Advanced Organic Chemistry (Chapter 1) sh.Javanshir Applications of HMO calculations n...

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


1-5-Qualitative Application of (MOT)

Molecular Orbital Diagram for Methane


Consider methane.

VSEPR gives 4 sp3 hybrid orbitals.

2s

2p

sp3


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.


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


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


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?


Consider methane in a cubic frame of reference (above) a)Atomic orbitals of carbon

b) Molecular orbitals of methane


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)


های اوربیتال بر تاثیری چه ساختار در تغییردارد؟ مولکولی

مولکولی- اوربیتال اختالل )) PMOTنظریه

1-6-Application of Molecular Orbital Theory to Reactivity and Stability


Perturbation Molecular Orbital Theory (PMOT)

Mutual Perturbation

Frontier Orbital Control

Highest Occupied Molecular Orbital (HOMO)

Lowest Unoccupied Molecular Orbital (LUMO)

Symmetry


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.


Fig. 1.28- PMO description of interaction of ethylene andformaldehyde with an electrophile E+ and a nucleophile Nu−.


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


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.


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.


Gilman reagent


• 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:


H

?

H

Symmetry Requirement

Do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another?


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


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


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


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


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


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


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.


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


NC

R

R

H

RR

NC R

RH

R

R

Electron donation from nitrogen lone pair to C-H * orbital.

*


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.

*


Fluoromethanol shows a preference for the gauche conformation


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 Applications of HMO calculations n...

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Transcript of Advanced Organic Chemistry (Chapter 1) sh.Javanshir Applications of HMO calculations n...