Lecture Notes D: Molecular Orbital Theory Orbital plotting applet: http://www.mpcfaculty.net/mark_bishop/hybrid_frame.htm Images below from: http://employees.oneonta.edu/viningwj/chem111/hybrids%20and%20pi%20bonding.jpg and http://winter.group.shef.ac.uk/orbitron/ 1) Atomic orbitals From: http://www.chemcomp.com/journal/molorbs.htm Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 1 of 14
2) Bond formation Bonding between two 1s orbitals in H 2 1s 1s H 2 versus He 2 (definition of bond order) 1s 1s 1s 1s Bond order = (number of bonding electrons number of antibonding electrons)/2 If atom B is more electronegative than atom A A B Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 2 of 14
E Chemistry 09-107: Honors Chemistry 3) Sigma ( ) versus pi ( ) bonding Sigma bonding between s orbitals Sigma bonding between p orbitals Sigma bonding between s and p orbital E Pi bonding E Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 3 of 14
4) Names of hybrid orbitals Steric number Geometry Hybrid orbitals 2 linear sp 3 trigonal sp 2 4 tetrahedral sp 3 5 trigonal bipyramidal dsp 3 6 octahedral d 2 sp 3 5) SN=4, sp 3 hybrid orbitals CH 4 NH 3 H 2 O Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 4 of 14
6) SN=3, sp 2 hybrid orbitals H 2 CCH 2 Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 5 of 14
7) SN=2, sp hybrid orbitals HCCH HCN Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 6 of 14
8) Practice molecules For each of the bonds in the following molecules, give the atomic orbitals that lead to those bonds and the bond type (sigma, pi, etc.) Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 7 of 14
9) Resonance: Butadiene 1.83 1.33 1.83 4 * E 3 * 2 1 Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 8 of 14
10) Resonance: Polyacetylene (the first conducting polymer) * long chain limit empty conduction band filled valence band E g =2 2-1 E width =2 2 + 1 Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 9 of 14
11) Resonance: Benzene http://www.cem.msu.edu/~reusch/virtualtext/react3.htm Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 10 of 14
12) Terminal heavy (non-hydrogen) atom: Diatomic molecules Orbitals on a terminal atom do not hybridize For B 2, C 2, N 2 For O 2, F 2 2p 2p 2p 2p 2s 2s 2s 2s Why does the order switch for O 2 and F 2? pz orbital and 2s orbital are in same region of space, electron-electron repulsions raise the energy of the pz orbital. This effect is weaker for O 2 and F 2, since these more electronegative elements can better accommodate the electrons. Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 11 of 14
Exercise Draw the molecular orbital diagram for C 2 What is the bond order for C 2, C 2 + and C? C 2 a) 1 b) 1 ½ c) 2 d) 2 ½ C 2 + a) 1 b) 1 ½ c) 2 d) 2 ½ C 2 - a) 1 b) 1 ½ c) 2 d) 2 ½ Which of the following are paramagnetic? C 2 a) paramagnetic b) not paramagnetic C 2 + a) paramagnetic b) not paramagnetic C 2 +2 a) paramagnetic b) not paramagnetic C 2 - a) paramagnetic b) not paramagnetic C 2-2 a) paramagnetic b) not paramagnetic Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 12 of 14
13) Diatomic molecules: hydrides HF Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 13 of 14
14) Terminal heavy atoms in organic molecules Lecture Notes D Distributed on: Thursday, September 2, 2010 Page 14 of 14