Molecular Geometry Dr. V.M. Williamson Texas A & M University Student Version Valence Shell Electron Pair Repulsion- VSEPR 1. Valence e- to some extent 2. Electron pairs move as far away as possible to repulsive forces 3. Shape determined by repulsive position of electron pairs to give base or electron geo. MUST consider both lone pairs and bonds. 4. Molecular geo from base or electron geo! If no lone pairs (only bonds), then electron geo molecular geo 5. Molecular geometry describes the position of the bonded atoms around the central atom. Principal originator: R. J. Gillespie in the 1950 s 2 VSEPR: Definition of Terms " Central atom: bonded to more than one other atom " Regions of high electron density: bonding electron pair = region (multiple bonds count = region) lone pair of electrons = region " Electronic geometry: geometry of regions of high electron density around central atom " Molecular geometry: geometry of atoms around central atom 1
Determining Geometry 1. Count the regions of electron density from Lewis structure. (lone pairs and single, double or triple bonds are just 1 region.) 2. Determine the "base" or electron geometry. 3. Determine the number of electron regions that are bonded to another atom. 4. Determine the actual molecular geometry from the "base" or electron geometry 4 VSEPR: Electronic Geometries " To minimize repulsions, situate electron pairs as far apart as possible O 109.5 109.5 VSEPR 6 2
4 regions 5 regions 6 regions 7 Electronic Geometries in 3-D Copyright 1995 by Saunders College Publishing VSEPR: Space Considerations " Lone pairs occupy space than bonding pairs H O >109.5 H <109.5 3
10 VSEPR: Multiple Bonds " Multiple bonds treated as single bonds but they occupy space Predict 120 actually 117 Multiple Bonds 12 4
13 Electronic vs Molecular Geometry " When nonbonding or lone electron pairs are present, electronic and molecular geometries are not identical SF 4 CH 4 SF 4 CH 4 Copyright 1995 by Saunders College Publishing Electronic and Molecular Geometries 4 regions of Electron Density 15 5
5 regions of electron density 16 5 Regions of Electron Density 6 Regions of Electron Density 17 18 6
19 VSEPR: When There is More Than One Central Atom " Determine molecular geometry about each central atom to obtain complete structure of the molecule H H H C C H H H Tetrahedral Carbons Copyright 1995 by Saunders College Publishing VSEPR: Another Example 109 120 lone pair caffeine bonding electrons 7
VSEPR: A Look at Caffeine Copyright 1995 by Saunders College Publishing Predict the shape and the approximate bond angles for the atom numbered 1 and 2. (A) linear, and tetrahedral, (B), 109.5 and, 109.5 (C) angular, and square planar, (D), 109.5 and, 109.5 23 Bond Polarity and Molecular Polarity " Bond polarity can be determined by examining the difference in EN for the 2 atoms in the bond " Molecular Polarity is the sum of the bonds in the molecule. This sum depends on the!! 24 8
25 Measurement of Molecular Polarity Molecular Polarity from Bond Polarity and Geo 27 9
28 Molecular Polarity: Symmetrical Molecular Geometries µ total = 0 Symmetry and Polarity CH 4 CClH 3 30 10
31 Dipole Moment " A measure of the polarity of a molecule " Defined as the product of the magnitude of the partial charges δ + and δ and the distance d separating these charges " Represented by symbol µ in unit of debye (D) dipole moment + - dipoles δ H δ Cl µ = 1.07 D polar molecule Dipole Moment: An Application " The dipole moment of a molecule can be used to provide, indirectly, information on molecular geometry Y X Y Linear Y X Bent Y Y X Y Y X Y Net dipole = 0 Nonpolar Net dipole > 0 Polar 11
Polarity of Water See demo Copyright 1995 by Saunders College Publishing Why is water attracted to this comb? (Try it at home J) 35 36 12
Why polarity?? " Solubility!! " See demo " Likes dissolve " Ionic and polar covalent substances dissolve in polar solvents! Think of ionic as polar extreme!! " Nonpolar substances dissolve in nonpolar solvents " What will dissolve in water? In CCl 4? 37 Valence Bond (VB) Theory (orbital overlap model) " A bond is formed by the overlap of an orbital from 1 atom with an orbital from another atom. This theory allows for mixing of orbitals (hybridization) to fit the shape and energies observed in the molecule. Principal originator: L. Pauling in the 1930 s & 40 s Hybrid Orbital Formation Explains why Carbon will form 4 bonds Carbon = 2s 2 2p 2 Copyright 1995 by Saunders College Publishing 13
40 41 42 14
Why does BeCl 2 form? Be is 2s 2 : 2s 2p In order to make 2 bonds, need 2 places so: Hybrid Orbitals: Linear Geometry 180 degrees 44 Hybrid Orbitals: Trigonal Planar Geometry Why does BF 3 form? B is 2s 2 2p 1 : 2s 2p In order to make 3 bonds, need 3 places so: 15
120 degrees 46 P is 3s 2 3p 3 : 3s 3p To make PF 5 Note: empty, unhybridized d orbitals 90 & 120 degrees 47 S is 3s 2 3p 4 : 3s 3p To make SF 6 Note: empty, unhybridized d orbitals 90 degrees 48 16
Electronic Geometries and Hybridizations of Central Atom High Electron Density Regions Electronic Geometry Hybridized Atomic Orbitals Hybridization 2 linear 1 s, 1 p sp 3 trigonal 1 s, 2 p s sp 2 planar 4 tetrahedral 1 s, 3 p s sp 3 5 trigonal 1 s, 3 p s, 1 d sp 3 d bipyramidal 6 octahedral 1 s, 3 p s, 2 d s sp 3 d 2 50 Predict the hybridization at the numbered atoms in the following molecule. (A) sp, (B), sp3 (C), sp2 (D) sp3, 51 17
Predict the hybridization for # : (A) (B) (C) (D) 52 Sigma (σ) Bond Formation Sigma (σ) Bond Formation 18
55 Pi (π) Bond Formation σ and π bonding 57 19
C 2 H 4 OR 58 Hybrid Orbitals: Compounds with Double Bonds 60 20
Triple Bonding 61 How many sigma and pi bonds are in: Molecule (b)?? (A), 1 (B), 1 (C), (D), 1 Molecule (c)?? (A), 2 (B), 3 (C), 2 (D), 3 62 Benzene C 6 H 6 63 21
Benzene C 6 H 6 64 Benzene C 6 H 6 65 66 22
67 " VSEPR = Repulsion between e- pairs such that distance was maximized and effect of repulsion minimized determines. " Valence Bond = atomic orbitals overlap to form bonds. Atomic orbitals can form. " Both FAILED to explain:! why O 2 was attracted to a magnet (was paramagnetic)! Or why SO 2 had 2 equivalent bond that are an intermediate between a double and single bond. 68 Liquid N 2 and O 2 with a Magnet 69 23
70 Molecular Orbital Theory " MO theory = molecular orbital theory. Assumes that new MOLECULAR orbitals are formed from the atomic orbitals of the bonding atoms. These molecular orbitals can reflect the geom. of the molecule, but are hard to visualize. 71 Consider H 2 Bonding molecular orbitals (MO s) = e- spend most of time between the two nuclei. Antibonding MO s = e- spend most of time on the outside of nuclei. 72 24
MO Energy Level Diagrams H 2 through N 2 NEW filling order σ * 2p 2p π * 2p σ 2p 2p Energy 2s π 2p σ * 2s Molecular orbitals 2s 1s σ 2s σ * 1s Atomic orbitals 1s σ 1s MO Energy Level Diagrams O 2 through Ne 2 NEW filling order σ * 2p 2p π * 2p π 2p 2p Energy 2s σ 2p σ * 2s Molecular orbitals 2s 1s σ 2s σ * 1s Atomic orbitals 1s σ 1s How Is O 2 Paramagnetic? σ * 2p π * 2p 2p π 2p 2p Energy σ 2p σ * 2s Oxygen AO s 2s σ 2s 2s 1s σ * 1s 1s O 2 MO s σ 1s 25