MO and VB approaches to chemical bonding in diatomics and Huckel theory for conjugated systems - One Line Questions

1. What is the energy of the lowest unoccupied molecular orbital (LUMO) in butadiene (CH2=CH-CH=CH2) according to Hückel theory, assuming α=0 and β=-2.0 eV? — -2.0 eV
2. What is the bond order of H2+ according to MO theory? — 0.5
3. According to MO theory, the ground state electron configuration of B2 is σ2s² σ*2s² π2px¹ π2py¹. What is its bond order? — 1
4. What is the value of the overlap integral (Sij) between two p orbitals on non-adjacent carbon atoms in Hückel theory? — 0
5. The Hückel rule for aromaticity states that a planar, cyclic, conjugated system is aromatic if it contains (4n+2) pi electrons. For n=1, how many pi electrons are required for aromaticity? — 6
6. Benzene (C6H6) is a classic example of a molecule well-described by Hückel theory. How many pi electrons does it have? — 6
7. What does the term 'Hückel molecular orbital (HMO) method' specifically refer to? — A simplified LCAO-MO method for conjugated pi systems, making specific approximations for integrals.
8. An antibonding molecular orbital in MO theory is typically associated with: — A node between the nuclei, leading to repulsion.
9. In the Hückel MO diagram of benzene, the bonding molecular orbitals are: — One non-degenerate (lowest energy), one doubly degenerate, and one triply degenerate.
10. In Molecular Orbital (MO) theory, what is the fundamental principle behind the formation of molecular orbitals? — Atomic orbitals combine linearly to form molecular orbitals that span the entire molecule.
11. The Valence Bond theory's explanation of the bonding in diatomic molecules like O2 often involves: — Overlap of atomic orbitals to form localized sigma and pi bonds.
12. Which of the following best describes the Linear Combination of Atomic Orbitals (LCAO) approximation used in MO theory? — Each molecular orbital is a linear combination (sum or difference) of atomic orbitals.
13. In VB theory, the concept of atomic orbital overlap is crucial for: — Describing bond strength and directionality.
14. Which of the following diatomic molecules is predicted to be unstable by MO theory due to a bond order of zero? — He2
15. Which diatomic molecule has a bond order of 1.5 according to MO theory? — Li2
16. Pi (π) bonds in VB theory are typically formed by: — Lateral overlap of p orbitals.
17. What is a bonding molecular orbital characterized by in MO theory? —
18. For diatomic molecules from N2 onwards (like O2, F2), the energy ordering of molecular orbitals changes due to: — Interaction (mixing) between σ2s and σ2p orbitals, and between π2p and π*2p orbitals.
19. Which of the following is a limitation of the simple Hückel theory? — It does not consider sigma bonding.
20. What is the approximate energy difference between the bonding and antibonding MOs formed from two identical atomic orbitals? — It is roughly twice the resonance integral (2β).
21. According to MO theory, why is O2 paramagnetic? — It has unpaired electrons in antibonding pi orbitals.
22. In VB theory, sigma (σ) bonds are formed by: — Head-on overlap of atomic orbitals.
23. Hückel theory simplifies the calculation of pi molecular orbitals by setting β = 0 for non-adjacent atoms. This is a direct consequence of: — Assuming that the resonance integral is zero for non-bonded atoms.
24. What is the fundamental difference in how MO and VB theories treat electrons in a molecule like methane (CH4)? — MO theory describes delocalized molecular orbitals, while VB theory uses localized hybrid orbitals.
25. Which of the following is a key difference between MO and VB theories regarding electron distribution? — VB theory localizes electrons between specific atoms, while MO theory delocalizes them over the molecule.
26. For a linear conjugated system with n carbon atoms, how many pi molecular orbitals are formed according to Hückel theory? — n
27. Which statement accurately reflects the prediction of MO theory for the magnetic properties of O2? — O2 is paramagnetic because it has unpaired electrons in antibonding orbitals.
28. In MO theory, the combination of two atomic orbitals can lead to: — One bonding and one antibonding MO.
29. Which of the following features is characteristic of the Valence Bond theory's description of the C=C double bond in ethene? — One sigma bond formed by head-on overlap of sp2 hybrid orbitals and one pi bond formed by lateral overlap of unhybridized p orbitals.
30. For a homonuclear diatomic molecule like N2, which atomic orbitals are typically involved in forming sigma and pi molecular orbitals? — 1s, 2s, and 2p orbitals.
31. In Valence Bond (VB) theory, what is the primary concept for bond formation? — Overlap of half-filled atomic orbitals on adjacent atoms.
32. What type of atomic orbitals are considered in the Hückel approximation for pi systems? — Unhybridized p orbitals perpendicular to the plane of the pi system.
33. Hückel theory is specifically applied to: — Aromatic and conjugated pi systems.
34. The Pariser-Parr-Pople (PPP) method is an extension of Hückel theory that accounts for: — Electron-electron repulsion (electron correlation).
35. What is the Hückel approximation for the interaction between p orbitals on the same carbon atom (Coulomb integral)? — Hii = α
36. The VB theory approach to H2 involves: — The overlap of the 1s atomic orbitals of the two hydrogen atoms.
37. The resonance integral (β) in Hückel theory describes: — The interaction energy between pi electrons on adjacent carbon atoms.
38. The delocalization energy in Hückel theory refers to: — The difference in energy between the delocalized pi system and a hypothetical localized system.
39. The concept of resonance in VB theory is used to describe: — Molecules where a single Lewis structure is insufficient to represent the bonding.
40. In Hückel theory, the Coulomb integral (α) represents: — The energy of an electron in an isolated p orbital on a carbon atom.
41. The Highest Occupied Molecular Orbital (HOMO) in benzene, according to Hückel theory, is: — One of the doubly degenerate orbitals.
42. What does the bond order in MO theory signify? — Half the difference between the number of bonding and antibonding electrons.
43. The Hückel approximation assumes that the overlap integral between p orbitals on non-adjacent atoms is zero. This implies: — There is negligible pi electron interaction between non-adjacent atoms.
44. What role do hybrid orbitals play in Valence Bond theory? — They are formed by mixing atomic orbitals to achieve better overlap and explain molecular geometry.
45. In Hückel theory, what is the fundamental assumption regarding pi electrons in conjugated systems? — They are delocalized over the entire conjugated framework.
46. In Hückel theory, the energy levels of the pi molecular orbitals for a linear polyene with n carbon atoms are given by E_k = α + 2β cos(kπ / (n+1)), where k = 1, 2, ..., n. What is the energy of the HOMO for butadiene (n=4)? — α + 2β cos(2π/5)
47. In the context of MO theory for diatomic molecules, what is the order of energy for molecular orbitals formed from 2s and 2p atomic orbitals in elements like Li2 and B2? — σ2s < σ*2s < π2p < σ2pz < π*2p < σ*2pz
48. In the MO diagram for O2, what is the electronic configuration of the valence shell? — σ2s² σ*2s² σ2pz² π2px² π2py² π*2px¹ π*2py¹
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