MO s in one-dimensional arrays of like orbitals

Transcription

1 MO s in one-dimensional arrays of like orbitals There will be as many MO s as orbitals in the array. Every molecular orbital is characterized by a specific pattern of nodes, The lowest energy orbital will have zero nodes, the next highest will have one node, the next two, etc. The highest energy molecular orbital will have n-1 nodes, where n is the number of atomic orbitals involved. When symmetrical placement of a node causes it to fall on an atom, then that atom does not contribute to the molecular orbital. E

2 MO patterns in some common systems Allyl π system Benzene π system X 3 5+, 3 5., 3 5 -

3 MO s and reactions of allyls + X - X Three parallel p atomic orbitals + D + D + D ation Neutral (radical) Anion Three parallel p atomic orbitals generates three B type molecular orbitals Formed in equal amounts because the terminal atoms contribute equally to the occupied OMO in the anion. The red and blue carbons represent different carbon isotopes

4 Some extended solids black phosphorus diamond, silicon red phosphorus white phosphorus graphite

5 Large molecules, network solids and metals Very large numbers of atoms results in an even larger number of MO s In extended solids and metals it is useful to consider the MO s of similar composition/energy as bands The density of states in such bands is highest in the middle of the band and lowest at the extremes Depending upon the band gap the solid may be an insulator, a semi-conductor, or a conductor onductivity of semiconductors can be increased by temperature or addition of donor and acceptor atoms p s

6 Density of states in network solids and metals p s lithium diamond p calcium p s silicon s

7 Semiconductor band gaps ompound E gap, ev ompound E gap, ev α-sn 0.0 dte 1.6 InSb 0.2 Se 1.8 PbTe 0.2 u 2 O 2.2 Te 0.3 InN 2.4 PbS 0.3 ds 2.6 InAs 0.4 ZnSe 2.8 ZnSb 0.6 GaP 2.9 Ge 0.7 ZnO 3.4 GaSb 0.8 SrTiO Si 1.1 ZnS 3.9 InP 1.3 AlN 4.6 GaAs 1.5 Diamond 5.4

8 Large molecules, network solids and metals Addition of atoms to silicon that have an extra electron result in electrons in the conduction band; an n-type semiconductor Addition of atoms to silicon that have fewer electrons results in vacancies in the filled band and conduction through movement of the holes ; a p- type semiconductor

9 Light-emitting diodes

10 Single crystal silicon

11 The zochralski Method

12 Why do second row elements form many multiply bonded compounds whereas heavier elements generally do not? 3.35 Å vs vs Si 2 Si 2 n silicon (diamond) graphite N N vs O O vs S S S S S S S S P 4

13 Single and double bond dissociation energies* for some p-block elements, kj mol-1 - = N-N N=N O-O O=O -O =O N-O N=O -S =S inc. for π Si-Si Si=Si P-P P=P S-S S=S Si-O Si=O P-O P=O S-O S=O inc. for π *Modification of table 4.8 from Norman Scaled 3-D contour maps of silicon (left) and carbon p orbitals. The large size of the silicon p orbital results in poor overlap and low electron density both leading to low π bond strength.

14 Observations on bond dissociation energies Decrease in σ bond energies in 2 nd period, carbon through fluorine is a result of lone-pair repulsions on adjacent atoms. Low 3 rd period homonuclear π bond energies result from poor overlap of large 3 p atomic orbitals. Large σ bond energies for 3 rd row bonds with oxygen are largely due to ionic contributions. The maximum is for silicon because its electronegativity is smallest (so it forms the most polar bond with oxygen). Third row π bond energies increase on going from silicon to sulfur because the decrease in size of the element results in greater overlap of p orbitals. Whereas there are clear examples of S-O multiple bonding (σ + π), i.e., SO and SO 2, there are few examples of p π -p π bonds for Si and P. In fact compounds such as ( 3 ) 2 Si=O do not exist because the σ bonded polymer -[( 3 ) 2 Si-O]- (dimethylsilicone) is much more stable. There are a number of Si and P compounds that can be considered to have d π -p π bonds, i.e., SO 3 2-, SO 4 2-, PO 4 3-.

15 Synthesis of first isolable compound containing silicon-silicon double bond Space-filling model of mesitylene; used for substituent West, R., et al., Science 1981, 214, 1343

16 Polynitrogen: single-bonded nitrogen allotrope Scientists have eagerly studied the behavior of molecular nitrogen under high pressure for nearly 20 years, searching for evidence of polymeric nitrogen, a single-bonded form of nitrogen with a structure similar to that of diamond... ompressing N 2 above 110 gigapascals (about 1.1 million atm) and 2,000 K produces a form of nitrogen in which each atom is connected to three neighbors by single covalent bonds. Polymeric nitrogen is expected to be highly unstable because the N N single bond is relatively weak (160 kj per mole). Decomposition to N 2 and formation of the much stronger triple bond (954 kj per mole) would be so exothermic that polynitrogen should have an energy capacity more than 5x that of the most powerful nonnuclear energetic materials. Polymeric phase of nitrogen has a diamond-like structure. Figure from english/pri0804.htm hemical & Engineering News, August 2, 2004, p. 10