Class Summary 10/15/04

CHM 688 Mid-Term Examination

Monday, 10/25/04 IN CLASS

10/15 Hückel Treatment for Larger Systems

“Frost Mnemonics” get energy levels for n-annulenes

E_j = a + m_jb m_j = 2 cos(2pj/n)

Even n j = 0, ±1, ±2, …, ±n/2

Odd n j = 0, ±1, ±2, …, ±(n-1)/2

For acyclic systems m_j = 2 cos(pj/n+1)

Classification of Hydrocarbons (Alternate/Non-alternate hydrocarbons (star/non-star)

Alternate hydrocarbons have MO’s symmetrically distributed about a

Even Alternate Hydrocarbons (EAH) have same # above and below

Odd Alternate Hydrocarbons (OAH) have sane # above and below, and one on a

Non-Alternate Hydrocarbons do NOT have symmetric MO arrangements

Electron Density

In neutral Alternate Hydrocarbons, each carbon has a total of 1 e^-.

Charge in OAH is NOT evenly distributed, but reflects the properties of the NBMO

Sum of coefficients around a node = ZERO

Using the star/non-stars, one get easily get the coefficients and charges of the NBMO.

References Mentioned

Frost, A.A., Musulin, B. Mnemonic device for molecular-orbital energies J. Chem. Phys. (1953), 21, 572-3.

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