Upload
jordan-henry
View
217
Download
0
Embed Size (px)
Citation preview
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Applications of HMO calculations
delocalization energy (DE) total pi energy compared to that of a localized reference system
charge density for a given carbon atom, coefficient squared gives electron density in each MO
2 densityelectron jrjr cnq n = nbr of electron
density Charge1 rq
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
1-5-Qualitative Application of (MOT)
Molecular Orbital Diagram for Methane
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Consider methane.
VSEPR gives 4 sp3 hybrid orbitals.
2s
2p
sp3
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Although the four bonds of methane are equivalent according to most physical and chemical methods of detection (e.g., neither the nuclear magnetic resonances (NMR) nor the infrared (IR) spectrum of methane contains peaks that can be attributed to different kinds of CH bonds), there is one physical technique that shows that the eight valence electrons of methane can be differentiated.
In this technique, called photoelectron spectroscopy, a molecule or free atom is bombarded with vacuum ultraviolet (UV) radiation, causing an electron to be ejected. The energy of the ejected electron can be measured, and the difference between the energy of the radiation used and that of the ejected electron is the ionization potential of that electron.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Methods for Construction of MO Diagrams
a) Photo electron Spectroscopy (Ionization Potential; up to 20 eV, for valance electrons)
b) Electron Spectroscopy for Chemical Analysis (ESCA); Binding Energy for core electrons
UV or X-Ray Source:
Binding Energy = Photon Energy – K.E. of The Emitted Electron
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Photoelectron spectroscopy (PES)
ie Ivmh 2
2
1
hv: the energy of the incident photon
Ii : the ionization energy for ejection
of an electron from an orbital i
Koopmans’ theorem
iiI
i : the orbital energy of the ejected electron
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
LCAO Description of Methane
Photoelectron spectroscopy shows indeed two different ionization energies for methane.
Photoelectron Spectroscopy
ESCA spectrum of methane.
So why are there two valence ionizations separated by almost 10 eV?
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Consider methane in a cubic frame of reference (above) a)Atomic orbitals of carbon
b) Molecular orbitals of methane
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Molecular Orbitals of CH4
Cx
y
z
H(2)H(1)
H(3)
H(4)
4 H
s1+s2+s3+s4
s1+s2-s3-s4
s1-s2+s3-s4
s1-s2-s3+s4
a1
t2
t2
t2
C
a1 (2s)
t2 (2px, 2py, 2pz)
t2
a1
-22.3 eV
-11.7 eV
-13.5 eV
2a1
3a1
-25.7 eV(-23, PhES)
1t2
2t2
-14.8 eV(-14, PhES)
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
های اوربیتال بر تاثیری چه ساختار در تغییردارد؟ مولکولی
مولکولی- اوربیتال اختالل )) PMOTنظریه
1-6-Application of Molecular Orbital Theory to Reactivity and Stability
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Perturbation Molecular Orbital Theory (PMOT)
Mutual Perturbation
Frontier Orbital Control
Highest Occupied Molecular Orbital (HOMO)
Lowest Unoccupied Molecular Orbital (LUMO)
Symmetry
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Double BondReaction with Reaction with ElectrophilesElectrophiles
Reaction withReaction with NuclephilesNuclephiles
ReactiveNot reactive
Less ReactiveReactive
H
H
H
H
O
H
H
H2C CH2
H2C O
LUMO
HOMO
LUMO
HOMO
Fig. 1.27: Relative energy of the and * orbitals in ethylene and formaldehyde.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Fig. 1.28- PMO description of interaction of ethylene andformaldehyde with an electrophile E+ and a nucleophile Nu−.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Substitution Effect: Amino and CH2- Groups
H2C CH2
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
H2CHC NH2 H2C
HC CH2
MO energy levels with a-donor substituent.
الکتروفیل برابر در فعالیت افزایش
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Substitution Effect: Formyl and Ethylenyl Groups
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
H2C CH2 H2C CH
HC O H2C
HC
HC CH2
MO energy levels with a-acceptor substituent.
O O
0.580.48
-0.30-0.58
0.59-0.39
-0.48
0.51
HOMO LUMO
Fig. 1.29: Orbital coefficient for the HOMO and LUMO of acrolein.J. Am. Chem. Soc. 95, 4094 (1973)
اوربیتال کربن LUMOدر اتمدارایضریببزرگتریاستوکرباتمباترجیحاهادوستهستهمیدهندواکنشن
•In this case, the MOs resemble those of butadiene. Relative to butadiene, however,the propenal orbitals lie somewhat lower in energy because of the more electronegative oxygen atom. This factor also increases the electron density at oxygen at the expense of carbon.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Les réactions entre Nu et E mous ou entre Nu et E durs sont plus rapides que les réactions entre Nu mous et E durs, vice-versa.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Gilman reagent
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
• Frontier orbital theory also provides the framework for analysis of the effect that the orbital symmetry has on reactivity.
• One of the basic tenets of PMO theory is that the symmetries of two orbitals must match to permit a strong interaction between them. This symmetry requirement, used in the context of frontier orbital theory, can be a very powerful tool for predicting reactivity.
• As an example, let us examine the approach of an allyl cation and an ethene molecule and ask whether the following reaction is likely to occur:
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
H
?
H
Symmetry Requirement
Do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another?
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
HOMO
H2C CH2 LUMO C C
H
H H
HH
Fig. 1.30: MOs for ethylene and allyl cation.
Bonding interactionAntibonding interaction
LUMO of allyl cation
HOMO of ethylene
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Comparison of FMO interactions of ethene with an allyl anion and ozone.
Another case where orbital symmetry provides a useful insight is ozonolysis.
very fast
not observed
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Bonding interaction
LUMO of allyl cation
HOMO of butadiene
Bonding interaction
C
H2C CH2
C
CH2H2C
H
C
H2C
H2CC
CH2
CH2
C
H
HHHH
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Interaction Between and System - Hyperconjugation
Hückel Approximation:
Orthogonally of and Framework
sp3 carbon atom as subsistent
VBT: Hyperconjugation
electron donation from sp3 alkyl group to system
C
H
H
C
H
H
H
H
C
H
H
C
H
H
H
H
شدن مزدوج فوق
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
The two hydrogen AOs of the methyl groups are not in the nodal plane of the bond and can interact with 2pz of C-2
C
H
H
C
HH
H
H
C
H
H
C
H
HH
H
Eclipsed Staggered
Ab initio (STO-3G): Barrier energy = 1.5 - 2 kcal/mol
H
Interaction between hydrogen 1s orbitals and carbon 2pz orbitals stabilize the eclipsed conformation of propene.
H
More stable
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
C C
H
H
H
H
H
H C C
H
H
H
H
H
H
Staggered Eclipsed
3 kcal/mol
More stable
Hyperconjugation was found to contribute nearly 5 kcal/mol of stabilization to the staggered conformation, whereas electron-electron repulsion destabilized the eclipsed conformation by 2 kcal/mol.
Pophristic, V.; Goodman, L. (2001). Nature) 411:565
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
preference for staggered versus eclipsed conformations
• A first step in doing so is to decide if the barrier is the result of a destabilizing factor(s) in the eclipsed conformation or a stabilizing factor(s) in the staggered one.
• The main candidate for a stabilizing interaction is delocalization (hyperconjugation). The staggered conformation optimizes the alignment of the and ∗orbitals on adjacent carbon atoms.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Heteroatom Hyperconjugation (Anomeric Effect) in Acyclic
Molecules• If one atom with an unshared electron pair
is a particularly good electron donor and another a good acceptor, the n→ ∗ ∗contribution should be enhanced
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
NC
R
R
H
RR
NC R
RH
R
R
Electron donation from nitrogen lone pair to C-H * orbital.
*
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
This interaction is readily apparent in spectroscopic properties of amines. The C−H bond that is antiperiplanar to a nitrogen unshared electron pair is lengthened and weakened. Absorptions for C−H bonds that are anti to nitrogen non bonded pairs are shifted in both IR and NMR spectra. The C−H vibration is at higher frequency (lower bond energy) and the 1H signal is at higher field (increased electron density), as implied by the resonance structures. There is a stereoelectronic component in hyperconjugation.
The optimal alignment is for the C−H bond that donates electrons to be aligned with the ∗ orbital. The heteroatom bond- weakening effect is at a maximum when the electron pair is antiperiplanar to the C−H bond, since this is the optimal alignment for the overlap of the n and ∗ orbitals
NC
R
R
H
RR
NC R
RH
R
R
Electron donation from nitrogen lone pair to C-H * orbital.
*
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Fluoromethanol shows a preference for the gauche conformation
Advanced Organic Chemistry (Chapter 1) sh.Javanshir
Hyperconjugative stabilization is expected to have at least three interrelated consequences:
(1) altered bond lengths; (2) enhanced polarity, as represented by
the charged resonance structure; and (3) an energetic preference for the
conformation that optimizes hyperconjugation.
Advanced Organic Chemistry (Chapter 1) sh.Javanshir