1. CHEM 433 - 9/10/09 I. Introductory Topics C. Intermolecular Forces (handout, 18.1.-18.5) Qualitative/Conceptual Issues —> Types of IMF —> Physical basis / strength factors —> Recognizing IMF Read: Sections 18.1-18.5, IM forces handout HW Due: M

2. Two questions to guide our discussion: 1)What are “IMF”? 2)How are they different from Intramolecular forces (aka “Bonds”)? “Water Dimer” An O-H Covalent Bond An “H-Bond” (IMF)

4. A 2-minute H-Bonding Case Study: “HF Dimer”or (HF) 2 R(HF) = 0.919 Å  E(H-Bond) = 3.6 kcal/mol (15.1 kJ/mol) R(H…F) = 1.834 Å (non bond R is ~ 2.7 Å !!!) Donor Bond lengthens: R(HF, “A”) = 0.924 Å Charge rearrangement: - more polarity - partial e- transfer (0.05 e - ) A B Data are from a quantum chemical computation… (MP4/aug-cc-pVTZ)

5. Three key questions for identifying IMF? 1) Are ions present? 2) Are polar molecules present? 3) Are there molecules with OH, NH of FH bonds? What type of IMF are present in the following liquids? CH 3 OH, C 6 H 6, KBr(aq), Kr DISP D-D H-B DISPIon-D DISP D-D H-B DISP D-D H-B