Applications of HMO calculations

1. Applications of HMO calculations n = nbr of electron delocalization energy (DE)total pi energy compared to that of a localized reference system charge densityfor a given carbon atom, coefficient squared gives electron density in each MO

2. 1-5-Qualitative Application of (MOT) Molecular Orbital Diagram for Methane

3. 2p sp3 2s Consider methane. VSEPR gives 4 sp3 hybrid orbitals.

4. 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.

5. 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

7. Photoelectron spectroscopy (PES) hv: the energy of the incident photon Ii : the ionization energy for ejection of an electron from an orbital i Koopmans’ theorem i: the orbital energy of the ejected electron

8. LCAO Description of Methane Photoelectron Spectroscopy Photoelectron spectroscopy shows indeed two different ionization energies for methane. ESCA spectrum of methane. So why are there two valence ionizations separated by almost 10 eV?

9. Consider methane in a cubic frame of reference (above) a)Atomic orbitals of carbon b) Molecular orbitals of methane

10. Molecular Orbitals of CH4

11. 1-6-Application of Molecular Orbital Theory to Reactivity and Stability • تغییر در ساختار چه تاثیری بر اوربیتال های مولکولی دارد؟ - نظریه اختلال اوربیتال مولکولی PMOT) )

12. PerturbationMolecular Orbital Theory (PMOT) Mutual Perturbation Frontier Orbital Control Highest Occupied Molecular Orbital (HOMO) Lowest Unoccupied Molecular Orbital (LUMO) Symmetry

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

15. Substitution Effect: Amino and CH2- Groups افزایش فعالیت در برابر الکتروفیل

16. Substitution Effect: Formyl and Ethylenyl Groups • 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. در اوربیتالLUMO اتم کربن b دارای ضریب بزرگتری است و هسته دوست ها ترجیحا با اتم کربن b واکنش میدهند.

19. 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.

20. Gilman reagent

22. 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:

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

25. Another case where orbital symmetry provides a useful insight is ozonolysis. not observed very fast Comparison of FMO interactions of ethene with an allyl anion and ozone.

27. Interaction Between s and p System - Hyperconjugation HückelApproximation: Orthogonally of p and s Framework sp3 carbon atom as subsistent VBT: Hyperconjugation electron donation from sp3 alkyl group to p system فوق مزدوج شدن

28. The two hydrogen AOs of the methyl groups are not in the nodal plane of the p bond and can interact with 2pz of C-2 H I n t e r a c t i o n b e t w e e n h y d r o g e n 1 s o r b i t a l s a n d c a r b o n 2 p o r b i t a l s s t a b i l i z e t h e e c l i p s e d c o n f o r m a t i o n o f p r o p e n e . z More stable

31. 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

32. 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 sand s∗ orbitals on adjacent carbon atoms.

33. Heteroatom Hyperconjugation (Anomeric Effect) in AcyclicMolecules • If one atom with an unshared electron pair is a particularly good electron donor and another a good s∗ acceptor, the n→ s ∗ contribution should be enhanced

35. 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 sC−H bond that donates electrons to be aligned with the s∗ 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 s∗ orbitals

36. Fluoromethanol shows a preference for the gauche conformation

37. Hyperconjugative stabilization is expected to have at least three interrelated consequences: • altered bond lengths; • enhanced polarity, as represented by the charged resonance structure; and • an energetic preference for the conformation that optimizes hyperconjugation.