lecture 13 10/10/2024 Bond Cleavage & Types: heterolytic bond cleavage: the 2 e-'s that make the bond go w/ one of the atoms (ex. 2 e-'s in σ go to Cl) upon separation costs 474 kcal/mol homolytic bond cleavage: each of the 2 e-'s go to each of the atoms; get two neutral atoms back (ex. I e- to K and I e- to Cl -> K Cl) costs 427 kcal/mol ionic bonds are strong to break until in polar solvent that stabilizes ions upon heterolytic cleavage -> bond broken (E put in) but then bonds formed (E gained) nb orbitals: unused AOs, do not hurt bonding (unlike antibonds) MOs for a Triatomic Molecule (Trihydrogen): H-H-H Sym. model, 3 val. e-'s 1s 1s 1s = 1s orbitals = 0 = 2 nodes = 1 node E = 0 nb nodes 2 e-'s # of AOs mixed = # of MOs made Since the third H. has an available 1s orbital, e-'s will tend to share with the empty spot most advantageous allows e-'s to see all nuclei (no nodes, all in-phase) 1st and 3rd out-of-phase ; bond lengths same Ø effect on bonding regardless of 2nd atom’s phase sym -> thru nucleus ↳ middle AO unused (not in B.O.) one singular MO overall (e- spends half time around each end but never middle AO) bond: 2e- shared btw 2 nuclei ; but here, 2e- shared btw 3 nuclei B.O. ½/pair if e- removed from σnb -> length change "glue" if e- put in σ* -> longer B.O. only involves 2 atoms ↳ ½ time spent w/ pair more nodes, ↑ energy - worse in 3 atoms than 2 desirable absence of e- density, btw. nuclei Additional Notes molecular nb orbitals do not show up in diatomics molecules; only nb orbitals ↳ linear four-atom molecules B.O. can vary btw pairs of the same molecule if one of two e-'s in σ excite to σnb, B.O. as a result of one less e- in σ :1/2-> 1/4 Tetrahydrogen (Tetratomic): H-H-H-H H4 2H2 0 = = destabilizes E = * + + + + = more bonding than antibonding ; favorable to AOs e-'s can no longer occupy AOs (used to form MOs), if AO or MO nbs at low entropy/temp, H4 slightly better than 2H2 bc of σ delocalization localized e-: not confined to a single atom/bond ↳ close in E delocalized e-: confined to a single atom/bond Diagrams Includes diagrams with circles and various lines representing electron orbital diagrams for trihydrogen, tetrahydrogen and beryllium hydride. MO diagram showing energy level differences for tetrahydrogen. Beryllium Hydride (BeH2) (Triatomic w/p orbitals) 4AOs: 1s, 1s, 2s, 2p. 4 val e-. H-Be-H 1s, 2s, 2p mirrored shape nodal plane but no antibonding

CHEM 1810 - Lecture 13

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