Post on 31-Mar-2015
College 2
Struktuur van Biomolekulen
Wat is ψ(r,t)?
Since we cannot say exactly where an electron is, the Bohr picture of the atom, with electrons in neat orbits, cannot be correct.
Quantum theory describes an electron probability distribution:
39.1 Quantum-Mechanical View of Atoms
39.3 Hydrogen Atom Wave Functions
The wave function of the ground state of hydrogen has the form:
The probability of finding the electron in a volume dV around a given point is then |ψ|2 dV.
39.3 Hydrogen Atom Wave Functions
The ground state is spherically symmetric; the probability of finding the electron at a distance between r and r + dr from the nucleus is:
This figure shows the three probability distributions for n = 2 and = 1 (the distributions for m = +1 and m = -1 are the same), as well as the radial distribution for all n = 2 states.
39.3 Hydrogen Atom Wave Functions
Moleculen:Born-Oppenheimer Approximation
Nuclei fixed in a frame: use04
1
k
Schrödinger equation: rr
R
ke
Rr
ke
Rr
ke
m eeere
222
22
2/2/2
T Vb Va VR
A B
Solutions, for the case of only one potential:
2/1 RrAs
2/1 RrBs
Max Born Robert Oppenheimer
Symmetric and anti-symmetric wave functionsin H2
+
Bs
Asr 11
2
1 Bs
Asr 11
2
1 andDefine:
Two hydrogenic wave functions for 1s orbital: 2/Rrae
BAMO ss 11 BAMO ss 11
Bonding moleculaire orbitalen
Antibonding moleculaire orbitalen
It is not difficult to see that
represents an anti-bonding molecular orbital
BAMO ss 11
Diatomaire moleculen, H2
Fig. 2.1.5. A molecular orbital energy level diagram for orbitals constructed from (1s, 1s)-overlap, the separation of the levels corresponding to the equilibrium bond length.
Fig. 2.1.6. The ground electronic configuration of H2 is obtained by
accomodating the two electrons in the lowest available orbital (the bonding orbital.
Diatomaire moleculen, He2
Fig.2.1.7. The ground electronic configuration of the four-electron molecule He2 has two bonding electrons and two antibonding electrons. It has a higher energy than the separated atoms, and so He2 is unstable.
σ- and π-molecular orbitals
Fig. 2.1.9. (a) the constructive interference leading to the formation of a 2p-bonding orbital and (b) the corresponding antibonding MO.
Fig. 2.1.10. (a) The interference of 2px- or 2py-
AO’s leading to the formation of a 2p-bonding orbital and (b) the corresponding antibonding orbital. Note that for the -orbitals the contribution to the binding energy of a molecule is relatively small
Energy level diagrams
Fig. 2.1.11. The molecular orbital energy level diagram for (2p, 2p)-overlap. While simple overlap considerations suggest the order in (a), the order often found in practice is that shown in fig 2.1.12
Fig. 2.1.12 Variation of the - and -orbital energies of Period 2 homonuclear diatomics.
S en p overlap
Fig. 2.1.13. Overlapping s- and p-orbitals. (a) End-on overlap leads to non-zero overlap and to the formation of an axially symmetric -bond. (b) Broad-side overlap leads to no net accumulation of electron density in the internuclear region.
A measure of the extent to which two orbitals overlap is the overlap integral S
BABA dS *
Heteronuclear diatomairemoleculen: e- verdeling asymmetrisch
BBAAMO cc
The range of bond types, from nonpolar through polar to ionic is captured in MO theory by writing the LCAO as
The proportion of ψA in the bond is cA2, and the proportion of ψB
is cB2.
A nonpolar bond has cA2 = cB
2 and a pure ionic bond has one coefficient zero (so that A+B- would have cA = 0 and cB=1). A polar bond would have unequal, nonzero coefficients.
Hoe deze coefficienten te vinden??The variation principle, states: If an arbitrary wavefunction is used to calculate the energy, then the value obtained is never less than the true energy.
Variatieprincipe
ddHE **
0 AcE 0 BcE and
Hybridization and the Structure of Polyatomic Molecules
Fig. 2.1.16. Hydrogen and lithium atomic energy levels: H1s overlaps with both Li2s and Li2p, and the resulting MO can be viewed as arising from the overlap of H1s with a (Li2s,Li2p)-hybrid orbital. Li1s is a core orbital and plays only a minor role in the bonding.
Welke AO’s te combineren in een MO
Li 2p ligt te dicht in energie bij Li 2s en H1s om te negeren,
Variatie berekening geeft: sHpLisLi 187.0229.0241.0
Hybridization and the Structure of Polyatomic Molecules
sHpLisLi 187.0229.0241.0
Fig. 2.1.17. (a) A cross-section through the (Li2s, Li2p)-hybrid showing the accumulation of amplitude on one side of the nucleus. (b) The H1s-orbital overlaps the hybrid strongly, and a stronger bond is formed than with Li2s alone.
Deze coefficientenoptimale compromis
Hybridization and the Structure of Polyatomic Molecules
Li 2p ligt te dicht in energie bij Li 2s en H1s om te negeren,
Variatie berekening geeft: sHpLisLi 187.0229.0241.0
Simpele atomic orbital overlap idee weg…?
Zie de AO’s van Li als een hybride AO:
sHLihybride 187.05.0
pLisLiLihybride 258.0281.0
Hybridization and the Structure of Polyatomic Molecules
Waarom hebben moleculen bepaalde vormen? H2O driehoek, NH3 pyramideCH4 tetrahedral, CO2 linear?
H2OO:
Dus een basis set vanmet 4 elektronen te verdelen over deze bindingenoverlap elke H1s met een O2p, resulterend in 2 σ-bonds, met elk 2 e,dus:
11222 22221 yxz pppss
BAyx sHsHpOpO 1,1,2,2
22222 221 BAzpss
Maar: hoek van 90o, in werkelijkheid 104o…
Hybridization and the Structure of Polyatomic Molecules
Fig. 2.1.19 (a) p and p’ can be expressed as linear combinations of px and py but the combinations are not orthogonal. (b) the orthogonal hybrids, h and h’, obtained by mixing 2s-character into p and p’.
In the MO description of H2O we aim to construct two O-H bonds that are chemically equivalent, but spatially distinct.
H2O
Fig. 2.1.20 Three orthogonal AO’s hybridize to give three orthogonal hybrids. While the first two are chemically equivalent and each bind an H-atom, the third (dark) is different and in the case of H2O contains 60% 2s-character.
Ignoring the O2s contribution, there is no promotion energy, and moderately good (s,p)-overlap. When O2s-hybridization is allowed, forming the two equivalent hybrids h and h’, the bond strength increases because the overlap improves, but a promotion energy is required because the 2s-electrons now take part in the bonding. The actual shape of the molecule, which is found by minimizing the total energy is a compromise between strong bonding and promotion energy.
Finally, the electronic configuration of the H2O molecule is given by . The two electron pairs that are put in the third hybrid and 2pz0 are called ‘lone pairs’.
2221,'
21,
2 2''1OBA zshsho phs
More hybrids
Bepaal aan de hand van het aantal equivalente bonds de hybridisatie.
Bv drie equivalente bonds met (s,p2) hybridisatie, met (2s, 2px, 2py) als set
yxiiisp
yxiisp
xisp
pps
pps
ps
22
12
6
123
1
22
12
6
123
1
2223
1
2
2
2
More hybrids
)22(2
1
)22(2
1
xiisp
xisp
ps
ps
BeH2 ground state electronic configuration of the Be-atom is 22 21 ss
Koolstof atoom: 222 221 pss
De buitenste 4 valentie electronen kunnen 4 equivalentesp3 hybride orbitalen vormen:
zyxisp
zyxiiisp
zyxiisp
zyxisp
ppps
ppps
ppps
ppps
22224
1
22224
1
22224
1
22224
1
3
3
3
3
Lone e- pairs
NH3
N: atoomgetal 7, dus:
Maak vier sp3 hybrides
3 maken een σ bond met H, met elk 1 e van N, 2 e- over in 4e hybrid = lone electron pair
1112 2222 zyx ppps
221,'
21,'
21,
2 ''1 hsCBA shshsho
Lone pairs: H-bonding
Molecular orbital theory
Molecules with extensive π-bonding systems, like benzene or for instance the photosynthetic pigments chlorophyll a, b-carotene, are not described very well by valence bond theory, because the π-electrons are often not localized in a single bond, but instead are delocalized over the whole molecule.
Fig.2.1.24 Bonding in ethylene. (a) In the plane of the nuclei: the formation of a bond between the carbon atoms 1 and 2 using sp2 hybrid orbitals and -bonds between the 4 H 1s electrons and the remaining sp2 orbitals on each C-atom. (b) Perpendicular to the plane of the 6 nuclei: formation of a -bond between the two 2p-orbitals (that were not involved in the sp2-hybrids.
Voorbeeld ethyleen
Π Electronen houden het molekuul vlak!
Huckel theorieπ electrons do not interact with one another, and so the many-electron wavefunction is just a product of one-electron molecular orbitals. Furthermore it assumes that the structure of the molecule is given by the σ-framework, plus some simplifications gives….
Fig.2.1.25. Hückel molecular orbitals for ethylene. The carbon nuclei are represented by dots, and the nodal planes for the MO’s are represented by the dashed lines.
Excitation energy is 2β (resonance integral)Highest occupied molecular orbital is called HOMO, the lowest unoccupied molecular orbital is called LUMO