  2.5 Å and very strong (25-30 kcal/mol)

H-bonds in Proteins I. H-bonds contributing to structure and folding II. H-bonds contributing to catalysis

 -helix i+4, i+3, i+5 Dipole moment + charge at N-terminii, - charge at COOH Negative molecules (PO 4 2- groups) bind at N-terminii

 -sheets Antiparallel  -sheets atoms in a ring H-bonds linear Stability? Parallel  -sheets 12 atoms in a ring H-bonds not 180  Stability?