Chemistry 1, Fall 2013 Lectures 1516 Chemistry 1 Fall 2013 Lectures 1516 Quantum Mechanics of the Covalent ond LISTEN UP!!! WE WILL E COVERING SECOND PRT OF CHPTER 14 (pp 676688) FIRST You will go CRZY unless you concentrate on the material presented in lecture and homework for chapter 14 animations and links see: http://switkes.chemistry.ucsc.edu/teaching/chem1/www_other_links/ch14_links.htm 1 2 why do atoms form bonds to become molecules? full quantum mechanical treatment solving the Schrödinger equation lonely separated H atoms experiment theory 432 kj/mol bond energy bond length: bond energy: 74 pm 432 kj/mol 74 pm 431.679 kj/mol 74 pm bondlength happy, covalently bonded H atoms QM rules!!! 3 4 molecular orbital (approximation) interaction of atomic orbitals to form molecular orbitals the orbitals for electrons in molecules are described by combinations of atomic orbitals (a.o.s) on the atoms involved in the bond these orbitals (wavefunctions) are called molecular orbitals (m.o.s) our MISSION will be to: understand the nature of the m.o. s, their energies and their electron densities (Y 2 ) fill the m.o. s with covalent bonding electrons to give ground and excited configurations (states) understand the properties of diatomic molecules (bond strength, bond length, and magnetic properties) in terms of these electron configurations and orbital properties 5 atomic orbitals [waves] on the atoms involved in a bond can interact [wave interference] in two ways: add (constructive interference) subtract ( interference) 6 1
Chemistry 1, Fall 2013 Lectures 1516 constructive and interference of ao s constructive interference to form bonding molecular orbital constructive interference of 1s aos (contours) constructive interference of 1s aos (elevation) interference of 1s aos (contours) interference of 1s aos (elevation) originally from: http://www.wellesley.edu/chemistry/chem120/mo1.html 7 The atomic orbitals can add (constructive interference) to form a bonding molecular orbital. Properties of bonding orbital (from constructive interference of a.o.s) the bonding molecular orbital has a lower energy than the two contributing atomic orbitals the electron probability cloud ( 2 ) has a greater electron density between the nuclei than would noninteracting atoms 8 interference to form antibonding molecular orbital constructive and interference of 1s orbital waves The atomic orbitals can subtract ( interference) to form an antibonding molecular orbital. Properties of antibonding orbital (from interference of a.o.s) the antibonding molecular orbital has a higher energy than the two contributing atomic orbitals the electron probability cloud ( 2 ) has a lower electron density between the nuclei than would noninteracting atoms (notice node) Figure 14.25 interference constructive interference 9 10 1s molecular orbitals in hydrogen molecule from handouts for chapter 13 (Dickerson, Gray, Haight) lower electron density higher energy 1s * 1s : cylindrically symmetric around internuclear axis (x) *: antibonding ( interference) lower energy higher electron density 11 1s: from 1s a.o. s 12 2
Chemistry 1, Fall 2013 Lectures 1516 molecular orbital energy diagram (figure 14.28) mo diagrams for He 2 and He2 (fig. 14.30, 14.29) 2 e s configuration: 1s 2 H 2 bond order= ½ (20)=1 (single bond) 13 He 2 (3e s) He 2 (4e s) configuration: ( 1s ) 2 ( * 1s ) 1 configuration: ( 1s ) 2 ( * 1s ) 2 bond order =(21)/2 = 0.5 bond order =(22)/2 =0 no covalent He 2 molecule observed 14 when will two a.o. s interact to form an m.o.?? homonuclear diatomic molecules of the second period two a.o. s must have similar energy (for homonuclear diatomics 1s 1s, 2s 2s, 2p 2p, etc, also 2s 2p to some extent) the two a.o. s must have nonzero overlap (be able to have net constructive and interference; see in a moment) the 1s atomic orbitals on the two atoms interact to give 1s and * 1s molecular orbitals the 2s atomic orbitals on the two atoms interact to give 2s and * 2s molecular orbitals the degree of stabilization of the bonding m.o. and the degree of destabilization of antibonding m.o. depend on the extent of the interaction (overlap) between a.o. s 15 although the 2s has a lower energy than an 2s atomic orbital, the energy of the 2s is higher than the * 1s 16 resulting energy level diagrams for Li 2 and e 2 (fig 14.34, extra) how p x p x, p y p y and and p z p z interact 2s and * 2s differ in energy text uses xdirection for interatomic direction all 2p atomic orbitals have the same energy 2 nd row atoms 1s and * 1s have nearly same energy Li 2 6e s ( 1s ) 2 ( * 1s ) 2 ( 2s ) 2 b.o. = (42)/2=1 e 2 8e s ( 1s ) 2 ( * 1s ) 2 ( 2s ) 2 ( * 2s ) 2 b.o. = (44)/2=0 no covalent e 2 molecule observed 17 18 3
Chemistry 1, Fall 2013 Lectures 1516 Zumdahl figure 14.35 (interaction among 2p a.o.s on different atoms molecular orbitals from atomic porbitals (simple story) from interactions of the six porbitals (3 each from two atoms), six mo s will be formed these 2p mo s will have higher energy than the 2s and * 2s (2p ao s have higher energy than 2s) only the interactions (p x p x, p y p y, and p z p z ) occur (in the simple story) sideon interactions endon interaction 19 endon porbitals (p x p x ) have greater interactions than sidebyside porbitals (p y p y, and p z p z ) 20 how p x p x interact ( endon ) 2p x ao on DD 2p x ao on 2p x ao 2p x ao on on SUTRCT constructive interference interference no node yo node 2p mo * 2p mo from Zumdahl (fig. 14.36) energy constructive 21 22 from handouts (DGH) (endon from 2p x a.o.s) how side on, p y p y and and p z p z interact 2p x constructive node perpendicular to bond 2p x 2p y ao on Ḇ 2p y ao on constructive interference 2py mo 2px * 2px 2p y ao on interference 2p y ao on * 2py mo 23 24 4
Chemistry 1, Fall 2013 Lectures 1516 from Zumdahl (fig. 14.36) p y p y = and * sideon porbitals from 2p y a.o. s (from DGH, see handout) energy constructive 2py * 2py 25 : one nodal plane (which includes internuclear axis) 26 why p x and p y orbitals DO NOT interact text uses xdirection for interatomic direction 2p x and 2p y atomic orbitals DO have the same energy (meets criterion #1) UT constructive _ 2p x ao _ 2p y ao energy of mo s from porbitals (simple case), figure 14.37 endon (p x p x ) interaction is stronger than sidebyside: E 2p < E 2p and E *2p > E *2p there are two pairs of sidebyside patomic orbitals (p y p y and p z p z ): the pairs ( 2py, 2pz ) have the same energy and ( * 2py, * 2pz ) have the same energy 2p 2p This order applies to O 2,F 2, and Ne 2 no net overlap; no net interference; no interaction27 28 life is complicated: 2s 2p x interactions (and 2p x 2s ) summarizing (fig. 14.38 and 14.40) In some atoms, the 2s and 2p orbitals are sufficiently similar in energy that constructive and interactions occur between 2s and 2p x on differing atoms 2p ao s will make contributions to the 2s mo s and 2s ao s will make contributions to 2p mo s the resulting energy level scheme: applies to 2, C 2, and N 2 2s 2p s bonding (constructive ) stabilizes s* 2s 2s 2p s antibonding ( ) destabilizes s 2p with 2s2p mixing figure 14.40 29 simple O 2, F 2, Ne 2 2, C 2, N 2 30 5
Chemistry 1, Fall 2013 Lectures 1516 know properties of 2,C 2, N 2, O 2, F 2, and their ions (fig. 14.41) properties of 2,C 2, N 2, O 2, F 2, and their ions (Silberberg fig. 11.21) 31 32 mo s and properties of homonuclear diatomic molecules (fig 14.41) N 2 diamagnetic O 2 paramagnetic mole cule Li 2 e 2 ( 2s ) 2 ( * 2s ) 2 ( 2 2s ) 2 ( * 2s ) 2 ( 2p ) 2 C ( 2 2s ) 2 ( * 2s ) 2 ( 2p ) 4 N 2 O 2 F 2 Ne 2 configuration b.o ond energy (kj/mol) ond Length (pm) P or D ( 2s ) 2 1 105 267 D 0 0?? 1 290 159 P 2 620 131 D ( 2s ) 2 ( * 2s ) 2 ( 2p ) 4 ( 2p ) 2 3 942 110 D ( 2s ) 2 ( * 2s ) 2 ( 2p ) 2 ( 2p ) 4 ( * 2p ) 2 2 495 121 P ( 2s ) 2 ( * 2s ) 2 ( 2p ) 2 ( 2p ) 4 ( * 2p ) 4 1 154 143 D ( 2s ) 2 ( * 2s ) 2 ( 2p ) 2 ( 2p ) 4 ( * 2p ) 4 ( * 2p ) 2 0 0?? 33 Joanna and Steve http://app.jackyoutube.com/video/kcgeev8qul/liquid%20nitrogen%20vs.%20liquid%20oxygen:%20magnetism.html#_ 34 third row heteronuclear diatomic molecules just like second row but using 3s and 3p orbitals Cl 2 (14 VE s) N O ( 3s ) 2 ( * 3s ) 2 ( 3p ) 2 ( 3p ) 4 ( * 3p ) 4 H F 35 36 6
Chemistry 1, Fall 2013 Lectures 1516 heteronuclear diatomic molecules: heteronuclear diatomic (NO, fig. 14.43) N O same rules for homonuclear m.o.s apply UT now: same a.o.s on two atoms will not have the same energy (still, a.o.s with similar energies combine to form m.o.s) greater 2p on N observations NOT predicatble from Lewis structures NO bond stronger than double bond; b.o= 2.5 unpaired electron resides to a greater extent on N energy of O a.o. s LOWER (but ~similar) to N a.o. s the two a.o. s will NOT contribute equally to a given mo greater 2p on O need to be told use light atom energy scheme 37 11 valence e s 38 heteronuclear diatomic (HF fig 14.45) H F delocalized bonding (p 688): NOT on midterm more H1s than F2p x 1s on H and 2p x on F have similar energies and interfere to form and * mos the occupied has a greater contribution from 2p x on F leading to H F dipole moment H F P 688 Delocalized bonding will be covered after we study hybridization (lectures 1718) and will NOT be on midterm #2 and Z more 2p x on F than 1s on H 1s on H will NOT interact with 2 py or 2 pz on F (no overlap) y,z perpendicular to HF bond P 692 Spectroscopy later (lectures 1920) 6 valence e s 39 40 the floating frog the magnet 41 42 7
Chemistry 1, Fall 2013 Lectures 1516 the frog the frog s OK!!! The Frog That Learned to Fly (Molecular Magnetism and Levitation) originally from: http://www.hfml.ru.nl/pics/movies/frog.mpg 43 44 bond order End of Lectures 1516 bond order = ½ [ no. of bonding electrons no of antibonding electrons] 45 46 Zumdahl fig. 14.33 1s and 1s have little overlap; 1s and * 1s have similar energies 2s and 2s have greater overlap; 2s and * 2s have greater energy difference (splitting) 47 8