Understanding and Predicting Interference

In the same way, the antibonding (asymmetric) combination of the orbitals will interact and form two combined orbitals however, this time the sulfur-dominated orbital is the lower energy orbital. 

Another formulation of this rule looks at the phase of the coefficients on the terminal sulfur atoms. When the coefficients on the two terminal atoms have the same sign between consecutive orbitals (with both orbitals either having the same phase on the terminal atoms or opposite phase), we predict an interference effect [8-10]. One caveat is that this method can fail for even numbers of units with interference features connected in series, for example, two weta-substituted benzene rings together. In this case, the HOMO and LUMO may not indicate that there is interference, when in fact there is. Similarly, method 2 describes well the cyclic systems without side groups, but it may fail to correctly predict interference effects when a cyclic system has side groups that form part of the conjugated system. 

The problem with all these explanations/predictions based on molecular orbitals is that most of us cannot calculate orbitals in our heads. If we have to perform the calculation, it is not so much additional work to calculate the 

This final predictive method is clearly the more versatile it does not require calculation and odd-membered rings pose no challenge. If one were to remember 

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This chapter begins with an introduction to electron transport in organic molecules and how to understand the measurements/calculations. Next, we explore how to understand and predict interference effects and, finally, we go beyond topology and explain how interference effects can be controlled chemically. 

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